Lusitanian Basin - Springer Link

Int J Earth Sci (Geol Rundsch) (2009) 98:1949–1970 DOI 10.1007/s00531-008-0359-3

ORIGINAL PAPER

Sr-isotope stratigraphy of the Upper Jurassic of central Portugal (Lusitanian Basin) based on oyster shells Simon Schneider Æ Franz T. Fu¨rsich Æ Winfried Werner

Received: 29 December 2007 / Accepted: 10 August 2008 / Published online: 13 September 2008 Ó Springer-Verlag 2008

Abstract Strontium isotope stratigraphy was performed on oyster shells from the Late Jurassic of the Lusitanian Basin (central Portugal). This represents the first approach to obtain numerical ages for these strata. The new chronostratigraphic data provide a more precise age determination of several units. After a basin-wide hiatus sedimentation in the Late Jurassic is proven in the Cabo Mondego and Cabaços formations to resume as early as the Middle Oxfordian. The Alcobaça formation can be placed in the latest Late Oxfordian to Late Kimmeridgian, while data from the upper part of the Abadia Formation indicate an Early to Late Kimmeridgian age. The Farta Pao formation ranges from the latest Kimmeridgian to the latest Tithonian. The largely synchronous Sobral, Arranho´ I, and Arranho´ II members are overlain by the late Early to Late Tithonian Freixial Member. The brief, local carbonate incursion of the Arranho´ I member marks the Kimmeridgian–Tithonian boundary. Oysters are shown once more to be suitable for strontium isotope studies. Their calcitic shells are often unaffected by diagenesis. In particular for

S. Schneider (&) W. Werner Bayerische Staatssammlung fu¨r Palaöntologie und Geologie, Richard-Wagner-Strasse 10, 80333 Munich, Germany e-mail: [email protected] F. T. Fu¨rsich Institut fu¨r Geologie und Palaöntologie, Julius-Maximilians-Universita¨t Wu¨rzburg, Pleicherwall 1, 97070 Wu¨rzburg, Germany F. T. Fu¨rsich Geozentrum Nordbayern, Friedrich-Alexander Universita¨t Erlangen, Loewenichstr. 28, 91054 Erlangen, Germany W. Werner GeoBioCenter, LMU, Munich, Germany

marginal marine Jurassic and Cretaceous strata, where belemnites are usually absent, oysters may serve as a valuable tool for isotope stratigraphy. Keywords Oysters Strontium isotopes Stratigraphy Upper Jurassic Portugal

Introduction Despite of more than 150 years of geological research, started by Sharpe in 1850, the stratigraphy and subdivision of the Upper Jurassic rocks of the Lusitanian Basin (central Portugal) are still muddled. This is mainly due to the lack of (bio-) stratigraphic markers. Large parts of the Upper Jurassic succession of the area consist of marginal-marine strata that often formed in low-salinity regimes. Consequently, the classical biostratigraphic indicators of the Jurassic, i.e. ammonites, are scarce or absent from those rocks. Moreover, ammonite stratigraphy is hampered by the palaeogeographic position of the Lusitanian Basin, which had a restricted access to the open Proto-Atlantic. Moreover, the basin occupied an intermediate position between the Boreal and Tethyan faunal provinces, resulting in a largely endemic ammonite fauna (e.g. Leinfelder et al. 2004). As ammonites are scarce or lacking, other macrofossils, such as bivalves and gastropods, were used besides lithological characteristics for stratigraphically subdividing the basin fill (Choffat 1885a, 1885–1888, 1887, 1893a, b, 1901, 1914; Sharpe 1850). However, most of these macrofossils were facies-controlled and therefore of only limited biostratigraphic value. The resulting scheme, refined by subsequent workers, is still partially in use. Moreover, the lithostratigraphic subdivision of the Upper Jurassic strata of the basin is far from resolved. For

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example, the general progradation of the coast line towards the south resulted in most lithological units being diachronous. Additionally, the area is divided into several subbasins by a set of salt domes that started to rise quickly during the Late Jurassic due to syn-rift extensional tectonic activity (Rasmussen et al. 1998; Alves et al. 2002, 2006). Micropalaeontological studies (Ramalho and Rey 1969, 1975; Helmdach 1971, 1974; Ramalho 1971; 1981) revealed that foraminifera, dasycladaceans and ostracods may provide certain stratigraphic information on some of the Upper Jurassic strata. However, as these organisms were facies-controlled, the resulting schemes are not applicable for the entire basin. Likewise, local palynostratigraphic approaches have not led to an integrated concept for large parts of the Upper Jurassic basin fill (Mohr and Schmidt 1988; van Erve and Mohr 1988; Mohr 1990; Sousa Pereira de Castro 1997, 1998; Barroń and Azeredo 2003). During the last few decades, sedimentological and seismic studies produced models for the tectonic and depositional history of the Lusitanian Basin (Rasmussen et al. 1998; Alves et al. 2002, 2006; Carvalho et al. 2005). For the Late Jurassic, two detailed sequence-stratigraphic concepts exist (Leinfelder 1993; Leinfelder and Wilson 1998; Pena dos Reis et al. 2000). Despite these concepts, large-scale correlation is still limited. From this discussion it becomes obvious that a set of sound stratigraphic markers are essential in order to clarify the time-relationships between different strata, the evolution of the basin, and the temporal and spatial distribution patterns of the fauna. The present study provides numerical ages for a large part of the marine-influenced Upper Jurassic strata of the central and northern Lusitanian Basin. The data are derived mainly from the oyster genera Praeexogyra, Actinostreon, and Nanogyra. Belemnites were available only from a single locality. 87/86Sr-isotope values of these organisms were measured and ages derived from correlation with the global Sr-curve (Howarth and McArthur 2003).

Geological overview The Lusitanian Basin stretches along the western margin of central Portugal (Fig. 1). Including the shelf areas, it extends for approximately 300 km in length and 180 km in width (e.g. Montenat et al. 1988; Hill 1989). Onshore, the Lusitanian Basin reaches from Aveiro in the north to the Serra d’Arrabida south of the Tagus River. To the east it is bordered by basement rocks of the Iberian Meseta. To the west crystalline remnants of the former basin margin can be found on the Berlenga Horst (Carvalho et al. 2005).

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Int J Earth Sci (Geol Rundsch) (2009) 98:1949–1970

Fig. 1 Geological overview. NSB northern Lusitanian Basin, CSB central Lusitanian Basin, SLB southern Lusitanian Basin, MRSB Monte Real Subbasin, BSB Bombarral Subbasin, ASB Arruda Subbasin, TSB Turcifal Subbasin. Numbers indicate sample localities. For details see Table 3

Structurally, the Lusitanian Basin is an evaporite-prone rift-basin situated at a passive continental margin (e.g. Wilson 1975; Wilson et al. 1989; Alves et al. 2002). The basin developed during three major rifting phases, i.e., in the Triassic, the Liassic, and the Late Oxfordian (Alves et al. 2002, 2006). The basin is partitioned into three sectors by major faults (Nazare´ and Tagus faults) of supposed Variscan origin (Carvalho et al. 2005). During the Late Jurassic, partitioning into different subbasins started, when the rise of salt domes along lineaments of Variscan origin was triggered by rapid rifting. Onshore, the northernmost Monte Real Subbasin (=northern Lusitanian Basin, NLB) is separated from the central Lusitanian Basin (CLB) by the Nazare´ fault (Tresniowski 1958). The CLB is partitioned into at least three sub-basins, (1) the Bombarral Subbasin, (2) the Arruda Subbasin and (3) the Turcifal Subbasin. For the CLB and SLB, seismic and well data do not provide significant information on further palaeogeographic subdivision in the off-shore area (Boillot et al. 1975; Alves et al. 2006). Consequently, the coastal parts between Nazare´ and the Torres Vedras fault, which are separated


from the CLB by a diapir chain, cannot be directly linked to one of the subbasins. Nevertheless, these successions may be interpreted as marginal strata of corresponding structures to the west. The southern Lusitanian Basin (SLB) is equivalent to the Lower Tagus Subbasin. The Upper Jurassic portion of the basin sediment infill, formed during megasequence J3 (?late Early Oxfordian to Berriasian), is up to 2,700 m thick (Fig. 2; Alves et al. 2002). The entire Lusitanian Basin dips to the SSW. Consequently, the transition from marine to terrestrial sedimentation as a result of infilling of the basin and/or regression during the Late Jurassic gradually shifted southwards. This is expressed by several large-scale diachronous lithostratigraphic units. Continuous marine sedimentation from the Oxfordian to the Cretaceous is recorded only in the southwestern part of the basin (e.g. in the Sintra area; Ellis 1984). The lithostratigraphic subdivision of Late Jurassic strata used herein largely follows Rasmussen et al. (1998). Formations and members in small letters have not yet been formally established. The Oxfordian-Kimmeridgian stage boundary is interpreted sensu gallico herein (see Meleńdez and Atrops 1999 for details). The subdivision of the Kimmerdigian follows Enay (1997). After an erosional gap produced during the BathonianLate Callovian/Early Oxfordian, the Late Jurassic sedimentation started in the ?latest Early/Middle Oxfordian all across the Lusitanian Basin (Mouterde et al. 1979; Azeredo et al. 1998, 2002). In the SLB and CLB, the Middle Oxfordian Cabaços Formation marks the onset of the Oxfordian transgression. Deposition starts with lacustrine to brackish-water calcareous mudstones (Ellis et al. 1990; Azeredo et al. 2002). In the southern part of the basin (e.g. in the type region) the upper portion of the unit is

