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Sequence stratigraphy and depositional setting of the Pliensbachian and Toarcian marly limestones in the Lusitanian Basin, Portugal

Duarte L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

157-158

EN

The Pliensbachian and Toarcian series in the Lusitanian Basin (Portugal) are generally dominated by hemipelagic deposits, represented by marl/limestone alternations that are very rich in nektonic and benthic fauna. These sediments are included in the following four formations: Vale das Fontes, Lemede, S. Giăo and, partially, Póvoa da Lomba. The weak lateral facies variation, generally observed at the basin scale, suggests that these sediments were deposited in an epicontinental extensional basin on a homoclinal carbonate ramp controlled by eustatic fluctuations and regional tectonics. Considering the Late Triassic – Late Callovian large cycle, the sediments correspond to the maximum transgressive facies which can be widely observed throughout the succession. A detailed studied of several stratigraphic sections in terms of sedimentological, geochemical and paleontological analysis shows that the Pliensbachian-Toarcian series is subdivided into two second-order sequences (SP and ST). The Pliensbachian succession shows a typical second-order transgressive/regressive sequence, with a dominant marly deposition at the base and a calcareous dominant facies at the top. The basal discontinuity of the SP is particularly well observed in the western part of the basin, dating roughly from the Sinemurian/Pliensbachian boundary. The series shows a large transgressive phase, ending in the middle-upper part of the Margaritatus Zone (around Subnodosus/Gibbosus subzones boundary) associated with an organic-rich deposition verified at the basin scale. During the Spinatum Zone the sedimentation returned to a calcareous regime very rich in benthic macrofauna. The upper discontinuity of the SP observed in the whole basin dates from the lowermost Polymorphum Zone (intra-Mirable Subzone). The base of ST (Polymorphum Zone) corresponds to an abrupt flooding event, through a generalised marly accumulation in the whole basin. However, around the Polymorphum-Levisoni interval, an important tectonic activity occurred, responsible for a great sedimentary change with special facies features in some positions of the basin. The dominance of marl observed at the top of the Levisoni Zone marks the maximum peak transgression of the Toarcian second-order sequence, showing some evidence of pelagic deposition, with thin-shelled bivalve-rich (Bositra sp.) horizons. The Upper Toarcian – Lower Aalenian succession shows a regressive trend, ending ST with an upward increase of calcareous and bioclastic content, including ahermatipic corals in the eastern sectors. The upper discontinuity dates from the Opalinum Zone and shows different sedimentary records across the basin. ST it is subdivided into four third-order depositional sequences (St1 to St4), each bounded by regional discontinuities, recognized over most parts of the Lusitanian Basin.

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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.

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Microbial mounds from the restricted-marine Sinemurian succession of S. Pedro de Moel (Lusitanian Basin, Portugal): preliminary palaeoecological interpretation

Azeredo A., Duarte L., Cabral M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

