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Recent data on the ostracod fauna from the Lower and Middle Toarcian of Peniche, Portugal

Pinto S., Cabral M., Duarte L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

204-205

EN

At Peniche region, western Portugal, a rather continuous marly limestone succession ranging in age from the Sinemurian to the earliest Aalenian crops out. The ostracod fauna from a 74 m thick packet within this succession has been studied, encompassing deposits that range from the topmost Pliensbachian (topmost Spinatum Biozone) to the Middle Toarcian (Polymorphum, Levisoni, Bifrons and basal Gradata biozones). The studied levels belong to Lemede (2 m) and Cabo Carvoeiro Formations. From the 47 samples collected in the marly layers, a single one is azoic. Sample numbers and bed subdivision are according to biozonation in Duarte (1995), which considers 5 packets: topmost Spinatum, Polymorphum, Levisoni, Levisoni+Bifrons, Bifrons+basal Gradata. The systematic study of the ostracods has been undertaken, and some palaeoecological aspects have also been addressed, namely hydrodynamics (carapace/valve ratio), bathymetry and oxygenation, based on marker genera/groups. Around 8000 individuals were obtained (1 individual =1 carapace or 1 valve), which are commonly badly preserved. At least 58 species belonging to 22 genera have been identified until now. The top of Spinatum Biozone displays both high diversity (8-15 species/sample) and abundance (222-402 individuals/sample) of ostracods. The dominant species are from the genus Ogmoconcha, Ogmoconchella and Liasina, associated with Polycope, Paracypris, Ledahia, among others. The Polymorphum Biozone shows high diversity (3-22 species/sample) and variable abundance (1-150 individuals/sample). The genera Ogmoconcha, Ogmoconchella and Liasina still dominate and, from the first levels of the biozone, heavily ornamented species of Kinkelinella are present; the genus Isobythocypris occurs for the first time. In Levisoni Biozone, the ostracod diversity (1-4 species/sample) and abundance (1-150 individuals/ sample) are low, in contrast with previous biozones. Ogmoconcha and Ogmoconchella disappear, and the genus Cytherella appears for the first time, becoming dominant together with Liasina in the first levels of the biozone. In the upper part, Kinkelinella dominates and at the top Bairdiacypris and Cytheroptheron occur for the first time. Levisoni+Bifrons shows that ostracod diversity is moderate (5-11 species/sample) and abundance is high (60-310 individuals/sample). Species from the genera Bairdiacypris and Kinkelinella dominate, Cytherella and Patellacythere are common. Bifrons+basal Gradata shows higher diversity (3-18 species/sample) than previous unit and variable abundance (16-468 individuals/sample). Bairdiacypris, Cytherella and Kinkelinella dominate, and Macrocypris and Trachycythere(?) occur for the first time. All of the recognized species have been found in formations of a similar age in Western Europe, and are benthic marine species, except the genus Polycope (pelagic). Most of the species indicates a deep marine setting, with variable oxygenation levels, from normal to low (predominance of Metacopina and Cytherella), for the all section. The water energy was also variable, being stronger at the topmost Spinatum and Polymorphum biozones, within which ostracod valves are dominant, and lower at the Levisoni and Levisoni+Bifrons biozones, within which ostracod carapaces clearly dominate.

2

Pliensbachian – lowermost Toarcian chemostratigraphy (elemental and TOC) of the Peniche section (Lusitanian Basin, Portugal)

Veiga de Oliveira L., Duarte L., Rodrigues R.