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composed of marine marly limestones (Moitinho de Almeida et al. 1958; Ruget-Perrot 1961). The Cabaços Formation is conformably overlain by the late Middle to Late Oxfordian marine calcareous Montejunto Formation (Ruget-Perrot 1961; Gomes 1962; Camarate França et al. 1964a, 1964b). In the NLB, both Cabaços and Montejunto formations are replaced by the Cabo Mondego formation (Witt 1977; Rasmussen et al. 1998), which is composed of a heterogeneous series of marine carbonates and marls, lacustrine marls and anhydrites (Alves et al. 2002). In the northern part of the basin, a more proximal, siliciclastic-influenced setting is evident even at this early stage. The Oxfordian-Kimmeridgian transition is marked by an abrupt change from calcareous to siliciclastic sedimentation in the entire Lusitanian Basin caused by a major rifting event (e.g. Wilson 1979; Leinfelder and Wilson 1998). Along the north-south gradient, three lithological units replace each other. In the SLB and large parts of the CLB, the Abadia Formation, a basinal succession composed of turbiditic sandstones and deep-water marls, has been deposited. The still calcareous basal part of the formation in the Montejunto area (sensu Leinfelder and Wilson 1998), named Tojeira member (cf. Mouterde et al. 1973) is Late Oxfordian in age (Atrops and Marques 1988b). It is followed by coarse-grained conglomerates and sandstones of the Cabrito member, which is overlain by the Abadia marls (Mouterde et al. 1973). Locally, crinoid-coral-algal build-ups, the so-called Serra Isabel level, occur approximately 40 m below the top of the Abadia marls. According to Atrops and Marques (1988a, 1988b), the Abadia Formation (including the Tojeira member and the Amaral Formation; see below) ranges from the Upper Oxfordian to

Fig. 2 Traditional lithostratigraphy of the Upper Jurassic strata of the Lusitanian Basin (modified from Rasmussen et al. 1998)

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the basal Tithonian. Taking the general facies development of the area into account, this conclusion seems doubtful. Leinfelder (1993) suggested that part of the ammonites came from overlying strata of similar lithology. Therefore, he proposed the Abadia Formation to extend into the middle Upper Kimmeridgian (Leinfelder 1993; Leinfelder and Wilson 1998). The Amaral Formation directly rests on the Abadia Formation in the CLB. In large areas this unit is composed of shallow marine oolites and sandstones (the Lima pseudalternicosta beds of Choffat 1901; Hill 1989). Especially in the Arruda Subbasin these facies are replaced by coral limestones. While Atrops and Marques (1988a) place this unit in the Early Tithonian, Ellis et al. (1990) and Leinfelder and Wilson (1998) report a Late Kimmeridgian age for the Amaral Formation. North of Bombarral, the Abadia and Amaral formations are replaced by the earliest Kimmeridgian to Early Tithonian Alcobaça formation, a succession of 300–400 m (Camarate França and Zbyszewski 1963; Marques et al. 1992) of marginal-marine, brackish and terrestrial limestones and siliciclastics. Contemporaneous with the aforementioned formations fluvial sandstones of the Lourinha˜ formation were deposited in the northernmost sector of the basin. With time, the Lourinha˜ formation (sensu Hill 1989; including the Bombarral formation of Leinfelder and Wilson 1989; Manuppella 1998) gradually expanded southward to the Torres Vedras area and covered the entire NLB and large parts of the CLB. Stratigraphically, the unit spans the entire Kimmeridgian and Tithonian (e.g. Ramalho 1971; Helmdach 1971, 1974; Witt 1977). In the south, the Lourinha˜ formation is interfingering with a succession of rather heterogeneous marginal-marine, brackish and limnic-fluvial strata, the Farta Pao formation. Originally, this unit comprises the former Pteroce´rien (Choffat 1885a, 1901) (=Arranho´ formation of Leinfelder 1993), and the overlying Freixial formation (Leinfelder and Wilson 1989; Leinfelder 1993). Later, the Praia Azul member of the coastal area south of Santa Cruz, which was assigned to the Lourinha˜ formation by Hill (1989) and the Sobral formation were also included in the Farta Pao formation (Leinfelder 1993; Leinfelder and Wilson 1998). At the same time, the Sobral formation was partially also considered a basal member of the Lourinha˜ formation (Pena dos Reis et al. 1996; Leinfelder and Wilson 1998). Herein, we propose four members for the Farta Pao formation, which are briefly described in the following, as a large number of samples for isotopic analyses was collected from these strata. (1)

The Sobral member, a unit largely composed of brackish-water deltaic sandstones, is by far the most widespread lithologic unit of the Farta Pao formation. From its type area in the western part of the Arruda

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(2)

(3)

(4)

Subbasin it can be traced into the Turcifal Subbasin of the Torres Vedras region, and along the Atlantic coast from Santa Cruz (where it forms part of Hill’s (1989) Praia Azul member) in the south to Praia do Areia Branca west of Lourinha˜ in the north. In the northern part of its range it is directly overlain by the Lourinha˜ formation (Manuppella 1998). The Arranho´ I member is composed of nodular micritic limestones yielding a giant species of Protocardia, which is confined to this member. Leinfelder (1986) suggested that a temporal stop of influx of sediment from the hinterland may have led to the deposition of these pure carbonates. Typical exposures of this member are found on the high grounds between Arruda dos Vinhos and Bucelas. At Santa Cruz, it forms a thin carbonate interval (within the Praia Azul member of Hill 1989), which is characterized by giant Protocardia. The Arranho´ II member is a succession of finegrained siliciclastics and marls of marginal-marine to limnic-fluvial origin. Locally, coral reefs and associated shallow-water carbonates may occur. The occurrence of the bivalve Myophorella lusitanica is clearly limited to this member. North of Bucelas, these strata are widely distributed. They also form part of the Praia Azul member of Hill (1989) at Santa Cruz and a large part of the coastal section south of the Rio Sizandro. The Freixial Member is largely composed of limestones and marine and terrestrial sandstones which are increasing in abundance towards the top. Although restricted to sandy limestone facies, the bivalve ‘‘Trigonia’’ freixialensis may serve as index fossil for the Freixial Member. Outcrops of the Freixial Member are limited to the southern Arruda Subbasin.

As proposed by Leinfelder (1986), there is a general development from the siliciclastic Sobral to the calcareous Arranho´ I member, which is followed by the siliciclastic Arranho´ II member, and the mixed carbonate-siliciclastic Freixial facies in the area south of Arruda dos Vinhos. However, this succession may be largely diachronous, representing laterally changing environments within a large marginal-marine lagoonal system. It is not possible to clearly separate these units biostratigraphically. Traditionally, stratigraphic evaluation of these strata has been based on foraminifera and algae (Ramalho 1971, 1981). However, the occurrence of these organisms is strongly faciesrelated and provides only limited stratigraphic information within such a heterogeneous lithological framework. Vice versa, their absence from other levels of the succession should be considered insignificant. As a whole, the Farta Pao formation ranges from the latest Early Kimmeridgian


to the end of the Tithonian based on microfossils (cf. Ramalho 1971, Leinfelder 1986).

Material For analysis of the strontium isotope composition 89 oyster specimens of the species Praeexogyra pustulosa, Actinostreon solitarium, and Nanogyra nana, and three belemnites of the genus Hibolites were collected (Fig. 3). Oysters mineralize a primarily low-Mg-calcitic shell usually comprised of prismatic and foliate layers (Carter 1990). Hence, they are much less affected by diagenetic processes than aragonitic shells (e.g. Arthur et al. 1983; Veizer 1983b; Jones et al. 1994b). Additionally, oysters occur in a wide range of Late Jurassic marginal marine strata of Portugal. Where they occur, they are usually abundant, and except for the small Nanogyra shells, they are relatively large and therefore easy to sample. Moreover, as fixosessile epibionts, they directly record the strontium isotope composition of the water column in their shells. For these reasons, oysters have already been used for strontium isotope stratigraphy in several studies (e.g. Jones et al.

