An Early Jurassic oligohaline ostracod assemblage within the marine carbonate platform sequence of the Venetian Prealps, NE Italy (original) (raw)
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Journal of the …, 2002
Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios (87Sr/86Sr) and carbon-isotope ratios (δ13Ccarb and δ13Corg) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (ä18O) in carbonate, sulphur-isotope ratios (δ34S) in carbonate-hosted sulphate, nitrogen- isotope ratios (δ15Norg) in organic matter, osmium-isotope ratios (187Os/188Os) in black shales and neodymium-isotope ratios (143Nd/144Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ13C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium- isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon- isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.
Journal of South American Earth Sciences, 2017
Ostracods are microcrustaceans that inhabit different aquatic environments and are frequently used in paleoecological interpretations and biostratigraphic studies. The Barreirinhas Basin, Northern of Brazil, contains a well-preserved ostracod assemblage in its sedimentary rocks of early and late Cretaceous age, which have been so far poorly studied. This paper contains the first taxonomic identification of the ostracod assemblages, as well as the elaboration of paleoenvironmental and biostratigraphic inferences, for the Cretaceous of the Barreirinhas Basin. The studied material consists of 147 samples from the wells 1-MAS-1A, 1-MAS-3A, 1-MAS-4A and 1-MAS-14A. 495 specimens were recovered, distributed between 40 species, 16 genera and 9 families, including three new species. Based on previously established biozones for the Sergipe basin, two biozones were identified: the Nigeroloxoconcha aff. Nigeroloxoconcha sp. GA A 22 Range Zone, of lower Cenomanian age; and the Brachycythere sapucariensis Interval Zone, of Turonian to middle Coniacian age. Finally, three distinct ostracod assemblages were defined: Assemblage 1, dominated by Conchoecia? species; Assemblage 2, well diversified but with low abundance; and Assemblage 3, with cold water ostracods such as Krithe. Based on the ostracod assemblages identified, a middle neritic, platformal paleoenvironment was inferred for the studied interval.
The Campo Pisano Formation of southwestern Sardinia is represented by an offshore carbonate succession spanning the latest Early to late Middle Cambrian. Paleogeographically, the fauna is characteristic of western Perigondwana, and indicates faunal relations to France, Spain, and Turkey. Microfaunal paleoecology reflects drowning of an isolated carbonate platform at tropical latitudes. Sessile epibenthic filter feeders dominate at the base, succeeded upward by sessile, epibenthic, suspension feeders. Upsection, a shift in the ratio of faunal groups indicates increasing replacement by mobile epibenthos. Autochthonous faunal elements decline near the top where allochthonous taxa become important. In the basal portion of the formation the faunal succession indicates relatively shallow neritic habitats with a moderate influx of suspended sediment, followed by a period of slightly deeper neritic conditions. Probably a shallow bathyal environment was established at the transition to the overlying siliciclastic Cabitza Formation. The bathymetric and ecofacies changes in the Campo Pisano Formation are interpreted as having been caused by a discontinuous rise in eustatic sea-level, probably accompanied by subsidence due to tensional tectonics. The depositional environment was that of a distal open-marine shelf or ramp without strong relief. This interpretation is supported by the lack of significant lateral changes in the fossil assemblages, and an upward deepening of facies. The applied micropaleoecological approach is proven a powerful tool in regional stratigraphy and in the reconstruction of sedimentary realms in the Mediterranean Cambrian, particularly when high-resolution biostratigraphic data and diagnostic sedimentary characteristics are sparse.
Bollettino della Societa Paleontologica Italiana 01/2008;
Ostracod assemblages were analysed from three samples, from Lower Pleistocene sediments cropping out near Furnari, NE Sicily. Samples were collected from gravely sandy coral-rubbles along a palaeofault scarp and from silts and clayey silts proximally and distally located to the scarp foot. The ostracod assemblages indicate a palaeoenvironment in the upper bathyal zone, between 200 and about 600 m depth. Differences in composition, species richness and specimen abundance of autochthonous palaeocommunities are mainly due to substrate grain size and food availability, partly relating to the presence of deep-water coral build-ups along the fault. Allochthonous assemblages indicate a considerable down-slope displacement, seemingly by floating and subsequent settling.
