Sergio A. Marenssi

The upper carboniferous postglacial transgression in the Paganzo and Rio Blanco basins (northwestern Argentina): facies and stratigraphic significance

Abstract During the early Late Carboniferous (Namurian–early Westphalian), an important postglacial transgression took place in the basins of western Argentina. The regional distribution, lithological characteristics, facies arrangement, and age of the transgression are analyzed herein. The postglacial transgressive event was studied in the Rio Blanco and Paganzo basins. Seven regionally extensive facies were recognized. Laminated mudstones were deposited during the maximum flooding stage, including some thin marls and black limestone beds interpreted as condensed levels. Laminated mudstones with dropstones facies, due to iceberg melting, point out to deglaciation processes. Bouldery and pebbly diamictites mainly represent gravity flow deposits, which, in some cases, are associated with interbedded sandstones and mudstones sequences interpreted as turbidites. Sandstones with large-scale cross-bedding represent high constructive Gilbert-type deltas. Finally, coarsening and thickening upward sequences result from the progradation of mouth bars deposited in delta-front environments. On the basis of the facies arrangement, three major postglacial facies associations were recognized: open marine, transitional, and continental-dominated. Palynological assemblages suggest a Namurian to early Westphalian age for the postglacial transgression.

A roadmap for Antarctic and Southern Ocean science for the next two decades and beyond

Abstract Antarctic and Southern Ocean science is vital to understanding natural variability, the processes that govern global change and the role of humans in the Earth and climate system. The potential for new knowledge to be gained from future Antarctic science is substantial. Therefore, the international Antarctic community came together to ‘scan the horizon’ to identify the highest priority scientific questions that researchers should aspire to answer in the next two decades and beyond. Wide consultation was a fundamental principle for the development of a collective, international view of the most important future directions in Antarctic science. From the many possibilities, the horizon scan identified 80 key scientific questions through structured debate, discussion, revision and voting. Questions were clustered into seven topics: i) Antarctic atmosphere and global connections, ii) Southern Ocean and sea ice in a warming world, iii) ice sheet and sea level, iv) the dynamic Earth, v) life on the precipice, vi) near-Earth space and beyond, and vii) human presence in Antarctica. Answering the questions identified by the horizon scan will require innovative experimental designs, novel applications of technology, invention of next-generation field and laboratory approaches, and expanded observing systems and networks. Unbiased, non-contaminating procedures will be required to retrieve the requisite air, biota, sediment, rock, ice and water samples. Sustained year-round access to Antarctica and the Southern Ocean will be essential to increase winter-time measurements. Improved models are needed that represent Antarctica and the Southern Ocean in the Earth System, and provide predictions at spatial and temporal resolutions useful for decision making. A co-ordinated portfolio of cross-disciplinary science, based on new models of international collaboration, will be essential as no scientist, programme or nation can realize these aspirations alone.

High latitude Eocene climate deterioration: evidence from the northern Antarctic Peninsula

Clay mineral, sedimentological and geochemical data show that the northern Antarctic Peninsula (Seymour Island, La Meseta Formation) experienced a climatic deterioration from very warm, non-seasonally wet conditions at the end of the Palaeogene global optimum (early Middle Eocene ∼47 Ma) to a latest Eocene (post ∼34 Ma) regime that was cold, frost-prone and relatively dry. During the middle Middle Eocene there was an episode of strongly seasonal wet conditions, after which the climate was generally cool and humid. Overall, Eocene climatic trends in the Antarctic Peninsula mirror those recorded at ODP sites in the Southern Ocean. There is no evidence for glacial deposition on Seymour Island during either the latest Eocene (Submeseta) or the short Early Palaeocene (Sobral) cold episodes. Local Eocene climatic changes were registered by alterations in depositional and sedimentary characters of the La Meseta Formation, and reflect regional southern high latitude Palaeogene climatic mileu modulated by local tectonic events.

