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?

AGUIRRE M; RICHIANO S.; FARINATI E.; FUCKS E.

QUATERNARY INTERNATIONAL

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2011 vol. 233 p. 113 - 129

1040-6182

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 (4e5m) and exceptionally rich inmolluscan assemblages (ca. 75e90% 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 fromboth areas, the aimswere: 1) to establish quantitative grades of taphonomic alteration ofMactra from four lithostratigraphical units at Bahía Samborombón and identifywhich 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 about5400 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 Quaternarymarine 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 productmainly of long after-death exposure along the Taphonomically Active Zone in high to moderate wave energy conditions.