Search

A middle Eocene Crassostrea sp. reef near Río Turbio, southwestern Patagonia (Argentina), represents the earliest record of an oyster reef associated with estuarine facies in the southern hemisphere, and also one of the few known worldwide occurring in Paleogene rocks. The reef grew in an outer estuary environment subject to periodic changes in salinity and may have reached a maturing phase. The Río Turbio reef – by its dimensions, geometry, and substrate lithology– would have been located in a tidal channel convergence area. This reef provides new evidence suggesting that estuaries served as refuges for Crassostrea populations allowing them to disperse into fully marine environments many times throughout the Cenozoic. [ABSTRACT FROM AUTHOR]

Abstract: Oligocene siliciclastic shallow marine sediments of the San Julián Formation in southeastern Patagonia (Argentina) are interpreted as a deepening–shallowing cycle that represents a depositional sequence in which lowstand system tract deposits are not present. There are no significant compositional differences in the assemblages, all are dominated by the large oyster Crassostrea? hatcheri. However, four types of shell concentrations can be distinguished by differences in stratigraphic, sedimentologic, palaeoecological, and taphonomic features. These four types of shell concentrations are found in distinct positions within the depositional sequence: as a transgressive lag at the early phase of the transgressive system tract (TST), at the top of the TST, at the early phase of the high stand system tract (HST), and at the top of the HST. Oyster-dominated concentration in the early TST has a distinct basal erosional surface (ravinement surface), and the shells are completely disarticulated, fragmented and abraded with a chaotic orientation, suggesting reworking of previous deposited sediments, and deposition in a shoreface environment. Concentration at the end of the TST is a hiatal concentration formed from continued sea-level rise in the context of backlap deposition. It is characterized by high percentages of articulated oysters in life position, with high grades of bioerosion and encrustation. This concentration was accumulated during times of high rates of production of biogenic hard parts and low sedimentation rates, below fair weather wave base in an offshore environment. Concentrations over the maximum flooding surface in the early HST are in situ event concentrations (Census assemblages), characterized by high percentages of individual articulated adult specimens and clusters of juvenile oysters with well-developed endolithic bioerosion and encrustation, formed by rapid sedimentation in an offshore to lower shoreface environment. Concentrations at the top of the HST are multiple-event concentrations formed by densely fossiliferous deposits with completely disarticulated, convex-up oysters with low grades of abrasion and high grades of bioerosion and encrustation, intercalated with repeating beds with benthic colonizers. They were originated below fair weather wave base, in an offshore to lower shoreface environment, by sporadic high-energy events. The basic pattern of condensed deposits formed in the context of onlap, backlap, downlap, and toplap can be applied to the studied deposits. However, some differences are recorded, and these are attributed mostly to differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters. [Copyright &y& Elsevier]

Abstract: A diverse assemblage of mosasaurs was recently recovered from the Jagüel Formation (late Maastrichtian) exposed at three localities of northern Patagonia (Río Negro, Argentina). Four taxa (three mosasaurines and a plioplatecarpine) have been identified, and three of these marine reptiles can be identified at lower taxonomic levels: Mosasaurus sp. aff. M. hoffmanni, Plioplatecarpus sp., and Prognathodon sp. These occurrences are significant because they represent the first diagnostic material at generic level exhumed from Patagonia and include one of the youngest mosasaurs found worldwide. One of the specimens described herein was found only 1.5m below the Cretaceous/Tertiary boundary. Only mosasaurs from Antarctica found within a meter of the boundary are known to occur higher in the geologic section. [Copyright &y& Elsevier]

The fossil record of chondrichthyans in Argentina is still poorly known. The aim of the present contribution is to describe teeth representing a chondrichthyan assemblage from Man Aike Formation (late Eocene) exposed in the Lago Argentino area, at Santa Cruz province, Argentina. This report includes Striatolamia macrota, Macrorhizodus praecursor, Carcharias sp., a possible member of the family Pristiophoridae, for the Argentine territory. Previous Eocene reports of Striatolamia macrota from Argentina are based on incorrect determination of specimens. The shark association, especially the abundance of lamniforms, may be indicative of litoral and shallow (less than 30 m depth) temperate to cold waters, as suggested by micropaleontological content. • For the first time a late Eocene chondrichthyan assemblage is described from Santa Cruz province, Argentina. • Striatolamia macrota , Macrorhizodus praecursor , Carcharias sp., and Pristiophoridae are reported. • Species here described are frequent in fish faunas from the North Atlantic seas. [ABSTRACT FROM AUTHOR]

