Ana C. Azerêdo

Geometry and facies dynamics of Middle Jurassic carbonate ramp sandbodies, West-Central Portugal

Abstract The Middle Jurassic succession of the Lusitanian Basin (Portugal) represents a carbonate ramp depositional system. For most of the time, the shallow-water facies defined a microtidal barrier-island complex, where prominent oolite-dominated sandbodies developed. Detailed facies analysis of these sandbodies allowed the recognition of their elementary depositional facies, which are described and whose relative importance is estimated. Sandbody geometry, hydrodynamic controls and ramp evolution are discussed. Carbonate sediment production and redistribution, coupled with inner ramp aggradational and progradational trends, have interacted in a such a way that ramp geometry persisted for a longer period of time than usually recorded in these systems.

New charophytes from the Upper Jurassic of the Lusitanian Basin (Portugal)

In the Lusitanian Basin (west-central Portugal), Upper Jurassic (Oxfordian) sediments were investigated in order to identify palaeontological assemblages of charophytes. Systematic studies were undertaken on specimens obtained from four field sections (Pedrógão, Vale de Ventos, Memória and Valverde). These studies revealed the presence of new forms of Porocharaceae (Porochara pedunculata n. sp) and of forms previously unknown in this region (Auerbachichara cf. saidakovskyi), as well as P. raskyae, P. minima, P. fusca, P. sulcata, P. kimmeridgensis, Aclistochara longiformis and Porochara sp. Comparison of the charophyte palaeofloras recognized in all of the studied sections has allowed the definition of three different assemblages which, coupled with other data, help to correlate these successions of Early (?) to Middle (Late?) Oxfordian age.

BIO-SEDIMENTARY SIGNATURES OF HIGH-FREQUENCY SALINITY/SUBAERIAL EXPOSURE CHANGES: EXAMPLES FROM THE OXFORDIAN OF PORTUGAL (CABAÇOS FORMATION)

The detailed sedimentary and micropalaeontological analysis of a complex association of continental to marginal-marine deposits from the Oxfordian of Portugal (Cabacos Formation) has allowed the recognition of high-frequency, subtle changes in the environmental conditions. The main factors controlling the palaeobiological responses to such minor-scale fluctuations were also identified. Two factors have shown to be especially significant: subaerial exposure duration and frequency (estimated by assigning type of features to the exposure index) and salinity trends, as suggested by the sedimentary and fossil records. In the west of the basin, salinity fluctuations were much stronger and more frequent (fresh- brackish-restricted marine-hypersaline), and subaerial exposure more marked for longer periods, than in the east of the basin. The microfossil assemblages, as a whole, but in particular the ostracod faunas, show differences in abundance, diversity, dominant species, degree of intrageneric and intraspecific variations, both along the successions and between west and east. The western populations seem to have been much less stable, which suggests that high-frequency changes in salinity (more than its absolute values) and degree of exposure were the most important controls on the palaeobiota. PDF

The late Sinemurian carbonate platform and microfossils with Tethyan affinities of the Algarve Basin (South Portugal)

SummaryA Lower Jurassic shallow-platform carbonate succession recognized at Alte, in the Algarve Basin (South Portugal), bears a typically Tethyan microfossil assemblage, which had not been documented before in Portugal and allows better dating of the succession. This is mainly composed of intertidal-supratidal sediments, with infrequent subtidal sediments and event deposits, suggesting a marginal-marine, peritidal depositional setting. The microfossil assemblage comprises benthic foraminifera, calcareous algae, porostromates, ostracods andIncertaesedis. As regards to biostratigraphy, the most important are the larger imperforate foraminifera, which includeLituosepta recoarensis, Paleomayncina termieri andPseudopfenderina butterlini?, among other. The vertical distribution of these species indicates that the succession may be confidently dated as Upper Sinemurian, most probably as topmost Sinemurian. The persistence ofL. recoarensis at the top of the sucession indicates that these deposits do not reach the Carixian. The dasyclads are the most important group in the microflora, in particular the typical Liassic speciesPalaeodaysacladus mediterraneus. Also most abundant isThaumatoporella parvovesiculifera. As regards to the Algarve Basin Lower Jurassic shallow-platform formations, it is only in the levels bearing these assemblages that a biostratigraphical constraint is found. The recognition of these Tethyan assemblages also reinforces the knowledge of their palaeogeographical distribution and of the correlative depositional settings in the southern Iberian Peninsula, allowing further comparisons with the North-African region.