1953

1994a, b; Veizer et al. 1999; McArthur et al. 1994, 2000b; Denison et al. 2003; Fu¨rsich and Thomson 2005; Wierzbowski and Joachimski 2007). Belemnites grow a solid endoskeleton that is composed of low-Mg-calcite. Therefore, they have been subjected to various geochemical studies, including strontium isotope stratigraphy (e.g. McArthur et al. 2000a, 2000b, 2004; Price and Gro¨cke 2002; Price and Mutterlose 2004). Although no close relatives of belemnites exist today, they are thought to have lived as pelagic organisms because of their body shape (Rexfort and Mutterlose 2006). Obviously, they were restricted to fully marine habitats. Because of these two requirements, they are rare elements in the Late Jurassic sediments of the Lusitanian Basin. The fossils were collected from three types of localities, i.e., (1) natural outcrops along the coast, (2) quarries and road cuts in the interior, or (3) ploughed fields and weathered natural surfaces. All specimens chosen for isotope analysis were collected from horizons that contain characteristic macrobenthic associations, which permit a sound classification of salinity levels (Fu¨rsich 1981; Fu¨rsich and Werner 1984, 1986; Yin et al. 1995). These levels are important for evaluating the reliability of the strontium isotope data, as the Sr-isotope composition may have been affected by freshwater influx (e.g. Holmden and Hudson 2003). A list of the sampled horizons is given in Table 3.

Analytical methods Sample preparation

Fig. 3 Upper Jurassic oysters from Portugal. a Praeexogyra pustulosa, Sobral da Lagoa, Alcobaça fm., Late Kimmeridgian; scale bar 1 cm. b, c Nanogyra nana, Serra Isabel, Abadia Fm., Early/Late Kimmeridgian; outer and inner side; scale bar 0.5 cm. d Actinostreon solitarium, Salgados, Alcobaça fm., Early Kimmeridgian; scale bar 1 cm

The fossils were cleaned with a brush and water. Marly sediment was removed by immersion in hydrogen peroxide. The clean shells were cut into half using a diamond saw and polished slices were produced of all specimens. The remaining shell material was ground into sub-millimetre-sized fragments using an agate mortar and pestle. The fragments were dry-sieved and the best-preserved shell pieces were selected from the 0.5–1 mm residue under the binocular microscope. Nanogyra shells were too small to produce sufficient sample material from a single specimen. Consequently, two (060920/1 B/C), respectively three specimens (060920/2 A/B/C) were lumped in one sample. The chemical treatment of the samples was carried out in a class 100 clean laboratory. Ultrapure multiple distilled acids were used in order to minimize the procedural Sr blank (usually less than 100 pg per sample). The oyster and belemnite fragments were immersed for about 10 min in 0.5 N hydrochloric acid to remove traces of diagenetic calcite, washed in ultrapure water, and dried.

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Cathodoluminescence analysis To assess the state of preservation of the shells, the polished slices were examined by cathodoluminescence microscopy (Technosyn 8200 MK II; 15–22 KV; 400– 550 lA; University of Erlangen, Geocenter Nordbayern). Colour images of all slices were produced using a Zeiss digital microscope camera. From a total of 89 oyster specimens, five shells were excluded from further analyses because of overall luminescence. A single overall luminescent oyster (sample number 050921/1) was kept for comparison with the unaltered material. AAS (atomic absorption spectrometry) Aliquots of the flake samples were dissolved in aliquots of 2.5 N hydrochloric acid in order to determine absolute contents of Mn, Fe, Sr, and Mg by flame AAS (PerkinElmer AAS 3300; LMU Munich, Department of Earth and Environmental Sciences). Replicate analysis of standards revealed reproducibility better than ±5% (1 r) of the concentrations reported for Mn, Fe, and Mg. Apart from a few extremely small samples, reproducibility for Sr was generally better than ±12%. Oysters containing more than 250 ppm of manganese (i.e., 7 out of 84 specimens) were excluded from strontium isotope analysis except for a single altered specimen that was kept for comparison. Usually, not more than two specimens from each sampled horizon were chosen for further analysis. 87

Sr/86Sr analysis

For strontium isotope analysis aliquots of 5–10 mg of the samples were dissolved in 0.5 ml of 9 N HNO3. Purification and accumulation of Sr was achieved by ion-chromatographic column separation using a Sr-specific crown-ether resin (Sr-SpecÒ; method modified from Horwitz et al. 1992; Pin and Bassin 1992; Horn et al. 2005). Strontium was loaded on a mixture of TaCl5, HF, HNO3, H3PO4 and H2O (Birk’s solution) on single-band W filaments. Strontium isotope ratios were measured in static mode on two thermal ionisation mass spectrometers (Thermo Finnigan MAT 261) at the Bayerische Staatssammlung fu¨r Palaöntologie und Geologie (BSPG) and the LMU Munich, Department of Earth and Environmental Sciences (LMU). Measured isotope values were normalized for mass fractionation using the naturally invariant value for 88Sr/86S of 8.37521 and the exponential fractionation law. Accuracy and precision of the mass spectrometer runs were controlled by analysing reference material SrCO3 NIST SRM 987. During the period of analyses the mean value was 0.710252 ± 0.000022 (1 SD, n = 97) (for BSPG lab) and 0.710273 ± 0,000038 (1 SD, n = 23) (for LMU lab). For matching of the oyster and

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belemnite data with the SIS look-up table (Howarth and McArthur 2003), 87Sr/86Sr ratios were adjusted to a NIST SRM 987 value of 87Sr/86Sr = 0.710248.

Results Cathodoluminescence analysis The three investigated belemnite rostra are totally nonluminescent (Fig. 4a). Only few oyster specimens are totally non-luminescent (10), while a large part of the specimens (26) exhibits a twofold situation. The outer layers of the shell that were in direct contact with the surrounding sediment often show bright or dull luminescence, and in most oyster specimens (48) diagenesis has also affected parts of the foliate shell. However, at least large parts of the inner, foliate shell layers are non-luminescent. The non-luminescent portions are large enough for sampling and differ significantly from luminescent parts in showing the original translucent foliate shell structure (Fig. 4b–d). In contrast to the prismatic outer shell layers, the original appearance of foliate shell material can largely be evaluated by binocular microscopy. Only six shells were completely luminescent (Fig. 4c). AAS (atomic absorption spectrometry) The fossils that were examined by AAS contain 300– 1,100 ppm Sr, 400–5,500 ppm Mg, 100–4,600 ppm Fe, and 20–1,000 ppm Mn (Table 3). 87

Sr/86Sr analysis

The 87Sr/86Sr ratios of the samples range between 0.706827 and 0.707157. The derived ages range from *159.50 (Middle Oxfordian) to 146.49 (Late Tithonian). Belemnites and oysters from the same horizons (i.e. from Serra Isabel) produced similar Sr isotope ratios. The single altered specimen yielded a value of 0.707410, indicating a derived age of 134.58 my (Hauterivian). All data are listed in Table 3.