Provenance, environmental and paleogeographic controls on sandstone composition in an incised-valley system: the Eocene La Meseta Formation, Seymour Island, Antarctica

Abstract The Eocene La Meseta Formation is the youngest exposed unit of the back-arc James Ross Basin, Antarctic Peninsula, cropping out in Seymour (Marambio) Island. The formation comprises 720 m of clastic sedimentary rocks of deltaic, estuarine and shallow marine origin. It was subdivided into six unconformity-based units (Valle de Las Focas, Acantilados, Campamento, Cucullaea I, Cucullaea II and Submeseta Allomembers) grouped into three main facies associations. Facies association I represents valley-confined deposition in a progradational/aggradational tide-dominated and wave-influenced delta front/delta plain environment. Facies association II includes tidal channels, mixed tidal flats, tidal inlets and deltas, washover and beach environments. Facies association III represents nonconfined tide- and storm-influenced nearshore environments. La Meseta Formation sandstones are quartzofeldspathic with some hybrid arenites (glauconite and carbonate bioclasts-rich). Sandstone detrital modes are subdivided into two distinctive petrofacies: the low quartz petrofacies (petrofacies I, Q 12%), interpreted to retain the original provenance signal, and the high quartz petrofacies (petrofacies II, Q>55% and L 1.4) is clearly dominated by volcanic-derived clasts; it developed at times of high sea level and/or during later stages of the valley fill, when an “energy fence” at the shoreline prevented delivery of sediment from the Antarctic Peninsula, thus enhancing the relative participation of local volcanic sources.

Eustatically controlled sedimentation recorded by Eocene strata of the James Ross Basin, Antarctica

Abstract The Eocene La Meseta Formation is an unconformity-bounded unit that records the geological evolution of the James Ross Basin, NE Antarctic Peninsula, during a period of decreasing tectonism and a lull in volcanic activity. This unit represents a composite incised valley, filled with deltaic, estuarine and shallow-marine deposits showing a landwards facies shift that indicates deposition during an overall sea-level rise. The six unconformity-based internal units (Valle de las Focas, Acantilados, Campamento, Cucullaea I, Cucullaea II and Submeseta allomembers) are interpreted to represent minor-scale regressive-transgressive events. Geological, palaeontological and new strontium isotopic ages allow the correlation of base-level changes with second- and third-order eustatic sea-level fluctuations. The base of the La Meseta Formation is correlated with a global 56 Ma lowstand in sea level followed by a main episode of flooding between 54.3 and 52.4 Ma. The base of the Cucullaea I Allomember is correlated with the well-known late Ypresian (49 Ma) lowstand, and the base of the Submeseta Allomember with the 36 Ma lowstand. Correlation of Eocene sea-level fluctuations in the northern Antarctic Peninsula with the global sea-level curve strengthens the concept of global syncroneity of the eustatic sea-level curve.

Record of Late Miocene glacial deposits on Isla Marambio (Seymour Island), Antarctic Peninsula

Abstract We report and describe two new small diamictite outcrops on Isla Marambio (Seymour Island), Antarctic Peninsula. These rocks rest on an erosional unconformity on top of the Eocene La Meseta Formation and are unconformably covered by glaciomarine rocks of the ?Pliocene–Pleistocene Weddell Sea Formation. The lithology, fossil content and isotopic ages obtained strongly suggest that the rocks belong to the Hobbs Glacier Formation and support a Late Miocene age for this unit. Additionally, the dated basalt clast provides the oldest age (12.4 Ma) for the James Ross Island Volcanic Group recorded up to now. The here described diamictite cannot be confidently correlated with a glaciomarine unit previously assigned to the Late Eocene–Lower Oligocene taken as proof that initial expansion of ice on Antarctica encompassed the entire continent synchronously in the earliest Oligocene. However, it is now evident that there are likely to be more, short but important, stratigraphic sequences of key regional and Antarctic wide interest preserved on the plateau of Isla Marambio.

A new Eocene crab (Crustacea, Decapoda) from Seymour Island, Antarctica

A new xanthid crab, Tumidocarcinus foersteri n. sp. is described from the La Meseta Formation on Seymour Island, Antarctica. The fossils were obtained from the Allomember Submeseta of Late Eocene age. As other representatives of the genus Tumidocarcinus were only known from New Zedand and Australia, this finding provides new insights on the palaeobiogeography of high latitude faunas during the Early Tertiary.