Volumina Jurassica

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2006

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Vol. 4, no. 1

217-218

EN

In Portugal, the Peniche section constitutes one of the most continuous series of Lower Jurassic. This study is based on a detailed chemostratigraphic analysis of the Pliensbachian - lowermost Toarcian marly limestones, belonging to the Vale das Fontes, Lemede and Cabo Carvoeiro formations. 196 samples of limestones and marls were analyzed in terms of minor and major elements (Fig. 1). Total organic carbon (TOC) was determined in 233 samples. Besides the stratigraphic distribution of these geochemical parameters, the aim of this work is to perform a discussion about biogenic influx and aluminosilicate phasefluxes. The Al concentrations are a good indicator of detrital flux and good correlations between them signify aluminosilicate affiliation. In Peniche, K (0.98), Si (0.96) and Ti (0.98) show, in all section, excellent correlation with Al2O3. Mg (0.82), Ba (0.80), Cr (0.78), Li (0.80), Na (0.74), Sc (0.80), V (0.81) and Zr (0.86) correlate well with Al2O3 but other secondary factors, beyond the detrital flux, affect the resultant concentrations. The Pearson's coefficients between Al2O3 and CaO is strongly negative (-0.98), suggesting divergent behaviours for this elements. Al2O3 and Fe2O3 correlate moderately (0.61). However, the correlation coefficients calculated for each stratigraphic unit show variable values. Lemede Formation and the Members Marls and Limestones with Uptonia and Pentacrinus (MLUP), Lumpy Marls and Limestones (LML), to the Vale das Fontes Formation, and CC1, to the Cabo Carvoeiro Formation, show high correlation coefficients between Al2O3 and Fe2O3 (0.87, 0.91, 0.88 and 0.97, respectively). On the other hand, the Member CC2 has moderate correlation (0.67) and the Member Marls and Limestones with Bituminous Shales (MLBF) has weak correlation (0.20). These variations suggest that the elemental inputs change during the Pliensbachian - lowermost Toarcian, in the Peniche area. Ba enrichment is considered an indicator of high flux of biogenic material and high surface-water productivity. But, in the studied section, correlation between Al2O3 and Ba is high (0.80) and Ba lacks any correlation with TOC (0.17). Thus, in the Peniche region, the distribution of Ba is dominated by the original detrital flux and transported to the basin, mainly, as a constituent of clays. The mainly TOC values in the Pliensbachian - lowermost Toarcian of the Peniche section are below 2. But the MLBF (Ibex to Margaritatus zones) represents a high TOC interval with concentrations up to 15%, correlated with the 2nd order flooding interval, well know in the Lusitanian Basin.

3

Calcareous nannofossils biostratigraphy and oxygen isotope composition of belemnites: Pliensbachian – lowermost Toarcian of Peniche (Lusitanian Basin, Portugal)

Veiga de Oliveira L., Duarte L., Rodrigues R., Perilli N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

219-219

EN

The Lusitanian Basin is located in the western Iberian margin, opened during the Triassic.The Lower Jurassic is particularly well represented at Peniche, which exhibits a continuous seriesof carbonate sediments, more than 450 m thick and aged between Sinemurian and Toarcian.In lithostratigraphic terms it corresponds to the Agua de Madeiros, Vale das Fontes, Lemede and CaboCarvoeiro formations. In this study, 145 m thick section (from the Jamesoni to the Levisoni ammonite zones),was analyzed in terms of calcareous nannofossils biostratigraphy and oxygen isotopes of belemnite rostra.The nannofossil biozones NJ4a, NJ4b, NJ5a (Pliensbachian; upper part of Jamesoni to Spinatumammonite zones), NJ5b (uppermost Pliensbachian - lowermost Toarcian; upper part of Spinatumto Levisoni ammonite zones) and NJ6 (lowermost Toarcian; upper part of Levisoni ammonite Zone) wereidentified based on proposed NW European schema and correlated with ammonite zones. Additionally,the secondary biostratigraphic events were registered which will be useful to refine the nannofossilsbiozonation: the first occurrences (FO) of Biscutum grande and B. finchii were found in the upper part ofthe NJ4a biozone (lower part of Margaritatus ammonite Zone); the FO of Lotharingius frodoi wasidentified at the same stratigraphical level as L. hauffii; the FO of L. sigillatus was found in the upper partof the NJ5a biozone (Spinatum Zone); the first common occurrence (FCO) of Calyculus spp. was recognizedin the NJ5b base, near the Pliensbachian/Toarcian boundary; the FO of Carinolithus spp. was identifiedwithin NJ5b biozone, correlated with the lower part of the Levisoni ammonite Zone and below the extinctionlevels of Calcivascularis jansae and B. grande which are other nannofossil secondary events.The oxygen-isotope profile of the Peniche section seems to reflect primary signals and can be usedto interpret the sea water paleotemperatures variations. In the Early Pliensbachian the temperature showsa gradual cooling trend (NJ3 and the lower part of the NJ4a; Jamesoni ammonite Zone). Afterward,there is a warm period (NJ4a and NJ4b; Jamesoni to lower part of Spinatum ammonite Zone) correlatedwith high TOC values interval (up to 15%), suggesting a relative sea level rise and concomitant high surfacewaterproductivity. In fact, the Margaritatus ammonite Zone corresponds, in the Lusitanian Basin,to 2nd-order flooding interval. In the Late Pliensbachian and Early Toarcian (NJ5a and lowermost partof NJ5b; Spinatum to Polymorphum ammonite zones), the isotopic values show slight variations.However, they suggest a small cooling trend in the upper part of Spinatum ammonite Zone and a warmtendency in the lower part of Polymorphum ammonite Zone.