Discussion Sample preservation Only well-preserved, diagenetically unaltered calcitic fossils are considered to preserve initial isotope signals. We consider the oysters and belemnites that were used for Sr isotope analysis to be unaltered, based on the following criteria:


Fig. 4 Cathodoluminescence images. a Cross-section of Hibolites sp. (050903/1). Rostrum totally non-luminescent. Sediment infill of alveole brightly luminescent. b Section of large Praeexogyra pustulosa (051005/3A) with borings of lithophagid bivalves. Large non-luminescent shell portions. Dull luminescence in layers directly

(1)

(2)

(3)

Polished slices of all specimens were investigated for cathodoluminescence. Generally, luminescence in calcitic skeletons, activated by Mn2?, is indicative of diagenetic alteration (e.g. Barbin 2000; van Geldern et al. 2006). All three belemnite rostra and 11% (n = 10) of the oyster shells proved to be overall nonluminescent. At least large shell areas of 83% of the oyster specimens (n = 74) are also non-luminescent. From the oysters exclusively shiny, flaky, largely translucent pieces of foliate shell material without iron or manganese staining were picked, as these shell portions usually are best-preserved (see section above; Denison et al. 2003). Generally, well preserved oyster material can easily be distinguished from altered portions by binocular microscopy. From the belemnites solely clean, brightly shining, translucent, angular fragments showing radial arrangement of calcite prisms were chosen (e.g. McArthur et al. 2000b). The concentrations of Mn, Fe, Sr and Mg provide significant information on the diagenetic state of calcitic fossils. While contents of Mn and Fe usually increase during diagenesis, fossils become depleted in Sr and Mg by the same process (e.g. Brand and

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in contact to sediment. c Section of Praeexogyra pustulosa (060919/ 12B). Totally luminescent specimen with alternating layers of bright and dull luminescence. d Longitudinal section of Actinostreon solitarium (051004/2A) embedded in sediment. Minor dull luminescence in outer layers; inner layers non-luminescent. Scale bar 0.5 mm

Veizer 1980; Veizer 1983a, 1983b; Jones et al. 1994b; McArthur 1994). Empirically derived cut-offs for Mn and Fe in fossil oyster shells and belemnite rostra are available from several studies (Table 2). We have tested all specimens that passed the cathodoluminescence test. The three specimens of Hibolites retain values of Mn, Sr and Mg that are typical for well preserved belemnites (e.g. Saelen and Karstang 1989; McArthur et al. 2004, 2007a, 2007b). Specimen 050903/1 contains 214 ppm Fe, which is higher than the cut-offs applied in several studies (Table 2). Nevertheless, an equal number of scholars uses higher cut-offs (Table 2). We consider this specimen unaltered, as it does not differ from the other two specimens in showing original radial arrangement of clear, translucent prismatic calcite and in being overall non-luminescent. All oysters from the Lusitanian Basin retain trace element values that are well within the ranges known from extant oysters (Table 1). Many extant oyster species are prone to environments of reduced salinity usually found in the vicinity of river mouths. Correspondingly, their shells may display a wide range of trace element contents

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123 Ostrea edulis Pycnodonte Gryphaea Gryphaea, Aetostrea

Ozhigova (1992)

Ozhigova (1992)

Anderson et al. (1994) Jones et al. (1994a)

Western Interior Delaware

Pycnodonte Texigryphaea Crassostrea virginica Oysters Alectryonia Ceratostreon Ostrea

McArthur et al. (1994)


Carriker et al. (1996)

McArthur et al. (2000b)

Denison et al. (2003)



Western Interior

Texas, Oklahoma

Texas, Oklahoma

Texas, Oklahoma

Antarctica

Western Interior

Western Interior

Exogyra Lopha


Western Interior

Western Interior

Great Britain Great Britain

Russian Platform

Crimea

Black Sea

Japan

Black Sea

Delaware

Delaware

Russian Platform

Delaware

Delaware

–

Georgia –

Locality


Crassostrea

Ostrea edulis

Ozhigova (1992)

Ostrea

Crassostrea gigas

Ozhigova (1992)


Crassostrea virginica

Ozhigova (1992)


Crassostrea virginica Crassostrea virginica

Pycnodonteids

Ozhigova (1985)


Crassostrea virginica



Ostrea Crassostrea virginica


Crassostrea virginica Crassostrea

Pilkey and Harriss (1966) Milliman (1974)

Milliman (1974)

Taxon

Author

Cretaceous

Cretaceous

Cretaceous

U.Cretaceous

Extant

Cretaceous

Cretaceous

Cretaceous

Cretaceous

Cretaceous

Cretaceous

Oxfordian M.JurassicCretaceous

Cretaceous

Extant

Extant

Extant

Extant

Extant

Extant

Cretaceous

Extant

Extant

Extant

Extant Extant

Age

1,000–1,466

917–1,186

960–1,080

600–800

2,600–4,200

695

750–780

1,150

680–850

340–1,100

740

207–473 –

–

–

–

–

–

4,103–6,075

3,520–5,561

470–1,000

380–1,152

1,240–2,580

700–1,900

924–1,081 700–1,600

Sr (ppm)

120–440

103–810

1,444–2,258

60–300

600–2,600

630

92–190

395

105–230

315–445

1,300

60–15,512 61–3,626

700–6,000

50–2,600

30–100

300

200

2,791–4,782

28–1,235

200–3,400

20–37

28–37

7–750

159–432 159–432

Fe (ppm)

Table 1 Synoptic chart showing literature data on trace element contents (Sr, Fe, Mn, and Mg) of fossil and extant oysters

14–70

14–102

208–429

50–300

100–1,600

140

46–79

69

35–105

365–515

465

8–73 5–665

30–1,100

20–500

21–37

150

40

604–1,854

1,145–2,303

1,499–2,064

400–600

1,250–1,680

1,770

1,650–2,100

1,250

720–840

615–1,500

865

406–675 –

1,100–2,300

1,600–5,200

2,300–2,400

1,900

1,800

1,747–2,327

320–739

\20–673 429–858

1,300–2,300

183–1,450

148–252

1,900–8,000

1,320–2,540 1,100–2,700

Mg (ppm)

100–1,600

141–961

130–178

3–200

18–50 18–50

Mn (ppm)

Foliated and dense opaque material



Including altered specimens Including altered specimens

Bulk shell; no check for diagenesis

Bulk shell; organic parts not removed




Prismatic shell

Foliated shell

Bulk shell; no check for diagenesis

Prismatic shell; natural and experimental conditions

Foliated shell; natural and experimental conditions

–

Estuarine environment –

Remarks

1956 Int J Earth Sci (Geol Rundsch) (2009) 98:1949–1970

Including altered specimens 440–3,369

724–1,940 28–157 129–385 308–639 U.Jurassic Actinostreon solitaria This study

Portugal

1,784–2,252 20–58

22–997 102–4,623

254–353 667–757

535–971 U.Jurassic

U.Jurassic

Praeexogyra pustulosa This study

Portugal

Nanogyra nana This study

Portugal

– Liostrea Wierzbowski and Joachimski (2007)

Poland

BajocianBathonian

490–675

75–249

\ 25–95

Foliated and dense opaque material 380–3,870 18–976 17–2,824 565–1,702 Cretaceous Ostreids Denison et al. (2003)

Texas, Oklahoma

Foliated and dense opaque material 1,123–3,070 8–826 83–2,224 944–1,289 Cretaceous Texigryphaea Denison et al. (2003)

Texas, Oklahoma

Foliated and dense opaque material 2,005 114 749 1,171 Cretaceous Ilymatogyra Denison et al. (2003)

Texas, Oklahoma

Taxon Author

Table 1 continued

Locality

Age

Sr (ppm)

Fe (ppm)

Mn (ppm)

Mg (ppm)

Remarks


1957

originating from the heterogeneous load of river waters (e.g. Carriker et al. 1980, 1991, 1996). This may also hold true for the Late Jurassic oysters from Portugal but cannot be generally assumed. Therefore, the results from the Portuguese oysters were compared to literature data of fossil oysters (Table 1). Manganese and iron contents in oysters are typically higher than in belemnites (Jones et al. 1994b). This is also the case for the material from the Lusitanian Basin. In contrast, the concentrations of Sr and Mg are not as low as observed by Anderson et al. (1994) in Jurassic Gryphaea. They range considerably below those reported for Cretaceous oysters by Denison et al. (2003), but are comparable to those observed in well-preserved Cretaceous oysters by McArthur et al. (2000b). The average Sr value derived from Actinostreon (554 ppm) from Portugal is significantly lower than those observed in Praeexogyra (725 ppm) and Nanogyra (721 ppm). Most likely, Mg and Sr contents are controlled by specific vital effects (Anderson et al. 1994) and/or changing Sr/Ca ratio of the sea water through time (e.g. Veizer et al. 1999; Steuber and Veizer 2002). Compared to the broad range of values of Mn and Fe observed in extant oysters, the cut-offs proposed by different researchers are relatively strict (Table 2). The cut-off of 100 ppm Mn accepted by most scholars is met by 63% of the oysters from Portugal (Fig. 5). McArthur et al. (1994) and Denison et al. (2003) considered oysters retaining up to 300 ppm Mn to preserve initial 87/86Sr values. This cut-off is met by 93% (=75 samples) of the Late Jurassic oysters from Portugal (Fig. 5). Considering the histogram shown in Fig. 5, we have set the cut-off for this study right below the distributional minimum, i.e., at 250 ppm Mn. Proposed cut-offs for Fe range from 100 ppm (Price and Gro¨cke 2002) to 1,000 ppm (Anderson et al. 1994) (Table 2). Most likely, the Late Jurassic marginal-marine waters of the Lusitanian Basin contained significantly higher concentrations of Fe (and Mn) than sea-water in open-marine areas as they were directly influenced by freshwater run-off (Fe content of ocean/river water *0.7/700 ppm; Sarmiento and Gruber 2006). Additionally, oysters, although living epifaunally, may be more directly influenced by substances released from siliciclastic sediments than free-swimming belemnites. Applying again the distributional minimum criterion, the cut-off level in this study has been fixed at 700 ppm Fe (Fig. 6). Of 11 oysters containing more than 700 ppm Fe, six had previously been excluded from Sr isotope analysis because of high Mn concentrations. As the Mn concentrations of the remaining five samples are low (e.g. only 22 and 23 ppm for samples 050904/2B and 050913/3B), we decided to include these specimens in further analysis. It should be noted that, although high concentrations of Mn and Fe are indicative of diagenetic alteration, low