Eocene basilosaurid whales from the La Meseta Formation, Marambio (Seymour) Island, Antarctica

Abstract. Basal fully aquatic whales, the basilosaurids are worldwide known from Bartonian—Priabonian localities, indicating that this group was widely distributed during the late middle Eocene. In the Northern Hemisphere, fossils of basilosaurids are abundant, while records in the Southern Hemisphere are scarce and, in some cases (i.e., Antarctica), doubtful. The presence of basilosaurids in Antarctica was, until now, uncertain because most of the records are based on fragmentary materials that preclude an accurate assignment to known archaeocete taxa. Here we report the findings of mandibles, teeth, and innominate bone remains of basilosaurids recovered from the La Meseta Formation (TELM 4 Lutetian—Bartonian and; TELM 7 Priabonian), in Marambio (Seymour) Island (James Ross Basin, Antarctic Peninsula). These findings confirm the presence of Basilosauridae in the marine realm of Antarctica, increasing our knowledge of the paleobiogeographic distribution of basilosaurids during the middle—late Eocene. In addition, one of these records is among the oldest occurrences of basilosaurids worldwide, indicating a rapid radiation and dispersal of this group since at least the early middle Eocene. Resumen. El registro fósil de los basilosáuridos está bien documentado durante el Bartoniano—Priaboniano en varias localidades del mundo, lo cual indica que este grupo estaba ampliamente distribuido durante el Eoceno medio tardío. En el Hemisferio Norte, el registro fósil de este grupo es abundante, a diferencia de lo que ocurre en el Hemisferio Sur donde es escaso y, en algunos casos (i.e., Antártida), dudoso. La presencia de basilosáuridos en Antártida es incierta ya que la mayoría de los registros están basados en materiales fragmentarios, lo cual imposibilita su asignación a algún grupo de arqueocetos. En la presente contribución se describen restos de basilosáuridos correspondientes a mandíbulas, dientes aislados y un hueso pélvico, recuperados de la Formación La Meseta (TELM 4 Lutetiano—Bartoniano; TELM 7 Priaboniano), Isla Marambio (Seymour), (Cuenca James Ross, Península Antártica). Este hallazgo confirma la presencia de Basilosauridae en la Antártida, contribuyendo al conocimiento de la distribución paleobiogeográfica de este grupo durante el Eoceno medio—tardío. Finalmente, uno de estos registros se encuentra entre los basilosáuridos más antiguos conocidos, indicando un rápida radiación y dispersión de este grupo al menos desde el Eoceno medio temprano.

REVISIÓN DEL GÉNERO RETROTAPES DEL RÍO (BIVALVIA: VENERIDAE) EN EL EOCENO DE LA ANTÁRTIDA

Abstract REVISION OF GENUS RETROTAPES DEL RÍO (BIVALVIA: VENERIDAE) IN THE EOCENE OF ANTARCTICA. The validity of the genus Retrotapes del Río is discussed and Retrotapes newtoni (Wilckens), R. antarcticus (Sharman and Newton) and R. robustus (Stilwell and Zinsmeister), from the Eocene strata of the Antarctic Peninsula (La Meseta Formation), are described and illustrated. The stratigraphic range of R. robustus is extended from the Acantilados allomember to the Cucullaea I allomember. The morphogeometric analysis performed, allows the recognition of ontogenetic variations of R. antarcticus and R. robustus, as well as to shed light on their taxonomic status. The present is the oldest record of Retrotapes, a taxon that would have originated in the Antarctic region, from where it would have dispersed northwards to lower latitudes during Miocene times when it was widely represented in the Patagonian seas.

Borers and encrusters as indicators of the presence of hermit crabs in Antarctic Eocene gastropods shells

Abstract The community of encrusting and boring organisms developed on shells of the gastropod Antarctodarwinella ellioti from the lower section of the La Meseta Formation (Eocene) exposed on Seymour (Marambio) Island, Antarctic Peninsula, allows inference that the shells were inhabited by hermit crabs. A Chi-square Independence Test revealed that the community - dominated by polychaetes and bryozoans - shows preference for the aperture interior area of the shell. A subsequent Cochran Q Test indicated that the differences in frequency of encrusting and boring organisms as counted on the different interior sectors of the aperture were statistically significant. Thus, polychaetes, boring bryozoans, and encrusting bryozoans, do not show the same frequency in each interior sector of the aperture; they are more frequent on the columella (P < 0.0001, P < 0.01 and P < 0.001 respectively). Encrusting bryozoans also appear to show a preference - albeit not as high as on the columella - for the outer lip. This community of boring and encrusting organisms and their distribution on the shell confirms that the shells were inhabited by hermit crabs. The community is similar to that described in Recent hermitted shells from mid-latitude temperate water environments, suggesting that such communities were already established in the Eocene.