a dominant tectonic signal in high-frequency ... - Geology Journal

Journal of Sedimentary Research, 2009, v. 79, 389–415 Research Article DOI: 10.2110/jsr.2009.038

A DOMINANT TECTONIC SIGNAL IN HIGH-FREQUENCY, PERITIDAL CARBONATE CYCLES? A REGIONAL ANALYSIS OF LIASSIC PLATFORMS FROM WESTERN TETHYS DAN BOSENCE, EMILY PROCTER,1 MARC AURELL,2 ATEF BEL KAHLA,3 MARCELLE BOUDAGHER-FADEL,4 FRANCESCA CASAGLIA,1 SIMONETTA CIRILLI,5 MOHAMMED MEHDIE,6 LUIS NIETO,7 JAVIER REY,8 RUDOLPH SCHERREIKS,9 MOHAMED SOUSSI,3 AND DAVID WALTHAM1 1

Department of Earth Sciences, Royal Holloway University of London, Egham, TW20 0EX, U.K. 2 Departamento de Estratigrafia, Universidad de Zaragoza, Zaragoza, Spain 3 Departe´ment de Geólogie, Universite´ de Tunis El Manar, Faculte´ des Sciences de Tunis, C.P. 2092, Tunis, Tunisia 4 Geological Sciences, University College London, Gower Street, London, WC1E 6BT, U.K. 5 Dipartimento Scienze della Terra, Piazza Universita, 06100 Perugia, Italy 6 Departe´ment de Geólogie, Universite´ Ibn Tofail, BP 133, Kenitra, Morocco 7 Departamento de Geologıá, Facultad de Ciencias, Universidad de Jaeń, 23071 Jaeń, Spain 8 Departamento de Geologıá, E.U.P. Linares, Universidad de Jaeń, 23071 Jaeń, Spain 9 Geologische Staatssammlung, Luisenstrasse 37, 80333, Munich, Germany e-mail: [email protected]

ABSTRACT: Meter-scale, peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation. Three conceptual models, or a combination thereof, are commonly invoked to explain cycle formation; eustasy, tectonics, or autocyclicity. These three models are tested with a large new dataset from different Early Jurassic plate margins from western Tethys. Study of seven logged sections from Spain, Italy, Greece, Tunisia, Morocco, and Gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a Sinemurian time slice. Cycle types are diverse and include shallowing-upward cycles (parasequences) but also deepeningupward and diagenetic cycles (high-frequency sequences) and subtidal cycles. Numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section. Statistical tests (runs tests, time series, and bundling) all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends. Assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle-forming mechanism. However, the variability in numbers of cycles per section, thickness variations of the sections, cycle type variability, and randomness of stacking patterns all favor a pulsed, tectonic control for the creation and filling of accommodation space. This conclusion is further supported by evidence that has largely arisen during the course of this study of syndepositional extensional tectonics in the Sinemurian on these rifted Tethyan margins. Although tectonics appears to be the dominant control, superimposed eustasy and/or autocyclic processes cannot be discounted.

INTRODUCTION

The origin of stacked, high-frequency, peritidal carbonate cycles has been a major unresolved problem in stratigraphy since the first detailed studies on these features in the early to mid 20th century (e.g., Sander 1936; Merriam 1964). Three main process-based conceptual models of their genesis have been recognized in many reviews of this topic (e.g., Hardie 1986; Pratt et al. 1992) to explain their formation: the Milankovitch model and the tectonic model (which together are defined as allocyclic controls), and the autocyclic model. The Milankovitch model proposes that orbitally driven, eustatic changes repeatedly generate changes in accommodation space within which sedimentary cycles can accumulate (e.g., Fischer 1964; Goldhammer et al. 1987; Strasser et al. 1999; Fischer et al. 2004). The tectonic model relies on high-frequency, synsedimentary fault movements to generate these changes in accommodation space (e.g., Hardie 1986; Cisne 1986; De Benedictis et al. 2007), or the ‘‘tectonic dictator’’ of Fischer (1964, p.146). The autocyclic model

Copyright E 2009, SEPM (Society for Sedimentary Geology)

proposes that the migration of tidal flats and islands combined with basin-margin subsidence generates shallowing-upward peritidal cycles (Ginsburg 1971; Pratt and James 1986; Burgess et al. 2001). Whilst many workers have favored one particular mechanism for cycle formation, Bosellini (1967) provided an all-encompassing model that combined all three mechanisms. In contrast, Wilkinson et al. (1997) argued that the cycles may be more apparent than real and that statistical analysis of some published logs indicates that they cannot be distinguished from randomly stacked facies successions. Whilst the problem of whether cycles exist or not can be addressed through careful recording of successions followed by statistical tests, there remain major differences in the approaches, the methods used, and the resulting interpretations that have been made in this field. Much work has been driven by conceptual models, with little attempt to disprove one or more of the three possible mechanisms for cycle generation from preserved cycle types or stacking patterns. In a significant review paper

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on this topic, Lehrmann and Goldhammer (1999, p. 192), conclude that ‘‘From the stratigraphic record, there is no simple key to determining whether parasequences were driven by autocyclicity or allocyclicity.’’ However, Strasser (1991) indicated that autocyclic processes can only generate shallowing-upward cycles bounded by flooding surfaces but cannot account for cycles that show subaerial exposure of subtidal facies at cycle boundaries. Subsequently, symmetric deepening then shallowing cycles were shown to form in modeling experiments on autocyclic processes (Burgess 2006). Considerable effort has been undertaken to date or to apply time-series analysis to ancient peritidal carbonate cyclic successions to test for Milankovitch-band periodicities, but this requires assumptions concerning the length of time of cycle formation (typically assumed to be equal), or constant accumulation rates, neither of which can be established from ancient, shallow-water carbonate successions (cf. Pratt et al. 1992). A renowned case study of the problems associated with this approach comes from the Latemar Platform of the Dolomites, where earlier studies used stage-level dating and numbers of preserved cycles (Goldhammer et al. 1987), and later, time-series analysis of logged successions (Hinnov and Goldhammer 1991) as evidence for eccentricityand precessional-forced (i.e., Milankovitch band) sedimentary cycles. Subsequent work using more precise (single-zircon U-Pb) dating and biostratigraphic marker beds indicates that the cycles are of a much higher frequency than Milankovitch-band orbital controls (Brack et al. 1996; Zu¨hlke 2004), casting doubt on the interpretation of eustatic drivers there. The presence of 5:1 bundling of either cycle thicknesses (e.g., Goldhammer et al. 1987) or of thickness and facies trends (Strasser et al. 1999), has also been used as an indicator of orbitally forced cycles, or of variable subsidence rates (Fischer 1964). However, few workers, with the notable exception of Lehrmann and Goldhammer (1999), apply the required statistical tests to demonstrate the presence of such bundling or stacking trends. In addition, numerical forward modeling of cycle thickness trends produced from a wide range of environmental parameters indicates that with superimposed cycle frequencies large sections of stratigraphy are missing (missed beats), and that 5:1 bundling may not be common (Walkden and Walkden 1990), and in some cases results from aliasing (Procter et al. 2006; Procter 2008). The regular preservation of peritidal cycles has been used as an argument against the possibility of repeated fault movement generating the accommodation space required for cycle formation (Koerschner and Read 1989). However, evidence from neotectonic studies suggests that extensional faults may generate the right amount of accommodation at the required frequency for the formation of stacked peritidal cycles (De Benedictis et al. 2007). Whilst the complexities of cycle formation in shallow marine carbonates has been extensively studied over the last 70 years the review above indicates the outstanding areas of debate concerning the origin of these characteristic features of shallow-water carbonate platform stratigraphies (cf. Pratt et al. 1992). What can be confidently stated is that peritidal carbonate cycles are formed by repeated, high-frequency (and variable amplitude) changes in accommodation that might be generated by eustasy, fault movement, or tidal-flat or island progradation, on a carbonate shelf undergoing regional subsidence. The underlying driving mechanism was noted by Joseph Barrell over 90 years ago; ‘‘the making of a sedimentary series is conditioned by an oscillating but progressive rise in base level’’ (Barrell 1917). The object of this paper is to tackle this problem through the study of platform-top cyclicity within one time-slice but from a number of different shelves and plate margins so that global (i.e., eustatic) sea-level controls can be isolated from local relative sea-level controls (tectonic and autocyclic). If a eustatic control predominates, then its expression should be identifiable in all sections. If local controls predominate (as in the tectonic and autocyclic models), then each section will, to varying extents,

have a different style of cyclicity or stacking pattern. As far as we are aware such a wide, regional analysis of cyclicity from different plates and plate margins within one time slice has not been attempted before even though there are studies that report peritidal carbonate cycles that are correlatable over long distances, but within one plate (e.g., Osleger and Read 1991; Colombie´ and Rameil 2007) as well as from platform-margin to slope to basin transitions (Pasquier and Strasser 1997; Pittet and Strasser 1998). The main outcome of this work is that cycle stacking and cycle types vary from section to section and so a eustatic control on cycle formation is considered unlikely, that the sedimentology of some of the developed cycles argues against autocyclicity, but that the cycle types and their irregular stacking, together with the extensional tectonic setting of the sections, support an overriding tectonic control on cycle formation. THE SINEMURIAN DATASET FROM WESTERN TETHYS

Search for a suitable regional dataset based on continuously exposed sections led to the Lower Jurassic of western Tethys. Here, cyclic, platform-top carbonates are commonly exposed in remnants of the Alpine mountain chains surrounding the present-day Mediterranean Sea. These outcrops represent remnants of large epicontinental or epeiric seas that covered shelf areas of present-day north Africa and western Europe as well as isolated platforms of the Umbria–Marche area of the Apennines and the Pelagonian platform of present-day Greece (Fig. 1). Many of these areas accumulated shallow-water carbonates from the Late Triassic (Hallam 1975; Thierry 2000) with the progressive onlap and accumulation through the Early Jurassic transgression (Hardenbohl et al. 1998; Hallam 2001). This pattern is set against a background of regional subsidence and extensional tectonics resulting in the progressive westward basin formation in western Tethys during the Jurassic (Fig. 1; Ziegler 1990). Previous studies of the origin of the cyclicity within Lower Jurassic peritidal carbonates from this region both conclude that there is an orbital control of cycle development. Crevello (1990, 1991) carried out extensive studies on very well-exposed outcrops of Lower Jurassic shallow-water carbonates in the High Atlas of Morocco and established from stacking patterns and time-series analysis that cycles originated from long-term and short-term eccentricity and precession-driven sealevel changes. Walkden and de Matos (2000) also concluded from the nature of the cycle boundaries, the interpreted time duration of the cycles and comparison with documented cyclicity within time-equivalent basinal facies within the UK that the dominant control on cyclicity developed in the Lower Jurassic of UAE and Oman is eccentricity-related orbital forcing. A different view comes from a broader perspective in that the Early Jurassic is regarded to be within a greenhouse period when highfrequency, orbitally forced, eustatic changes are expected to be of low amplitude (Goldhammer et al. 1990; Wright 1992; Read 1995). With the expected meter-scale amplitude of eustatic changes the question arises as to whether the resulting low rates of sea-level change can impart a recognizable signature to cycles, or, whether other mechanisms predominate. Lehrmann and Goldhammer (1999) in their analysis of 64 datasets of shallow-water cyclic carbonates throughout the geological record suggest that Jurassic cycles show high levels of autocyclicity, as opposed to recording orbital forcing. This results in relatively high levels of randomness in facies occurrence and in cycle stacking patterns. In a previous study of the tectonically isolated (klippe) Rock of Gibraltar Bosence et al. (2000) concluded that either autocyclicity or eustatic changes could account for cyclic Lower Jurassic carbonates. Finally, there is evidence for syndepositional extensional faulting in the continental margins and microplates of western Tethys that increases in intensity from the Early through to Middle Jurassic (Fig. 1; Bernouilli and Jenkyns 1974; Ziegler 1990; Thierry 2000). If this were the case then