123

1958 Table 2 Synoptic chart showing literature data on diagenetic alteration cut-offs for Sr, Fe, and Mn (noted in ppm) in oysters and belemnites (selected references)


Author

Taxon

Sr

Fe

Mn

Anderson et al. (1994)

Diverse

\1,000

\100

Jones et al. (1994a)

Belemnites

\150

\30

Jones et al. (1994a)

Oysters

\150

\30

Jones et al. (1994b)

Belemnites

\150

–

Jones et al. (1994b)

Oysters

\150 (300)

–

McArthur (1994)

Diverse

\100

\100


Diverse

\500

\300

Ditchfield (1997)

Belemnites

\300

\100


Oysters

586

298

305

McArthur et al. (2000a)

Belemnites

1,221

210

58


Belemnites

932

251

88

McArthur et al. (2000c)

Belemnites

970

1,330

81

van de Schootbrugge et al. (2000) Price and Gro¨cke (2002)

Belemnites

\300

\30

Belemnites

1,136

\100

\100

Denison et al. (2003) Holmden and Hudson (2003)

Diverse Oysters

603

\300 (500) –

\300 –


Belemnites

937

193

20

Price and Mutterlose (2004)

Belemnites

358

\150

\100


Belemnites

1,021 (690)

\150

\20

Wierzbowski and Joachimski (2007)

Belemnites

[1,150

\200

\30

Wierzbowski and Joachimski (2007)

Oysters

\250

\100

Fig. 6 Histogram showing the distribution of Fe in the Upper Jurassic oyster shells

Fig. 5 Histogram showing the distribution of Mn in the Upper Jurassic oyster shells; proposed cut-off level for diagenetic alteration at 250 ppm

concentrations of the same elements, however, do not inevitably prove the opposite. Whereas Mn and Fe are easily mobilized by fluids under reducing conditions, these elements are far less soluble under oxidizing conditions,

123

resulting in the precipitation of diagenetic calcite that is not enriched in Mn and Fe (e.g. Jones et al. 1994b; van Geldern et al. 2006; Wierzbowski and Joachimski 2007). However, the rocks yielding the fossils utilized in this study usually contain considerable amounts of organic matter (e.g. plant debris in sandy limestone or sandstone from the Alcobaça formation and Sobral member; dark, bituminous limestone from Cesareda or Cabo Mondego; clays and marls containing plant debris and framboidal pyrite from the Arranho´ II member). Consequently, diagenesis under oxidizing conditions can be widely excluded (Jones et al. 1994b).


1959

Fig. 7 Scatter-plots of Mn and Sr over Fe (a), 87/86Sr over Mn (b), c and 87/86Sr over Fe (c), all showing no significant correlation between variables

(4)

(5)

There is no relationship between Fe and Sr or Mn, or between 87/86Sr and Fe or Mn (Fig. 7), indicating that diagenetic assimilation did not occur (McArthur et al. 2000b). All specimens that were considered unaltered, produced 87/86Sr ratios that range within the limits given for the Late Jurassic by Howarth and McArthur (2003). Additionally, the derived ages are in good accordance with the general lithostratigraphic concept of the Late Jurassic Lusitanian Basin. In many cases, measured 87/86Sr ratios for fossils from the same horizon are within the range of analytical reproducibility (Table 3). With a few exceptions, considerable variation among 87/86Sr ratios within specimens from a single sediment layer or discontinuous ratios within stratigraphic sections may be attributed to timeaveraging and/or associated minor salinity fluctuations (Palmer and Edmond 1989; Bryant et al. 1995; Reinhardt et al. 2003; see Salinity effects). This may hold true especially for fossils picked from ploughed fields or weathered surfaces. Furthermore, the single fully-luminescent specimen (containing 306 ppm Mn) that was analysed yielded a far too high 87/86Sr ratio, indicating a Hauterivian (Lower Cretaceous) age. This deviation is clearly diagenetic in origin, emphasizing again that the applied criteria are successful in unmasking diagenetic alteration.

Salinity effects As stated above, the Sr isotope ages may have been influenced to a certain degree by freshwater influx (e.g. Palmer and Edmond 1989; Bryant et al. 1995). Nevertheless, the detection limit for these deviations is around 20% salinity or below for most modern river waters (e.g. Andersson et al. 1992, 1994; McArthur et al. 1994; Bryant et al. 1995; Reinhardt et al. 2003). As the Sr/Ca ratio of the Late Jurassic oceans was insignificantly lower than that of modern oceans (Steuber and Veizer 2002) the detection limit for freshwater deviation may have been similar during this time interval. Most of the sampled faunal associations are attributed to habitats of salinity higher than 20% (Fu¨rsich and Werner 1986). A few samples (C3-25A, B; 051005/1B, C; 060924/7; 060924/3 A, B; 050927/5A, B) were derived from low diversity associations, which may indicate mesohaline conditions. However, the isotope ages derived from these samples correspond well with those derived from adjacent horizons and localities. Consequently, we consider the Sr

123

1960


Cabo Mondego formation

Fig. 8 Time-spans of lithostratigraphic units as indicated by Sr isotope values. Measured values (grey boxes) and calculated errors (brackets). Cabaços and Cabo Mondego formations: Lower limits coincide with point of inflection in global Sr curve (Fig. 9), and represent relatively rough time estimates. Abadia Formation: Lower limit may extend further down, as only samples from the upper part of the unit could be measured

signals derived from the Late Jurassic fossils as largely unaffected by freshwater dilution. Stratigraphic interpretation For the oldest Late Jurassic strata, belonging to the Middle Oxfordian, the error derived from the SIS look-up table (Howarth and McArthur 2003) is considerable as the 87/86 Sr curve reaches a point of inflection at this time interval (Fig. 9). Consequently, the ages derived for the samples from the Cesareda-Serra d’El Rei plateau and for part of the samples from Cabo Mondego are relatively rough estimates. Nevertheless, these values represent the first firm chronostratigraphic data for the onset of sedimentation in the Lusitanian Basin during the Late Jurassic, as dating of the basal layers of the Cabaços and Cabo Mondego formations was previously largely based on the occurrence of a single dasycladacean species, i.e. Heteroporella lusitanica (Ramalho 1971).

We have also tried to fix the onset of sedimentation in the Late Jurassic at the type locality of the Cabo Mondego formation. The Middle/Late Jurassic unconformity at Cabo Mondego is directly followed by fossiliferous sandstones containing oyster-coral patch reefs (Ruget-Perrot 1961; Witt 1977; Wilson 1979; Azeredo et al. 2002). For the two Sr values derived from these oysters no data are given in the SIS look-up table (Howarth and McArthur 2003), as the global Sr-isotope curve reaches a point of inflection in the time interval between 159 and 160 Ma and errors become too large for accurate dating. Nevertheless, at least a Middle Oxfordian age (plicatilis zone) may be derived for the two basal samples. The three samples from higher up in the section range from the Late Oxfordian (bimammatum zone) to the Early Kimmeridgian (hypselocyclum zone). Abadia Formation The only primarily calcitic fossils that were available from the Abadia Formation stem from the Serra Isabel level approximately 40 m below the top of the formation. We have sampled belemnites (Hibolites sp.) and oysters (Nanogyra nana) from two different levels. The derived ages vary from 152.3 to 153.4 Ma and indicate an Early to Late Kimmeridgian age that corresponds to the divisum and eudoxus zones. The Sr isotope values thus point to a slightly younger age than that proposed in the literature (Leinfelder 1993; Leinfelder et al. 2004; Schmid and Werner 2005: hypselocyclum/divisum zones) based on ammonites. The assumption of Atrops and Marques (1988a, 1988b) that the Abadia Formation ends in the Early Tithonian, cannot be confirmed by our data. Moreover, the Late Kimmeridgian to Late Tithonian ages derived for the superimposed Farta Pao formation (see section below) clearly point to a Kimmeridgian age for the top of the Abadia Formation. Alcobaça formation

Cabaços Formation At the Cesareda-Serra d’El Rei plateau we have sampled two layers from the middle part of the Cabaços Formation, based on the sections and descriptions provided by Oertel (1952) and Azeredo et al. (2002). Unfortunately, the relative errors of the measurements from these two samples are comparably large. Nevertheless, a Late Oxfordian to Early Kimmeridgian age (155.3–154.4 Ma; bimammatum-platynota zones) may be derived. The initial transgression at Cesareda, represented by brackish water sediments below the sampled marine horizons most likely occurred during the Middle Oxfordian (Azeredo et al. 2002).

