Tectonic control of facies architecture, sequence ... - Earthdoc

Basin Research (2004) 16, 235–257, doi: 10.1111/j.1365-2117.2004.00231.x

Tectonic control of facies architecture, sequence stratigraphy and drowning of a Liassic carbonate platform (Betic Cordillera, Southern Spain) P. A. Ruiz-Ortiz, n D. W. J. Bosence,w J. Rey,z L. M. Nieto, n J. M. Castro n and J. M. Molina n Depto. de Geolog|¤ a, Facultad de Ciencias Experimentales, Universidad de Jae¤n, Spain wDepartment of Geology, Royal Holloway University of London, Surrey, UK zDepto. de Geolog|¤ a, E.U.P. Linares, Universidad de Jae¤n, Spain n

ABSTRACT This paper develops a tectono - stratigraphic model for the evolution and drowning of Early Jurassic carbonate platforms.The model arises from outcrop analysis and Sr isotope dating of successions exposed in the Betic Cordillera in southeastern Spain. Here, an extensive Early Jurassic (Sinemurian) carbonate platform developed on the rifted Tethyan margin of the Iberian Plate.The platform was dissected by extensional faults in early jamesoni times (ca. 191 Ma) and again in late ibex times (ca.188 Ma) during the Pliensbachian stage. Extensional faults and fault block rotation are shown to control the formation of three sequence boundaries that divide the platform stratigraphy (the Gavilan Formation) into three depositional sequences.The last sequence boundary marks localised drowning of the platform and deposition of the deeper water Zegri Formation, whereas adjacent platforms remain exposed or continue as the site of shallow-marine sediment accumulation.This study is based on mapping, facies analysis and dating of platform carbonates exposed in three tectonic units within the zone: Gabar, Ponce and Canteras. Facies analysis leads to the recognition of facies associations deposited in carbonate ramp environments and adjacent to synsedimentary, marine, fault scarps. Sr isotope dating enables us to correlate platform-top carbonates from the di¡erent tectonic units at a precision equivalent to ammonite zones. A sequence stratigraphic analysis of sections from the three tectonic units is carried out using the facies models together with the Sr isotope dates.This analysis indicates a clear tectonic control on the development of the stratigraphy: depositional sequences vary in thickness, have wedge- shaped geometries and vary in facies, internal geometries and systems tracts from one tectonic unit to another. Criteria characterising depositional sequences and sequence boundaries from the Gabar and Ponce units are used to establish a tectono - stratigraphic model for carbonate platform depositional sequences and sequence boundaries in maritime rifts, which can be applied to other less well- exposed or subsurface successions from other sedimentary basins. Onlapping transgressive and progradational highstand systems tracts are recognised on dip slope ramps. Falling stage and lowstand systems tracts are developed as thick breccia units in hangingwall areas adjacent to extensional faults. Sequence boundaries vary in character, amplitude and/or duration of sea-level fall and persistence across the area. Some boundaries coalesce onto the Canteras unit, which remained as a relatively positive area throughout the early Pliensbachian (Carixian).The carbonate platform on the Ponce tectonic unit drowned in the latest Carixian (davoei biozone). However, the adjacent tectonic units remained emergent and developed a long-lived sequence boundary, indicating tectonic subsidence as the major cause for platform drowning.The stratigraphic evolution of this area on the rifted southern Iberian margin indicates that a widespread restricted shallow-water carbonate platform environment accumulating peritidal carbonates evolved with faulting to a more open-marine setting. Sr dating indicates that this transition took place around the Sinemurian^Pliesbachian boundary and it was driven by local fault-related subsidence together with likely post-faulting regional subsidence.

INTRODUCTION Correspondence: P. A. Ruiz-Ortiz, Depto. de Geolog|¤ a, Facultad de Ciencias Experimentales, Universidad de Jae¤ n, 23071 Jae¤ n, Spain. E-mail: [email protected] r 2004 Blackwell Publishing Ltd

Sequence stratigraphic models have commonly been applied to successions in which widespread regional, or

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P. A. Ruiz-Ortiz et al. possibly eustatic, sea-level changes are thought to have controlled the accumulation of marine shelf successions. This has led to the use of sequence stratigraphic surfaces and systems tracts in regional correlation (Vail et al., 1984; Graciansky et al., 1993). Their application to stratigraphic analysis in tectonically active areas is less common and few studies have been undertaken on tectonically active marine carbonate successions. Developing the facies models proposed by Leeder & Gawthorpe (1987) for carbonate sedimentation on and around extensional fault blocks, Bosence (1998), Bosence et al. (1998) showed how tectonic movements on fault-blocks could generate and control the evolution of accommodation space and the critical relationship between this space and the location of the subtidal carbonate factory. Tectonically generated carbonate depositional sequences and sequence boundaries show characteristically wedge- shaped geometries with the diachronous development of systems tracts (contra van Wagoner et al., 1988). Although footwall sites were emergent and developing a sequence boundary, hangingwall dip slopes were subsiding, being £ooded and accumulating transgressive systems tracts (TST). Recently, these models have been developed by Brachert et al. (2002) in their study of carbonate sequence stratigraphy, facies and geometries across relatively small- scale footwall blocks and adjacent half-graben basins from the Miocene of SE Spain. In this paper, we extend these concepts to a more regio nal, and therefore more broadly applicable study where outcrops are not continuous and surfaces cannot be mapped out continuously. During Early Jurassic times,

the Southern Iberian Continental Margin (SICM) was dominated by widespread carbonate platforms (Figs 1 and 2). Today, these platforms outcrop in the Subbetic zone of the Betic Cordillera, southern Spain, as the Gavila¤ n Formation (Fig. 1). Localised outcrop studies have shown the occurrence of coeval deepening and shallowing successions in these marine carbonates that have been interpreted as having accumulated over extensional faultblocks (Ruiz-Ortiz & Vera, 1992; Rey, 1993, 1995a, b, 1997; Vera, 2001). In this study, we integrate data from a number of these localities from the Murcia and Almeria provinces of Spain (Fig. 3). Because of the lithologically diverse stratigraphy preserved on these fault-blocks and the lack of suitably precise biostratigraphic markers, we use Sr iso tope dating for correlation. The new dating enables a sequence stratigraphic framework to be established. Facies analysis, local mapping of stratigraphically signi¢cant surfaces and our dating indicate that the stratigraphy can be divided into depositional sequences de¢ned by sequence boundaries. Extensional fault movements control the geo metry, thickness variation, facies occurrence and system tract development within depositional sequences and the occurrence and timing of tectonically controlled sequence boundaries. The revised interpretation indicates a complex tectonostratigraphy for the area during the Pliensbachian (Carixian^ early Domerian) at the early stages of rifting of the SICM.We show the existence of synchronous fault movement in the area during the jamesoni ammonite biozone previously interpreted to be of intra-Sinemurian age (Garc|¤ a-Herna¤ ndez et al.,1986 -1987,1989; Andreo et al.,

Fig. 1. (a) Geographical location of the Betic Cordilleras in the southern Iberian Peninsula. (b) Simpli¢ed geological map. (c) Palinspastic reconstruction of the South Iberian Continental Margin (SICM) for the latest Jurassic. Modi¢ed from Garc|¤ a-Herna¤ ndez et al. (1989).

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r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

Liassic carbonate platform

Fig. 2. (a) Early Liassic palaeogeographic map with position of palinspastic cross- sections, 1-2 and 2-3-4, in (b); modi¢ed fromVera (2001). (b) Sketch cross- sections of the Southern Iberian Continental Margin just after the early Pliensbachian faulting.

1991). More localised movement then occurred towards the middle of the ibex ammonite biozone, and regional differential fault-activity occurred during late ibex^davoei times. The latter event led to localised drowning of this already fault-dissected carbonate platform. This paper details the sedimentary response to faulting within the platform, and the dating and sequence stratigraphic analysis provide an integrated and consistent interpretar 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

tion of the development of a complex carbonate platform stratigraphy. This paper introduces the study area and research methods and then details the nature of the sequence boundaries, depositional sequences and their facies within the Lower Jurassic Gavila¤n Formation prior to a discussion of the controls on the accumulation of the stratigraphy and presentation of a tectonostratigraphic model.

237

P. A. Ruiz-Ortiz et al.

Fig. 3. (a) Geological map of the studied region with positions of localities studied in this paper. (b) Geological map of the Gabar unit. (c) Geological map of the Ponce and Canteras units. Maps based on 1 : 50 000 scale IGME geological maps: 952 (Ve¤ lez Blanco) and 932 (Coy).