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FIG. 1.— Paleogeographic map, facies associations, and paleolatitudes for western Tethys during the Early Jurassic (late Sinemurian). Abbreviations and approximate locations of logged sections (black stars) and other carbonate platforms (black ellipses): AK, High Atlas (Arghbalou n’Kerdous section); AP, Apennine carbonate platform (Monte Bove section); A, Apulia; AT, Atlas Trough; B, Subbetic platform (Loma Prieto section); C, Calabria; CH, Corsican High; CP, Corbie`res Provence platform; D, Dalmatia; G, Gibraltar; IB, Iberian Basin (Almonacid de la Cuba section); MA, Middle Atlas; P, Pelagonia (Chalkis section); TP, Tisza Plate; T, Tunisian Trough (Jebel Aziz section); UM, Umbria Marche Basin (After Thierry 2000).

different locations would be expected to have different thicknesses and stacking patterns. Therefore, a tectonic control on cycle formation cannot be discounted, as it might be for more stable, continental-interior settings. The selection of the studied sections was entirely based on where timeequivalent, very well-exposed cyclic carbonate successions could be found on different plates and plate margins. Fortuitously, the time slice and locations that met these purely methodological criteria contain peritidal cycles that might have originated as a result of the three different models for cycle development. Location and Age Control Seven sections have been selected, dated, and logged to provide the dataset for this study (Appendix and Data Archive, see Acknowledgements section for URL). Selection of sections was not a trivial matter and involved reconnaissance and grading of possible outcrops, followed by dating and the erection of a new foram-based biostratigraphy for the Early Jurassic (Boudagher-Fadel and Bosence 2007). Sections from platform-margin environments (if present) were avoided because these are likely to have a poor record of cyclicity (cf. Crevello 1990) due to accumulation in sand shoal environments with significant depositional relief. Similarly, no totally emergent (terrestrial) or slope and basin sections have been included in this study. Six new sections were identified, dated, and analyzed as part of this study: Loma Prieto, Betics, Spain; Almonacid de la Cuba, Iberian Basin, Spain; Mt Bove, central Apennines, Italy; Chalkis, Evvoia, Greece; Jebel Aziz, Dorsales range, Tunisia; and Aghbalou n’Kerdous, High Atlas, Morocco (Figs. 1, 2; for details of locations and evidence for their age see Appendix 1). One section (Gibraltar) was the subject of previous work (Bosence et al. 2000) and is summarized again here for completeness, even though this section is not of the quality of exposure or completeness of record of the others. These sections are essentially complete and only very minor gaps of few centimeters occur over the hundred or so meters logged in each section (e.g., Fig. 3). Such high-quality sections proved essential for the detailed analysis of cycles that requires all beds and their contacts to be visible and accessible. The carbonate rocks within this region have undergone Alpine deformation and are folded and faulted; where minor faults are found these have been worked around, or correlated across, to re-establish continuity. The only section (in addition to Gibraltar) where this has not been possible is at Loma Prieto in the Betics, and here the Fischer plot

construction assumes an average cycle thickness for any cycles truncated by minor faults, or the preserved cycle thickness, if this is greater than the average thickness (cf. Bosence et al. 2000). To ensure consistency, a common methodology of data recording, and interpretation of facies and cycles, the logging was carried out by the first author working in collaboration with other authors with extensive experience of the local and regional geology. The logging, facies, and cycle interpretation therefore pools the expertise of 10 different researchers across the region. The only exception to this is the Monte Bove section, where the lower quarter was jointly logged (DB and FC) and the remainder by Casaglia (2003/4). The logs are at a scale of 1:50, sometimes 1:25, so that the features of each bed and each surface could be recorded (e.g., Fig. 3). An important part of this work has been establishing the age of the sections. Traditionally, these Tethyan carbonates have proved difficult to date, but some success has been achieved using foram biostratigraphy and 87 Sr/ 86Sr dating (for details see Appendix). New biozones, based on small benthic foraminifera (Boudagher-Fadel and Bosence 2007) enables us to recognize the Hettangian–Sinemurian boundary, the early, mid, and late Sinemurian, and the Sinemurian–Pliensbachian boundary for the first time (Fig. 2). In addition, pristine skeletal calcite (from brachiopods, crinoids, and oysters) has been collected wherever they are present, checked for alteration using CL luminoscope and C and O isotopes, and analyzed for 87Sr/ 86Sr using the mass spectrometers at the Royal Holloway Isotope Laboratories. Age assignments (Table 2; Fig. 2) have been achieved by taking the corrected ratio (together with minimum and maximum 2 3 standard deviation values) plotted against time on Gro¨cke’s (2001) graph of 87/86Sr variations through the Early Jurassic and using the stage ages of Gradstein et al. (2004). Many of the successions become more open-marine up-section, and skeletal material suitable for dating occurs mainly near the tops of the sections. The logs are therefore hung from these strontium dates which identify the Sinemurian– Pliensbachian boundary, together with the less precise, but more numerous, benthic foram biozones (Fig. 2). Facies and Facies Associations Because of the freshness of exposure, or nature of weathering, most facies can be assigned and logged in the field. However, about 900 thin sections were studied for lithological, biostratigraphical, and more detailed facies analysis. In general, the facies observed in the logged

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FIG. 2.—Summary of lithostratigraphy, biostratigraphy, and strontium isotope dates (from Table 2) for Sinemurian sections studied. Time scale is from Gradstein et al. (2004) and foraminiferal biozones are from Boudagher-Fadel and Bosence (2007). Biozones identified as present in each section are labeled in respective stratigraphic columns. For details of stratigraphy see text and appendix.

sections conform to those previously observed and interpreted from Upper Triassic–Lower Jurassic Tethyan peritidal carbonate successions (e.g., Fischer 1964; Colacicchi et al. 1975; Demicco and Hardie 1994; Bosence et al. 2000; Ruiz-Ortiz et al. 2004; Wilmsen and Neuweiler 2008). The facies (Table 1, Fig. 3) are assigned to four broad paleoenvironmentally based facies associations established from their grain composition, biotas, structures, and textures: shallow open-marine (SMFA), shallow restricted-marine (SRMFA), intertidal (IFA), and subaerial (SFA) (Table 1). Most facies associations occur in each of the measured sections, but some facies have more restricted distributions (Table 1). These Tethyan carbonates have varying degrees of dolomitization. The commonest form of dolomite is fine grained and texture-preserving, is restricted to intertidal and supratidal facies (e.g., fine crystalline dolomite of Qing et al. 2001) and does not present a problem with recognition of depositional facies. Sections with extensive texture-destroying dolomite have been avoided for this study, but, where present (e.g., Jebel Aziz, Tunisia), meter-scale pods of such dolomite have been worked around whilst logging to achieve a continuous stratigraphic record. In addition to facies and thicknesses, the logs record transitional bed boundaries and sedimentary surfaces such as sharp and erosional bed boundaries, marine flooding, emergence, and subtidal omission (e.g., Fig. 3). The latter two surfaces comprising facies Nc, Lc, and F-Hg (Table 1) and represent significant gaps in the record. Cycle Definition With experience from previous local studies and thin-section petrography for this study, most facies and bed surfaces could be confidently

identified in the field. Similarly, paleoenvironmental interpretations of lithologies and cycle identification could also be made whilst logging. The term cycle is used here to describe commonly repeated meter-scale sedimentary successions. In all cases, the cycle boundary is taken as the most pronounced erosion surface and/or facies shift at the accommodation minima, whether this be a subaerial exposure surface, an intertidal facies association within subtidal facies, or a subtidal omission surface within deeper-water subtidal facies. Cycles are almost exclusively of meter scale and are commonly bounded by surfaces of emergence associated with non-Waltherian facies shifts, and hence do not follow simple, shallowing-upward facies trends (Fig. 3). However, in detail the cycles are made up of different arrangements of deepening- or shallowing-upward facies associations and bounded by different surfaces, such as subaerial exposure or marine flooding surfaces. Not all of them conform to the definition of a parasequence (Van Wagoner et al. 1988) inasmuch as not all are bounded just by flooding surfaces. In the usage of Lehrmann and Goldhammer (1999) some cycle types are parasequences and others are high-frequency sequences. The revised definition of parasequences by Spence and Tucker (2007) encompasses the range of cycle types found here. Ordered Cycles or Random Successions? Because of the important debate about whether or not interpreted facies trends and cycles can be shown to differ from random arrangements of facies (Wilkinson et al. 1997), all of the logged successions have been analyzed using upward-transition matrices and Markov chain analysis (Davis 2002). These analyses statistically test the

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FIG. 3.—Portions of logged sections to illustrate scale and detail of logging, data recorded, sample locations, interpretations made, and nature of different cycle types a, b, c, d present. (Heights in meters are from base of measured sections).

hypothesis that upward facies transitions are in commonly occurring patterns (cycles) that can be distinguished from a random succession of facies (Fig. 4). Each upward shift from one facies association to another, or to an erosion surface and/or facies shift, is counted and their frequency of occurrence is displayed (e.g., Fig. 4). Facies associations and one surface type (erosion surface and facies shift) were chosen because they represent interpretations of lithologies or surfaces that are common in all of the sections and use commonly accepted paleoenvironmental interpretations (Table 1). Because there are five paleoenvironmental indices, and, because like-to-like upward passages are not recorded, there are twenty possible upward transitions for each of the sections. Of these, only a small number are common in each section and some are very frequent as vertical transitions (Fig. 4). Embedded Markov chain analysis uses the chi2 test to assess whether the transitions are significantly different from a random vertical arrangement of facies transitions (Fig. 4). This is expressed as the P value; being the probability that the recorded series of upward transitions in a dataset could occur by chance in a random vertical arrangement of facies. When all the transitions from all the logged sections are analyzed (cf. ‘‘composite section’’ of Wilkinson et al. 2003) the probability that these 1541 transitions might occur in a random arrangement is essentially zero (2.8 3 102129) (Fig. 4). The very common upward transitions that are significantly different from a random arrangement are the transitions from restricted marine FA to intertidal FA or erosion surface and/or facies shift, and where subaerial FA are overlain by erosion surfaces and facies shifts. These are the shallowing-upward cycles bounded by marine flooding surfaces (Figs. 3, 4). The results from the individual sections are that P values are also all very small numbers (8.9 3 10252 to 1.2 3 1025), indicating that for each of the measured sections the probability that the observed facies association transitions are distinct from randomly stacked facies is very

remote. The results concur with analyses of Wilkinson et al. (2003) that show with increasing number of facies transitions, Markovian order is more likely to be discerned. In addition the small number of faciesassociation states increases the likelihood of order within these successions. The two sections that were identified in the field as having a larger proportion of different cycle types (Jebel Aziz, Tunisia and Chalkis Qu., Greece) than the other sections also have largest P values. However, these are still small (1.7 3 1028 and 1.9 3 1025 respectively) compared to the critical value of 19.7. Cycle Types Five different types of sedimentary cycles are recognized in the measured sections, including one facies alternation, labeled a to e (Fig. 3):

N

N

a cycles are asymmetric, shallowing-upward cycles that may pass from open-marine to subaerial facies associations, or, for example, restricted-marine to intertidal. Cycle boundaries are marked by erosional surfaces at accommodation minima and are overlain by marine flooding surfaces (Figs. 3A–C, 5A, D, 6). They therefore conform to the definition of parasequences (cf. Van Wagoner et al. 1988; Lehrmann and Goldhammer 1999). Such cycles develop from normal regression, and filling of accommodation space that might result from either autocyclic or allocyclic mechanisms. They are the commonest cycle type recorded in this work and overall account for 71% of the cycles within the dataset, as is also found to be the case from many parts of the geological record (e.g., Lehrmann and Goldhammer 1999; Strasser et al. 1999). b cycles are asymmetric cycles that show a deepening-upward facies trend. Cycle boundaries are marked by an erosion surface (e.g., Fig. 5B), overlain by subaerial or intertidal facies (SFA, Table 1) and

SMFA

PBw

PBp-g

Bif Brf Tf-b Onf

IBp-r F-Hg

SRMFA PSw

PSp-g

Ir

Peloidal bioclastic wackestone

Peloidal bioclastic packstone–grainstone

Bivalve floatstone

Brachiopod floatstone

Thaumatoporella floatstone–boundstone

Oncoid floatstone (–rudstone)

Intraclastic bioclastic packstone– rudstone

Firmgrounds to hardgrounds

Shallow Restricted Marine F A

Peloidal shelly wackestone

Peloidal shelly packstone–grainstone

Intraclastic rudstone

Abb.