123

We have analysed 22 specimens of Praeexogyra pustulosa from different localities and horizons of the Alcobaça formation. Seventeen specimens come from outcrops in the Bombarral Subbasin, while five specimens stem from the coastal section of Consolaçaõ. Additionally, we have sampled seven specimens of Actinostreon solitarium from the coastal sections of Salir do Porto and Saõ Martinho do Porto. Based on the ages that were derived from the Sr isotope values, the Alcobaça formation ranges from the latest Oxfordian (planula zone) to the Late Kimmeridgian (lower beckeri zone). An identical time span was reconstructed for the coastal section of Salgados (samples

Euhaline Euhaline

Cabo Mondego (29)

Cabo Mondego (29)

Cabo Mondego (29)

Cabo Mondego (29)

Cabo Mondego (29)

Serra Isabel (1)

Serra Isabel (1)

Serra Isabel (1)

Serra Isabel (1)

Serra Isabel (1) Consolaçaõ (28)

Salir do Porto (24)

Salir do Porto (24)

Salir do Porto (24)

Salir do Porto (24)

Saõ Martinho do Porto (25) Saõ Martinho do Porto (25) Saõ Martinho do Porto (25)

Barrio (22)

060925/1 A

060925/1 B

060925/2 A

060925/2 B

060925/4

050903/1

060920/1 A

060920/1 B/C

060920/2 A/B/C

060920/2 D

051004/1A

051004/1f

051004/2A

051004/2B

Sao Martinho

051006/2A

051004/4C

051004/4B

C. Forte

Cesareda (17)

050924/2

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Euhaline

Cesareda (17)

050924/1

Salinity regimes

Locality (numbers refer to Fig. 1)

Sample number

Alcobaça (M)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (?L)

Alcobaça (L)

Abadia (U)

Abadia (U)

Abadia (U)

Abadia (U)

Abadia (U)

Cabo Mondego (?U)

Cabo Mondego (?M)

Cabo Mondego (?M)

Cabo Mondego (L)

Cabo Mondego (L)

Cabaços

Cabaços

Formation/ member

Table 3 Sample and locality data, and results of AAS and Sr isotope analysis

Praeexogyra pustulosa

Actinostreon solitarium








Hibolites sp.

Nanogyra nana

Nanogyra nana

Hibolites sp.

Hibolites sp.








Sample type

152.7 ± 2 153.3 ± 6

0.706948 ± 7

152.0 ± 2

153.5 ± 3

151.6 ± 4

153.4 ± 3

154.3 ± 4

153.6 ± 4

154.2 ± 4

153.6 ± 6

153.4 ± 7

152.8 ± 9

153.4 ± 6

152.3 ± 7

152.7 ± 5

153.8 ± 7

155.3 ± 11

1,087

724

1,035

1,081

1,125

832

1,292

850

832

5,419

2,252

1,784

2,689

5,495

2,063

1,248

943

633

*159.5 ± 30 156.9 ± 17

733

1,181

1,270

Mg (ppm)

*159.5 ± 30

154.4 ± 16

155.3 ± 10

Numerical age

0.706927 ± 16

0.706978 ± 7

0.706921 ± 7

0.706998 ± 15

0.706923 ± 7

0.706897 ± 7

0.706916 ± 7

0.706900 ± 7

0.706918 ± 16

0.706925 ± 21

0.706946 ± 30

0.706923 ± 16

0.706978 ± 26

0.706950 ± 18

0.706913 ± 17

0.706876 ± 13

0.706857 ± 18

0.706827 ± 11

0.706834 ± 13

0.706894 ± 29

0.706878 ± 12

Sr/86Sr ± 2r (abs.)

87

702

559

447

639

535

552

551

635

786

1094

757

667

1,049

1,129

613

602

587

615

622

535

516

Sr (ppm)

254

129

209

135

385

213

299

218

169

147

353

246

121

214

874

263

181

169

440

118

231

Fe (ppm)

53

44

66

28

70

53

61

48

62

18

58

20

20

24

108

55

44

51

110

32

55

Mn (ppm)

Int J Earth Sci (Geol Rundsch) (2009) 98:1949–1970 1961

123

123 Euhalinebrachyhaline Brachyhalinemesohaline Brachyhalinemesohaline

Euhaline Euhaline

Consolaçaõ (28)

Consolaçaõ (28)

Consolaçaõ (28)

Sobral da Lagoa (27)



Vestiaria (23)

Vestiaria (23)

Fonte Santa (18)

Fonte Santa (18)

Salgados (26)

Salgados (26)

Salgados (26)

Consolaçaõ (28)

Chiqueda (21)

Chiqueda (21)

S Carrascal (20)

W Carrascal (19)

050926/2

C3-25-A

C3-25-B

060923/2A

060923/2B

060923/2C

060924/3A

060924/3B

051005/1B

051005/1C

051002/1

051002/2A

051002/2B

050928/1A

051005/3A

051005/3B

060924/7

051005/2A

Euhaline

Brachyhalinemesohaline

Euhaline

Euhaline

Euhaline

Euhaline








Euhaline

Barrio (22)

051006/2B

Salinity regimes


Sample number

Table 3 continued

Alcobaça

Alcobaça

Alcobaça

Alcobaça

Alcobaça

Alcobaça (?U)

Alcobaça (?U)

Alcobaça (?U)

Alcobaça (?U)

Alcobaça (?U)

Alcobaça (U)

Alcobaça (U)

Alcobaça (U)

Alcobaça (U)

Alcobaça (U)

Alcobaça (M)

Alcobaça (M)

Alcobaça (M)

Alcobaça (M)

Formation/ member




















Sample type

154.2 ± 8

0.706897 ± 7

0.706939 ± 14

0.706930 ± 7

0.706886 ± 7

0.706989 ± 13

0.706920 ± 7

0.706905 ± 7

0.706923 ± 7

0.706910 ± 7

0.706910 ± 7

0.706957 ± 15

0.706937 ± 18

0.706972 ± 16

0.706946 ± 16

0.706986 ± 15

0.706923 ± 14

0.706921 ± 12

154.3 ± 4

153.0 ± 5

153.2 ± 3

154.8 ± 5

151.8 ± 4

153.5 ± 3

154,0 ± 4

153.4 ± 3

153.9 ± 4

153.9 ± 4

152.5 ± 4

153.0 ± 6

152.2 ± 4

152.8 ± 5

151.8 ± 4

153.4 ± 6

153.5 ± 5

151.8 ± 4

0.706905 ± 7 0.706996 ± 18

154.0 ± 4

Numerical age

0.706903 ± 16


87

631

711

719

589

1,566

622

738

651

852

687

591

557

919

944

1,244

663

847

742

889

Mg (ppm)

727

661

643

746

764

653

642

764

690

702

683

725

696

771

734

703

687

793

666

Sr (ppm)

167

149

204

209

505

162

272

376

193

170

173

287

405

235

204

166

189

227

459

Fe (ppm)

64

38

55

73

137

24

46

116

69

81

61

73

110

91

81

83

69

132

78

Mn (ppm)


Brachyhalinemesohaline Euhalinebrachyhaline Brachyhalinemesohaline

Chaõ da Cruz (4)

Chaõ da Cruz (4)

E Arranho´ (3)

Santa Cruz (15)

Santa Cruz (15)

Boiero (5)

Santa Cruz (15)

Bemposta/Serra de Alrota (7)




Lameiro das Antas (6)

Lameiro das Antas (6)

NNW S. de Alrota (9) Quinta´ (11)

050915/1B

050915/2

050918/3

050927/5A

050927/5B

050917/2

050927/2

Riff B

Riff A

050910/1

050914/4

050912/5A

050912/5B

050920/2

050925/1

Rio Sizandro (16)


Chaõ da Cruz (4)

050915/1A

050911/1

Euhalinebrachyhaline

A dos Arcos (2)