GEOLOGICAL SETTING The External Zones of the Betic Cordillera (Fig.1) comprise sedimentary rocks deposited in the SICM during the Alpine tectonic cycle (Mesozoic to Early Miocene). To the north^northwest of the Cordillera, Prebetic para-autochthonous units are overthrust by southerly derived Subbetic allochthonous units. The Subbetic is divided into Intermediate, External, Median and Internal domains or subzones, from northwest to southeast, respectively (Figs 1 and 2), based largely on the occurrence of di¡erent thrust units (e.g. Garc|¤ a-Herna¤ ndez et al., 1980; Vera, 2001). The early Liassic was characterised by an extensive carbonate platform that occupied much of the SICM at this time (Fig. 2). Di¡erences among the Subbetic domains became more marked from the early Domerian (mid-Pliensbachian), re£ecting continued maritime rifting on this margin (Vera, 2001). The earlier extensive carbonate platform was dissected and subdivided with at least two large sectors that continued with shallow-platform sedimentation. The northern area was attached to the continent and comprises the Prebetic, Intermediate Domain, External Subbetic and northern Median Subbetic zones (Fig. 2b). The southern, more o¡shore area, includes the southern Median Subbetic and the Internal Subbetic (Fig. 2b). These two large sectors, with shallow-platform sedimentation, were separated by a central belt of irregularly occurring, intraplat-

238

form basins with hemipelagic sedimentation. The southern platform, the main focus of this paper, was, in turn, composed of a series of smaller platforms with varied morphology. New dates in this paper indicate that prior to the jamesoni zone peritidal carbonates of a similar nature accumulated throughout the area. From jamesoni to davoei zones fault-blocks accumulated di¡erent carbonate successions according to their tectonic history and palaeogeo graphic position. Regionally, from the earliest Domerian (start of the late Pliensbachian) onwards, the northernmost sectors (Prebetic Zone) continued as platform settings, whereas southern sectors (Subbetic) evolved to more pelagic and/or open-marine environments. This study examines outcrops of the Gavila¤ n Formation (Lower Liassic) from three tectonic units from the southern Median and the Internal Subbetic (Figs 2 and 3).The Sierra Gabar (Internal Subbetic) sections are located in Almer|¤ a province, and the Ponce (Median Subbetic) and Canteras (Internal Subbetic) sections further north in Murcia region (Fig. 3). Figure 2b shows the interpreted palaeogeographic position of the studied tectonic units as deduced from their stratigraphy, present tectonic position and the subzone that they represent from the Subbetic. The Ponce and Canteras units are anticlinal structures with axes oriented N^S to N101E. The easternmost limb of the Ponce unit is overturned and thrust over the Canteras unit (Fig. 3), which has a vertical eastern limb. The Canteras unit overthrusts Cretaceous and Tertiary rocks located in more r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257


Fig. 4. Logs, panels and correlations of the stratigraphic sections studied (for locations of sections and horizontal scales, see Fig. 3).

southern and eastern areas. According to Paquet et al. (1974), the thrusting occurred in the lateTortonian.

METHODS Fieldwork, facies and biostratigraphic analysis Fieldwork was carried out to log critical sections and construct detailed two -dimensional stratigraphic panels of r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

some localities (e.g. Figs 4, 7 and 8). Facies, structures and sequence stratigraphic surfaces have been mapped out on the southwestern face of Sierra Gabar (Figs 3 and 4). A similar two -dimensional cross- section has been mapped out from a photomosaic at Barranco del Pardo (Figs 4, 7 and 8). At Loma Prieto, a near continuous roadside cut exposes a one-dimensional section, whereas at Canteras an exposure with good lateral continuity has been studied. The 1 : 50 000 scale IGME geological maps of the Gabar (952, Ve¤ lez Blanco) and Ponce-Canteras (932, Coy) have

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Fig. 5. 87/86Sr isotopic values for the early Liassic (circles) based on belemnites and correlated with ammonite zones and subzones (after Grocke 2001). Stage ages from Palfy et al. (2000), and subzone ages based on equal duration of each subzone between each stage age. Domerian biozones from Braga (1983). Our analyses (from Table 1) are plotted as triangles. Note that the standard error of all measurements is within the size of the symbols used and for our data are given in Table 1 together with the mean ages. Ages determined by plotting highest and lowest 2 sd error values for 87/86Sr and ¢nding equivalent oldest and youngest ages on horizontal time scale as per dashed lines given for sample 200-2.

Table 1.

13/12

Sample no.

Carbon, 18/16 Oxygen and 87/86 Sr isotopic values for brachiopod and matrix samples from the Gavila¤ n Formation d13C PDB

Loma Prieto (Ponce) 200-2 1.41 200-8 2.94 201-1-1 1.51 Barranco del Pardo (Ponce) A-2 2.37 B-2 2.23 B-2 (matrix) 0.25 C-5 2.31 D-3 1.73 D-3 (matrix) 1.48 Sierra de Gabar G-2 2.71 G-4 4.54 G-4 (matrix) 0.50

d18O PDB

87

Sr/86Sr corrected

Age

2.74 1.66 6.78

0.707328 10 (mid-value)

189.2 (188.3^190.3) (luridum-brevispina)

0.707313 10

189.2 (188.3^190.1) (luridum-brevispina)

3.54 2.80 3.89 2.33 4.43 2.84

0.707337 11 0.707328 10 0.707411 9 0.707310 9 0.707282 9 0.707277 9

189.9 (188.7^191.2) (valdani-taylori) 189.3 (188.4^190.4) (valdani-brevispina)

1.67 2.19 1.69

0.707356 10 0.707322 9 0.707513 9

189.1 (188.1^190.0) (luridum-brevispina) 188.7 (188.0^189.3) (luridum-masseanum)

190.2 (189.0^191.4) (masseanum-taylori) 189.2 (188.3^190.2) (luridum-brevispina)

For locations of samples see Fig. 4. Ages determined from mid-Sr value and maximum and minimum Sr value (2 sd) compared with ages of ammonite subzones as illustrated in Fig. 5

been revised and modi¢ed where necessary (Fig. 3). About 160 samples were collected for thin- section study to assist facies analysis. Macrofauna are generally scarce in the

240

Gavila¤ n Formation and ammonites are very scarce (Garc|¤ aHerna¤ndez et al., 1979). Benthic foraminifera are present within the Lower and Middle Liassic, but they do not r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

Liassic carbonate platform appear to be su⁄ciently di¡erentiated to allow more than stage-level age assignments at best.

Sr isotope dating Sr isotope stratigraphy is based on the essentially synchronous variations in the 87Sr/86Sr ratio in the world’s oceans due to variations in continental and sea£oor weathering, which show a simple falling trend through the Liassic (Fig. 5; Jones et al., 1994; Jenkyns et al., 2002). Brachiopods are known to precipitate their calcite shells with stable C, O and Sr isotopes in equilibrium with ambient sea water (Veizeretal.,1999). If such shells can be shown to be diagenetically unaltered, then 87Sr/86Sr ratios can be obtained from sampled shells and used for dating against a 87 Sr/86Sr curve where ages are known (e.g. Fig. 5; Gr˛cke, 2001; Jenkyns et al., 2002). For this study, we have collected brachiopod shells from three study areas in the Gavila¤ n Fm (Gabar, Barranco del Pardo and Loma Prieto; Figs 3 and 4). Samples were initially screened by light microscopy and cathodoluminescence to determine the degree of textural preservation of the shells. The diagenetically unaltered samples showed an absence of luminescence and were further checked by analysing d13C and d18O to ascertain any diagenetic alteration from early Jurassic sea water values (Table 1). Sr isotopic analyses on unaltered shells were performed in accordance with standard techniques using the mass spectrometers in the Department of Geology, Royal Holloway University of London. In addition, matrix samples were also analysed for d13C, d18O and 87Sr/86Sr for comparison with the diagenetically unaltered shells (Table 1). Previous work on Early Jurassic d13C values was undertaken by

Jenkyns etal. (2002), who recorded data derived from belemnites in northwest Europe and showed a mid-Sinemurian positive excursion to around 14 followed by a negative excursion to about 0 to 1 at the Sinemurian^Pliensbachian boundary. This negative trend in values is not recorded in our, rather closely space samples but otherwise the brachiopod data fall within their Sinemurian to Pliensbachian range of values. Our d18O values (Table 1) fall within the range reported by Jones et al. (1994) for the Jurassic with the exception of sample 201-1-1. Jenkyns etal. (2002) present d18O values of between 0 and 4 for belemnites from the Sinemurian to Pliensbachian and our brachiopod samples fall within this range. The screening for diagenetic alteration using cathodoluminescence and d13C and d18O values indicates that the brachiopod analyses that we present have all, except for one sample, preserved their original Early Jurassic seawater values. One brachiopod sample (201-1-1) has a relatively light d18O value but the Sr ratio is very similar to samples 200-2 and 200-8.These values suggest some alteration of 201-1-1 that occurs about 25 m higher in the section than the 200 samples. The 87Sr/86Sr ratios for all the brachiopod valves decrease up to the measured sections, which is consistent with the global trend at this time (Jenkyns et al., 2002). Geological ages are obtained by taking the corrected 87Sr/86Sr ratio (together with minimum and maximum 2 sd values) (for details of these, seeTable 1) and plotting the equivalent zone or subzone on the 87 Sr/86Sr ratio vs. time graph (Fig. 5). This graph is based on Sr ratios of belemnites and oysters from European successions sampled at ammonite subzone level (Gr˛cke, 2001; Jenkyns et al., 2002). The zones and subzones are dated using the recent stage ages of Palfy et al. (2000) and assume that subzones are of equal duration (Fig. 5).