Shallow Marine Facies Association

Name

B, AK,

G, B, IB, AK, T.

G, B, IB, AK, T.

All sections except AP and P.

IB, T

G, B, IB, AP, P, T.

B, P, AP, T, AK.

.P.

B.

B, AK.

G, B, IB, P, AP, T, AK.

B, IB, P, AP.

Occurs in all sections measured

Occurrence

Dm-thick massive beds, local imbrication.

Massive beds, typically 1–2 m thick. Locally lg-scale x-stratification.

Dm thick massive beds.

Dm thick beds, channel bases, and 1–2 m thick massive beds (common at AP). Mm to cm thick Fe-stained bed surfaces and associated burrow linings commonly overlain by intraclastic or bioclastic layers.

Massive beds (1–2 m thick) local normal grading of oncoids over erosional bed base.

Dm thick beds, massive.

Massive beds, 0.5 to 1.0 m thick.

Typically 1 m thick, massive.

Typically massive beds (1–3 m thick). Also plane bedded and cross-stratified.

Dm bedded, Massive.

Structures

Intraclasts (cm to dm size) of locally common facies (6 black pebbles) in PSw or PSp-g matrix.

Peloidal and minor fragmented molluscs and/or ostracods.

Restricted marine bioclasts. Locally, nonmarine clasts (black pebbles). Peloidal and minor fragmented molluscs and/or ostracods.

Microbial textures (micrite tufts, filament tubes) and peloids in irregular concentric layers to form cm-size oncoids in PBw or PBp-g matrix. Intraclasts (cm to dm sized) of locally common facies (6 black pebbles) in PBw or PBp-g matrix. PBw, PBp-g or Onf with Fe-stained upper surface and/or burrow linings. Locally with encrusting Exogyra.

Thaumatoporella constructions in PBw or PBp-g matrix.

Marine, commonly photic-zone marine bioclasts. Locally shallow marine or non-marine non-skeletal grains (ooids and black pebbles). Peloidal with variable abundance of bioclasts of molluscs, Thaumatoporella, Cayeuxia, Palaeodasycladus, benthic forams, echinoderms, and ostracods. Peloidal (locally with intraclasts, ooids, oncoids, and Favreina) with variable abundance of bioclasts of molluscs, Thaumatoporella, Cayeuxia, Palaeodasycladus, benthic forams, echinoderms, brachiopods and ostracods. Lithiotid, megalodontid or ostreid bivalves (locally in situ) in PBw or PBp-g matrix. Brachiopods in PBw or PBp-g matrix.

Components

Interpreted as deeper-water shelf facies (below FWWB) as at TT and IB occurs as facies transitional to drowning Moderate to high energy events (bed bases or channel bases) or ambient conditions at AP. Cessation of sedimentation and progressive submarine lithification of substrates. Associated coarse bioclastic sediment accumulation. Shallow subtidal shelf; restricted marine. Subtropical photozoan. Low-diversity assemblage in sheltered, shallow, restricted marine environment. Platform interior to lagoon. Low diversity assemblage in moderate energy, shallow, restricted marine environment. Platform interior/ lagoon. Moderate to high-energy events (bed and channel bases) in restricted shallow marine setting.

As for PBw or PBp-g above but abundance of brachiopods suggests most open marine facies Interbedded with PBw or PBp-g above and probably similar conditions

As for PBw or PBp-g above.

Mud-supported texture with normal marine biota indicates deeper or more sheltered setting than facies below. Grain-supported texture with locally preserved planar and cross stratification. Photic zone grains indicates moderate energy shelf (above wave base?).

Shallow subtidal shelf; normal marine. Subtropical photozoan.

Interpretation

TABLE 1.— Description and interpretation of facies and facies associations from all localities (G, Gibraltar; B, Lomo Prieto, Betics; IB, Almonacid da Cuba, Iberian Basin; AP, Monte Bove, Apennines; P, Chalkis Quarry, Pelagonia; T, Gebel Aziz, Tunisia Trough; AK, Aghbalou n’Kerdous, High Atlas).

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Occurrence

B. IB, P, T. P, T.

Ms FPm-w FPSp Ir SFA Nc Lc Eps Fe-Br I-r Rsh

Stromatolite

Fenestral peloidal mudstone– wackestone

Fenestral peloidal bioclastic packstone Intraclastic rudstone

Subaerial Facies Association

Nodular calcrete

Laminar calcrete

Evaporite Pseudomorphs

Fe-rich crackle breccia

Intraclastic rudstone

Red shale

AK

P, T, AK

IB, T, AK.

IB, AK.

B, AP, P, T, AK.

B.


G, B, IB, AP, T AK.

G, B, P, T, AK.

G, B, IB, AP, P, T, AK.

Ml


Microbial laminite

Abb. IFA

Name

Intertidal Facies Association

Laminated and locally burrowed. , 20 cm thick.

Cm-size nodules in cream-khaki clay over brecciated and/or microkarstic surface Laminar, sheet cracks, brecciation, tepee structures, fenestrae. Calcite and/or sediment-filled pseudomorphs of evaporite nodules and crystals. Unbedded coarsely crystalline Fe carbonate plus breccia and laminites. Erosional base within calcretes.

Erosional base normal grading. Dm thick beds.

Dm to m thick beds with crinkly lamination and fenestrae, local mud-cracks and flat pebble concentrations. As above but organised into cm high domes (10–20 cm across) Early sediment-filled geopetal fenestrae with minor Fe staining and brecciation. As above.

Structures

TABLE 1.— Continued. Components

Intraclasts (locally black pebble), pisoids, peloids in pack-grainstone matrix. Clay minerals and quartz. Locally with pseudomorphs of evaporite nodules

Calcite, dolomite, dedolomite crystalline masses.

Anhydrite nodules, anhydrite and gypsum crystal pseudomorphs.

Intraclasts, peloids, pisoids.

Peloidal and micritic with coated and bioclastic grains. Intraclasts (locally black pebble) in packgrainstone matrix. Peloids, pisoids, and intraclasts with various authigenic carbonate and sulfate minerals Micrite nodules with cracks, coated grains, intraclasts

Peloidal and micritic.

As above.

Irregular microbial (tufts, vugs, filament molds) and peloidal fabrics.

Skeletal, microbial, and non-skeletal grains (intraclasts and peloids)

Interpretation

Intervals of increased siliciclastic supply into carbonate margin. Flood plain or coastal plain.

Brecciated surfaces altered in a vadose environment. Related to subaerial exposure. Erosional events within paleosols.

Subaerial exposure in semi-arid to arid environment. Subaerial exposure in arid environment.

Subaerial exposure in humid to semihumid environment.

Erosional events/surfaces in intertidal setting. Supratidal setting in humid to arid climatic settings

As above.

As above.

As above.

Intertidal setting in subtropical environment with associated photozoan carbonates Partially sediment-filled, and early cemented fenestrae, with local mud cracks indicate intertidal setting.

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FIG. 4.—Markov-chain analysis of upward transitions of facies associations and surfaces in logged successions (Fig. 2). The Tables in A to G record the number of upward transitions from one facies association or surface (left column) to the overlying unit (top row). Table A contains all the data pooled from all the sections, and B to G are data from each individual measured section. The flow diagrams (B to G) indicate the commonly occurring facies transitions within the sections with their frequency values (as percentage of each upward transition). These are several to many standard deviations away from the random value. The results of the embedded Markov-chain analyses for the entire dataset and each individual section are also given (for discussion see text).

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then a deepening-upward facies trend (Fig. 3B; cycles 12, 17, and 22, Fig. 5B, C). These cycles are essentially the same as cycles described, illustrated, and interpreted in detail by Fischer (1964, his figs. 4, 6, and 7) as ‘‘Lofer cyclothems’’ from the Triassic of the Alps. Subsequent workers (Goldhammer et al. 1987; Satterley and Brandner 1995) interpreted these as shallowing-upward cycles (a type, above) but without the detailed evidence provided in Fischer’s original (1964) work. Cycles with this arrangement of facies and surfaces cannot be produced by an autocyclic mechanism and require an absolute sealevel fall, or uplift (i.e., an allocyclic mechanism) to subaerially expose the subtidal facies. Cycle boundaries are commonly expressed in subaerial calcretes with associated erosional, locally microkarstic, surfaces. Such surfaces may represent missed beats of relative change in sea level. They are overlain by intertidal then subtidal facies in a deepening-upward trend. Erosion developed during the transgressive phase is common between the intertidal and subtidal facies (Fig. 5B, C) and this is interpreted as resulting from shoreface erosion, or ravinement during transgression (cf. Wright 1986), and not as a cycle boundary. This surface does not conform to the definition of a cycle boundary (above) in that it does not represent the accommodation minima. c cycles have a symmetric arrangement of facies with a transgressive phase, an interval of maximum marine flooding, followed by a regressive phase capped by an erosional surface (Fig. 3B cycle 13 and 3C cycle 25). Intertidal and subaerial facies associations are generally more fully developed in the regressive phase, with thick calcretes with tepee structures in the Monte Bove section. The cycles are symmetric in terms of facies arrangement even though there is variability in the thickness of the deepening or shallowing phases of different cycles (cf. Crevello 1990). Some c cycles do not have a significant erosion surface or facies shift at the cycle boundary. In this case the cycle boundary is still taken at the accommodation minima indicated by the shallowest preserved facies. c cycles, like a cycles, may be generated by either autocyclic or allocyclic mechanisms inasmuch as the preservation of the transgressive phase depends on the balance among generation of accommodation space, production, and erosion rates. d cycles are asymmetric and comprise a single sedimentary facies with a subaerial diagenetic overprint affecting its upper surface. In the Sinemurian examples, microkarstic or dolomite breccias typically alter underlying marine facies (Fig. 3C cycle 27). The supratidal or subaerial facies is then cut by an erosion surface and facies shift to another subtidal unit. These are similar to the ‘‘diagenetic cycles’’ of D’Argenio et al. (2004 and references therein) and Hardie (1986) in which subtidal facies are repeatedly subaerially exposed and altered but lack an intervening intertidal facies that would result from simple regression. These cycles may only be generated by an absolute sealevel fall driven by eustasy, or by tectonic uplift, and again may hide missed beats of relative sea-level change in the succession. e cycles, or, alternations, occur where only one facies or facies association is preserved that is cut by a subaerial exposure surface or other surface of accommodation minima. This surface is then overlain by another, or the same, facies or facies association. These alternations are included here and in the logs as cycles (e.g., Fig. 3B, cycles 11 and 21) for completeness because they may represent, as with the cycles above, repeated fluctuations in meter-scale accommodation space. This cycle type also includes those that are purely subtidal in origin (Fig. 3D). These have a deepening-up facies trend (contra ‘‘subtidal cycles’’ of Osleger 1991) within the open marine facies association with a superimposed omission surface or hardground. This is followed by another deepening-upward facies trend within this facies association. The combination of subtidal omission surfaces and deepening-upward trends repeated through many cycles (Fig. 3D) is interpreted to result from a relative fall followed by relative rise of sea

