Molluscs Palaeoenvironments and Palaeoclimates during Late Pleistocene and Mid- Holocene highstands along Bahia Vera-Camarones (Patagonia)

AGUIRRE, MARINA L.; RICHIANO, SEBASTIÁN; NEGRO SIRCH, Y.; TRUCCO, M.I.

Conferencia; PUGLIA 2003. FINAL CONFERENCE PROJECT IGCP 437; 2003

The Quaternary sea level history along the Patagoniancoastal area has been explained by several factors (i.e.,eustasy, sediment supply, wave action, current energy). Sealevel changes allowed the preservation of a complex offossiliferous coastal deposits known in the local literatureas Feruglio´s Marine Terraces (MT, III to VI). Especiallywell preserved are those along the southern ChubutProvince, between Bahía Vera and Camarones (44.2º S-45ºS) in response to global sea-level highstands and minorneotectonic events. They were documented early in theliterature (d´Orbigny, 1834-1847; Darwin, 1846; Feruglio,1933-1950) and by subsequent authors (Codignotto et al.,1992; Rostami et al., 2000). Their rich biogeniccomposition consists mainly of molluscs and associatedmacrofauna (balanids, terebratulid brachiopods,polychaetes, bryozoans). In marine sedimentary depositsoriginally described as MT III (+ 44m, Camarones), IV (+22-29m, Punta Pescadero and Camarones) and V (+ 16-18m, Bahía Vera, Punta Pescadero, Camarones) of LatePleistocene age, and as MT VI (+ 8-14m, Punta Lobería,Punta Pescadero, Camarones) of mid-Holocene age, boththe gastropod and mainly the bivalve taxa recorded havenot as yet been sufficiently studied.Preliminary results for selected samples (10fossiliferous localities and 4 modern collecting sites alongthe adjacent nearshore) indicate that molluscs (42 taxa; 25gastropods and 17 bivalves) represent 91% of the totalfaunal content.The molluscan assemblages show variations incomposition, distribution, diversity patterns andmorphology of individual taxa (such as differences in sizeand intraspecific morphological variations), bothlatitudinally and chronologically, likely linked toenvironmental controls. Faunal differences, geographicallyand between the Late Pleistocene (MTIII, IV, V), mid-Holocene (MTVI) and present (Magellanean Province),suggest that changes of substrate nature, water energy,salinity and especially SST, are the main controllingfactors.The molluscan assemblages integrate parautochthonousshell concentrations which can providepalaeoenvironmental signals complementary of detailedsedimentary and modern geochronological work performedin the area (Schellman and Radtke, 2000).The most characteristic taxa recorded are Patinigeradeaurata, Fissurella spp., Crepidula protea, C. aculeata, C.cf. unguiformis., N. isabelleana, Trochita pileus,Buccinanops spp., Pareuthria plumbea (Philippi),Acanthina monodon (Pallas) and Trophon spp.(Gastropoda); Brachidontes purpuratus, Protothacaantiqua, Clausinella gayi, Mactra aff. patagonica, Ostreacf. tehuelcha. New records for the Late Pleistocene (MTIVand V, MOIS7/9 - 5) are C. aculeata, N.isabelleana, B.purpuratus, A. tehuelchus (Bivalvia).Apart from qualitative compositional differences, anoutstanding observation relates to the dominance andgreatest size of Tegula atra and P. antiqua in thePleistocene (especially within MTIV) and of Trophon spp.,A. atra, B. purpuratus in the Holocene (MTVI) or modernnearshore.They are appropriate for new analysis, especially interms of palaeobiogeographical and palaeoclimate changesto present. An attempt to establish the typical molluscanfauna and, on this basis, to characterize the nearshoreconditions during marine episodes since at least the long