An understanding of paleonvironmental and paleoceanographic evolution of the Southwestern Atlantic Ocean during the Palaeogene is prevented by the lack of precise tools to date and correlate the sedimentary units. Palynological samples collected in the upper portion of the Man Aike Formation, which crops out southern Lago Argentino area, in the southwest of the Austral Basin (50°21′45″S–72°14′30″W), contain well preserved marine organic dinoflagellate cysts, which are potentially important biostratigraphic and paleoenvironmental indicators. Herein we describe the composition of the Man Aike Formation's dinoflagellate cyst assemblages and compare them to similar assemblages collected in the same basin in the upper member of the Río Turbio Formation using Compositional Statistical Analysis. The dinoflagellate cyst assemblages from the upper part of the Man Aike Formation are highly correlated to the assemblages from the upper member of the Río Turbio Formation from outcropping sections (51°31′13″S–72°15′11″W) and with the lower part of sediment cores drilled by Yacimientos Carboníferos Fiscales in the Río Turbio Formation area. These dinoflagellate cyst assemblages show a very low correlation with the assemblages from the upper part of the Yacimientos Carboníferos Fiscales's cores. The comparison of our results with the high-resolution Southern Pacific Ocean dinoflagellate cyst zonation for the late Palaeocene to late Eocene allow us to date some of the dinoflagellate events recorded in formations of southwestern Patagonia. The assemblages from the Man Aike Formation and the lower part of the upper member of the Río Turbio Formation relate to the zones SPDZ11 and SPDZ12 and are assigned to the mid-middle Eocene (late Lutetian to early Bartonian). The biostratigraphy proposed herein constrains the age of the Man Aike Formation and equivalent units based on calcareous microfossil data, mollusks affinities and 87Sr/86Sr isotopic values to an age ranging between ∼42 and 39 Ma. The assemblages from the upper part of the upper member of the Río Turbio Formation relate to the SPDZ13 Zone, which corresponds to the late Eocene (early Priabonian). The dinoflagellate cyst assemblages indicate that they were under the influence of relatively warm, marine open waters in an inner-shelf environment for the lower part of the sections. In the upper part of the sections, the assemblages suggest shallow marine waters associated with coastal areas and high trophic levels, possibly as a result of freshwater inputs. [ABSTRACT FROM AUTHOR]

Abstract: Integration of sedimentologic, ichnologic and sequence-stratigraphic data supports that the upper member of the Río Turbio Formation records sediment accumulation in tide-dominated, outer-estuarine and coastal-plain environments. Outer-estuarine assemblages comprise facies that were deposited in channel-thalweg, subtidal-sandbar complex, oyster-reef and reef-margin environments. Coastal-plain facies assemblages include distributary-channel, floodplain, salt-marsh, swamp, and swamp margin deposits. Twelve ichnogenera assigned to soft-, stiff- and firmground suites were identified. Surfaces demarcated by autogenic and allogenic expressions of the Glossifungites Ichnofacies were recognized. The trace fossils Thalassinoides, Psilonichnus, and Bergaueria were documented within the autogenic stiffground suites, whereas Gastrochaenolites was found in both autogenic and allogenic firmground suites. Allogenically produced surfaces occur on a regional scale, creating major discontinuities, while minor autogenically produced surfaces extended laterally only over a few meters. Integrating the data derived from the substrate-controlled ichnofacies with the vertical and lateral distribution of the paleoecological data provided a more thorough understanding of the importance and scale of the erosional discontinuities. Estuarine and coastal-plain deposits contain low-diversity trace fossil assemblages inferred to have been produced by opportunistic, impoverished marine infauna, indicative of brackish-water settings. The overall distribution of ichnocoenoses within the lower portion of the upper member of the Río Turbio Formation appears to have been controlled largely by the salinity gradient, with other factors such as energy and sedimentation rate playing a more limited role. [Copyright &y& Elsevier]

Abstract: The San Julián and Monte León formations (“Patagonian”) are exposed along the Atlantic coast of Patagonia, whereas in the west equivalent rocks are known as Centinela Formation. Sixteen 87Sr/86Sr measurements on the oyster Crassostrea? hatcheri (Ortmann) from the San Julián and Centinela formations and an 40Ar/39Ar-date from a whole-rock sample from the Centinela Formation yielded ages that allow more precise correlation between the two areas. 87Sr/86Sr measurements from the San Julián Formation yielded ages between 23.83 and 25.93Ma, while for the Centinela Formation the ages ranged between 21.24 and 26.38Ma. The 40Ar/39Ar analysis of a sample of the Centinela Formation yielded an age of 20.48±0.27Ma. The age data suggest a late Oligocene (Chattian) age for the San Julián Formation and the lowermost beds of the Centinela Formation (northernmost exposures). The Monte León Formation along the East coast and the entire section of the Centinela Formation in the southern area – and the middle and upper beds of this same unit in the northernmost localities – were deposited at the end of the Oligocene and early Miocene (Chattian–Burdigalian). The invertebrate fauna present in these units shows diverse preservation patterns that makes these fossils, especially the mollusks, not useful – at this stage – for correlation purposes. [Copyright &y& Elsevier]