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

Abstract

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.