LATE PLEISTOCENE DEEP-WATER SCLERACTINIA OF THE EWING TERRACE, MIDDLE SLOPE OFF NORTHERN ARGENTINE CONTINENTAL MARGIN

CECILIA LAPRIDA; RICARDO GARBEROGLIO; NATALIA GARCÍA CHAPORI; ROBERTO A. VIOLANTE

Ciudad de Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

The Argentine continental margin (ACM) is the Atlantic part of the cold temperate Magellan Province. Its northern border is the Rio de la Plata estuary which coincides with the border of the Brazil-Malvinas Confluence Zone. The southern border is the Antarctic Convergence. Although biogeographic marine provinces are defined mainly by the distribution patterns of shelf fauna, they are also fairly accurate delineators for upper and middle slope fauna (200?1300 m). In general terms, the cold temperate upper slope Argentinean fauna is highly distinctive with very few species that also range to the tropics. For example, of the twenty modern scleractinian species known from off Argentina, 13 are endemic (Cairns and Polonio, 2013). This renders stony corals to be useful paleobiogeographical, paleoecological and paleoceanographic indicators. Deep-sea sediment cores are the main source of palaeoceanographic data. However, macrofossils are not actually frequent in deep-sea cores, although they can be found as fossil concentrations in outer shelf cores. In the ACM, these fossil beds use to be hydraulic concentrations constituted mainly by bivalves and gastropods (cf. Urien and Ewing, 1974). Fossil deep-sea corals have been sporadically mentioned from sediment cores from the shelf and slope off Buenos Aires, northern and southern Patagonia, and off Cape Horn (Squires, 1961). More recently, fossil colonial scleractinians from 1000-1300 m water depth were mentioned by Violante et al. (2010) and Bozzano et al. (2011). Nevertheless, they have been never studied in detail. These findings from the northeastern Argentinean slope are analyzed here. Our main objective is to identify for the first time these fossil Scleractinia retrieved from the ACM and briefly discuss biostratigraphic and paleoceanographic aspects. Scleractinians specimens were retrieved from two sediment cores in the outer border of the Ewing terrace in the middle slope. The core SHN-T300 is a 219 cm long core located at 37°48.4´S - 54°20.1´ W at 1300 m water depth. The sampling location is presently near the transition between the Antarctic Intermediate Water (AAIW) and the Underlying Upper Circumpolar Deep Water (UCDW). Core SHN-T372 is a 115 cm long core located at 38°00.3´ S - 54°24.9´ W at 1208 m water depth, also in the transition between AAIW and UCDW. Colonial Scleractinia found belong to Bathelia candida Moseley, 1881 (Figure 1). A matrix-supported fossil concentration of B. candida was found in the uppermost 23 cm of core SHN-T372 in olive-grey massive mud. In core SHN-T300, eight discrete levels of this coral in the upper 90 cm of the core were observed. Although B.candida is a well-known, modern deep-sea cold water coral inhabiting the southwestern south Atlantic, it has been never described as fossils before. Micropaleontological analyses of coral-bearing levels indicate a typical upper Pleistocene assemblage dominated by transitional species in a bathyal context. In core SHN-T300, echinoderm spines, benthic and planktonic foraminifera, ostracods, radiolarians and sponge spicules were found accompanying corals between 55 and 68 cm. A moderately diverse foraminiferal assemblage composed of Globorotalia truncatulinoides malvinensis, G. inflata, G. puncticulata, G.hirsuta, and G. scitula, Globigerina bulloides, Globigerinita glutinata, Neogloboquadrina pachyderma, was found at 63 cm. Typical bathyal ostracods belonging to Macropyxis, Krithe spp., and Echinocythereis sp. were also found. In core SHN-T372, a diverse assemblage of ostracods Krithe spp. Macropyxis sp., Henrihowella sp, Bythocypris sp., Xestoleberis sp. and Hemicytherura sp. has been identified. Nowadays, B. candida is distributed offshore southern South America from southern Brazil to southernmost Argentina, and in the southeast Pacific ocean, offshore southern Chile (Cairns, 1982; Cairns et al., 2005; Kitahara et al., 2009). Bathymetric distribution from 500 to 1600 m depth roughly coincides with the location of the AAIW. It is considered a species needing a hard substrate to settle and usually forms small aggregates or ?coral gardens? in Patagonia (Muñoz et al., 2012). Fossil Bathelia were concentrated in the Ewing terrace mostly around the head of the Mar del Plata submarine canyon. This canyon originates at ~1000 m water depth cutting transversally into the middle slope reaching the foot of the lower slope (Violante et al., 2010). The canyon´s head ends upslope at the foot of an erosive steep slope that marks the transition between La Plata and Ewing terraces (Preu et al., 2013). Cores containing B. candida are located at the foot of this erosive steep slope, and include pebbles and rock pieces ranging from fine gravel to blocks >25 cm. Taphonomic analysis of fossil concentrations allow to infer that in SHN-T300 and SHN-T372 cores, B. candida forms parauthochtonous assemblages formed by hydraulic agents and transported short distances after definitive burial. These fossil concentrations located in a reduced area indicate dense population occupying hard or consolidated substrate favorable for the development of ?Bathelia candida gardens? in the transition between La Plata and Ewing terraces during the upper Pleistocene. The location of Bathelia-bearing samples in the landward side of the Ewing terrace at the foot of the erosive steep slope is indicating dense flows relate with mass wasting deposits. These deposits were favored by the nearness of Mar del Plata canyon acting as a funnel during phases of paleoceanographic reorganization and the final morphosedimentary configuration of the Ewing terrace during the late Pleistocene.