77-77

EN

In the Lusitanian Basin the Sinemurian corresponds to a marginal- to restricted-marine succession, representing the early stage of sea flooding in the recently formed basin, following a Late Triassic rifting event. Except for its topmost part, the Sinemurian succession belongs to the Coimbra Fm., composed of dolostones, dolomitic limestones and limestones. This unit is dominated by peritidal facies towards the eastern, more landward zone of the basin, whereas in the W (as at S. Pedro de Moel) more distal (though not quite deep) facies occur. At S. Pedro de Moel the Coimbra Fm. is well exposed and the section displays a succession mostly composed of argillaceous and/or dolomitic limestones and fossiliferous/skeletal limestones (bivalves, gastropods, ostracods), interbedded with a few marly levels. Although fossil remains are commonly present in variable amounts, some beds seem to be azoic, others exhibit rare, deformed burrows and very rare ostracods. Undulating, irregularly bounded, laminar levels are common and, locally, centimetre to decimetre-thick concentrated skeletal/fossil layers occur. However, towards the middle part of the section, well-preserved, dome-shaped stromatolites occur, in clear contrast with the under- and overlying bedded deposits. Towards the top of the succession, the calcareous/clay ratio increases. This section is still under study, so we only make a preliminary palaeoenvironmental approach here. The microbial mounds have an average maximal thickness of 0.75 m and show different fabrics, sometimes within the same mound: laminated, stromatolitic crusts; clotted, peloidal micrite; micritic and sparitic threads; degraded, tuft-like filamentous structures; enhanced fenestral-like porosity; dense, slightly darker micrite. These features suggest that they were formed through hardening of calcified cyanobacterial and other microbial films, whose early disintegration also would have contributed with autochthonous mud for the mounds. However, a more detailed study is clearly needed. It is known from the literature that microbialites, as a whole, may form in a wide range of environmental conditions, though some associations or particular morphologies may give us more accurate ecological information. However, a crucial basic condition is a very low to low background sedimentation. In the present case, it is suggested that the stromatolites grew under low-energy, restricted water-circulation and low rate of mixed terrigenous and calcareous mud deposition. A likely nutrient-poor substrate, the existence of terrigenous material and, maybe, slight (?)hypersalinity would have inhibited a more significant development of epibenthic and heterotrophic organisms, favouring the microbial community. The low energy prevented physical erosion which, coupled with the absence of predators, allowed the development of the well-defined dome morphologies. In contrast, towards the top of the succession a more open setting prevailed, with better water-circulation, probably better oxygenation and somewhat higher sedimentation rate with dramatic decrease of clay material. Though most of the upper deposits are still low-energy ones (biomicrite mudstones and wackestones, with a few ostracods, gastropods, bivalves, rare hyaline forams), skeletal/intraclastic/peloidal packstones and rarer grainstones occur more frequently, attesting for the less protected environment. These conditions did not allow the continuation of microbial growth.

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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.

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Ammonite horizons at the basal Bathonian zone (Parvum Subzone) in Cabo Mondego, Portugal

Fernandez-Lopez S., Henriques M., Manglod Ch.

Volumina Jurassica

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2006

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Vol. 4, no. 1

161-161

EN

The ammonite succession at the Bajocian/Bathonian boundary in the Cabo Mondego region provides one of the most complete biostratigraphical records so far recognized in the Iberian Plate. Lower Bathonian ammonite fossil assemblages are composed of Submediterranean taxa. Parkinsonids characterizing the Northwest European Province, as well as phylloceratids and lytoceratids characterizing the Mediterranean Province, are very scarce. The basal Bathonian zone (Zigzag Zone) established for NW Europe areas, belonging to the Northwest European Province, can be identified in the Lusitanian Basin. The Lower Bathonian boundary may be established by the lowest occurrence of the dimorphic group Morphoceras (M) + Ebrayiceras (m), although morphoceratids are scarce. The Zigzag Zone can be characterized as composed of two subunits (Parvum and Macrescens subzones) represented in diverse European basins of the Submediterranean Province. The revision of previous collections from the classical section and new field samplings of two other separate sections on Cabo Mondego allow to distinguish the lowest subzone of Bathonian (Parvum Subzone, Zigzag Zone). Along up to ten metres of thickness, over forty successive assemblages have been recognized in the Parvum Subzone. The lowermost subzone of the Bathonian yields common perisphinctids (40%), oppeliids (25%) and hecticoceratids (20%), being the most abundant genera: Planisphinctes (m) + Lobosphictes (M), Oxycerites (M) + Paroecotraustes (m) and Nodiferites (m) + Zeissoceras (M). Ammonite fossil assemblages of the Parvum Subzone may be grouped into two successive biohorizons. The lower biohorizon, beginning with the lowest occurrence of Morphoceras (M) + Ebrayiceras (m), has been characterized by perisphinctids of the dimorphic couple Bigotites gr. diniensis Sturani (M) + “Bigotites” acurvatus (Wetzel) in Torrens (m), although they are scarce. The upper biohorizon, through 1.5-2 m of thickening upwards beds, has been identified in the stratigraphic interval beginning with the lowest occurrence of Zigzagiceras (m) + Procerozigzag (M) and underlying the lowest occurrence of Morphoceras macrescens (Buckman). These two successive ammonite horizons are biochronostratigraphically equivalent to the subdivisions of the Convergens Subzone distinguished on the Digne-Barr˘me area (France). The occurrence of Bigotites gr. diniensis (M+m) in Cabo Mondego in the Parvum Subzone represents a new criterion for chronostratigraphical subdivision and chronocorrelation with the Digne-Barr˘me area, useful in understanding the evolution of the West Tethyan Perisphinctidae during earliest Bathonian.