Taphonomic analysis of fossil concentrations from the Monte León Formation (lower Miocene), Santa Cruz Province, Argentina

Abstract

The Monte León Formation (lower Miocene) carries many invertebrate fossil concentrations characterized by the abundance of well-preserved macro- and micro-mollusks which have been the object of systematic studies, but little information exists about its genesis. The aim of this work was to describe and analyse four fossil concentrations of this unit, exposed in the Cabeza de León section, in order to reconstruct the taphonomic history of the included bioclasts and identify the environmental conditions and the depositional processes involved in their origin. Sedimentological characteristics of the concentrations allowed, in addition to field observations, the collection of washable samples that were processed and analysed in the lab. Study of the palaeontological characteristics of these samples increased by over 150% the number of taxa and allowed the recognition of three polytypic concentrations with mixed size spectra, and one paucitypic concentration revealing a dominance of adults, suggesting the influence of diagenetic processes. The sedimentological and stratigraphic characteristics of the fossil concentrations suggest a low-to moderate-energy shallow marine environment, with interspersed hydraulic events favouring sediment input, tractive reworking and amalgamation. Taphonomic features suggest that bioclasts experimented different histories of burial, exposure, and reworking that favoured temporal and within-habitat time averaging. Final burial was product of storm events that favoured the preservation of aragonite shells and the mixing of infaunal and epifaunal shells with other transported shells from shallower environments.