Fig. 6. Chronostratigraphic diagram for each tectonic unit. Sr ages from this study (Table 1; Fig. 4). Stage ages from Palfy et al. (2000). r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

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SEQUENCE STRATIGRAPHY AND FACIES ASSOCIATIONS Stratigraphic framework In the Subbetic and particularly in the study area (Fig. 3b, c), Jurassic carbonates deposited before the early Do merian are referred to as the Gavila¤ n Formation (Van Veen, 1969).The Gavila¤ n crops out to varying extents in all three tectonic units and comprises three members (Lower or M1, Middle or M2 and Upper or M3) as described by Nieto et al. (1992) and Rey (1997), and that are best represented in the Ponce unit as described below. The M1 Lower Member of the Gavilan Fm throughout the Subbetic outcrops comprises dolostones, formed from the transformation of peritidal carbonates. M2 is composed of packstones and grainstones with ooids, peloids or bioclasts as the main allochems. M3 is basically characterized by the common presence of crinoidal limestones. The correlation of these members from one section to another has been based, until now, on both lithostratigraphic and biostratigraphic criteria, biostratigraphy being focused on microfossils, mainly foraminifera (Nieto et al., 1992; Rey, 1993; Nieto, 1997), as ammonite are very scarce in the shallow water facies in this formation. In this work, sections have been correlated using both lithostratigraphic and biostratigraphic criteria and, in addition, in some sections Sr isotope dates provide more precise correlation (Table 1; Figs 4 and 5). In this paper, we focus on the Middle and Upper Members of the Gavila¤ n and show that its age extends from 190.2 Ma (G-2, 189.0^191.4) to 188.7 Ma (D-3, 188.0^189.3) equating to the brevispina subzone of the jamesoni zone to the valdani subzone of the ibex zone (Table 1; Figs 4 and 5). The sedimentary facies and sequence stratigraphy of the Middle and Upper Members of the Gavila¤ n Fm are described together with sedimentary breaks R1, R2 and R3 (Rey 1997), which de¢ne the upper and lower surfaces of these two members. These breaks are interpreted here as sequence boundaries and the members are shown to be unconformity-bound depositional sequences. Our data are from sections 1 and 2 within the Gabar unit, ¢ve sections from the Ponce unit (sections 3^7) and section 8 within the Canteras unit (Figs 3b, c, 4 and 6). The correlation of the R1 sequence boundary that separates the Lower and Middle Members is based on lithostratigraphic data and Sr ages of overlying M2 strata (Fig. 6). Lithostratigraphic correlation and our new Sr ages allow detailed correlation between the Gabar, Barranco del Pardo and Loma Prieto sections for the Middle Member (Figs 4 and 6). Correlation with M2 of the Canteras unit is based on mapping, lithologic and regional criteria (e.g. crinoidal grainstones are only found regionally within the M3 Member). In addition, the laterite level in this section at R112 is interpreted to be laterally equivalent to the black pebble levels at the R1 sedimentary break in Gabar (Figs 4 and 6). The R2 correlation has been made with lithostratigraphic criteria within the framework of our Sr dates. In

242

sections 5 and 7 of the Ponce unit, the correlation is based on Sr ages.This break coincides with the disappearance of the Lithiotis facies and the appearance of crinoidal packgrainstone facies (Fig. 4). At Gabar, the break is evidenced by the presence of a black pebble bed, at the highest part of M2, between the Lithiotis limestones and a level of mudmounds. The correlation of M3 between all sections has also been made using lithostratigraphic and regional criteria, principally, the occurrence of more open-marine crinoidal and cherty limestones (Fig. 4). The sequence boundary at the top of the Gavila¤ n Fm (R3) has been correlated using biostratigraphic and lithostratigraphic criteria. This boundary is between the limestones of the Upper Member and the ammonite-bearing marly limestones of the Zegr|¤ Fm. In the Gabar and Canteras units, this sedimentary break places the Gavila¤ n Fm against Middle and Upper Jurassic limestones, respectively. This surface is an important regional mapping horizon in the Jurassic of the Betic External Zones; it marks the transition from erosion-resistant limestones of the Gavila¤n Fm to less resistant lithologies of the Zegr|¤ Fm. Below, we describe the sequence boundaries (R1^R3) and the depositional sequences, or members (M2 and M3), from the three tectonic units.

Sequence boundary R1 This sedimentary break has been described in the central sectors of the Median Subbetic by Garc|¤ a-Herna¤ndez et al. (1986 -1987, 1989), who classed it as a discontinuity surface of likely intra-Sinemurian age. At Gabar (sections 1 and 2, Fig. 4), the upper strata of the Lower Member are faulted (throw of at least 45 m) and tilted with respect to the overlying rocks that seal the fault in an angular unconformity (Fig. 4 section 2; Rey,1997). Overlying the extensional fault, and in¢lling fractures in the underlying M1 Member, are breccias and their relationship with the fault and the termination of the fault at this level indicates that this extensional fault movement occurred prior to, and during, the early stages of accumulation of the overlying M2 strata. In the Sierra de Ponce unit and, speci¢cally in sections 3^5 (Barranco del Pardo sections, Fig. 4), R1is expressed as an angular unconformity between the Lower and Middle Members. The dolomites of the Lower Member are tilted and eroded and then onlapped by limestones of the Middle Member (Figs. 7a and 8). In the Canteras unit (section 8, Figs 6 and 7), the Lower Member (M1) of the Gavila¤ n Fm is a¡ected by fractures sealed by sediments of the Upper Member (M3), and no record of the Middle Member (M2) occurs. The R1 sedimentary break is included within an irregular palaeokarstic surface containing black pebbles and laterite remains (Rey, 1993; Nieto, 1997). The tilting of the underlying strata in the Gabar and Ponce units, the syndepositional fault and sedimentary/ tectonic breccia at Gabar, and probably at Canteras, the subsequent onlap of Middle Member limestones at Gabar r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257


Fig. 7. (a) Photo -mosaic of the outcrop on the NE side of the Barranco del Pardo (Fig. 3 for location) illustrating stratigraphic geometries and sequence boundary R1 of the Gavila¤ n Fm. For an interpretive sketch, see Fig. 8. (bb 5 basal bed of M2 member. b1^b3 lower, middle and upper grainstone bars. (b) Photo -panel of sequence boundary R2 and grainstone bars (b1 and b2) of the Upper Member (c) grainstone bar with cross-bedding (Upper Member of the Gavila¤ n Fm). (d) Normal (extensional) palaeofault displacing cm- sized laminations at the top of the Lower Member. (e, f) Palaeofaults with reverse (compressional) sense and low angle of dip.The location of photos (c^f) is shown in Fig. 8. r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

243

244

Green clays

Laminites

Micritic limestones with oncolites

Brachiopod rudstone

Grainstones with low-angle cross and parallel bedding

Cherty limestone

Grainstones with low-angle parallel bedding

Micritic limestones with gastropods Mytilus limestones

Gervilleioperna and Hippochaeta £oatstone

Lithioperna-Cochlearites £oatstone

F1

F2

F3

F4

F5

F6

F7 ( 5 F5)

F8

F10

F11

F9

Lithofacies

No

F.A. 2

F.A. 1 Subtidal-intertidal environments with low energy Subtidal lowenergy environments

Supratidal Palaeosoil

Environmental interpretation

Open platform with high energy

Orbitopsella praecursor GUMBEL Haurania amiji HENSON Siphovalvulina sp. Lituosepta compressa HOTTINGER Brachiopods Subtidal lowenergy Orbitopsella praecursor GUMBEL environments, locally Haurania amiji HENSON a¡ected by currents Miliolids Crinoids Subtidal open platform Echinoderms a¡ected by currents, Benthonic foraminifera reaching intertidal environment Sponges Hemipelagic environment Crinoids Subtidal open platform Echinoderms a¡ected by currents, Benthonic foraminifera reaching intertidal environment Gastropods Restricted lagoon. Very low energy Mytilus Restricted lagoon. Very low energy Gervilleioperna Restricted lagoon. Hippochaeta Very low energy

