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

AGUIRRE M. L.; RICHIANO, S.; FARINATI, E.; FUCKS, E. Y PISANO, F.

Granada, ESPAÑA

Congreso; 5ª Reunión sobre Tafonomía y Fosilización - 3rd Meeting on Taphonomy and Fossilization.; 2008

Rich skeletal concentrations were preserved along the coastal area of Argentina (South America) during the Late Quaternary as a consequence the last transgressive-regressive marine processes. Excellent examples of molluscan shell concentrations are recorded along the Bonaerensian littoral at Bahía Samborombón in the north and southwards at Bahía Blanca. Taphonomic and palaeoecological aspects of two abundant bivalves, Mactra isabelleana d´Orb. and Brachidontes rodriguezi (d´Orb.), of different habitats and life habits are useful palaeoenvironmental tools, allowing to compare the timing of removal from the taphonomically active zone and general physical parameters prevalent at the time of deposition. Molluscan skeletons belonging to 42 gastropod species and 50 bivalve species from fossiliferous deposits sampled at Pascua Formation (late Pleistocene; +6 m a.m.s.l.; > 35 ka B.P.; MIS 5 ?), Destacamento Río Salado Formation (early-mid-Holocene; +3 m a.m.s.l.; 7 ka B.P.; MIS1) and Las Escobas Formation (mid-Holocene; +3-8 m a.m.s.l.; 2.5-7 ka B.P.; MIS1) from Bahía Samborombón and in sediments of the Holocene transgression in the Bahía Blanca estuary (+6-8 m a.m.s.l.; 5-6.5 ka B.P.; MIS1) represent 80-95% of the whole faunal content. Cnidarians, bryozoans, polychaetes, cirripedes and decapods represent the associated invertebrate fauna (5-20%). In the marine Quaternary along Bahía Samborombón Mactra isabelleana is the dominat molluscan species (85-95%) while Brachidontes rodriguezi predominates in the Bahía Blanca deposits (75%). The main taphonomic attributes (articulation, fragmentation, abrasion, colour and luster, incrustation, bioerosion and dissolution) classified as three alteration grades (0, good; 1, fair; 2, poor) on shells of Mactra and Brachidontes, which are constant and dominant in the sedimentary units sampled (Aguirre and Farinati, 1999), were quantified and plotted as ternary taphograms (Kowalewski et al., 1995) for an objective regional comparison. Volumetric samples (5000 cc) from Bahía Samborombón 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 Member [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 Samborombón), where only 39 shells were recorded. Sand shell 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 Samborombón is poor (100% grade 2) for all the samples analysed and fragmentation is similar for all the units (> 50% grade 0). Outer and inner abrasion, as well as colour and luster, show similar grades both in Destacamento Río Salado Fm and Canal 18 Mb of Las Escobas Fm. (100% grade 0), but poor in the remaining units (0-30 % grade 0), except for the modern sample with intermediate values. It is very unusual to identify shell specimens altered by bioerosion and incrustation 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%, grade 1-2) in the internal area. Fragmentation is fair to poor (50%, grades 1-2) but better in the inner sector of the ridge (50%, grade 1-0). Bioerosion and incrustation is good (0-20%, grade 0) in all the samples. Abrasion and dissolution are poor (> 80%, grade 2) in both sectors. Taphonomic signals confirm the general palaeoenvironmental interpretations proposed based on palaeoecological data for the units concerned. Despite the shell architecture (Mactra 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 infralittoral and Brachidontes inhabits intertidal 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 scarce or absent in both areas. This is due to different causes for each unit sampled. In the ridges, low degree of bioerosion 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 incrusting and/or carnivorous organisms, they could have been absent. Alternativelly, a sudden death of the infaunal shells probably prevented any biological alteration. The rich parautochthonous shell concentrations in the deposits analysed and compared confirm a typical littoral marine environment characterized by moderate to high wave energy but with multiepisodic 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 palaeoecological analysis of dominat taxa in the marine Quaternary of Argentina with different ecological requirements and life modes/habits are able to provide reliable palaeoenvironmental information both in high and low energetic facies.