Several stratigraphic studies have proved that Early Toarcian carbonate platforms and basins suffered a sharp sedimentary and palaeoecologic evolution worlwide reflecting the onset of dramatic environmental perturbations in the atmosphere/ hydrosphere system. An upper Liassic slope carbonate succession cropping out in the Gran Sasso range (Abruzzi, central Italy) and pertaining to the transition between the Latium-Abruzzi carbonate platform and the Umbria-Marche basin has been studied in this light.
Palaeogeography Palaeoclimatology Palaeoecology, v. 77, p. 145-177., 1990
Distinctive sedimentological, geochemical, isotopic and micropalaeontological response characterise the nature of sediments deposited under hypoxic/anoxic bottom conditions during the latest Cenomanian to earliest Turonian of most of the world's basins. The composition of the biota is a direct response to overall environmental conditions. The recognition of distribution patterns of low-oxygen tolerant microfauna is, therefore, of primary importance in evaluating and reconstructing the regional palaeoceanographic settings where such events took place. A case-study is presented from two coeval stratigraphic sequences, the Sergipe basin (northeastern Brazil) and the western Anglo-Paris basin (southern England).The associated benthonic foraminiferal assemblages from oxygen depleted environments consist mostly of “opportunistic”, r-selected species. Basic patterns in community trophic structures appear to have primarily dictated the composition of the biota. The assemblages from outer shelf to upper slope oxygen depleted environments are commonly characterised by: (1) low-diversity; (2) limited intraspecific variation; (3) predominance (c. 40–60% of the total foraminiferal assemblage) of epifaunal, deposit feeding, plano/concavo-convex, trochospiral, calcareous-hyaline morphotypes (gavelinellids, conorboidids, rosalinids and bagginids); (4) variable numbers (c. 10–30%) of infaunal, bacterial/detrital scavengers, tapered and elongate morphotypes (turrilinids, bolivinids, buliminellids, fursenkoinids, polymorphinids and caucasinids); (5) epifaunal/shallow infaunal, deposit feeding, lenticular morphotypes (coiled vaginulinids) (c. 5–19%); (6) variable numbers (c. 10–30%) of agglutinated specimens (lituolids, discamminids, eggerellids, textulariids, trochamminids, ammodiscids, tritaxiids and valvulinids); and (7) dominance (about 60–80%) of minute (c. 100–200 μm in maximum diameter, width or length) calcareous-hyaline taxa during widespread dysaerobic/quasi-anaerobic episodes. Highly oxygen depleted bottom conditions are marked by the absence of benthonic microfauna. Minor taxonomic differences between the two studied areas are mostly due to palaeobiogeographic provincialism.
Palaeobiodiversity and Palaeoenvironments, 2021
The Kimmeridgian Alcobaça Formation of the Lusitanian Basin forms a mixed carbonate-siliciclastic unit between basinal deposits of the Abadia Formation, and fluvial-terrestrial strata of the Lourinhã Formation. This study presents >2.5 km of detailed logs of nine outcrop sections of the Alcobaça Formation in its type region. Eight of these sections encircle the Caldas da Rainha Diapir, which was a prominent, emergent, passive salt diapir during the time of deposition. Palaeoenvironments of the unit form a complex mosaic of low-to high-energy, carbonate-or siliciclastic-dominated shallow shelf settings; coastal embayments and lagoons; and coastal plains with rivers, lakes and playas. In the strata, abundant microfauna is often joined by a rich macrofauna, usually dominated by bivalves. Locally, corals, calcareous sponges or oysters form meadows or patch reefs. These autochthonous to parautochthonous remnants of former communities are assigned to 35 benthic macrofaunal associations. The integration of palaeoecological analysis of these associations with microfaunal and sedimentological data provides constraint on their salinity ranges, which range from slightly hypersaline to freshwater. Frequent temporal and spatial salinity fluctuations are attributed to variations in relative sea-level, salt tectonics or climate. The NNE-trending Caldas da Rainha Diapir induced pronounced facies differentiation. Predominantly, non-marine siliciclastic facies in the northwest and carbonate to siliciclastic, marine to brackish facies in the southwest are contrasted by shallow-marine carbonate facies east of the diapir. Comprehensive exposure and well-preserved fossils make the Alcobaça Formation an excellent showcase to demonstrate how biofacies analysis can help to disentangle the interplay of climate changes, sea-level fluctuations and salt tectonics. Based on the improved characterisation of the unit, the Alcobaça Formation is formally defined, and seven members are established.