Algae

Without biota

Biota

Gervilleioperna are centimetre to decimetre size, with a planar upper side and a convex lower one. Hippochaeta are large globular forms These lithiotis are large with planar and subrectLithioperna-Cochlearites angular shells.They are isolated in a micritic matrix

Wackestone with small triangular shells of Mytilus

Mudstone / wackestone with bioclast of gastropods

Occasional ripples at the top. Grainstone with peloids, ooids, crinoidal fragments, echinoderms and some benthonic foraminifera

Mudstone with sponge spicules

Occasional ripples at the top. Grainstone with peloids, ooids, crinoidal fragments, echinoderms and some benthonic foraminifera

The brachiopods are well preserved but are not in life position.The matrix, occasionally calcarenitic, contains benthonic foraminifera and miliolids

Mudstone or wackestone with benthonic foraminifera

Green clays with small nodules of micritic limestones Mudstone with algal structures, frequently planar, but sometimes with a lower stromatolitic level

Facies association Description

Table 2. Facies and facies associations described and interpreted from the Middle and Upper Members of the Gavila¤ n Fm

Middle Member (M2)

Gavila¤ n Fm members



Disorganised £oatstone/rudstone lithiotis

Black pebble conglomerate or breccia Mud-mound

Oolitic grainstones

Crinoidal grainstones

F13

F14

F16

F17


Cherty limestones

Marls and marly limestones

F19 ( 5 F6)

F20

F18 5 F5, F7 Peloidal packstone-grainstones

F15

Lithioperna-Cochlearites £oatstone to rudstone

F12

F.A.3

Grainstone of oolites with minor proportions of crinoids, benthonic foraminifera and oncolites Crinoidal grainstone with small amounts of oolites, bivalves and some intraclasts Packstone or grainstone of micritized grains such as abraded shell fragments, corals, ooids and mud clasts Mudstone containing sponge spicules and occasional bioclasts. Nodular chert is frequent Mudstone with peloids, some bioclasts and belemnites

Mudstone with some bioclasts of bivalves and black pebbles

Levels with a large number of black clasts up to 10 cm size. Some clasts are Lithiotids

The shells form colonies, with a base formed of shells in horizontal position. Upwards they evolve towards a vertical position The Lithiotids are the most frequent clast lithology forming lumachelles. Black pebbles also occur

Belemnites

Sponges

Corals

Crinoids Benthonic foraminifera Bivalves

Bivalves

Lithioperna-Cochlearites Corals Gastropods Lithioperna-Cochlearites Gervilleioperna Mytilus Gastropods Reworked Lithiotids

Hemipelagic environment Hemipelagic-pelagic environment

Inner^ outer platform transition.Very low energy High- energy open platform High- energy open platform Low- energy platform

Supratidal environment

Open platform with high energy; a barrier context Open platform with moderate to high energy

Zegr|¤ FM

Upper Member (M3)


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Fig. 8. Sketch taken from the photograph in Fig. 7a of the outcrop located to the NE of the Barranco del Pardo; labelling lithologies, facies, lithostratigraphic and sequence stratigraphic features, and location of photographs (c^f) in Fig. 7. Note locations of measured sections 5 and 6.

and Ponce all indicate a phase of extensional tectonism controlling the formation of this sequence boundary. Andreo et al. (1991) proposed two reasons for this surface being of late Sinemurian age: (1) The occurrence of a benthic foraminiferal association at the top of the Lower Member with Lituosepta recoarensis CATI and Lituosepta compressa HOTTINGER and the absence of Orbitopsella praecursor (GMBEL) and Haurania deserta HENSON. (2) The palaeontological and lithological change between the algal laminite facies of the Lower Member and the Lithiotis limestones of the Middle Member. The latter facies is considered to be of Pliensbachian (Carixian) age in other sectors of the Betic Cordillera (Mart|¤ nezGarrido & Rivas, 1988) and in other Tethyan domains (Bosellini, 1972; Broglio -Lo¤riga & Neri, 1976; Sherreiks, 2001). Our Sr dating has been undertaken on the beds immediately above this surface at Gabar (from within the Lithiotis limestones), Barranco del Pardo (sample A) and Loma Prieto (Sample 200; Table 1; Figs 6 and 7). These provide dates within the jamesoni to ibex zones of the early Pliensbachian (Carixian). Therefore, assuming a late Sinemurian age for the top of the Lower Member, following Andreo et al. (1991), a jamesoni to ibex zone (earliest Pliensbachian or Sinemurian/Pliensbachian boundary) age seems likely for the R1 sequence boundary (Fig. 6).

The Middle Member (M2) of the Gavila¤n Formation Facies and facies associations From the ¢eld logging and thin section work 14 facies are described in the Middle Member of the Gavila¤ n Fm.These are grouped into two Facies Associations: FA.1 and FA.2

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(F1^F14, Table 2; Fig. 10). The lower Facies Association (FA.1) in Loma Prieto and in the eastern part of the Barranco del Pardo (sections 5^7, Fig. 4) occurs as a cyclic succession of facies organised in metre- scale elementary sequences (Fig. 9a). The lower beds of this sequence comprise shallow subtidal^intertidal facies (F3^F5) that are usually capped by intertidal laminite facies (facies F2). Locally at Loma Prieto, the top of these elementary sequences contains green clays with micritic nodules (F1). In the lower part of the Middle Member towards the west at Barranco del Pardo, there is a lateral change of facies to grainstones (F5) in section 4 (Fig. 4), and then in section 3 to cherty limestones (F6, Fig. 4). At Barranco del Pardo, a basal bed of dolomitized laminites (F2) occurs (bb, Fig. 8), which is a¡ected by both normal and reverse micro faults (Figs 7d^f and 8). The facies association that characterises the upper part of the Middle Member (FA.2) includes Lithiotis limestones (facies F9^F14, Table 2) with bioclastic grainstones (facies F7 equating to F5 above) and intercalations of micritic limestones with gastropods (F8;Table 2; Fig.9b).The facies present in this association appear, generally, without a clear cyclic arrangement, although some vertical and lateral trends can be described. In Loma Prieto (section 7, Figs 5 and 9b), two types of elementary cyclic sequences are present, similar to those previously described by Rey et al. (1990) in nearby sections: Type 1 ^ Micritic limestones with gastropods (F8) or grainstones (F7) at the base, followed by a succession of Mytilus (F9) then Gervilleioperna rich limestones (F10), and ending with a brecciated level with black pebbles (F14) (Fig. 9b). Type 2 ^This cycle type di¡ers from the former one only in its upper part. The black pebble beds are lacking, and the cycles end with Lithioperna-Cochlearites r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257


Fig. 9. Elementary sequences of Middle Member (M2) of the Gavila¤ n Fm. For details, see text.

Fig. 10. Sedimentary facies models for westerly facing ramps deduced from facies analysis of the Middle and Upper Members of the Gavila¤ n Fm. SeeTable 2 for facies description.


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P. A. Ruiz-Ortiz et al. ba¥estones and rudstones (F12) evolving from initial horizontal to vertical orientations towards the top (Fig. 9b). In the Loma Prieto section, FA.2 is well developed and shows a general vertical trend from type 1 cycles dominating in the lower part, to a higher proportion of type 2 cycles up- section. In the Barranco del Pardo, Lithiotis limestones are weakly developed and mainly composed of disordered shells, with few in life position. In section 6 (Fig. 4), there is a vertical transition from a predominance of Gervilleioperna (facies F10) in the base to Lithioperna-Cochlearites (facies F11 and F12) towards the top. Laterally, there is a gradual reduction in the content of Lithiotis towards the west and none are found in section 3 (Fig. 4). At the scale of the whole study area there are some marked lateral di¡erences, such as the lack of the Middle Member (M2) in the Canteras unit section (section 8, Fig. 4) and, by contrast, the great thickness of Lithiotis limestones (F9^F12) developed in section 1 at Gabar (Fig. 4). In this latter outcrop, FA.2 occurs overlying the Lower Member (M1) of the Gavila¤ n Fm and FA.1 is not present. However, some lateral changes in thickness and facies from the NW to the SE sectors exist. To the NW (section 1), F9 and F10 facies are predominant, whereas F11 and F12 facies are better developed in the SE at section 2. Between these sectors, in the central part of the Gabar outcrop the M2 Gavila¤ n Member thins over the footwall of the palaeofault and a breccia occurs containing pebbles from both the Lower Member (laminite facies, F2, Table 2), and the Middle Member facies (e.g. Lithiotis limestones), black pebbles and reddish- coloured micritic clasts (Fig. 4, section 2). Interpretation of facies Facies Association 1 is interpreted as having developed in shallow-marine and peritidal environments (Fig. 10a). The oncolite-bearing micritic limestones (F3) and brachiopod rudstones (F4) were generated in subtidal, lowenergy environments (cf. Rey, 1993; Nieto, 1997), while the cross- and parallel-laminated bioclastic grainstones (F5) are interpreted as deposited in an inter-subtidal, openwater setting a¡ected by currents. The juxtaposition of higher- with lower- energy facies (above) may have been through tidal channels (Fig. 10a). The microbial laminites (F2) are typical of subtidal^intertidal conditions with low energy (cf. Bosence et al., 2000).These four facies are organized in metre- scale, shallowing-upward sequences, with a decrease in energy towards the top (Figs 4 and 9a). The micritic nodules in clay (F1) are interpreted as nodular calcrete palaeosoil facies and in Loma Prieto indicate that supratidal conditions were reached in this unit associated with shallowing-upward sequences. To the west, more open-platform conditions occurred with the deposition of moderate-to -highenergy grainstone facies, F5 (Fig. 10a) and the local development of brachiopod rudstone