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level rather than local changes in current regime. Being subtidal, these cycles can be generated only by allocyclic controls. The Logged Cyclic Sections The logged sections are presented as modified Fischer plots (Fischer 1964; Sadler et al. 1993) to illustrate cycle thickness variations. Because cycles typically fill to sea level, accommodation trends are illustrated on these plots, although these values are relative because cycle thicknesses have not been decompacted. Some sections have wholly subtidal portions, in which case thickness variations do not relate to accommodation space (cf. Osleger and Read 1991), but these are shown here for consistency. The plots are also used to document the cycle-by-cycle variations in cycle types and occurrences of paleoenvironmentally significant lithologies and biotas (Fig. 6). The characteristic facies associations found within each measured section (see also Table 1) and interpretations of cycle trends are given below: Loma Prieto, Betics (Fig. 6).—This 117.5-m-thick succession comprises 98 cycles, of which shallowing-upward (a) cycles are by far the dominant (87%) cycle type (Figs. 3A, 5A, D, 6). A small number of cycles are recorded that are much thicker than average (Fig. 6, cycle numbers 36– 41). These are types a and c, suggesting that preservation of the transgressive phase in these cycles relates to large increases in accommodation space. Several long-term trends can be seen on the Fischer plot (Fig. 6); initially a falling trend marking thinner than average cycles through to cycle 25, and then an abrupt rise with the 5 unusually thick cycles in which the thickness is developed in marine facies associations. This is followed by a level trend with bundling of cycles in irregular sets of up to 9 cycles per bundle (e.g., cycles 50 to 59). No regular 4:1 or 5:1 bundling by cycle thickness or by facies is observable in the section. Two possible examples of such bundling occur in cycles 25 to 35, where thinner cycles (with restricted marine facies) pass up section to thicker cycles with more marine facies, but this is not repeated elsewhere in the succession. The subtidal facies are typically peloidal shelly packstones–grainstones (PSp-g) or, less commonly PBp-g (Table 1). These pass up to intertidal microbial laminites (Ml) with locally developed fenestrae. Intertidal facies pass up to subaerial nodular calcretes (Nc) (Figs. 3, 4, 6, Table 1). Both of these latter facies occur throughout the succession and show no correlation with decreasing accommodation as represented by falling or level trends in the Fischer plot (Fig. 6). Open-marine allochems (foraminifera, echinoderm plates, and brachiopods) occur locally throughout the succession. When viewed against evolution of accommodation space, as approximated by the envelope of the Fischer plot (Fig. 6), there appears to be no relationship between marine allochem abundance and cycle thickness, indicating that increased accommodation space did not translate to deeper waters but that space was filled with sediment as it was generated. Almonacid de la Cuba, Iberian Basin (Fig. 7A).—This continuous section comprises 42 cycles; 24 peritidal followed by 18 subtidal cycles. The lowest 8 are a cycles comprising restricted marine facies (PSp-g, PSw), and intertidal Ml facies. These cycles are thinner than average, suggesting long-term highstand to falling-stage conditions, or more rapid sedimentation rates (Burgess 2001). Subaerial facies include peloidal packstones with anhydrite pseudomorphs (Eps) and Fe-rich crackle breccias (Fe-Br). Both these facies indicate more arid conditions than were prevailing in the southern Iberian margin at this time where nodular calcretes were developing. Cycle 9 contains a well-developed intertidal channel fill with basal erosion surface, lenticular geometry, and inclined, lateral-accretion surfaces with anhydrite pseudomorphs. Succeeding a

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cycles become more open marine, with foraminifera, echinoderm plates, brachiopod debris and ooids, all of which shallow-up to Ml intertidal facies. These cycles are thicker than average, indicating that pulses of accommodation gain coincide with marine flooding, or slower sedimentation rates in an autocyclic system. Above cycle 24 the cycles are subtidal e cycles (Fig. 3D). Cycle boundaries are marked by omission surfaces (FHg, Table 1) with prominent burrowing (Thalassinoides) and local oyster encrustation and Fe mineralization. These are overlain by intraclast and skeletal-rich rudstones and floatstones that fine upwards through peloidal bioclastic packstones (PBp) to PBw (Table 1). These facies trends are interpreted as deepening-upwards and subsequent shallowing generates sea-floor erosion and colonization by firmer-substrate macrobenthos. Because these cycles do not fill to sea level they cannot be used as indicators of accommodation. The cycles are interpreted to have formed in a mid ramp environment and in just one case (top cycle 30) there is shallowing into the intertidal zone (desiccation cracks in wackestone). Much of the overlying Pliensbachian comprises shelf carbonates of the Rio Palomar and subsequent formations (Go´mez et al. 2003), indicating that this platform does not show complete drowning like many others of this age from the Tethyan area (Fig. 2). Monte Bove, Central Apennines (Fig. 7B).—The 51 high-frequency cycles recorded in this complete section are mainly a cycles with just three symmetric c cycles. Uniformity is also the motif of the facies that comprise these cycles. The subtidal portions are dominated by meterthick beds of massive, intraclastic bioclastic packstones–rudstones (IBpr) with a fully marine biota (Fig. 7B) and common oncoids. Both of these facies indicate moderate- to high-energy, open-marine conditions as might be expected in isolated platforms sited within the Tethys Ocean (Fig. 1; Bice and Stewart 1990; Santantonio 1993, 1994). This is the most open-marine succession within this study, with records of ammonites and corals (Casaglia 2003–4). Intertidal facies are characterized by fenestrate Ml and FPm-w facies (Table 1), and supratidal facies have well developed sheet cracks, meter-scale tepee structures, reddish vadose silts, and dissolution vugs that penetrate through the underlying marine beds. Neptunian dikes are also common in the succession (Casaglia 2003–4), indicating synsedimentary tectonics, but only one is recorded cutting through the logged section (Fig. 7B). The first 11 cycles are thinner than average and comprise a and just 2 c cycles that shallow-up to mainly intertidal environments; supratidal facies, where present, are relatively thin. Two thick cycles (11 and 14) largely comprise subtidal IBpr facies with intertidal and supratidal caps. The subsequent falling and rising limbs through to the end of the section show no major changes in cycle type or compositional trends. Chalkis Quarry, Evvoia, Greece (Fig. 8).—The record from this continuous, cyclic, peritidal section shows long-term fall then rise, suggesting low-order highstand and falling stage followed by a transgressive phase, or variations in accumulation rates. The 101 meterscale cycles (average 1.06 m thick) show variability in style, with a cycles being the most common (49%) but with significant proportions of b (15%), c (24%), and d (12%) cycles (Fig. 8). Neither the cycle types nor the lithologies and biotas appear to show any relation to the long-term trends in the Fischer envelope. The small numbers of thick (. 2 m) cycles show no evidence of more open-marine biotas. a cycles start with subtidal portions commonly with PSp-g but also PBp-g, Bif, Onf, and particularly well-developed Tf-b (Fig. 3B). These latter microbial fabrics are not found in any of the other sections. Generally, open-marine skeletal grains are absent. These facies grade up-section into fenestral, and locally oncoidal, Ml, intertidal facies, prior to erosion, marine flooding, and return to subtidal conditions (Fig. 3B). b cycles occur at intervals throughout the section. Cycle boundaries erosionally overly subtidal facies and are either microkarstic surfaces overlain by laminated, or

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brecciated, pink to brown micritic calcretes (Fig. 5C), or erosion surfaces overlain by Ml intertidal facies (Fig. 5B). These grade up-section into subtidal facies similar to those of a cycles. Symmetric c cycles start with erosion surfaces followed by intertidal laminites that grade up into marine facies that record the highest-energy and most open-marine waters found in this succession. Evidence for this comes from the occurrence of oncoid floatstones (Onf) and IBp-r facies, prior to shallowing up to intertidal fenestral laminites. Jebel Aziz, Dorsales Axis, Tunisia (Fig. 9A).—This 213-m-thick succession is characterized by relatively thick (average, 3.02 m) peritidal cycles (0–159 m, cycles 1–52) followed by 54 m (159–213 m) of a noncyclic, subtidal succession. The cycles comprise type a (58%) together with cycle types b (9%), c (29%), and d (4%) (Fig. 9A). The cycles develop on a low-order falling trend, with no visible bundling of cycles. The shallowing-upward a cycles are characterized by restricted marine subtidal phases passing up through fenestral laminites (Ml) to well developed laminar calcretes (Lc) with sheet cracks, brecciation, and internal sediment including vadose pisoids. In addition, five of the shallowing-upward cycles (cycles 1, 18–20) can be identified as channel fills with basal erosional surfaces, intraclastic rudstones (Ir) channel lags, and inclined laterally accreting beds which are commonly graded and locally slumped. These point-bar deposits shallow-up to intertidal and subaerial facies associations. A series of 4 a cycles at the top of the section show rapid increase in thickness to cycle 51 at 31.1 m thick, Fig. 9A). These comprise thick subtidal portions of peloidal–molluscan–dasyclad packstones, and oncoid floatstones and local oncoid rudstones associated with winnowed, subtidal surfaces. These are followed by 54 m of noncyclic, subtidal, oncoid-rich successions of packstones, floatstones, and rudstones that fine up to peloidal wackestones. The top of the section (and the Oust Fm) is marked by medium-bedded, dark micrites, with reddened partings and elongate chert nodules. These are interpreted as hemipelagic muds and mark the final drowning of this Sinemurian platform. Because of their location prior to platform drowning the 4 thick cycles at the top of the cyclic portion of the succession (cycles 48–51) appear to record increasing accommodation space that is filled to the intertidal zone (they do not exhibit subaerial facies) prior to the start of platform drowning. b cycles occur near the base and top of the section and start with erosional surfaces which are overlain by intraclastic rudstones containing clasts of intertidal or supratidal facies (generally the underlying lithology). These grade up-section to subtidal restricted marine PSp-g facies. c cycles exhibit facies typical of the Jebel Aziz a and b cycles (above) but arranged in a symmetric, deepening-up then shallowing-up trend. d cycles occur in the lower half of the section (Fig. 9A) and comprise a restricted marine subtidal unit that is superimposed and altered by brecciation and dolomitization that is associated with iron staining (Fe-Br, Table 1). The succession shows evidence of syndepositional tectonics in the form of neptunian dikes synsedimentary faults, and slump folds (Fig. 5E, G). Arghbalou n’Kerdous, southern High Atlas Morocco (Fig. 9B).—This second example from the Maghrebian margin is also thick (244 m), and also has relatively thick (average 3.26 m) peritidal cycles. The Sinemurian section could be thicker, because the late Hettangian biozone (S. gibraltarensis) has not been recognized at the base of the outcropping succession. This section was logged and the facies and cycles were described by Crevello (1990, 1991) but neither the nature nor the origin of the cyclicity in this section was not specifically discussed. The long-term Fischer envelope shows an initial decreasing trend followed by a major portion of the section dominated by a cycles of average thickness (Fig. 9B). A final rising then falling trend marks the upper third of the section. Counterintuitively, these final thinner-than-average cycles, like those at the base of the section, are marked by more open-marine biotas.

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These include oyster-rich floatstones (Bif, Table 1) in cycle 75 that give the consistent Sinemurian–Pliensbachian boundary Sr isotope date at the top of this section (Table 2). The initial d cycle has an open-marine subtidal portion with superimposed subaerial facies. This comprises a laminated red shale, which is not found in any other section and is taken to reflect the proximity of this site to the marginal clastics of the Saharan Craton (Fig. 1). Supratidal facies also occur as Fe-rich crackle breccia (Fe-Br), with centimeter-size, ellipsoidal nodules of coarse calcite spar set in reddened dolomitic intraclast floatstones. These are interpreted as pseudomorphs after anhydrite nodules (Eps, Table 1). This facies, which is not recorded from any other of the 6 sections studied, indicates an arid climatic setting. The lower to central part of the section is dominated by a cycles characterized by thick (commonly 2–5 m, but up to 14 m in cycle 65) subaerial sections of dolomite with laminar calcretes with sheet cracks and pisoids and particularly well-developed tepee structures (Fig. 5H). Thinner intertidal portions comprise fenestral microbial laminites. Skeletal allochems are mainly gastropods and (locally) dasyclad algae in PBp-g indicating restricted-marine conditions. Recognition of cycle boundaries in thick subaerial sections is based on the presence of subtidal facies, but the abundance of erosional surfaces within these thick subaerial calcretes suggests that there are many missed beats within this section. These equate to the ‘‘pisoid–ooid tepee-bedded cycles’’ of the restricted marine inner-platform facies belt of Crevello (1990; p. 119, 123, 147, and fig. 22). Open-marine biotas return in the last 10 thinner-thanaverage a cycles with benthic foraminifera of the Lituosepta recoarensis biozone. Main Ridge, Gibraltar (Fig. 10).—This section is described in detail in Bosence et al. (2000) and is included here only for comparison. Much of this section is now thought to be obscured by extensive building work on the peninsula. The cycle types used in this study were not recognized at that time and are therefore not included. The main feature of this section is the long-term rise and fall of the Fischer-plot envelope and the presence of more marine biotas (benthic foraminifera) in the rising limb and laminar calcretes in the falling limb. This relationship was not found in the six sections included in the present study. At the stratigraphic top of this (now overturned) section the platform drowns and the peritidal cycles pass sharply to a succession of hemipelagic mudstones and calciturbidites of the Catalan Bay Shale Fm (Fig. 2). REGIONAL ANALYSIS

