Mactra vs. Brachidontes in the marine Quaternary of Argentina: taphonomic and palaeoecological aspects.

AGUIRRE, M., RICHIANO, S., FARINATI. E., FUCKS, E., PISANO, M.F.

Granada

Congreso; TAPHOS´08 Quinta Reunión de Tafonomía y Fosilización. Third Meeting On Taphonomy and Fossilization.; 2008

Quinta Reunión de Tafonomia y Fosilización

Rich skeletal concentrations were preserved along the costal area of Argentina (South America) during the Late Quaternary as a consequence of the last transgreessive-regressive marine processes. Excellent examples of molluscs shell concentrations are recorded along the Bonaerensian littoral at Bahía Samborombóm, in the north, and southwards at Bahía Blanca. Taphonomic and palaeoecological aspects of the two abundant bivalves, Mactra isabelleana d´Orb. and Brachidontes rodriguezi (d´Orb.), of the different habitats and life habits are useful palaeoenvironmental tools, allowing to compare the timing of removal fronm the taphonomically active zone (TAZ) and general physical parameters at the time of deposition. Mollusc skeletons belonging to 42 gastropod species and 50 bivalve species from fossiliferus deposits sampled at Pascua Formation (late Pleistocene; + 6 m a.m.s.l.; + 35 ka. B.P.; MIS5?), Destacamento Río Salado Formation (early-mid Holocene; +3 a.m.s.l.; 7 ka. B.P.; MIS1) and Las Escobas Formation (mid Holocene; +3-8 a.m.s.l.; 2.5-7 ka. B.P.; MIS1) from Bahía Samborombóm, and sediments of the Holocene transgression in the Bahía Blanca estuary (+6-8 m a.m.s.l.; 5-6 ka. B.P.; MIS1) represent 80-95% of the whole faunal content. Cnidarians, bryozoans, polychaetes, cirripedes  and decapods represent the remaining associated invertebrate fauna (5-20%). Mactra isabelleana is the dominant mollusc species (85-95%) in the marine deposits of Bahía Samborombóm, while Brachidontes rodriguezi predominates in the Bahía Blanca deposits (75%). The main taphonomic attributes (articulation, fragmentation, abrasion, colour and luster, incrustation, bioerosión and dissolution) classified as tree alteration grades (0, good; 1, fair; 2, poor) on shells of Mactra and Brachidontes, which are constant and dominant in the sampled sedimentary units (Aguirre & Farinati, 1999) were quantified and plotted in ternary taphograms (Kowaleski et al., 1995) for an objective regional comparison. Volumetric samples (5000 cc) from Bahía Samborombóm provided 2200 shells of Mactra isabelleana. The samples were collected in sediments of the Pascua Formation (beach ridges), Destacamento Río Salado Formation (tidal flats), and Las Escobas Formation (Canal 18 Members, lagoon; 6-7 ka. B.P; and Cerro de la Gloria Member, beach ridges, 3-5 ka B.P.) Additional samples were collected along the present day beach at Punta Rasa (southern Bahía Samborombóm), where only 39 shells were recorded. Sand shells ridges on the northern coast of Bahía Blanca estuary (internal and external areas) provided 1800 shells of Brachidontes rodriguezi. Degree of articulation in Mactra isabelleana from Bahía Samborombóm is poor (100% grade 2) for all samples analysed and fragmentation is similar for all the units (› 50% grade 0). Abrasion of the outer and inner surfaces of shells, as well as colour and luster, show similar grades both Destacamento Río Salado Fm and Canal 18 Mb of the Las Escobas Fm. (100% grades 0), but poor remaining units (0-30% grade 0), except for the modern sample with intermediate values. It is very unusual to identify shell specimens altered by bioerosión and incrustatios which show high values (grade 0) for all the units. Degree of articulation in Brachidontes rodriguezi from Bahía Blanca is poor (› 80%, grade 2) in the external area and fair to poor (50%, grade1-2) in the internal area. Fragmentation is fair to poor (50%, grades 1-2) but better in the inner sector of the samples. Abrasion and dissolution are poor (› 80%, grade 2) in both sectors. Taphonomic signals confirm the general palaeoenviromental interpretations based on palaecological data for the units concerned. Despite the shell architecture (Mactra is harder than Brachidontes), the substrate (Mactra is infaunal in muddy substrates whilst Brachidontes is epibyssate on varied sandy-rocky bottoms), and the depth distribution Mactra is infralitoral and Brachidontes inhabits intetidal to supratidal settings), the high degree of abrasion and loss of original luster confirm higher energetic conditions and longer periods of exposure in the TAZ. Bioerosion, depending on other organism activity, is scare or absent in the both areas due the different causes for each unit. In the ridges, low degree of bioerosión is directly linked to strong tidal current and higher water energy that produce rapid transport and a sudden burial of the shells from the upper infralittoral to the final depositional environment. In the tidal flat and lagoon, where the low energy would imply a major activity of incrustation and/or carnivorous organisms, they could have been absent. Alternatively, a sudden death of the infaunal shells probably prevented any biological alteration. The rich parautochthonous shell concentration confirms a typical marine environment characterized by moderate to high wave energy but with miltiepisodic extraordinary storm events, responsible for the transport and accumulation of bioclasts and lithic material at different distances, not too far, from their original position. The taphonomic and paleoecological analysis of dominant taxa in the marine Quaternary of Argentina with different ecological requirements and life modes/habits are able to provide reliable palaeoenviromental information both in high and low energetic facies.