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facies (F4). These resulted from lower- energy environments, probably in the lee- side of grainstone shoals. The most westerly outcrops record deeper-water conditions that accumulated cherty, micritic limestones (F6). Facies Association 2 is interpreted as being deposited under shallow platform conditions, with development of a lagoonal area to the east (Fig. 10b). The Lithiotis limestones (F9^F11) have been interpreted as accumulating in both restricted and open-platform environments; Mytilus facies (F9) are considered to be deposited in low energy and low sedimentation rate environments (Rey et al., 1990; Rey, 1997). Gervilleioperna and Hippochaeta (F10) represent characteristic facies of protected lagoons, probably slightly restricted, leading to the development of monospeci¢c accumulations (Fig.10b). On the other hand, Lithioperna-Cochlearites facies (F11 and F12) have been interpreted as deposited in more open-marine environments, sometimes a¡ected by moderate-to -highenergy currents. The vertical transition in these facies from horizontal to vertical life positions has been interpreted to be a response to an increase in the sedimentation rate (Rey et al., 1990; Rey, 1997).The facies with reworked and fragmented shells (F13) is formed by erosion of bivalve colonies, and redepo sition, probably without noticeable transport, under moderate-to -highenergy open-platform conditions. Therefore, the facies association of the Loma Prieto outcrop is interpreted as having formed initially in a lagoonal, restricted environment (FA.1) that evolved in time towards more open-marine waters (FA.2).The facies asso ciations of the Barranco del Pardo outcrops are interpreted as deposited in more higher-energy, open-platform environments. Moreover, in both facies associations, a lateral facies change re£ecting more distal, open-marine conditions towards the west are clearly recorded in the Barranco del Pardo outcrops. In the north westerly section on Sierra Gabar (section 1), the lower- energy lagoonal facies are found and the southeasterly transition to Lithioperna-Cochlearites (SE of section 2) re£ects higher- energy waters and open-marine environments. Both areas are separated by a tectonically elevated footwall fault block in the central part of the Gabar unit where M2 is thinner (Rey, 1997).The breccia associated with this fault is derived from erosion of the footwall area, probably during and subsequent to emergence.To the more eastern part of the studied region, the only location studied (i.e. Canteras unit) indicates subaerial exposure that lasted the entire time span embraced by the Middle Member. Stratigraphic geometries, system tracts and age The lower and the upper surfaces of the Middle Member (M2) of the Gavila¤ n Fm are sedimentary breaks with localised subaerial exposure or sequence boundaries. The Member can therefore be classed as a depositional sequence (sensu van Wagoner et al., 1988). The internal architecture of the depositional sequence, the facies, their distribution and the geometry of the deposits can be deduced from a synthesis and correlation of the outcrops at r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257


Fig. 11. Detailed stratigraphy, environmental interpretations and sequence stratigraphic units and surfaces for sections located on the Ponce tectonic unit.

Fig. 12. Tectonostratigraphic model for the sequence stratigraphy of the Gabar, Ponce and Canteras units. Note 10 vertical exaggeration for two-dimensional panels and location of sections in Fig. 3. r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

249

P. A. Ruiz-Ortiz et al. Gabar, Barranco del Pardo, Loma Prieto and Canteras (Figs 4, 11 and 12). Following the intra-jamesoni faulting and erosion event, the Canteras unit continued to undergo subaerial erosion and sequence boundary development, whereas the Gabar and Ponce units accumulated carbonate sediments on fault-blocks undergoing di¡erential subsidence. This is evidenced by the di¡erent thicknesses of coeval strata exposed in these two tectonic units. In addition, at Gabar and at Barranco del Pardo the large two -dimensional outcrops indicate the wedge- shaped geometry of the depositional sequences and the laterally variable facies and environments (Fig. 12). The hangingwall region southeast of the fault at Sierra Gabar (section 2, Fig. 4) is ¢lled with breccia derived from erosion of the footwall area subsequent to fault movement. This breccia therefore represents ¢ll during erosion and sequence boundary development on the footwall and can be labelled as a combined falling stage and lowstand systems tracts (FSST and LST, Fig. 12). This is overlain by Lithiotis limestone that onlaps the breccias and also accumulates on the footwall area of the fault. Here, some15 m of Lithiotis limestones overlie the sequence boundary R1 (section 2, Fig. 4) that thicken northwestwards to about 50 m. These shallow-water deposits represent accumulation during a phase of increasing accommodation space, and, with little up- section change in facies, a TST overlying the FSST and LST is interpreted. Superimposed on this overall relative rise is a higher frequency relative rise and fall as re£ected in the small- scale type 1 elementary cycles of FA.2. The sections at Barranco del Pardo provide more continuous exposures, and stratigraphic geometries and facies allow system tracts to be identi¢ed. The basal bed (bb, Fig. 8) is a marine unit overlying and onlapping the sequence boundary (R1) eroded into the underlying peritidal dolomite unit of the Lower Member (M1). The basal bed is made up of sediment derived from erosion of shallower, eastern, areas and shows a brecciated appearance and syndepositional folds, although lightly obliterated by the dolomitization, which are evidence of gravitational sliding to the west. These folds are accompanied by updip extensional fractures (Fig. 7d) and down-dip compressional faults (Fig. 7e, f), which are genetically related to the slidefolds. The presence of these slides indicates deposition on a westerly dipping surface, indicating tectonic activity associated with the sedimentary break (R1) and the base of the Middle Member (M2). The basal bed at Barranco del Pardo is overlain by shallow marine and peritidal facies of FA.1 arranged in elementary sequences. These sequences pass up into FA.2 (Fig. 8). Progradational geometries are well displayed in these latter two units, indicating restriction of accommo dation space and an highstand system tract (HST) in this area (Figs 7, 8 and 12).The top of the prograding HST, and the end of the deposition of the Middle Member is de¢ned by a surface of local erosional truncation (sequence boundary R2).

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The age of this depositional sequence is well constrained by Sr dating of brachiopod levels at Barranco del Pardo (Figs 4 and 6). Here, four brachiopod-bearing levels show a consistent up section decrease in 87Sr/86Sr ratio equivalent to ages ranging from late jamesoni zone to midibex zone (sample A-2, 188.7^191.2 to sample D-3,188.0^ 189.3; Table 1; Figs 4^6).

Sequence boundary R2 This sedimentary break de¢nes the boundary between the Middle Member (M2) depositional sequence and the Upper Member (M3) depositional sequence (Figs 4, 6 and 8). This surface is only clearly de¢ned in the Barranco del Pardo, where it is marked by erosional truncation of M2 followed by retrogradational geometries in M3 (Figs 7, 8 and 11). Sr isotope dates obtained from brachiopods immediately below this surface (sample D-3 Table 1) give a mean age of 188.7 (mid-valdani subzone of ibex zone). The overlying beds have not been dated. An equivalent erosional surface is not evident at Loma Prieto and in this one-dimensional roadside section would be di⁄cult to identify and isolate from the minor exposure surfaces associated with each of the small- scale elementary cycles in FA.2. However, an increase in accommodation space is indicated in the section with the transition from Lithiotis limestones (Middle Member, M2) to crinoidal grainstones (M3) and we consider the base of this transition as the laterally equivalent R2 surface (Fig. 4, section 7). We have sampled and analysed brachiopods from this level (sample 201, Fig. 4, section 7), but the negative d18O values (Table 1) suggest alteration and they have the same Sr ratio as brachiopods from sample 200 some 25 m lower in the section (see Methods above). At Sierra Gabar there is no angular unconformity at the level, but a well-developed black pebble bed (F14) below the mud-mound level indicates a relative sea-level fall and local emergence.This is followed by an increase in accommodation space to accumulate ¢rstly mud-mounds (o10 m thick) and then more open-marine facies of crinoidal grainstones (F16). The bottom of the black pebble bed and the overlying strata are taken as equivalent to the R2 erosional truncation at Barranco del Pardo followed by the retrogradational geometries in M3 (Fig. 12). At Canteras, our correlations indicate that R2 combines with R1 to form a palaeokarst (Figs 4 and 12). The evidence of emergence and erosion at this level at Gabar and Barranco del Pardo followed by subsequent transgression and more open-marine and higher-energy environments suggests a regional relative, or eustatic, sea-level fall then rise. No tectonic features are associated with this break in the study area. However, the existence in adjacent regions of fractures ¢lled with crinoidal facies (Vera et al., 1988), which is likely to be coeval to the Upper Member of the Gavila¤ n Fm (see below), suggests that a tectonic control was also an important factor in sequence boundary development. r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257


