Search

Omission surfaces represent a gap in the sedimentary record on a particular environment, constituting an important tool for basin analyses. Nevertheless, not always these surfaces constitute key sequence-stratigraphic surfaces, and their correct interpretation is highly important for the recognition of genetically related strata. Documentation and interpretation of ichnological data from stratigraphic surface can refine the sedimentological model. The Puesto El Moro Formation (lower Upper Cretaceous) in the Austral-Magallanes Basin, southern Patagonia (Argentina), shows excellent examples of several omission surfaces stacked during a transgressive context associated with the beginning of the foreland stage. Each of these surfaces, interpreted as Tidal Ravinement Surfaces, contain Thalassinoides isp. and Sinusichnus isp., which represent opportunistic colonization by crustaceans into a predominantly firm substrate. In this case study, the vertical recurrence of substrate-controlled trace fossil suites is the result of high-frequency repeated coastal conditions during the transgression; the associated surfaces, which have limited lateral extension, have low potential for correlation at basin scale. [ABSTRACT FROM AUTHOR]

Abstract: Results of the systematic review and regional palaeobiogeographical context of Tegula atra, including morphometric, multivariate and cladistic analyses, show that it is a keystone species in the marine Quaternary of Argentina that can be used as Pleistocene biostratigraphical tool and paleoclimate-palaeoceanographical signal. While it was absent in warmer than present high sea-level episodes during the Miocene (“Entrerriense” transgression, ca. 10 Ma) and Pleistocene (MIS11), it exhibits an excellent and abundant fossil record within dominantly cool coastal settings exclusively during the Late Pleistocene (MIS9, 7 and 5) between Río Negro and southern Santa Cruz provinces (Patagonia). It first appeared in the SEP during the late Pliocene (cooling trend), dispersed during the Late Pleistocene into the SWA presumably by rafting on macroalgae along the Cabo de Hornos and Malvinas (Falkland) currents, but became extinct in the Mar Argentino (Magellan Malacological province) during the Holocene (amelioration trend). Its absence at present represents a climate change-driven range shift and independent evidence of palaeoceanographical changes after the LGM and at the Pleistocene/Holocene transition: changes in SST (ca. 2 °C higher), wind velocities (less), light (less), nutrient availability (less), extension and intensity of cold (less) and warm (increased) shallow water currents altering water masses and biogeographical boundaries. Altogether, these changes and the Holocene scenario were disadvantageous, causing direct effects on its physiology and survival, in turn preventing the occurrence of the associated macroalgae (Durvillaea antarctica) and its successful dispersal in the SWA or retraction to the cold Humboldt System waters. This study reinforces the importance of dispersalist models to explain the origin of key taxa, adding for a better understanding of molluscan taxonomic differences along the SWA and SEP margins of South America, with implications for future coastal scenarios. The distribution of T. atra across time is a new example of the strong linkage between earth history-climatic cycles-atmospheric and oceanic circulation and the late Quaternary biotic responses, showing a possible consequence of future climate change on nearshore communities. [Copyright &y& Elsevier]

Abstract: The molluscan content from three “marine terraces” (TM) preserved along 250 km of the Santa Cruz province provided 49 molluscan taxa (33 Gastropoda and 16 Bivalvia) and the first Quaternary records for Ataxocerithium pullum, Crepidula protea, C. aculeata, Adelomelon ferrusacii, Brachidontes rodriguezi, Aequipecten tehuelchus, Ostrea tehuelcha, and Solen tehuelchus. The mollusca assemblages suggest typical marine nearshore conditions similar to the adjacent modern littoral (Southwestern Atlantic), with mostly hard substrates, shallow and cold waters. The TMVI (+5–15 m a.s.l., ca. 8–1.2 ka A.P.) is mainly characterized by dominance of Brachidontes spp., Nacella spp. and Mytilus edulis, associated with Siphonaria lessoni. No evidence has been found for the TMVI of clear biotic responses or strong palaeobiogeographic effects during the time span of the Climatic Optimum (“Hypsithermal”, mid-Holocene) which has been documented for other coastal areas northward in Argentina, down to ca. 44–45° S. Scarce differences distinguish the TMV (+15–30 m a.s.l., ca. 108–137 ka A.P., MIS5?) and TMIV (+20–30 m a.s.l., ca. 242 ka A.P.) regarding the Holocene and the modern littoral. Within the TMV, Clausinella, Eurhomalea, Solen tehuelchus and Brachidontes cf. darwinianus (typically in warmer waters) are exclusive; in the TMIV, Trophon, Buccinanops, Brachidontes and Pectinidae are characteristic. Predominance of Trophon spp. (typically in cold Magallanic waters) in modern samples vs. scarcity or absence in the TM suggest slightly lower modern SST. Several taxonomic similitudes with the Pacific Sudamerican coast (e.g., Fissurella, Nacella, Tegula, Crepidula, Trochita, and Acanthina) could respond to cold currents helping cold-water immigrants to join southern Southamerican Atlantic after the opening of the Drake Passage. [Copyright &y& Elsevier]