The comparison of time-equivalent cyclic sections from different basins and plate margins is the major aim of this project, and this is achieved in two different ways. First, a qualitative comparison is made, based on the Fischer plots, presented above, together with the numbers of cycles per section, cycle types and cycle stacking patterns. Second, various statistical analyses are used to assess degrees of randomness or order within the cycle stacking patterns. Qualitative Aspects The Fischer plots provide a simple graphical way of comparing data, when presented at the same scale, together with cycle types and numbers of cycles per section (Fig. 11). The Fischer plots are correlated with the end-Sinemurian datum (Fig. 11A). This is the best age-constrained horizon because it combines strontium age control as well as the first appearance of Pliensbachian foraminifera (L. compressa biozone), or ammonites, in some sections. In one instance, Tunisia (and also Gibraltar but the dating here is not so well-constrained), this is a platformdrowning surface, but in other sections there is no major facies change at this interval. When all the plots are presented at the same scale, as

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measured by numbers of cycles, the bases of some sections have the appearance of being younger than other sections. Figure 11B therefore plots the same data to honor the strontium and foraminiferal age controls but without a common x-axis scale for the Fischer plots. Within the established time constraints and accepting the small number of sections measured (7), the numbers of cycles per section varies considerably (42–101), as do the thicknesses of the sections (117–213 m, Fig. 2), the numbers of cycles in the sections, and the average cycle thickness (Fig. 12). There is very little correlation (r 5 +0.13) between thickness of the section and the numbers of cycles within that section (Fig. 12A). There is however, a positive correlation (r 5 +0.61) between section thickness and average cycle thickness. Some Sinemurian sections are twice as thick as other sections and tend to be made up of thicker cycles (Fig. 12B). The thinner Gibraltar and Betics sections differ from the remaining thicker sections in that they are made up of a relatively large number of thinner cycles (Fig. 12B, C). This relationship, coming from a small number of data points, is not understood. Within the entire dataset, average cycle thickness varies only from approximately 1 to 3.5 meters (Figs. 11A, 12). The thickest cycles are recorded from a drowning succession within a platform (Jebel Aziz, Tunisia). However, this is not always found to be the case because the temporary drowning of the Iberian Basin platform (Almonacid da Cuba) shows thinner-thanaverage cycles (Fig. 11A). Cycle thickness in these situations will depend on the balance between generation of accommodation and rates of sediment accumulation. Visual assessment of the Fischer plots, whether they are plotted by cycle number or by age (Fig. 11), indicates no common long-term, loworder, trend of cycle thickness variation. Whilst there is some similarity between the plots from the Iberian Basin and the Apennines, in that they both start with a falling trend and then both have long-term rises and then falls, none of the other time-equivalent plots show these long-term trends, nor do they show any similarities to one other. The two sections that are in the closest paleogeographic position (Betics and Iberian Basin, Fig. 1) show different trends whichever way the Fischer plot is displayed; the time plots show the most similarity, especially if a lack of precision in the biostratigraphic ages are accepted, but the Betics section has twice as many cycles that are less than half the thickness of the cycles from the Iberian Basin. The Gibraltar section is different again, and, although close to the Betics today, is on a nappe of unknown paleogeographic provenance (see Fig. 1 for approximate location). On a smaller scale there is no graphical evidence in the Fischer plots of the persistent 4:1 or 5:1 bundling of cycle thickness trends in any of the sections that has been used by many authors as evidence for an orbital control on cycle development (e.g., Goldhammer et al. 1987). Proportions of cycle types are given in Figure 11A and this shows a clear dominance of shallowing-upward, or a, cycles. This dominance is particularly strong in the Betics, Apennines, and Morocco sections, where small numbers of d and c cycles also occur. Cycles from the Iberian Basin, Greek, and Tunisian sections show a large proportion of d, c, and b cycles. If the individual Fischer plots for these three sections are examined (Figs. 7A, 8, 9A) it can be seen that in Iberia there are a small number of symmetric, c cycles scattered through the section and that all of the deepening-upward, d, cycles are found in subtidal facies in the upper third of the section. In the Chalkis Qu section in Greece it is concluded (above) that there is no pattern to the occurrence of the different cycle types, nor is there any relation of cycle type to long-term accommodation trends. In the Jebel Aziz section cycle types are clustered, e.g., c and b cycles occur in a set near the base and again near the top, both on falling trends of the Fischer plot. In summary, there is a large degree of variation in the thickness of the sections, and the numbers of cycles, the thickness trends of cycles, and the cycle types found in each of the seven studied sections. The sections cannot be correlated using thickness trends or facies trends within the

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FIG. 6.—Fischer plot, cycle types, and occurrence of environmentally significant biota and lithofacies plotted for Loma Prieto section, Betics, Spain (for location see Appendix). Mean cycle thickness is 1.3 m. In the cycle-type record a blank entry indicates a minor fault in the section that truncates the upper or the lower surface of the cycle and no identification of cycle type can be made.

r FIG. 5.—Field photographs of typical facies, cycle types, cycle boundaries, and structures in Sinemurian peritidal carbonates from western Tethys. A) Shallowing-up a cycles from Loma Prieto Section, Betics, Spain (rucksack 50 cm high). Cycle boundaries at flooding surfaces (arrowed) on top of subaerial nodular calcretes and below thick beds of subtidal, peloidal bioclastic packstones. B) Detail of cycle boundary (arrowed) of deepening-up b cycles, Chalkis Q. Greece. Subtidal peloidal bioclastic wackestone (PBw) of underlying cycle is cut by sharp, erosional surface (arrowed). Surface is overlain by intertidal microbial laminite (Ml), passing up to subtidal largebivalve floatstone (Bif) through shoreface erosion surface (SES) (hammer head 20 cm long). C) Detail of cycle boundary of b-cycle Chalkis Qu. Greece with reddened, microkarstic pocket (arrowed) at cycle boundary passing up to intertidal microbial laminite (Ml) and then shoreface erosion surface (SES) to subtidal peloidal bioclastic packstone (PBw). D) Detail of facies in a single, thin, a cycle from Loma Prieto Section, Betics, Spain (hammer 33 cm high). Sharp cycle base (arrowed) is overlain by subtidal, peloidal bioclastic packstone–grainstone (PBp-g) passing up to intertidal, microbial laminite (Ml) and capped by subaerial nodular calcrete in cream colored clay (Nc). E) Neptunean dyke (margins arrowed) with light colored intraclasts, Jebel Aziz, Dorsales Range, Tunisia (hammer head 18 cm). F) Vadose pisoids in subaerial facies, Arbalou n’Kerdous section, Morocco (photo width 30 cm). G) Syndepositional slump fold, Gebel Aziz, Dorsales Range, Tunisia (hand lens 10 cm across). Note thinned and faulted limbs to fold. Overlying and underlying strata are not folded. H) Tepee structure in subaerial unit Arbalou n’Kerdous section, Morocco (hammer 33 cm long). Note undeformed beds below hammer. I) Overview of cyclic section Chalkis Q. Greece (car for scale). Dark beds are predominantly subtidal limestones, and lighter beds are intertidal and subaerial dolomite. Note continuity of beds and cycles over tens of meters.

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FIG. 7.— Fischer plot, cycle types, and occurrence of environmentally significant biota and lithofacies plotted for A) Almonacid da Cuba section, Iberian Basin, Spain, mean cycle thickness 3.2 m, and B) Monte Bove section (After Casaglia 2003–4), Sibillini Mts., Apennines, Italy. Mean cycle thickness 2.9 m. (For locations see Appendix and Fig. A1 in JSR’s Data Archive).

constraints of the isotopic and biostratigraphic ages. The points of commonality are rather few; the similar aged sections comprise meterscale, peritidal cycles with similar average thicknesses (1 to 3.5 m) and a predominance of shallowing-upward cycles. Each section was chosen to represent an interior platform-top paleoenvironment, which partially explains why many of the facies are similar in the different sections. Finally, the thicker sections tend to have thicker cycles and thicker cycles are found in the sections with the smaller number of cycles.

Because of this large variability in the sections there is no evidence for a regional process-based rhythm being preserved in each of these platforms on the margins of the Tethys ocean. Within the 6.9 Myrs. of the Sinemurian, the numbers of preserved cycles, the types of cycles, and the short- and the long-term changes in their stacking patterns shows no common pattern or signature. The maximum number of cycles preserved (101 cycles at Chalkis, Greece) within the 6.9 Myrs indicates a process of cycle generation with a repeat time of at the most 68 kyrs, if it were to have

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FIG. 8.—Fischer plot, cycle types, and occurrence of environmentally significant biota and lithofacies plotted for Chalkis quarry section, Evvoia, Greece (for location see Appendix and Data Archive). Mean cycle thickness 1.1 m.

had a regular beat preserved in the rock record. However, of course, periodicity cannot be proven, nor can the many gaps in the record at exposure surfaces be measured, because both require a resolution of dating (i.e., cycle by cycle) that is impossible to achieve at the present time. Quantitative Aspects Over the last 15 years, a number of statistical tests on cycle thickness data have been applied to assess whether stratal stacking is ordered or disordered (e.g., Drummond and Wilkinson 1993; Sadler et al. 1993; Davis 2002; Weedon 2003). This is seen as an important aspect of the analysis of peritidal successions because sections previously regarded as cyclic have been shown subsequently to have thickness trends and lithological patterns that are indistinguishable from random occurrences (Wilkinson et al. 1997). These tests can be divided into three different approaches: bundle testing, runs testing, and time-series analysis. These have been applied to the Tethyan Sinemurian dataset (Procter 2008): Bundle Testing.—The cycle bundles were calculated using a method consistent with those of Sadler et al. (1993), Sadler (1994), Drummond

and Wilkinson (1993; for ‘‘megacycles’’), and Lehrmann and Goldhammer (1999; for ‘‘parasequence sets’’). Bundles represent groupings of cycles which have a run of increasing cycle thickness to a maximum and then a run of decreasing cycle thicknesses. Bundle boundaries were taken at cycle thickness minima (where a cycle occurs with a thicker cycle below and above it in the section). The number of cycles between these boundaries is recorded, as are the number of cycles per bundle (Fig. 13). All of the sections have similar numbers of cycles per bundle, but in every section the number is well below the lowest Milankovitch ratio of 5:1 (precession:eccentricity). The degree of variation in the number of cycles per bundle is high, as shown by the standard deviation error bars and the high coefficients of variance. This supports the visual inspection of the Fischer plots (above) that there is little evidence in the logs for a 4:1 or 5:1 bundling of cycle thicknesses. Runs Testing.—Runs testing is used in order to determine whether an ordered or disordered vertical relationship exists in rock-cycle thickness patterns (e.g., Drummond and Wilkinson 1993; Sadler et al. 1993). It is based on the number of cycles in the section that are either thicker than,

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FIG. 9.— Fischer plot, cycle types and occurrence of environmentally significant biota and lithofacies for A) Jebel Aziz section, Dorsales range, Tunisia, mean cycle thickness 3.0 m, and B) Arghbalou n’Kerdous section, High Atlas, Morocco (for locations see Appendix and Data Archive) Mean cycle thickness 3.3 m

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TABLE 2.—Strontium isotope dates obtained from pristine skeletal material at five of the measured sections. Sample Gibraltar Brachiopods B10409 B85423 B85425 B85426 B85439 Lomo Prieto, Betics Brachiopod RA-01-154 (46 m) Crinoid RA-01-185 (80 m) Almonacid de la Cuba Brachiopod 33a (77 m) Brachiopod 33b (77 m) Brachiopod 33c (77 m) Mt Bove, Apennines Crinoid ossicle MB9S Arghbalou n’Kerdous Oysters (av. of 7 samples 245 m)