Upper Member (M3) of the Gavila¤n Formation Facies and facies associations Five facies types (F15^F19, FA.3, Table 2) are described from M3 and the Member can be identi¢ed in all the studied sections (Figs 4 and 6). Large and signi¢cant lateral thickness and facies changes are evident in M3 (Fig. 4). In the Barranco del Pardo (Figs 7 and 9), three cross-bedded, grainstone bars (F16 and F18) are intercalated in micritic limestones with nodular cherts (F19). The cross-bedding (Figs. 7c and 8) indicates a westerly £owing (N 270^290 E) current direction. The lower bar (b1, Fig. 8) is an oolitic grainstone (F16), with the proportion of crinoids increasing upwards to F17.The middle bar (b2 in Fig. 8) is a grainstone mainly composed of crinoid debris with some ooids, with crinoids dominating the lower part of the bar and the western sectors of the Barranco del Pardo outcrop. The upper grainstone bar (b3 in Fig. 8) varies in thickness from 5 to 10 m and mainly consists of oolitic grainstones (F16), with the proportion of crinoids again increasing towards the top. Westwards (section 3 in Fig. 4), these bars are replaced by peloidal pack-grainstones (F18) with coral patches. In the Canteras unit (Fig. 4, section 8), the M3 member directly overlies R11R2, sequence boundaries and the crinoidal facies (F17) is the only facies type present. In the Loma Prieto section 7 (Fig. 4), M3 is mainly composed of peloidal packstones-grainstones (F18) and lesser amounts of ooids, crinoids, benthic foraminifera and bioclasts (F16 and 17); locally, there are also sponge spicules and thin bivalves.The presence of nodular chert (F19) in this latter facies is related to the high concentration of sponge spicules in the sediment. In the Gabar unit (Fig. 4; sections 1 and 2), overlying the R2 sedimentary break, a relatively continuous level of mud-mounds occurs, forming the lower part of the M3 Gavila¤ n Fm. These are up to 10 m in thickness and 40 m across. The mud-mounds comprise massive micritic limestone (F15) with very scarce skeletal components preserved in the core of the mounds. The micrite shows thin lamination and possible cyanobacterial remains. Overlying this level, F17 (crinoidal) and F18 (peloidal) facies occur.

Interpretation of facies The oolitic and crinoidal grainstones (F16, F17) are interpreted as being deposited in highenergy, open-platform environments at shallow depths. The peloidal pack-grainstones (F18) are usually found in slightly lower- energy platform environments. The mudstones with occasional nodular chert and sponge spicules (F19) are interpreted to have been deposited in relatively deeper water environments (Fig. 10b). These facies indicate a westward transition at Barranco del Pardo into more distal and open-water positions (Figs 8 and 10).This relatively simple distribution indicates that the westerly sloping platform continues from jamesoni r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

times, which is in accordance with the regional and palaeogeographic data (Fig. 2; Rey, 1993; Nieto, 1997). In the Canteras unit, crinoidal facies represent highenergy, open-platform environments at shallow depths. In the other outcrop on the Ponce unit, Loma Prieto, outer-platform facies occur, but also with some packages of more open and relatively deeper facies with sponge spicules and chert. The mud-mounds of the Gabar area are lowenergy facies, and their context indicates an inner platform setting (cf. Holocene mounds of Florida Bay, Bo sence,1995).They are overlain by crinoidal (F17) and peloidal (F18) facies that represent the transition to openwater, outer-platform environments. The lateral distribution of the facies of the di¡erent outcrops does not ¢t the simple model that has been erected for the adjacent sections of the Barranco del Pardo.This is because sediment accumulation was occurring on di¡erent fault-blocks even if they are outcropping today in close proximity (e.g. Loma Prieto and Canteras).The apparently unique occurrence of the mud-mounds above an emergent sequence boundary (black pebble conglomerate) and beneath a thick (20 m) grainstone succession suggests accumulation in a very restricted shallow-marine environment that has not been preserved elsewhere.

Stratigraphic geometries, system tracts and age The Upper Member of Gavila¤ n Fm can be described as a depositional sequence as it is de¢ned by two unconformity surfaces (R2 and R3). It mainly comprises a well-developed TSTand there is no clear evidence of a period of high stillstand of sea level. At Barranco del Pardo the TST is expressed in the retrogradational stacking of the ooid bars (Fig. 11), in Loma Prieto by the aggradational stacking of shallow-marine grainstones followed by more distal cherty micrites and at Gabar by the upward transition from inner platform mud-mounds to more open-marine crinoidal grainstones (Fig. 12). The alternation of grainstones and cherty mudstones with spicules within the Ponce unit indicates cycles of sea-level change of a higher order than that of theTST. The extensive area of cherty mudstones at Barranco del Pardo represents the deepest water facies and indicates maximum £ooding of the platform at the top of the depositional sequence (Figs 11 and 12). The extent of this relative sea-level rise was such that the Canteras unit was submerged for the ¢rst time since M1 deposition.The TST marks the top of this depositional sequence and an HST does not appear to have been developed, or has been eroded, although no evidence for the last interpretation has been observed. This unit has not been dated in detail because of the absence of suitable calcitic fossils but a post mid-ibex zone age for the initiation of the sequence is obtained from the youngest date from the top of sequence M2 at Barranco del Pardo. A late ibex^davoei time is the probable age range for the depositional sequence, considering also the age range of the sequence boundary R3 (see below).

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Sequence boundary R3 In the Ponce unit, this break, which de¢nes the top of the Upper Member (M3) of the Gavila¤n Fm, is recorded at the base of a condensed level with abundant fossils (brachio pods, gastropods, belemnites and ammonites) separating the Gavila¤ n and the Zegr|¤ Fms (Fig. 4, section 7 and Fig. 6). In this level, Rey (1993) and Nieto (1997) found ammo nites (Fucciniceras isseli FUCINI, F. fucini D’ORB., Protogrammoceras celebratum FUCINI and Fieldingiceras ¢eldingii REYNES) that typify the lavinianum biozone (portisi subzone) of the mid-Pliensbachian (early Domerian). Also, at other locations in the SICM the basal sediments overlying this regional R3 surface have been dated as earliest Domerian (portisi ammonite subzone, Braga, 1983). In the Ponce unit, this surface is overlain by the Zegr|¤ Fm comprising pelagic marls and marly limestones. Because of the transition from underlying shallow-marine, cross-bedded grainstones to cherty mudstones and then a condensed pelagic interval, this surface is considered to indicate deepening waters and stratigraphic condensation on the Ponce unit (Fig. 11). Considering the age data available, mid-ibex zone from the top of the M2 sequence and earliest Domerian (portisi subzone) from beds overlying R3, it is considered that R3 is probably davoei zone in age (Fig. 6). However, more precise age data are necessary to rule out a latest ibex age for R3 and the extent or otherwise of a sedimentary break at this level in the Ponce unit. In the nearby Canteras unit, this sedimentary break is represented by a palaeokarstic surface with evidence of emersion such as the presence of bauxites ¢lling karstic cavities in the underlying M3 grainstones (Seyfried, 1978; Vera et al., 1986 -1987, 1988; Molina et al., 1991; Rey, 1993; Nieto, 1997). Locally, on this palaeokarstic surface, Upper Jurassic Ammonitico Rosso limestones directly overlie the Lower Member of the Gavila¤n Fm and the Middle and Upper Members are absent (Figs 4 and 11). In the Gabar unit, the stratigraphic break at the top of the Upper Member of the Gavila¤ n Fm corresponds to a sharp and irregular surface carved into the underlying crinoidal (F17) facies. Oolitic limestones of Middle Jurassic age overlie the R3 unconformity although, locally, some irregularities of the surface carved in the underlying M3 rocks and some cavities (karstic?) occur ¢lled with red marly-micrite limestones (Rey, 1993) (Fig. 4, section 2). This prominent regional stratigraphic marker level clearly has di¡erent origins in the di¡erent tectonic units. On the Ponce unit, there is a rapid relative sea-level rise indicated by the transition from shallow marine to pelagic facies, and a short period of non-deposition (with or without emersion) could be the cause of the likely absence of the davoei ammonite zone. The surface clearly marks drowning of the carbonate platform that, on the basis of current data, is either aTST ( 5 drowning surface with associated condensation) or a sequence boundary (time gap, non-deposition and condensation followed by £ooding). However, on the Gabar and Canteras units a relative sea-

252

level fall, emergence and karsti¢cation occurs (Fig.12) followed by a much later transgression.This opposite sea-level history on these di¡erent blocks indicates a strong tectonic control on the origin of this sequence boundary and evidence that the drowning of the platform in the Ponce unit was tectonically driven while adjacent blocks became subaerially exposed.