Revision of Late Tithonian charophytes from Lisbon and Sintra-Cascais (Portugal): taxonomy, biostratigraphy and palaeobiogeographical significance

The re-evaluation of the charophyte content of ‘Purbeckian’ sediments from several sections around Lisbon (Olelas and Brouco) and Sintra-Cascais (Murches, Atrozela and Malveira-Guincho) revealed new palaeofloral associations from the Late Tithonian of the South Lusitanian Basin (Portugal). These sections contain Globator rectispirale, G. aff. nurrensis, Nodosoclavator bradleyi, Clavator reidi, Clypeator cf. discordis, Porochara maxima, and newly described occurrences of P. jaccardi, Mesochara harrisi and nodosoclavatoroide utricles. These revised data reinforce the evidence for assigning most of the studied deposits to a Late Tithonian age, instead of the formerly accepted wider interval Tithonian to Early Berriasian (‘Purbeckian’). Population analysis and statistics were applied in order to better assess population variation of the different species. The results of this study are relevant as they contribute to improve the biostratigraphical definition of the ‘Purbeckian’ formations of Portugal and allow more accurate palaeobiogeographical interpretations within the central Tethyan domain, by comparison of the identified charophyte assemblages with documented Jurassic–Cretaceous transition charophytes from other regions.

Improving the Palaeoecological Understanding of the Lisboa–Sintra Region Cenomanian Units: The S. João Das Lampas Section

The S. Joao das Lampas section, in the Sintra region (Lusitanian Basin, Portugal), exhibits a highly fossiliferous marl–limestone Cenomanian succession and belongs to the Canecas Formation. In this work, we present data from ongoing studies on ostracods and microfacies to address palaeoecological issues within the regional framework. The fossil assemblage comprises ostracods (very abundant and represented mainly by brackish and marine littoral species), benthic foraminifers, dasycladalean and bryopsidalean algae, bivalves, gastropods, serpulids, and echinoderms. The succession consists predominantly of oyster-rich marls and marly limestones, interbedded with skeletal/microfossil-rich carbonate and marly layers and skeletal storm beds or lenses. Overall, the data point to a low- to moderate-energy marginal-marine setting with frequent variations in salinity.

Advances in the Stratigraphy and Geochemistry of the Organic-Rich Lower Jurassic Series of the Lusitanian Basin (Portugal)

Here we present the main results of a study of the high-resolution stratigraphy of the Lower Jurassic organic-rich marine series in the Lusitanian Basin, Portugal. In this basin, the Lower Jurassic consists mainly of an alternating marl–limestone succession (upper Sinemurian–Toarcian), with some intervals enriched in organic matter (upper Sinemurian and Pliensbachian). These Lower Jurassic carbonate units correspond to the Coimbra, Agua de Madeiros, Vale das Fontes, Lemede, and S. Giao/Cabo Carvoeiro formations. In the last few years, these units have been the subject of a multidisciplinary and integrated stratigraphic analysis, based on the study of a range of biotic and abiotic parameters. In our study, high-resolution analysis of these units in the main reference sections of the basin have allowed seven main themes to be developed, involving sedimentological and sequential analysis, ammonite biostratigraphy, macroinvertebrate palaeontology and palaeoecology, ostracod analysis, chemostratigraphy, organic and petrographic geochemistry, and gamma-ray analysis. Besides the novel results obtained in each scientific domain, the integration of all the collected data has improved our knowledge of not only the stratigraphy and sedimentary knowledge of the Lower Jurassic carbonates but also the hydrocarbon generation potential of the different Sinemurian and Pliensbachian units.

Middle Jurassic Tethyan–Boreal ostracod faunal links: a case study from the Callovian of Portugal

Late Callovian (athleta Chronozone) age sediments of Pedrógão, west coast Portugal, yielded a low diversity ostracod assemblage with relatively little in common with coeval NW European assemblages except at the weak level of comparative (cf.) species identification. Five new marine species are described (Neurocythere multiforma sp. nov., Palaeocytheridea (Malzevia)? dorsocostata sp. nov., Praebythoceratina parascrobiculata sp. nov., Praeschuleridea lisensis sp. nov., Virgulacytheridea posteroacuminata sp. nov.) and three previously described non-marine species recorded that so far are indigenous to Portugal (Septacandona azeredae, Sinuocythere pedrogaensis, Theriosynoecum helmdachi). Ostracod taxonomic links to western Tethys are evident but weak. Calcareous nannofossil assemblages were analysed for biostratigraphical control. Supplementary material: Sample data and raw ostracod counts are available at https://doi.org/10.6084/m9.figshare.c.3271394.v1