An integrative physical, mineralogical and ichnological approach to characterize underfilled lake-basins

Lakes are particularly sensitive to environmental fluctuations, which arerecorded in their facies and stratigraphy. Ephemeral lakes reveal their sensi-tivity to palaeoenvironmental changes in the overprinting of the sedimentaryfeatures in every single bed. Tetrapod-track taphonomic-modes and ichno-logical taphonomic-pathways can be used as sensitive indicators of environ-mental conditions of the track-bearing beds during deposition andimprinting. The Middle Triassic Cerro de las Cabras succession (CuyanaBasin, Argentina) provides an excellent opportunity to these environmentalindicators in an underfilled palaeolake. A model of ichnological preservationfor underfilled lake systems is proposed and the role of the ichnology recordin the sequence stratigraphy analysis is evaluated, based on the integrationof tetrapod-track modes, taphonomic-pathways of playa-lake ichnofauna,mineralogy and physical data. Soft-ground suites include those dominatedby invertebrate grazing traces and arthropod locomotion traces (Suite 1), andthose overprinted by horizontal-vertical dwelling burrows with tetrapodtracks preserved in taphonomic modes B and C (Suite 2). The firm-groundsuite (Suite 3) comprises tetrapod-tracks with the best preservation styles(modes A and B) along less abundant invertebrate dwelling and feedingtraces as found in Suite 2. Clay mineralogy (dominated by illite with subor-dinate smectite) suggests low plasticity of the layers, in agreement with low-relief deformation structures observed in tetrapod-track taphonomic-modes.The well-preserved track tetrapod features documented in the Cerro de lasCabras succession, together with the absence of pedogenic disturbance, tram-pling obliterating the footprints, and/or evidence of strong disturbance bywind, desiccation and/or precipitation, supports short periods of exposure ofthe imprinted surface particular to this succession. An integrated multiproxyapproach is proposed to evaluate the evolutionary interpretation and identi-fication of autogenic versus allogenic controls in underfilled lake-basin histo-ries. The observed aggradational-trend suggests an equilibrium between ratesof accommodation change and sediment supply, and that the basin-centredid not experience prolonged sediment-starved conditions.Fil: Mancuso, Adriana Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Krapovickas, Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Benavente, Cecilia Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Marsicano, Claudia Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentin

Three-fold nature of coastal progradation during the Holocene eustatic highstand, Po Plain, Italy – close correspondence of stratal character with distribution patterns

Although general trends in transgressive to highstand sedimentary evolution of river-mouth coastlines are well-known, the details of the turnaround from retrogradational (typically estuarine) to aggradational\u2013progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7\ub77 cal kyr bp. Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment-dispersal pathways within a radiocarbon-dated chronological framework. A three-stage progradation history was reconstructed. First, as soon as eustasy stabilized (7\ub77 to 7\ub70 cal kyr bp), rapid bay-head delta progradation (ca 5 m year 121), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beach-barrier deposits in the lower highstand systems tract (7\ub70 to 2\ub70 cal kyr bp) records the development of a shallow, wave-dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2\ub75 m year 121). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year 121) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the \u2018A\u2013P surface\u2019. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave-dominated to river-dominated deltaic system. Identifying the A\u2013P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone

Amplitude, frequency and drivers of Caspian Sea lake-level variations during the Early Pleistocene and their impact on a protected wave-dominated coastline

The Caspian Sea, the largest isolated lake in the world, witnessed drastic lake-level variations during the Quaternary. This restricted basin appears very sensitive to lake-level variations, due to important variations in regional evaporation, precipitation and runoff. The amplitude, frequency and drivers of these lake-level changes are still poorly documented and understood. Studying geological records of the Caspian Sea might be the key to better comprehend the complexity of these oscillations. The Hajigabul section documents sediment deposited on the northern margin of the Kura Basin, a former embayment of the Caspian Sea. The 2035 m thick, well-exposed section was previously dated by magneto-biostratigraphic techniques and provides an excellent record of Early Pleistocene environmental, lake-level and climate changes. Within this succession, the 1050 m thick Apsheronian regional stage, between ca 2·1 Ma and 0·85 Ma, represents a particular time interval with 20 regressive sequences documented by sedimentary and palaeontological changes. Sequences are regressing from offshore to coastal, lagoonal or terrestrial settings and are bounded by abrupt flooding events. Sediment reveals a low energy, wave-dominated, reflective beach system. Wave baselines delimiting each facies association appear to be located at shallower bathymetries compared to the open ocean. Water depth estimations of the wave baselines allow reconstruction of a lake-level curve, recording oscillations of ca 40 m amplitude. Cyclostratigraphic analyses display lake-level frequency close to 41 kyr, pointing to allogenic forcing, dominated by obliquity cycles and suggesting a direct or indirect link with high-latitude climates and environments. This study provides a detailed lake-level curve for the Early Pleistocene Caspian Sea and constitutes a first step towards a better comprehension of the magnitude, occurrence and forcing mechanisms of Caspian Sea lake-level changes. Facies models developed in this study regarding sedimentary architectures of palaeocoastlines affected by repeated lake-level fluctuations may form good analogues for other (semi-)isolated basins worldwide