DISCUSSION Srontium dating The Sr isotope dating of these shallow-marine Liassic carbonates has been pivotal to the correlation of the sections and the erection of a sequence stratigraphic framework. Previous benthic foraminiferal dating provides stage-level assignments, at best, and although they may be internally consistent, they have not been tied to standard ammonite zonal schemes. The Sr values from brachiopod valves screened using cathodoluminescence and stable C and O isotopes all fall within recently published curves based on Early Jurassic belemnites and oysters from Northwest Europe (Grocke, 2001; Jenkyns et al., 2002). Although this dating technique has been known for many years it is still not widely used in stratigraphy. This study indicates its value in being able to date non-ammonite bearing, shallow-marine Liassic carbonates to ammonite zone level. The main limitation of this technique in the study area is the limited occurrence of beds with large, well-preserved brachio pods. Our dating has been carried out on all brachiopodbearing levels found within the stratigraphy and this is limited by the occurrence of more open-marine facies containing brachiopods. No oyster levels were found to compare Sr values between the two fossil groups.

Ramp facies The Gavila¤ n Formation is varied lithologically and 19 facies are described in this paper. Most of these facies have been previously described from the Liassic of theTethyan area, particularly in southern Europe. Because many examples come from successions involved in Alpine tectonism large two - or three-dimensional exposures showing lateral facies relations are the exception rather than the rule. The large two -dimensional sections assembled for the Ponce tectonic unit allows reconstruction of a relatively steeply sloping westerly facing ramp. The ramp has an o¡shore-directed energy input as shown by cross-strati¢cation in grainstone bars and illustrates in over just 2 km the transition from inner ramp peritidal and lagoonal facies, to mid-ramp (open platform) bioclastic and oolitic pack to grainstones, to outer ramp (hemipelagic cherty micrites). The wedge- shaped geometry of the two depositional sequences within this ramp (Fig. 12) indicates tectonic tilting of this ramp to the west (see also below) and it is likely that the region represents a hangingwall dipslope ramp (sensu Burchette and Wright, 1992; Bo sence et al., 1998, Brachert et al., 2002) related to an original r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

Liassic carbonate platform east dipping extensional fault and footwall area to the east. The dip of the depositional slope at Ponce would have been less than 151, which is the present angle formed by R1 and R2. This angle is the combination of the original depositional slope, likely di¡erential compaction and subsequent tectonic tilting (Figs 7a and 8). The Canteras unit is a candidate footwall area (cf. Bo sence et al., 1998; Brachert et al., 2002) with its superimposed sequence boundaries and shallow-water Liassic facies (Fig. 12), but today it is separated by thrust faults from the Ponce tectonic unit and this correlation cannot be proved.The tectonic tilting of the platform, as indicated by the synsedimentary faulting, wedge- shaped depositional sequences indicates a tectonic control on the origin, and persistence, of this westerly facing ramp over about 4 million years prior to its drowning.

Lithiotis limestones These limestones are well known and widespread in the Tethyan region and are generally regarded to be of Pliensbachian age in the Subbetic (Mart|¤ nez-Garrido & Rivas, 1988; Rey et al., 1990; Rey, 1997), Apulia (Bosellini, 1972; Broglio -Lo¤riga & Neri, 1976) and Pelagonia (Sherreiks, 2001). They are considered to have accumulated in inner platform/lagoonal environments and this is consistent with their occurrence on the well- constrained facies model for the Ponce unit ramp presented. Here, they are di¡erentiated into facies that formed in more restricted to less restricted waters in a trend of three facies: Mytilus to Gervilleioperna-Hippochaeta to Lithioperna-Cochlearites, respectively. Such trends are recorded both vertically and laterally in the outcrops from Ponce and Gabar and are consistent with the adjacent facies within our facies model. In all three of the sections where this facies occurs it is within transgressive and highstand systems tracts, above cyclic peritidal facies, or a synsedimentary fault breccia and subsequently cut by a sequence boundary. Although the occurrence of this facies is shown here to be of early Pliensbachian age (jamesoni-ibex biozones), its presence presumably is also controlled by the occurrence of suitable lowenergy, inner-platform environments.

Tectonically driven sequence stratigraphy of Liassic carbonate platfrorms It has been known for about 30 years that the stratigraphy of the Early Jurassic carbonate platforms of western Tethys is a¡ected by synsedimentary block-faulting (Bernoulli and Jenkyns 1974). These authors also noted that synsedimentary neptunian dykes and sills had been reported throughout these platforms and that the block-faulting gave rise to a distinctive seamount and basin topography, illustrated in a frequently cited Fig. 8 (Bernoulli and Jenkyns 1974). Positive areas accumulated shallow-water platform carbonates or, following subsidence, deeper-water seamount facies and basins accumulated periplatform carbonates, slope facies and marls. This reconstruction was developed in more detail by Elmi (1990) for platforms r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

and basins developing on the rifted southeastern margin of the Massif Central (France), and by Winterer and Bosellini (1981) to explain the facies and palaeogeography of the rifted northern margin of the Apulian Plate in northern Italy. These authors describe the faulting and subsidence of platforms during the lateTriassic and the Early Jurassic, and following Winterer and Bosellini (1981), describe the platform submergence as diachronous. Previous work in the Betics by Ruiz-Ortiz and Vera (1992), Rey (1997) and Vera (2001), among others, presents a similar picture of Early Jurassic basin evolution. The tectonostratigraphic model developed for the sections we have examined in the Betics (Fig. 12) therefore supports this earlier, more general and widespread work. In addition, we provide more detailed information on how the internal stratigraphy of the platforms (facies, stratigraphic geometries and surfaces) is also driven by extensional tectonics.The dating and correlation of the sections have enabled a sequence stratigraphic analysis to be undertaken, which reveals a dominant tectonic control on the origin of the sequence boundaries and the depositional sequences. This work therefore provides a model for the interpretation of less well- exposed, or subsurface, examples and we list below what we regard as the main features of carbonate platforms growing in extensional tectonic settings. We also note that some of these features have also been recorded from the Miocene rifted platforms of the Red Sea. Although these are set in a di¡erent climatic regime and are mainly attached platforms with an associated siliciclastic supply, they have a number of features similar to theTethyan rifted platforms. Analysis of the outcrops at Gabar, Ponce and Canteras indicates that tectonically generated sequence boundaries on carbonate platforms are characterised by the following features: (1) Synsedimentary extensional faulting, tilting and ero sion (e.g. Sierra Gabar, Barranco del Pardo), (2) fault-related neptunian dykes/fractures and associated sedimentary breccias (e.g. Sierra Gabar), (3) lateral convergence into superimposed sequence boundaries (e.g. Ponce to Canteras units), (4) localised down-dip disappearance from proximal to distal ramp sites (e.g. Barranco del Pardo), (5) lateral change in duration/amplitude and character of relative sea-level fall resulting in lateral change from erosion surface overlain by black pebble conglomerate (Gabar unit) to down-dip disappearance (4 above) to deep weathered lateritic (R1in Canteras unit) or bauxitic (Molina et al., 1991) karst (R3 in Canteras unit). Depositional sequences accumulating in accommodation space generated by fault block rotation are characterised by: (1) variable stratigraphic thicknesses throughout the area. Although the original positions of the platforms cannot be established, within individual tectonic units thicknesses vary from 50 m to 15 m in less than 100 m

