Taphonomic comparison between two bivalves (Mactra and Brachidontes) from Late Quaternary deposits in northern Argentina: Which intrinsic and extrinsic factors prevail under different palaeoenvironmental conditions?

Quaternary records of unique skeletal deposits studied at two coastal areas of the Bonaerensean littoral, Bahía Samborombón in the north (at Puente de Pascua and Canal 15 area) and Bahía Blanca southwards, are thick (4–5 m) and exceptionally rich in molluscan assemblages (ca. 75–90% bivalve shells), indicating different high sea level episodes. Through taphonomical aspects of target bivalves (Mactra, Brachidontes) which differ in shell structure, life habit and habitat, abundantly present in the marine Late Quaternary from both areas, the aims were: 1) to establish quantitative grades of taphonomic alteration of Mactra from four lithostratigraphical units at Bahía Samborombón and identify which signature/s better discriminates palaeoenvironments; 2) test whether Mactra shows different alteration than does Brachidontes in response to differences in intrinsic features and ecological requirements; 3) compare isotaphonomic assemblages complementing independent geological and palaeoenvironmental interpretations. Attributes (articulation, fragmentation, abrasion, colour, encrustation, bioerosion) were classified for about 5400 shells from 11 samples. Bar plots, ternary taphograms and Q-Mode Cluster analysis discriminate beach, beach ridges and tidal flat environments and two taphofacies: Late Pleistocene and Holocene high energy units; and low energy Holocene units. Only abrasion and colour allow discrimination between high and low energy conditions. Rules cannot be applied to the processes of deposition of shells, at least for the Late Quaternary marine deposits along the Buenos Aires Province coastal area. Despite the harder (Mactra) or more fragile (Brachidontes) shell architecture, in softer (Mactra, muddy substrates, infaunal) or harder (Brachidontes, varied sandy-rocky bottoms, epibyssate) and deeper (Mactra, infralittoral) or shallower waters (Brachidontes, intertidal to supratidal), the taphonomic grades of alteration are similar, a product mainly of long after-death exposure along the Taphonomically Active Zone in high to moderate wave energy conditions.