87/86

Sr 6 2 3 sd

0.707504 0.707516 0.707550 0.707566 0.707570

6 6 6 6 6

12 10 11 10 11

Interpreted age 191.2 190.8 192.1 192.5 192.5

– 193.4 – 193.4 – 193.5 – 193.7 – 194.3

Ma Ma Ma Ma Ma

0.707416 6 9 0.707436 6 10

189.1 – 189.9 Ma 189.3 – 190.3 Ma

0.707417 6 11 0.707423 6 10 0.707420 6 9

188.7 – 190.0 Ma 189.9 – 190.0 Ma 189.9 – 190.0 Ma

0.707324 6 13

186.9 – 188.6 Ma

0.707393 6 10

188.3 – 189.8 Ma

or thinner than (as measured to the nearest centimeter), the average cycle thickness. In bundles interpreted to have formed with a superimposed precession and eccentricity orbital control, two runs may exist, the first showing successive increases in cycle thicknesses to a maximum and the next showing successive reductions in thickness to a minimum at the end of the bundle. The Z-test (Fig. 14; Sadler et al. 1993) makes a comparison between the observed number of runs (in cyclic sections, U) and the ¯ , Fig. 14). number that might occur randomly (U The Z-score of any section is positive if the number of runs observed is greater than the number that are produced randomly (expected number of runs). Conversely, negative Z-scores will be produced if the observed number of runs is less than the number that would be produced randomly

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(expected number of runs). The statistical threshold determining whether the number of runs observed is significantly different from the number of runs that would be produced randomly is plotted in Figure 14. Z . 1.96 is ordered, within the field of too many runs, Z , 1.96 is disordered, and Z , 21.96 indicates sections exhibiting order but with too few runs (cf. Sadler et al. 1993). The significant threshold value is the statistical norm of 2 standard deviations from the mean (s2U¯ is the expected standard error). All of the sections except Morocco have Z-scores that are indistinguishable from random. Morocco just lies within the ordered field but with too few runs, meaning that the bundle ratio is also low, with a few cycles making a bundle. Lehrmann and Goldhammer (1999) published Zscore analyses from a number of other cyclic sections, and the Z-scores of 11 out of 12 of their Mesozoic examples also plot within the random field with Z-score values (21.92 to 0.22) within the field of the scores for the Tethyan sections. This analysis is consistent with the visual assessment of the Fischer plots, and together they suggest that there is little evidence for systematic patterns of bundling within these Jurassic cyclic sections. Whilst Z-scores have been used by previous workers to assess randomness or order in stacking patterns, current numerical modeling of cycle formation by Procter et al. (2006) and Procter (2008) indicates that random z-score values can in fact occur in synthetic cyclic profiles with a known Milankovitch input, thus casting some doubt on the usefulness of this technique in discriminating between random and ordered thickness stacking patterns. These results are the subject of current work and will be published elsewhere. Time-Series Analysis.—The aim of using time-series analysis is to determine whether it is possible to recognize a regular frequency signal, or ordered periodicity from cycle stratigraphic stacking patterns. Time-series analysis of the cycle thickness data was undertaken using the fast Fourier transform method (FFT) (Weedon 2003).

FIG. 10.—Fischer plot and occurrence of environmentally significant biota and lithofacies for Main Ridge section Gibraltar (modified from Bosence et al. 2000). Note that cycle types were not recorded in this study. Breaks in the Fischer plot indicate faults in the section, and for plot construction either an average cycle thickness is used, or the measured cycle thickness is used, if this greater than the average cycle thickness.

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The time-series analysis is based on the cycle thickness data, which is a discrete data record (cf. Weedon 2003). The rationale for using cycle thicknesses as a discrete dataset is that the cycles (produced by precessional oscillation) will show successive thickness increments that represent the longer term eccentricity sea-level curve. Time-series analysis transforms the data into the frequency domain, and the resulting amplitude spectra are plotted against the number of cycles per bundle (Fig. 15). All the plots show random background variation or noise. None of the sections have prominent peaks indicating a 5:1 or 4:1 bundling and, of the peaks that are present, there is no consistency in their position between the six sections (e.g., Fig. 15; Iberian Basin and Betics). The time series analysis shows that there is no consistent pattern of bundling that could be attributed to a regular control on cyclicity, such as orbitally induced sea-level oscillations. In summary, the quantitative analysis of the Sinemurian cycle thickness data indicates that the cycles are arranged into small bundles with averages varying from 2.5 to 3.5 cycles per bundle (Fig. 15), although the coefficient of variance of cycles per bundle is high (Fig. 15). Runs testing of the bundling shows that all but one of the sections have Z-scores within the random field. Time-series analysis of the sections indicates that most do not show frequency peaks any higher than background values (Fig. 15) so that whilst some bundling exists it cannot be separated from a random effect. DISCUSSION AND CONCLUSIONS

It has variously been argued that meter-scale peritidal cycles may just be randomly stacked facies associations, or if cycles are present, they may be formed by high-frequency eustatic changes, or by fault-movements, or by the repeated progradation of tidal flats and islands, all operating within a subsiding shelf setting to provide the necessary accommodation space. Evidence for and against these different models for cycle formation is discussed below for the Sinemurian dataset of western Tethys. Cycles or Random Vertical Stacking of Facies? The results from the embedded Markov-chain analyses confirm the field interpretations that the likelihood of any of the sections having a random arrangement of facies associations and surfaces is extremely remote. All the sections have a dominance of one cycle type; shallowingupward, a cycles. Some sections have a mix of two or more cycle types. However, even with this mix of cycle types, there is little evidence of randomness or disorder as proposed by Drummond and Wilkinson (1993) and Wilkinson et al. (1997) for other datasets. To conclude, it seems clear that the sections are cyclic but that they are characterized by a range of different cycle types, not all of which have been widely recognized as occurring together.

r FIG. 11.— Summary diagrams to compare modified Fischer plots from each section. A) All Fischer plots (from Figs. 6–10) with numbers of cycles and departures from average cycle thickness at same scale for each section. Fischer plot envelopes are hung off the end Sinemurian section tops (see Fig. 3). B) Fischer plots scaled to fit early, middle, and late Sinemurian foraminiferal biozones but with cumulative deviation from mean cycle thickness at the same scale for each section. For convenience of display, the Fischer plots have been stretched evenly throughout their length to fit the known presence of the biostratigraphic zones in each section. Portions of plots have not been individually adjusted to fit biostratigraphic data for each biozone because this would imply a precision that is probably not justified by the biostratigraphic data (Boudagher-Fadel and Bosence 2007).

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FIG. 12.— Relations between section thickness, cycle thickness and numbers of cycles per section for seven studied sections. A) Thickness of measured Sinemurian section bears no statistical relationship (r 5 20.02) to numbers of cycles within those sections. B) Thicker sections tend to have thicker cycles (r 5 0.62). C) Thicker cycles tend to occur in sections with lesser numbers of cycles and thinner cycles in those with greater numbers of cycles (r 5 20.62).

Eustatic Changes? Eustatic changes driven by precession, eccentricity, or obliquity orbital fluctuations have repeatedly been presented as the mechanism for the formation of cycles similar to those described in this paper (e.g., Fischer 1964; Goldhammer et al. 1987; Read 1995; Lehrmann and Goldhammer 1999; Fischer et al. 2004). Evidence for this process has been sought in the Sinemurian dataset, but none of the commonly used criteria for its recognition are present, suggesting that this is not the overriding mechanism for cycle formation. In particular, visual examination of logs and Fischer plots indicates there is no evidence for a 4:1 or 5:1 bundling of cycle thickness trends or of facies. Z-scores of cycle thickness data indicate random stacking. Similarly, time-series analysis does not identify any non-random bundling within the commonly cited 4:1 or 5:1 ratio. The numbers of cycles within the 6.9 My of the Sinemurian varies from section to section within the range of 42 to 101 (Fig. 11B). If peritidal carbonate cycles were to equate one-to-one with orbitally driven sea-level cycles then their periodicity would be 6.9 My divided by the numbers of cycles per section (Fig. 11B). This equates to either 70, or 90, or 132, or 135, or 164 ky for these Sinemurian sections and, although overlapping, do not match with any orbital periodicities for the Early Jurassic (cf. Berger et al. 1989). The extensively developed subaerial exposure surfaces within these Tethyan cycles indicate numerous horizons that may represent missed beats on the platform top, which may explain the lack of bundling and match to reported orbital frequencies. However, variably developed exposure surfaces can also be generated in the tectonic or the autocyclic models (see below). Related to this is the question of rhythmic and non-rhythmic sedimentation expressed in Sanders Rule (Sander 1936), as translated by Schwarzacher (1975, p. 288) as ‘‘cyclicity in space

(which means stratigraphical thickness) indicates cyclicity in time but the absence of cyclicity in the stratigraphic record does not indicate the absence of time cyclicity.’’ This rule has been more recently supported by numerical forward modeling (Walkden and Walkden 1990; Procter et al. 2006) which illustrates the problems of achieving a complete one-to-one preservation of rhythms in the rock record from perfectly ordered rhythms of sea-level change. However, it would be unhelpful to follow this rule to provide an explanation for the Sinemurian dataset because this record, although ‘‘cyclic in space,’’ gives no indication, by any known metric, of ‘‘cyclicity in time’’ or that any rhythmic process was involved in cycle formation. In addition, this rule does not take into account other possible models of cycle formation that may operate. Two previous studies of Early Jurassic cyclic, peritidal successions from this region have proposed a high-frequency orbital control on cycle formation. Crevello (1990, 1991) examined a number of sections, mainly of Pliensbachian age, in the Central and High Atlas of Morocco and reported a range of different platform-top facies arranged in shallowingupward (locally ‘‘top-truncated’’), and symmetric cycles together with non-cyclic intervals. The Aghbalou n’Kerdous section (Figs. 1, 9B) logged by Crevello (1990; Section 27) showed small-scale cycles but is an outlying section from his study area and was not included in his analysis of cyclicity. His visual assessment of logs and Fischer plots of the Pliensbachian sections together with time-series analysis indicated an orbital control that produced a 20:5:1 bundling of rock cycles which were equated to superimposition of the long and the short eccentricity cycles with precessional cyclicity (Crevello 1990, 1991). Although of Early Jurassic age, all of his analyzed cyclic sections in the Atlas are all slightly younger than those studied for this paper and so a direct comparison cannot be made. The requirement in this study to work on Sinemurian-

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FIG. 13.— Average numbers of cycles per bundle in the six measured sections of this study. Error bars are the standard deviation of the number of cycles per bundle. Numbers adjacent to data points are the coefficients of variation expressed as the percentage of average cycle thickness.

aged successions was based on the fact that a number of platforms in the Tethyan region drown within the Pliensbachian (Fig. 2). In the other example, Walkden and de Matos (2000) record the cyclicity and omission surfaces within a peritidal succession of Hettangian to Pliensbachian age from the southwest Musumdam Peninsula, UAE. The section is well exposed, permitting a detailed study of facies, surfaces, and cycles, but the section is ‘‘poorly constrained biostratigraphically’’ (Walkden and de Matos 2000, p. 38). The cycles are described as shallowing-upward, and many are capped by subaerial exposure surfaces, including subaerial exposure of subtidal facies (G. Walkden, personal communication, 2008). By comparison with coeval basinal successions in NW Europe that are thought to exhibit Milankovitch band orbital cycles (e.g., Weedon 1993), the interpreted average cycle duration of 90 kyr, and the subaerial exposure surfaces at cycle boundaries, these cycles are interpreted to have formed under an orbital control with a frequency of 40 kyr (Walkden and de Matos 2000). The long-term trends in their Fischer plot is similar to that recorded here for Morocco, but both of these differ from other plots constructed from the new data from the Tethys region (Fig. 11). The types of cycles described from these two studies from carbonate platforms from the Early Jurassic in western Tethys are consistent with models for orbital control. However, this is not supported with precise dating of the periodicity of the cycles, or by bundling within the successions. In addition, a comparison with the time-equivalent shallowwater carbonate successions from the current dataset does not provide evidence for a eustatic driving force.