253

P. A. Ruiz-Ortiz et al. around synsedimentary faults at Gabar and from 45 to 15 m over 2 km at Barranco del Pardo (Fig. 12), (2) wedge- shaped geometries result from thickness changes in (1). Importantly, these may be repeated vertically in superimposed depositional sequences (e.g. Barranco del Pardo), indicating continued tilting rather than passive ¢lling of depositional relief (Fig. 12), (3) variability in facies and internal stratigraphic geometries. The M2 depositional sequence at Barranco del Pardo indicates geometries close to those proposed in the model of Brachert et al. (2002), including downstepping progradation geometries on the dipslope ramp. However, we also demonstrate a TSTonlapping an underlying erosional sequence boundary that in turn is overlain by a progradational HST. This HST is then truncated by the subsequent sequence boundary (Fig. 12). Because of the relatively steep slope of the ramp, this sequence boundary, and its likely origin through tilting rather than regional sea-level change, is restricted to a proximal location. However, this pattern cannot be seen regionally and the tectonic control in the Subbetic results in di¡erent systems tracts and facies in this same depositional sequence at Sierra Gabar. Here, syndepositional extensional faults result in the development of a thick FSSTand LST breccia followed by a thick onlapping TST in which shallowwater carbonates track an extended period of relative sea-level rise.There is no evidence of a decrease in accomodation space and development of an HST. The overlying depositional sequence (M3) in these two lo cations is quite di¡erent from M2 in both these locations, in that it has an extended TST with backstepping grainstone bars in the Ponce unit whereas shallow, restricted water mud-mounds are buried by more open-marine grainstones at Gabar (Fig.12). No HST is recorded and the sequence drowns on the Ponce unit, but becomes emergent on Gabar and Canteras.

or drowned platform carbonates. To a certain extent, this is also the case for this study from the Ponce unit, which drowned prior to accumulation of the earliest Do merian lavinianum biozone. The timing of drowning can only be constrained by the youngest underlying Sr dated platform carbonates of the mid-ibex biozone.With respect to the cause of drowning, the drowned Ponce unit described in this paper, it is considered most likely that tectonically induced rapid subsidence led to the drowning of shallow marine carbonates. The drowning surface shows stratigraphic condensation and transition into deeper water biotas. No major subaerial erosion surface has been recognised at the drowning surface before. The biotas are not unusual, suggesting that carbonate production was neither, stopped by exposure, or, reduced by palaeo -oceanographic causes, prior to drowning. This contrasts with the interpretation of the detailed record of drowning unravelled from the platforms of the central Apennines, Italy. Here, Santantonio (1993), Santantonio et al. (1996) and Morettini et al. (2002) describe a relatively thick (10’s of metres) drowning succession on a number of platforms each overlying peritidal carbonates of the Calcare Massicio.They consider the drowning to be synchronous within the region, albeit within a rather long time period (the Carixian ca. 5 Myr), and that it may be caused by environmental causes such as oceanic circulation, upwelling or nutrients rather than by tectonic subsidence. A fuller picture of the drowning episode in the Betics cannot be reconstructed as no equivalent slope or basinal sediments are known in the Betic Cordillera. Di¡erential tectonic movement is, however, clearly recorded by uplift of adjacent tectonic units into the subaerial environment and a synchronous deepening-upward facies trend (i.e. increased subsidence rates) in the drowning succession exposed at Loma Prieto on the Ponce unit (Fig. 12). This succession indicates increased subsidence into waters of lower production rates leading to sediment starvation. This resulted in the condensed interval rich in brachio pods, gastropods, belemnites and ammonites.While subsidence was occurring on the Ponce block, adjacent blocks were being uplifted and undergoing erosion.

Platform drowning The drowning of carbonate platforms is a relatively common process in the geological record despite the fact that open-marine carbonates normally accumulate at faster rates than rates of relative sea-level rise (Schlager, 1981). Causes for platform drowning include rapid sea-level rises from glacio - eustacy or tectonism, and environmental changes (e.g. temperature, nutrients) reducing rates of benthic carbonate production (Schlager, 1989; Erlich et al., 1990; Ruiz-Ortiz & Castro 1998). Many of the Liassic platforms that develop on the rifted margins of Tethys drown and are commonly onlapped by slope and basinal facies (Bernoulli and Jenkyns, 1974; Lemoine and Trumpy, 1987; Santantonio, 1993).The timing of drowning is rarely accurately recorded as the dating is normally obtained from the overlying ammonite-bearing facies rather than the drowning

254

Elementary depositional cycles In addition to the outcrop scale (third to fourth order?) depositional sequences described above, M2 contains smaller-scale (higher frequency) elementary sequences. These may also be labelled as parasequences as they are bounded by marine £ooding surfaces (sensu van Wagoner et al., 1988) or high-frequency cycles (cf. Bosence et al., 2000). These elementary sequences all tend to shallow-up to shallower marine facies or to emergent horizons with calcretes (Fig. 9). Such sequences could be generated by repeated local, tectonically driven increases in accommodation space (sensu Cisne, 1986), progradation of carbonate shorelines against a background of tectonic subsidence (sensu Ginsburg, 1971) or eustatic sea-level changes driven by Milankovitch scale climate changes related to the Earth’s orbital r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

Liassic carbonate platform cycles (sensu Fischer, 1964).Whereas extensional tectonism can be demonstrated as the driving mechanism for the larger scale depositional sequences, the origin of the elementary cycles is not clear at this stage and is currently being investigated by us (Bosence et al., 2002, 2003).There seems to be no a priori reason why, in this case, di¡erent driving mechanisms should not operate at di¡erent scales or whether the same control might operate at di¡erent scales in a hierarchical manner.

Regional geological evolution This work indicates that the extensive and sedimentologically rather uniform carbonates of the Lower Member of the Gavila¤n existed until the earliest Pliensbachian or Sinemurian-Pliensbachian boundary. At that time, the platform was dissected by extensional faulting and the tectonic units outcropping today indicate that they underwent different geological histories in the early Pliensbachian. The Canteras unit was emergent for much of the time; the Ponce unit underwent fault-related rotation and subsidence to accumulate a west facing ramp that eventually drowned, whereas the Gabar unit was spectacularly faulted and then underwent subsidence but at all times carbonate production was able to keep pace with this. The two depositional sequences detailed in this paper indicate an increase in marine conditions up section both within the sequences and from sequence to sequence.This is interpreted to re£ect more open-water conditions on the Subbetic platform generated by the localised faulting and likely post-faulting subsidence.

CONCLUSIONS (1) Two unconformity-bounded depositional sequences are di¡erentiated in the Gavilan Fm from the studied outcrops in the Murcia and Almeria provinces. These sequences correspond to Member 2 (M2) and Member 3 (M3) of the Gavilan Fm of previous authors and are separated by three sequence boundaries (R1^R3). (2) The sequence boundaries vary from fault block to fault block and within each fault block dependent on whether they occur in footwall or hangingwall sites. They vary from emersion and/or karsti¢cation surfaces on the footwall to erosional truncation or drowning surfaces in hangingwall sites. (3) Twenty facies types have been di¡erentiated, described and interpreted. They are organized into three facies associations. Fourteen facies within the M2 Member are organized into elementary shallowing-upward sequences or parasequences.The lateral facies evolution in the Barranco del Pardo outcrop records the transition, in a dipslope ramp setting, from shallow-water carbonate facies to more distal, open-marine conditions to the northwest. r 2004 Blackwell Publishing Ltd, Basin Research, 16, 235^257

(4) Dating and correlation have been assisted by Sr isotope analysis of brachiopod shells that have been rigorously screened for diagenetic alteration using cathodoluminescence and stable C and O isotopes. The results show a good correlation with the published 87Sr/86Sr ratio vs. time graph for the Early Jurassic of northwest Europe based on belemnite samples. (5) A tectonostratigraphic model is erected to illustrate the nature and evolution of facies and sequence stratigraphic units for these Early Jurassic carbonate platforms from the Betics (6) Comparisons are made with previous work on other Early Jurassic carbonate platforms from the western Tehtys area.These are also considered to have a strong tectonic control on their formation. A list of the main criteria that characterise the sequences boundaries and depositional sequences is provided for these early Jurassic platforms that formed in extensional tectono stratigraphic settings. (7) The two depositional sequences detailed in this paper re£ect the evolution of rifting in the Subbetic from a large semi-restricted epeiric platform to more openmarine conditions, on individual platforms during the Early Jurassic, as a consequence of the extensional processes a¡ecting the Iberian Plate Margin.

ACKNOWLEDGEMENTS This paper has been ¢nanced by Research Project BTE2000-1151 of the Spanish DGI and Royal Society grant to Bosence at Royal Holloway University of London. We are also grateful to the Working Group RNM-200 from the Junta de Andaluc|¤ a. We also thank A. Piedra and A. Carrillo at the Universidad de Jae¤n for the preparation of thin sections for this study, and to Liz Whittaker, Dr. Dave Lowry and Professor M.Thirlwall from the Department of Geology, Royal Holloway University of London for assistance with the Carbon, Oxygen and Strontium isotope analyses and to Frances Witkowski for assistance with the cathodoluminescence work.

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Manuscript accepted: 2 April 2004

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Sequence Stratigraphy-Facies Analysis and Stylolite-Fracture ...

Facies Analysis, Sedimentary Environment and Sequence ...