050918/7


Euhaline(brachyhaline)






Euhaline

Euhaline





Euhaline

W Carrascal (19)

051005/2B

Salinity regimes


Sample number

Table 3 continued

Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa

Arranho´ I Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II



Arranho´ I

Arranho´ II









Sample type

Sobral

Sobral

Sobral

Sobral

Sobral

Sobral

Sobral

Alcobaça

Formation/ member

0.707058 ± 12

0.707006 ± 14

0.707043 ± 11

0.707084 ± 20

0.707015 ± 11

0.707135 ± 10

0.707041 ± 11

0.707019 ± 13

0.707025 ± 27

0.707027 ± 7

0.707033 ± 16

0.707091 ± 7

0.706896 ± 7

0.707047 ± 11

0.707026 ± 11

0.707041 ± 10

0.707088 ± 9

0.707036 ± 10

0.706882 ± 7


87

150.1 ± 4

151.4 ± 4

150.5 ± 4

149.3 ± 7

151.2 ± 5

147.6 ± 5

150.6 ± 3

151.1 ± 4

150.9 ± 7

150.9 ± 2

150.8 ± 4

149.1 ± 5

154.3 ± 5

150.4 ± 4

150.9 ± 3

150.5 ± 3

149.2 ± 4

150.7 ± 3

155.0 ± 6

Numerical age

876

440

840

1,364

2,342

2,519

1,838

976

853

708

1,389

1,240

941

1,153

1,171

496

1,673

742

899

Mg (ppm)

627

644

735

769

971

823

808

791

799

776

705

725

646

849

743

762

786

751

618

Sr (ppm)

129

143

192

636

400

1,807

415

102

89

177

951

647

205

392

329

201

1,169

174

161

Fe (ppm)

134

151

46

79

82

138

63

174

96

165

118

225

40

94

69

75

127

49

39

Mn (ppm)

Int J Earth Sci (Geol Rundsch) (2009) 98:1949–1970 1963

123

123

Rio Sizandro (16)

Rio Sizandro (16)

S Serra de Alrota (8)

Santa Cruz (15)

Santa Cruz (15)

Santa Cruz (15)

Serra de Alrota (10)

Serra de Alrota (10)

N Chamboeira (13)

N Chamboeira (13)

Paços (12)

W Chamboeira (14)

W Chamboeira (14)

050925/2

050925/3

050920/1

050927/3A

050927/3B

050927/4

050920/5A

050920/5C

050904/2A

050904/2B

050921/1

050913/3A

050913/3B

Brachyhaline

Brachyhaline

Brachyhaline

Brachyhaline

Brachyhaline









Salinity regimes

Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa Praeexogyra pustulosa

Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II Arranho´ II

Freixial

Freixial

Freixial

Freixial







Arranho´ II

Freixial

Sample type

Formation/ member

0.707143 ± 16

0.707134 ± 24

0.707410 ± 13

0.707157 ± 19

0.706977 ± 17

0.707053 ± 15

0.707033 ± 11

0.707065 ± 7

0.707046 ± 7

0.707108 ± 7

0.707036 ± 11

0.707063 ± 21

0.707079 ± 11


87

147.2 ± 9

147.6 ± 13

134.6 ± 8

146.5 ± 11

152.0 ± 4

150.2 ± 5

150.8 ± 3

149.9 ± 3

150.4 ± 3

148.5 ± 3

150.7 ± 3

149.9 ± 7

149.5 ± 4

Numerical age

2,653

2,600

1,003

1,654

1,214

1,827

2346

1,320

1,264

616

1,367

1,119

1,524

Mg (ppm)

737

670

665

741

765

708

668

679

691

838

624

565

731

Sr (ppm)

853

550

392

1,185

474

554

424

678

189

159

267

174

278

Fe (ppm)

23

54

306

22

66

66

44

148

156

123

41

151

235

Mn (ppm)

Column 1 Sample numbers. Column 2 Localities and locality numbers (referring to Fig. 1). Column 3 Salinity regimes. Column 4 Formations or members; subdivision of formations spanning large time intervals is indicated in brackets where possible; L/M/U lower/middle/upper part. Column 5 Sampled organisms. Column 6 87Sr/86Sr values ± 2r (abs.). Samples 051006/2A and 051006/2B from Barrio have been measured on both mass spectrometres to allow for direct comparison of results. Column 7 Derived ages. Errors are derived from combination of standard deviation of measurements and relative errors of SIS look-up table. Columns 8–11 Concentrations of Mg, Sr, Fe, and Mn


Sample number

Table 3 continued



Fig. 9 Late Jurassic part of the global Sr curve (bold line) with error interval (thin lines) (redrawn from Howarth and McArthur 2003). Measured Sr-isotope values from Portugal are plotted with errors; vertical: 2r (abs.); horizontal: combination of standard deviation of measurements and relative errors of SIS look-up table

051002/1 and 2 of this study) based on foraminifers (Fatela 1990). Traditionally, the Alcobaça formation is thought to span the entire or at least most of the Kimmeridgian (e.g. Ruget-Perrot 1961; Witt 1977; Wilson 1979; Fu¨rsich and Werner 1986; Rasmussen et al. 1998; Alves et al. 2002 [scaling in fig. 2, p. 730 is inconsistent with the text]). According to Helmdach (cited in Fu¨rsich & Werner 1991), the ostracod fauna indicates an early Late Kimmeridgian age at least for part of the upper portion of the Alcobaça formation. All these results correspond well with the Sr isotope data. In contrast, Pena dos Reis et al. (1996) divided the Alcobaça formation into a lower part spanning the uppermost Oxfordian and lower Upper Kimmeridgian (which would also correspond well), and an upper part ranging from the uppermost Kimmeridgian far into the Berriasian. The two parts are separated from each other by the Amaral formation. However, the authors did not provide an explanation for this interpretation. Based on ammonites Marques et al. (1992) propose an Early Kimmeridgian to Early Tithonian age for the Alcobaça formation. However, no specimens are figured and the results of Marques et al. (1992) are partially contradicted by our data. We have sampled the oyster patch reef right below level 193 in the top part of the Vestiaria section of Marques et al. (1992), which has been attributed by them to the hybonotum zone (Lower Tithonian). The Sr isotope data of these samples (060924/3 A and B) indicate a Late Kimmeridgian age (eudoxus/acanthicum zones), which is supported by the ostracod data (Helmdach, pers. comm. 1991).

1965

The Consolaçaõ section was subject to detailed sedimentological and palaeontological analyses by Werner (1986). Summarizing earlier literature data with new results based on foraminifers, ostracods, and rare ammonites, he suggested a ?Late Oxfordian/Early Kimmeridgian to Late Kimmeridgian/?Early Tithonian age for the entire succession. We have tested oysters from the lower marinebrackish part of the section, which indicate Early to early Late Kimmeridgian ages, and therefore correspond well with the results of Werner (1986). This part of the section is considered a separate unit, termed ‘‘Gre´s, margas, calca´rios oolı´ticos e dolomitos de Consolaçaõ’’, and has been attributed to the Alcobaça formation by Manuppella (1998) and Manuppella et al. (1999). The ‘‘Gres, margas e arenitos da Praia da Amoreira-Porto Novo’’ have also been attributed to the Alcobaça formation by Manuppella et al. (1999). Because of its exclusively fluvio-terrestrial, mainly siliciclastic nature, this newly designed unit (including the Praia da Armoreira member of Hill 1989) is considered part of the Lourinha˜ formation herein. The stratigraphic position of the coastal sections of Saõ Martinho do Porto and Salir do Porto has been far from clear until now. Located at the northern and southern rim of the lagoon of Saõ Martinho, these two successions can easily be correlated. However, both are separated from the overlying strata by a fault (Camarate França and Zbyszewski 1963), which makes it difficult to correlate them with the adjacent Upper Jurassic units. On lithological grounds, the levels sampled for Sr isotope stratigraphy are attributed to the Late Oxfordian Pholadomya protei beds (Wilson 1979; Bernardes et al. 1991) or Montejunto Formation (Camarate França and Zbyszewski 1963) (=Cabo Mondego formation of this paper). However, there is no stratigraphic evidence for these assignments. Moreover, Camarate França and Zbyszewski (1963) state that the macrofauna of the strata at Saõ Martinho do Porto is most similar to that of the Alcobaça formation. The rather scattered Sr data indicate an Early to Late Kimmeridgian age for the sampled layers, also suggesting their assignment to the Alcobaça formation. Five relatively uniform values point to an Early Kimmeridgian age (*divisum zone). The significantly younger ages derived for two levels each represented by a single sample (051004/4C and ‘‘Saõ Martinho’’) differ too much from the remaining values to be attributable to salinity effects. Rather, they may reflect undetected diagenetic alteration. Farta Pao formation Sobral member The nine oyster samples from the Sobral member were derived from the Arruda Subbasin (five samples), the cliffs at Santa Cruz (two samples), and from Porto das Barcas southwest of Lourinha˜ (two samples).