Despite the lack of evidence within the Sinemurian dataset for an orbital control of cycle formation, high-frequency eustatic changes could account for the formation of each of the different cycle types found in this study. In particular, eustatic fall can explain cycle types b, d, and e, and eustatic changes could generate the subtidal cycles with deepening-upward trends. Spence and Tucker (2007) have suggested recently that if different frequencies of sinusoidal, sea-level cycles were operating to drive cycle formation, then preservation of facies within shallowing-upward cycles should not always be expected and different cycle types and cycle trends might characterize different portions of longer-term sea-level cycles. They propose trends of ‘‘peritidal cycle sets’’ generated by superimposed sinusoidal sea-level cycles; however, such trends cannot be recognized in any of the Sinemurian sections studied (Figures 6–10). On a larger scale, there are no common, low-order variations in accommodation between the seven logged sections (Fig. 11) that might relate to longer term (third order) sea-level changes. Hardenbol et al. (1998) propose that the Sinemurian lies within the transgressive portion of the Hettangian to mid Toarcian ‘‘major transgressive–regressive cycle.’’ This may be reflected in the seven sections that all show an overall aggradational style of accumulation, with little up-section variation of facies associations. However, there is evidence in most sections of an initial trend of decrease in cycle thickness, rather than the predicted increase, through the early Sinemurian. Similarly, there is little correspondence with their higher-order events such as their flooding surface within the turneri biozone, which approximates to the early part of the E. praevirguliana biozone of this work (Fig. 11B). Finally, there is

FIG. 14.— Z-scores, as calculated using equation of Sadler et al. (1993; see figure inset) for the six logged sections (for discussion see text).

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FIG. 15.— Time-series analysis (using Fast Fourier Transform) to illustrate amplitude frequency of possible cycle bundling for the six studied sections. Cycles per bundle are calculated as 1/(short-period cycles) per long-period cycle. This affects the data spacing on the x axis, leading to the longer-period waves seen at higher bundle numbers (for discussion see text).

no evidence for the five Sinemurian sequences (Si 1 to Si 5) that Hardenbol et al. (1998) also propose. To conclude, the seven logged sections do not show any consistent pattern of coeval changes in accommodation, cycle stacking, or occurrences of cycle types that would support an overriding control by high- or lower-frequency eustatic sea-level changes. Tectonically Generated Changes in Accommodation Space? The large variability in the long-term accommodation trends, the nonsystematic cycle stacking and thickness trends, the irregular variation in cycle types, and different numbers of cycles in each section all argue for a

less metronomic control of accommodation space on these seven carbonate platforms than that expected from a eustatic control. The setting of these platforms, on the extensional continental margins and isolated platforms of western Tethys in the Early Jurassic (Fig. 2), are consistent with accommodation space being generated by local syndepositional, extensional fault movement superimposed on longer-term subsidence. Therefore, although this process of cycle formation has been largely ignored, or sometimes refuted for some cyclic sections (e.g., Koerschner and Read 1989; Goldhammer et al. 1993) particularly for sections from stable continental interiors, it cannot be ruled out as a process for cycle formation in the Sinemurian of western Tethys, and perhaps elsewhere. The principles behind the hypothesis that synsedi-

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FIG. 16.— Tectono-sedimentary model indicating sedimentation in response to sea-level changes on a carbonate shelf on an extensional, subsiding basin margin. Relative sea-level (RSL) changes may be driven by: firstly, extensional fault movement in footwall and horst (slow rises interrupted by sharp falls) and hanging wall and graben (sharp rises and slow rises) sites; secondly, tidal flat and island progradation; and thirdly, by eustasy. Note: thickness changes in synrift strata, syndepositional faults may or may not (e.g., left-hand fault) break through to form submarine scarp, and predicted change in relative sea-level history if central fault propagates from A to B (see text).

mentary fault movements might generate the accommodation space for cycle formation were laid down by Cisne (1986), Hardie (1986), and Hardie et al. (1991) but have not been widely accepted. More recently, De Benedictis et al. (2007) re-examined this model integrating data from the literature on neotectonics which documents that active extensional faults have the right amounts of instantaneous throw (0.4 to 4.0 m), averaged slip rates (0.05 to 2.8 m/ky), and frequency of recurrence (0.5 to 40 ky) to provide the repeated phases of subsidence and uplift for the accumulation and subaerial modification of shallow-water carbonate cycles. The previous argument that syndepositional faults cannot generate the perceived rhythmic timing of cycle generation is not accepted here, because such rhythmicity in time cannot be substantiated with any known dating methods. Syndepositional hanging wall and graben sites are characterized by fault-related downdropping, or ‘‘jerky’’ subsidence (sensu Koerschner and Read 1989) or ‘‘yo’’ tectonics (sensu Hardie et al. 1991) that generate a staircase-like relative sea-level curve (Fig. 16; De Benedictis et al. 2007). Such a sea-level history is predicted to generate a and c cycles from the initial subsidence phase followed by a more quiescent phase when syndepositional rift-margin subsidence (cf. Cochran 1983; Dorobek 2008) would affect the region. Synthetic cycles with shallowing-up and deepening-shallowing paleobathymetric trends were generated from jerky subsidence in forward numerical modeling of De Benedictis et al. (2007). Shallowing-up (a) cycles were modeled when production rates were low, when little sediment accumulated following a rapid period of subsidence, and symmetric cycles (c) when production rates were set at higher values and sediment accumulated during the rapid subsidence. Conversely, footwall and horst sites experience coseismic uplift during extensional fault movement interspersed by phases of slower, synrift subsidence (Fig. 16). Such alternating uplift and subsidence equates to the ‘‘yo-yo’’ tectonics of Koerschner and Read (1989) and Hardie et al. (1991). The combination of rapid uplift followed by slow subsidence may result in subaerial exposure and cycle boundary formation followed by accommodation space for cycle accumulation of b and d cycles and also cycles or alternations of type e. Variable and more prolonged exposure may occur in footwall sites that remain above sea level. Similar synthetic

cycles were generated by numerical modeling using a stepped relative sealevel curve (De Benedictis et al. 2007) that produced d and e cycles (note that both of these are encompassed within e cycles in this earlier publication). Examples of accumulation of shallow-water carbonates capped by exposure surfaces in other footwall, synrift settings are given in recent reviews by Cross and Bosence (2008) and Dorobek (2008) and were modeled by Bosence et al. (1998). The differing sense of fault movement in hanging wall and graben sites (rapid subsidence followed by slower subsidence) compared to footwall and horst sites (rapid uplift followed by slower subsidence) (Fig. 16) therefore accounts for generation of the different cycle types reported in this paper. Up-section changes in cycle types might be driven by changes from hanging wall to footwall locations as faults propagate laterally (Fig. 16), or an irregular arrangement of cycle types would occur if throw is transferred irregularly from one fault to another within fault arrays. The irregular up-section occurrence of different cycle types in Figures 6– 10 suggests the latter mechanism rather than the former. In extensive 2-D and 3-D exposures the stratigraphic wedging of successive sedimentary cycles, which is the distinctive signature of growth strata adjacent to synsedimentary faults, provides evidence for cycle generation by this mechanism, as has been documented elsewhere in the geological record (Rosales et al. 1994; Bosence et al. 1998; Cross and Bosence 2008). However, because of Alpine tectonism, such outcrops are generally not available in this study, as will commonly be the case in other outcrop studies, and in most subsurface datasets. A notable exception are the Sinemurian sections in the Betic Cordillera (Ruiz Ortiz et al. 2004), where stratigraphic wedging is visible at outcrop and depositional sequences and sequence boundaries vary laterally with a tectonic control. Similarly, current work in progress in the Iberian Basin section (Almonacid de la Cuba) indicates local wedge-shaped patterns of cycle thickness variation and lateral cycle splitting (Ba´denas et al. 2007). Other evidence of synsedimentary, extensional fault movement for the Liassic of western Tethys comes from detailed local stratigraphic and tectonic studies as listed and referenced in Figure 17 for each of the areas in this study. This figure also provides the paleogeographic and paleotectonic setting of the extensional shelf areas and fault-bounded platforms, as also

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FIG. 17.— Paleogeographic map for western Tethys in the Sinemurian with text boxes listing syndepositional tectonic features recorded in the literature and in this paper for locations indicated (map after Thierry 2000). 20u paleolatitude indicated.

presented in Bernouilli and Jenkyns (1974), Thierry (2000), and Ziegler (1990) which characterize western Tethys at this time. A tectonic control for cycle generation cannot therefore be dismissed. The variation in cycle types, the long- and short-term stacking patterns, the variability of cycles from one location to another around western Tethys, together with the geological setting, are all consistent with cycle formation from fault-generated accommodation space. Autocyclic Mechanisms? An autocyclic mechanism of tidal-flat and island progradation (Ginsburg 1971; Pratt and James 1986) could explain a large proportion

of the sections logged here because shallowing-up, a, cycles are the commonest cycle type in all the measured sections (Fig. 11A). Similarly, variably developed exposure surfaces, recorded in many of the sections, could occur during variable lengths of subaerial exposure of supratidal flats. Tidal-flat progradation could also account for symmetric, c, cycles if a transgressive phase is preserved during platform flooding (cf. Burgess 2006). Other aspects of the measured sections that might also be indicative of an autocyclic mechanism are the variability in section thickness, in cycle thicknesses, and in stacking patterns within the sections (cf. Burgess et al. 2001; Burgess and Wright 2003). However, cycle types b and d require an absolute sea-level fall which does not occur in the autocyclic model. Similarly, the subtidal e cycles in the

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Iberian Basin section (Fig. 13A) cannot be generated by this model (cf. Osleger 1991). Also, it should be noted that a cycles, the most common cycle type in this dataset, can also be produced by high-frequency, eustatic changes (Goldhammer et al. 1987; Strasser et al. 1999) and by tectonically generated accommodation space (Hardie 1986; De Benedictis et al. 2007). Therefore, whilst an autocyclic mechanism might account for most of the cycles in the studied sections, the records from the Iberian Basin, Greece, and Tunisia have large numbers of cycle types (Figs. 7A, 8, 9A, 11A) that cannot be generated by this mechanism. Can a Unique Driving Force be Recognized? Research on high-frequency, peritidal carbonate cycles over the last 30 years has tended to be polarized into those workers who have promoted either a single driving mechanism, or the view that the cycles cannot be distinguished from random vertical stacking of facies. Whilst the latter view is not supported by the Sinemurian dataset it is difficult to totally exclude any one of the three main cycle-forming mechanisms. Since oceans have existed, orbitally induced sea-level changes would have occurred, tidal flats and islands would have prograded, and synsedimentary faults would have generated pulses of accommodation change in extensional continental margins. All three are viable mechanisms for cycle formation, but confusingly for the geologist they appear to generate cycle types that are remarkably similar in their facies trends and bounding surfaces. If there are three valid models for cycle generation, can any one of these be excluded based on this analysis of the western Tethys Sinemurian dataset? As argued above, none of the commonly used criteria for recognizing a eustatic signal for cycle formation are present, so it is here excluded as the major driving mechanism for cycle formation. The autocyclic model can account for the commonest cycle type found in the dataset, and the variability in stacking patterns reflect those generated by numerical forward modeling (Burgess et al. 2001; Burgess and Wright 2006). However, it cannot account for the origin of some of the cycle types that form significant portions of some of the sections. Tectonically generated accommodation space on the extensional shelf areas of western Tethys can account for the range of cycle types found and the large variability in their stacking patterns. If one overriding mechanism is to be identified as imparting a dominant signature to this dataset it is therefore believed to be the tectonic model (Fig. 16). Geological processes are, however, not that simple, and there is no evidence in this dataset that excludes the possibility that all three processes might be influencing cycle formation at the same time (Fig. 16) and as proposed much earlier for the Dolomia Principale of Italy (Bosellini 1967). Tidal-flat progradation as well as high-frequency eustatic sea-level changes were, in all likelihood, always taking place, but the critical question is: do they impart an identifiable sedimentological signature in the studied sedimentary record? There are no known criteria available to sedimentologists to recognize such combinations of cycle-forming processes. The dataset presented above can equally well be explained by tectonic forcing on its own, or a combination of tectonic forcing and autocyclicity and/or eustatic forcing. Is the Western Tethys Sinemurian Dataset a Unique Record? The results of the analysis of this dataset differ from all previous detailed analysis of cyclic peritidal successions. The question that then arises is whether this is due to the methods used, or, are there real differences in the stratigraphic record of cyclicity? The first part of this question cannot be answered yet because this is the first study of its type that compares cyclicity from one time slice but from a number of different plates and plate margins. Hopefully other such studies will follow, so that local versus regional, or even global signals can be convincingly