An integrated study combining sedimentological and ichnological data has been conducted in the lower wave-dominated deposits of the Calafate Formation. The study aimed to assess the variation in fair-weather and storm processes, tidal flux, and the coast configuration to correctly constrain sedimentary dynamics in complex depositional systems. The wave-dominated deposits comprise foreshore to upper shoreface (FA1), middle to lower shoreface (FA2) and offshore-transition deposits (FA3) moderately affected by storms. In FA1, the most proximal strata contain Macaronichnus isp. and occur within the foreshore, while the upper shoreface shows an increased ichnodiversity associated with the occurrence of the archetypal Skolithos ichnofacies evidencing heightened energies of the upper shoreface. FA2 display a fair-weather community assigned to the proximal Cruziana ichnofacies that shows a wider range and variability of living strategies, and a storm-related community represented by opportunistic colonizers which produced burrows assigned to Ophiomorpha nodosa. The monospecific occurrence of Thalassinoides isp. at the top of FA2 in CC02P section is assigned to the Glossifungites ichnofacies, developed on a firmground and autogenic surface. The ichnoassemblage in FA3 is attributable to the transitional Skolithos-Cruziana ichnofacies, that exhibits a slightly reduced diversity of the fair-weather community and a sparse bioturbation. The distribution and the general characteristics of the ichnological assemblages are consistent with deposition in fully marine conditions. The bioturbation intensity, trace-fossil diversity, maximum penetration, and burrow size distributions indicate fully oxygenated and clear waters, normal-marine salinity conditions, abundance and accessibility of food, and continuous and moderate sedimentation. Nevertheless, the ichnological signature in FA3 is related to minor physico-chemical stress produced by strong tidal currents originated during the southward expansion of the Magallanes embayment. • Integration of sedimentological and ichnological data allowed the differentiation between fair-weather and storms processes • The ichnological vertical trend shows a continuous of fully marine conditions along the shoreface depositional profile. • Tidal currents originated physico-chemical stress in FA3 deposits and later the tide-dominated deposits of the unit. [ABSTRACT FROM AUTHOR]

The Austral-Magallanes Basin covers the southernmost region of Patagonia, South America. South of Lago Argentino, the Calafate Formation documents part of the foreland stage of the evolution of this basin. Here, sandstones from the Calafate Formation were characterized using petrographical, X-ray diffraction (XRD) and geochemical data in order to establish their tectonic setting and sediment source areas. Based on our results, three intervals (lower, middle, and upper) were distinguished according to their composition. The lower and upper intervals are composed of felsic volcanic lithics, abundant quartz and orthoclase. A shift in the composition was observed in the middle interval, with an increase in the modal abundance of plagioclase as well as other accessory minerals such as pyroxene. In addition, XRD data reveal that the middle interval is dominated by smectite in the clay-fraction, whereas the lower and upper intervals present mixed-layer chlorite-smectite and chlorite assemblages. Similar differences are observed using major and trace elements: data from the lower and upper intervals point to source rocks of intermediate to acid composition, while the middle interval had source rocks of intermediate composition and a lower degree of weathering. With regard to the provenance studies, the three intervals have a strong affinity to intermediate compositions of the Tobífera Formation (El Quemado Complex) and with the diorite/andesite of the Patagonian Batholith, which implies an exclusive contribution from the western edge of the basin. However, the shift in the composition towards the middle interval indicates a sediment supply from the juvenile Andean arc, suggesting contemporaneous magmatic activity during the deposition of the Calafate Formation sandstones. This peak of magmatic activity may have been caused by an eastward migration of the arc, driven by a switch in the tectonic setting south of 48°S. • Petrographic and XRD analyses show changing compositional signatures throughout the unit. • Al 2 O 3 -(Na 2 O+CaO)–K 2 O diagram and CIA values indicate initial-moderately weathered sources. • Samples have Zr/Sc ratios of ~3 meaning that they are depleted compared to typical UCC. • In the Th/Sc–Zr/Sc and Co/Th–La/Sc diagram samples plot between the andesitic and granitic field. • Nb <20ppm and Zr <350ppm values indicate a tectonic affinity of a subduction-related volcanic arc. [ABSTRACT FROM AUTHOR]

This paper describes an integrated field, petrographic and geochemical study of the Puesto del Museo Formation, a poorly known unit that evidences a Cenozoic transgression at the southernmost Golfo San Jorge Basin. The most striking features of the Puesto del Museo Formation is its occurrence as restricted ~20 m-thick-outcrops composed exclusively of friable cross-stratified grainstones. We analyzed the sedimentary facies and facies associations in order to reconstruct the depositional setting. The petrographic characterization allows assessing the composition as a very homogeneous lithological succession dominated by skeletal grainstones. Geochemistry of trace elements (Mn, Sr, Rb, rare earth elements plus yttrium = REY), and stable isotopes (C and O) were used to characterize the grainstones compositionally, and to constrain environmental and climate conditions that controlled its deposition. The diagenetic environmental comprehension helped to the interpretations of the syn- and post-depositional conditions related to the grainstones. Finally, the stratigraphic position of the Puesto del Museo grainstones into the context of the basin is discussed. This succession can be correlated with the Patagonian marine event (i.e., Golfo San Jorge and Austral basins) that took place during the late Oligocene–early Miocene, related to an elevated global sea level controlled by glacio-eustatic conditions. • Puesto del Museo Fm can represent the southernmost mid-Cenozoic carbonates at the GSJB. • These grainstones deposits flooded a rocky shore composed of the Jurassic volcanites. • The cementation begun at the seafloor followed by vadose conditions. • Trace elements supported by facies analysis reveal shallow open marine environment. • Bioclasts and C-isotope may suggest it as part of the Patagoniense transgression. [ABSTRACT FROM AUTHOR]