123

1966

Unfortunately, the oyster specimens from Porto das Barcas are severely altered. Consequently, Sr isotope data for the northernmost parts of the Sobral member are missing. For six of the remaining seven samples the derived ages span 1.8 Ma and indicate a latest Late Kimmeridgian to middle Early Tithonian age (uppermost beckeri to semiforme zones), corresponding largely to the time span proposed by Leinfelder (1993). Leinfelder and Wilson (1998), in contrast, placed the greatest part of the Sobral member in the Late Kimmeridgian. Other age specifications in the literature are mostly based on Leinfelder’s (1986) initial interpretation, in which the Sobral member was largely placed in the Late Kimmeridgian (e.g. Manuppella et al. 1999; Schmid and Werner 2005; Yaguë et al. 2006). A single sample (050927/5A) yielded an aberrant earliest Early Kimmeridgian age, which does not fit in the lithostratigraphic framework and should be disregarded. Arranho´ I member The nodular micrites of this member in the Arruda Subbasin and the single carbonate shell bed cropping out south of Santa Cruz (part of the Praia Azul member of Hill 1989) can be correlated by the presence of a giant species of Protocardia, which occurs only at these levels. The ages derived for both localities are more or less identical (150.9 and 150.8 Ma). Obviously, the carbonate incursion of the Arranho´ I member may be interpreted as reflecting a supra-regional event and, together with the occurrence of the giant Protocardia species, marks the Kimmeridgian-Tithonian boundary, as already assumed by Leinfelder (1986). The interpretation of Leinfelder (1986), who suggested a temporal stop of hinterland sedimentation during deposition of the Arranho´ I member, seems doubtful in the light of the Sr isotope data for the Sobral and Arranho´ II members, which indicate contemporaneous deposition of siliciclastic sediments (see sections above and below). Moreover, the isotope data do not confirm Leinfelder and Wilson’s (1998) view that the Farta Pao formation was latest Kimmeridgian to Early Tithonian in age and that the basal Sobral member extended far into the Tithonian and is followed by the Arranho´ I member. Arranho´ II member Similar to the Alcobaça formation, the Arranho´ II member is a heterogeneous unit predominantly composed of shallow marine, brackish, and terrestrial siliciclastics, but with occasional intercalations of coral reefs and associated carbonates (Leinfelder 1986). The 17 oyster specimens that were analysed come from the southern Arruda Subbasin (11 specimens), the coastal section of Santa Cruz (part of Praia Azul member of Hill 1989: three specimens), and the coastal cliffs south of the Rio Sizandro (three specimens). The derived Sr isotope ages range from the latest Kimmeridgian (151.4 Ma = beckeri zone) to the late Early Tithonian

123


(147.6 Ma = admirandum/biruncinatum zone). Therefore, they can be correlated in part with the Sobral and Arranho´ I members (Fig. 8). This is in good accordance with the ages based on microfossils (Fu¨rsich 1981; Fu¨rsich and Werner 1986; Leinfelder 1986) and seismic mapping (Rasmussen et al. 1998), but contradicts in part results from sedimentologic interpretation (Wilson 1979) and sequence stratigraphy (Leinfelder 1993; Leinfelder and Wilson 1998), which attributed the entire Arranho´ to the Early Tithonian. The isotope results strongly contradict the model of Pena dos Reis et al. (1996), in which the Arranho´ is included in the Lourinha˜ formation, the latter assumed to have started in the late Early Tithonian. Freixial Member The Freixial Member is restricted to the southern part of the Arruda Subbasin, and to parts of the Turcifal Subbasin, and is obviously the youngest Jurassic lithostratigraphic unit of the area. The oyster specimens from Paços proved to be diagenetically altered. Three of the remaining four samples from the surroundings of Chamboeira indicate a Late Tithonian age, which corresponds to foraminiferal and dasycladacean data from the Freixial Member (Rey et al. 1968; Ramalho and Rey 1969, 1975; Ramalho 1971, 1981; Leinfelder 1986). The fourth sample (050904/2 A) suggests a Late Kimmeridgian age, which does not fit the lithostratigraphic framework and should be disregarded.

Conclusions Strontium isotope values of oyster shells and belemnite rostra from the Lusitanian Basin (central Portugal) produced sound numerical ages for several Upper Jurassic lithostratigraphic units (Fig. 2). Late Jurassic sedimentation started in the Middle Oxfordian, as can be demonstrated for the Cabaços Formation in the Bombarral Subbasin (CLB), and the Cabo Mondego formation in the Monte Real Subbasin (NLB). The Cabo Mondego formation extends into the Early Kimmeridgian, and therefore may be partially correlated with the Alcobaça formation further to the south. The Alcobaça formation, including the coastal section of Consolaçaõ, ranges from the Late Oxfordian to the Late Kimmeridgian. The enigmatic outcrops at Salir do Porto and Saõ Martinho do Porto can also be correlated with the Alcobaça formation and are attributed largely to the Early Kimmeridgian. Towards the south, the Alcobaça formation is replaced by the time-equivalent basinal Abadia Formation. In large parts of the Arruda and Turcifal subbasins, the Farta Pao formation forms the top of the Late Jurassic succession. It can be divided into three, largely contemporaneous lower members and a single upper member. The sandy Sobral member starts in the


latest Kimmeridgian and extends into the Early Tithonian. Sediments of the patchily distributed micritic Arranho´ I member were deposited around the Kimmeridgian-Tithonian boundary. The fine-siliciclastic Arranho´ II member with locally developed reefs also begins in the Late Kimmeridgian and extends nearly to the Late Tithonian. In the south, the Arranho´ II is finally overlain by the coarsesiliciclastic to calcareous Freixial Member of late Early to Late Tithonian age. The shift of facies belts from north to south with time is reconfirmed by the strontium isotope data. After a short, partially brackish phase fully marine sediments were deposited all over the basin from the Middle to Late Oxfordian onwards. While open marine carbonates were deposited in the south (Cabaços and Montejunto formations), the marginal marine Cabo Mondego formation of the NLB, though carbonate-dominated, soon turned brackish. Around the Oxfordian-Kimmeridgian transition, when se-di-men-ta-tion largely changed from carbonates to siliciclastics, the brackish-marine facies belt migrated southward, expressed in the Alcobaça formation. At the same time, basinal conditions persisted in the southern part of the Lusitanian Basin (Abadia Formation). Near the end of the Kimmeridgian, the basin was filled up, and brackish to marginal marine conditions, represented by the Farta Pao formation, had reached the southern CLB and persisted throughout the Tithonian. Likewise, the fluvial-terrestrial siliciclastics of the Lourinha˜ formation extended southward. By the Early Kimmeridgian, they had replaced the Cabo Mondego formation in the NLB. By the end of the Kimmeridgian they extended across the northern CLB, and by the end of the Jurassic they covered large parts of the CLB. Oysters have been proven once more to be suitable for strontium isotope stratigraphy. In most cases, the specimens from the Upper Jurassic of Portugal were largely unaffected by diagenetic alteration and were therefore thought to preserve initial strontium isotope ratios. Moreover, freshwater influx seems to have had no significant effect on the strontium isotope data. These facts make oysters one of the most valuable tools for chronostratigraphy in marginal Mesozoic sediments lacking index fossils. The present results provide a solid chronostratigraphic framework for the Upper Jurassic strata of the Lusitanian Basin that can be refined by further detailed studies. Acknowledgments This study was financially supported by the Deutsche Forschungs-Gemeinschaft (projects Fu 131/31-2 and We 1152/2-2). Official permission for sampling was granted by M. Ramalho (Lisbon). Logistic support was offered by O. Mateus (Lourinha˜). C. Helbig (Munich) prepared the slices of oyster shells. M. Joachimski (Erlangen) kindly provided access to the cathodoluminescence microscope and gave useful advice. K. Paschert (Munich) carried out the AAS measurements. E. Hegner, S. Ho¨lzl, A. Rocholl, and S. Rummel (all Munich) helped with chemical treatment of the samples and with mass spectrometry. The oyster photographs were

1967 taken by G. Janßen (Munich). We would also like to thank G. D. Price (Plymouth) and an anonymous reviewer for their constructive criticizm on the manuscript.

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