identified. Prior to this study, published work on the Early Jurassic cycles from this region argued strongly for an orbital control of cycle formation (Crevello 1990, 1991; Walkden and de Matos 2000), as has been published for the Triassic (e.g., Goldhammer et al. 1987; Hinnov and Goldhammer 1991) and for the Late Jurassic and Early Cretaceous (Pittet and Strasser 1998; Colombie´ and Rameil 2007; Badenas et al. 2004; D’Argenio et al. 2004) in this region. The only criterion for selection of the Sinemurian time slice were the existence of well-exposed, correlatable, cyclic successions from different plate margins. The Early Jurassic represents greenhouse Earth conditions (Sellwood and Valdes 2006) with likely short-term, small-amplitude (i.e., meter-scale, Read 1995; Hallam 2001) eustatic oscillations in contrast to icehouse periods with greater amplitude fluctuations. In greenhouse conditions synsedimentary fault movement would be expected to override the reduced rates of climatically induced sea-level changes. With the documented extensional tectonism of basin margins in the Early Jurassic of western Tethys, together with local studies, some undertaken in this project (Fig. 17), a dominant tectonic control of relative sea-level changes seems very likely. However, later in the Jurassic and Cretaceous the extensional regimes of the Lias in northwestern Europe shifted to the newly forming Alpine Tethys and Atlantic passive margins (Zeigler 1990), and this may have allowed autocyclic and/or eustatic mechanisms to dominate. Lehrman and Goldhammer (1999) also acknowledge the lesser forcing potential of eustatic sea-level changes during greenhouse times that might lead to conditions where autocyclicity would dominate. However, they do not explore the possibility that tectonically induced sealevel changes might also affect the temporal and spatial occurrence of cyclicity. This dataset demonstrates that the currently popular hypothesis of orbital control of shallow-water cycle formation cannot be applied to all parts of the geological record where cyclicity in peritidal carbonates occur. Indeed more extensive regional analyses should be undertaken if evidence for this model is being sought. An important implication arising from this is that cyclostratigraphy is of little value as a predictive chronostratigraphic tool in shallow-water carbonate successions in tectonic situations such as the Lower Jurassic of western Tethys because stacking patterns of cycles from one location cannot be used to predict stacking patterns in other areas or basin margins. ACKNOWLEDGMENTS

Funding for this project was provided by grants to Bosence from the Royal Society and from the Central Research Fund of the University of London; this support is gratefully acknowledged. Research in the Betic Cordillera by LN and JR was carried out within the project: CGL2005-06636-CO2-01/BTE (DGI, Science and Education Ministry of Spain) and the research group RNM-200 (Junta de Andalucıá). Procter carried out research with joint funding from the NERC, UK (NER/S/C/2003/11944) and Shell International, Rijswijk and this award is acknowledged with thanks. Research in Greece was carried out under permit from IGME, Athens and access to Chalkis Quarry with kind permission and assistance from the Halkis Cement Company of the Heracles Group, Chalkis. Thanks to Paul Crevello, Fritz Neuweiler, and Markus Wilmsen for assistance with locating suitable sections in Morocco. Finally our gratitude goes to the work carried out by our four reviewers, Peter Burgess, David Osleger, Andreas Strasser, and Paul Wright together with JSR Editors Maya Elrick, Gene Rankey, and John Southard for detailed comments and criticisms that significantly improved our earlier manuscripts. Figure A1 is available from JSR’s Data Archive, http://www.sepm.org/jsr/ jsr_data_archive.asp. REFERENCES

BA´DENAS, B., SALAS, R., AND AURELL, M., 2004, Three orders of regional sea-level changes control facies and stacking patterns of shallow platform carbonates in the Maestrat Basin (Tithonian–Berriasian, NE Spain): International Journal of Earth Sciences, v. 93, p. 144–162.

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BA´DENAS, B., AURELL, M., AND BOSENCE, D.W.J., 2007, Origin of high-frequency cycles on a Sinemurian epeiric platform; pulses of accommodation gain and lateral facies heterogeneities (NE Spain) [Abstract]: European Geosciences Union, General Assembly 2007, Vienna, Austria. BARRELL, J., 1917, Rhythms and the measurements of geologic time: Geological Society of America, Bulletin, v. 28, p. 745–904. BEL KHALA, A., SOUSSI, M., AND BOSENCE, D.W.J., 2007, Early Jurassic (Sinemurian) shallow carbonate platform and associated peritidal cycles of Atlas Tunisia: sedimentary characteristics and controlling factors (eustasy and/or tectonics?)[Abstract]: 13th Bathurst Meeting of Carbonate Sedimentologists, University of East Anglia, Norwich, U.K. Abstracts, p. 44. BERGER, A., LOUTRE, M.F., AND DEHANT, V., 1989, Astronomical frequencies for preQuaternary palaeoclimate studies: Terra Nova, v. 1, p. 474–479. 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APPENDIX

Gibraltar.—All details of these cliff, road-cut, and quarry sections and samples are given in Bosence et al. (2000). However, much of the area is now built over and may be inaccessible. Only the approximately 240-mthick Main Ridge section (Fig. 2) is used here for comparison. Latest Sinemurian is proven by Sr isotope dates (Table 2) but not by foram records (Boudagher-Fadel and Bosence 2007). Loma Prieto, Murcia Province, southern Spain.—The location of this road-cut section is along the forest road on the northern side of the hill of Loma Prieto in the Lower Member (M1) of the Gavilań Formation (Fig. 1A). A detailed map is given in Ruiz-Ortiz et al. (2004, their fig. 3). The section comprises peritidal texture-preserving dolomites and limestones with well-developed paleosols with green-gray clays and nodular calcrete. The top of this section is taken where brachiopod assemblages and a crinoid give essentially the same Sr dates of 189.1–190.3 Ma indicating the Sinemurian–Pliensbachian boundary (Table 2, Fig. 2). All three Sinemurian foraminifera biozones are present together with the earliest Pliensbachian biozone. Almonacid de la Cuba, northeast Spain.—This section is situated in the barranco to the north of the village of Almonacid de la Cuba on the southern limb of a northwest–southeast trending anticline. Most of the section corresponds to the logged section of the Cuevas Labradas Fm by Comas-Rengifo et al. (1999), the top of our section being equivalent to the 10 m level in their figure 2.2. The base of our section comprises dolomitized and brecciated beds exposed in the crest of a minor anticline within the barranco (Approx. 40 m level, fig. 2.1 in Comas-Rengifo et al. 1999). The section is continuous and is the only one of our sections to include some heavily brecciated and dedolomitized units (‘‘Fe-rich crackle breccia’’ Table 1) that always occur at the tops of cycles. These are

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interpreted as subaerial exposure surfaces. This section is also the only one to include thicknesses of subtidal facies that are organized into subtidal cycles (cf. Osleger 1991) that are part of the Rio Palomar Fm (Fig. 3; Gomez et al. 2003). The base of this Formation is 23 m below the top of our measured section and is marked by a prominent regional flooding surface. Brachiopods collected from the bed above this flooding surface provide strontium isotope ratios that give essentially the same Sinemurian – Pliensbachian boundary ages (188.7–190 Ma; Table 2, Fig. 2). This confirms the earlier ammonite dating for this horizon as earliest Pliensbachian Jamesoni Biozone (Radstockiceras complanosum in ComasRengifo et al. 1999). Note that the L. recoarensis biozone is not present in the upper part of the section despite an otherwise open marine biota. Possibly this represents a facies control and that these benthic foraminifera do not occur in these deeper-water facies (see Tunisia section below). Monte Bove South, Sibillini Mts., Central Apennines, Italy.—This section is located on Monte Bove South to the west of the top of the ski lift that starts at the Hotel Felicitas, in Fontignano (Fig. A1, available from JSR’s Data Archive. See Acknowledgments section.). The section, which is all within the Calcare Massicio, is logged from the track leading west from the ski lift and continues to the summit of Monte Bove South. Details are given in Casaglia (2003–4). The section comprises peritidal limestones with particularly well-developed subaerial facies with reddened horizons with meter scale tepee structures. It also contains fissures filled with fine sediment with locally abundant ammonites. A loose crinoid ossicle from mountainside scree has been dated at 186.9 to 188.6 Ma (early to mid Pliensbachian), and early to mid Sinemurian foraminifera biozones are identified in the lower two thirds of the section (Fig. 2). The top third of the logged section ends at the mountain top but late Sinemurian forams have not been detected. However, the combination of the foram biozones and the Sr date suggests that the entire Sinemurian is present. Chalkis Quarry, Evvoia, Greece.—This very large cement quarry to the north of the road on the Athens side of the new Chalkis bridge to Evvoia island (Fig. A1) provides a continuous but sometimes faulted section through Lower Jurassic carbonates of the Pantokrator Formation. The measured section (Fig. 2) comes from a relatively unfaulted portion of the quarry that in 2002 was to the west of a large vertical zone of faulted, brecciated, and dolomitized strata. The continuous 120 m section measured (some locally covered with speleothems) lies between two faults which repeat the Liassic succession. No material suitable for Sr

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isotope dating was found but a complete series of foraminifera biozones is present from the Hettangian through to the early Pliensbachian (Fig. 2). Jebel Aziz, Tunisian Dorsales, Tunisia.—This previously unlogged section in the Oust Formation (Soussi 2002) crops out as an isolated hill to the west of the town of Bir M’cherga, about 50 km south of Tunis (Fig. A1). The measured section lies between two quarries, an active one to the north and a disused one to the south, and starts above the fields at the base of the hill. The 245 m section continues unbroken to the crest of the hill where the overlying Zaghouan Formation crops out. These overlying hemipelagic mudstones have been dated at this locality by Faure´ and Peybernes (1986) by ammonites indicating an early Pliensbachian (Tropidoceras zone) age. The lower 180 m of this 240-m-thick section comprises the early to mid Sinemurian on the basis of the foraminiferal assemblages; a late Sinemurian interval is unproven at this locality because benthic forams are unidentifiable from the dolomitized portions of the section and are rare or absent in the deeper-water facies in the upper quarter of this section (cf. Almonacid de la Cuba section above). However, the section is believed to be complete because of the ammonite record from the overlying mudstones. Parts of the section are altered by meters-scale pods of a late-stage, texture-destroying dolomite, and these areas have been worked around to produce the log. Aghbalou n’Kerdous, southern High Atlas, Morocco.—This section crops out continuously in the wadi north of the village of Aghbalou n’Kerdous, between the towns of Goulmima and Tinerhir (Fig. A1, see Acknowledgments section). The log was measured on the northwestern side of the wadi along the irrigation channel and starts with the first continuous section above the fields to the north of the village. It equates to section 27 of Crevello (1990; Plate 21) where it is described as Sinemurian in age. The section is mainly within the Jebel Rat Formation, with the exception of the top few meters which we interpret to be the Aghbalou Formation as described in the Carte Geólogique du Maroc. It is dated here by a number of Sr isotope analyses from pristine oysters occurring at the top of the section indicating an earliest Pliensbachian age of 188.3–189.8 Ma (Table 2, Fig. 2). Diagnostic foraminifera are absent from the central part of this section, where intertidal and subaerial facies, with well-developed sheet cracks and tepee structures, dominate. However, the early and mid Sinemurian biozones are present at the base and the late Sinemurian and early Pliensbachian are present at the top of this measured section (Fig. 2; Boudagher-Fadel and Bosence 2007).