Stratigraphy, facies and depositional evolution of the Sauce Grande Formation (Upper Carboniferous): Sauce Grande Basin, Sierras Australes, Buenos Aires, Argentina

Abstract

The detailed analyses of the lithofacies (including their composition) and the depositional processes related to the Sauce Grande Formation, represent the first siliciclastic sedimentation at the Sauce Grande Basin. Two profiles were studied: the North profile (with 826 m) and the South profile (with 1095 m). The unit is composed of abundant diamictites [Dmm1, Dmm2, Dmg, Dms, Dms(r )], as well as sandstones (Sma, Smw, Sh, Sr, Sp), shales (Fm, Fl, Fd), and scarce conglomerates (Dcm, Dcg). The ciclicity was also analized, recognising three megacycles (lower, middle, and upper, with their respective lithofacies association). The lower megacycle allows a view of the Devonian paleorelief (Lolén Formation), and is composed by abundant diamictites, sandstones and scarce conglomerates. The middle megacycle contains sandstones and conglomerates and in the upper megacycle it is possible to see the diminuition in the frequence of diamictites (containing abundant dropstones) partially replaced by sandstones and shales (in this case, only in the north profile). In the lower and upper megacycles, the Dmm1, Dmm2 and Dms(r) facies, with decametric thickness and spread-out by hundred of meters, are the more conspicuous members, and are related to semiconsolidated or fluid cohesive debris flows, generated at proximal strong slopes. It is probable that the diamictites of the upper megacycle can be of the rain out type. The Dms(r) includes contortioned or oriented sand bodies and frequent shaly intraclasts. The conglomerates represent non coesive deposits leaved by hiperconcentrated flows or grain flows. The Sma sandstones were deposited by suspensive dense flows, while Smw represent fine and dense suspensive flows transitional to liquefact flows. The other sandstones represent wave or current action (Sr) or frontal accretion of transversal bars or sand dunes, by traccional currents. The beginning of the depositional processes was characterized by a rapid transgressive process during the Late Carboniferous under moderate subsidence conditions (eustatic processes). The lower megacicle is characterised by a slope-apron or debris-apron, localized near the continental talus, with strong aggradation and rare progradation. These conditions depend almost on the thermal regime of the cold base glaciers at South Africa and under a low sea tract. The middle megacicle represents a rapid phase of glacial advance, related to isostatic shallowing. The upper part of the apron were reworked by waves and currents, forming an almost emergent platform. During the upper megacycle a transgressive process occurr (high sea tract), related to the retreat of the Southafrican glaciers and producing the tastubian transgression that continues in the Piedra Azul Formation. The diminuition of sediments is visible by the sistematic reduction of the facies thickness, the major frequency of Dmm1 and Smw facies, and the interaction of shaly deposits. All these facies were deposited at low slope platforms, where the advance or retreat of the glaciers were recognised. Two glaciohorizons were identificated: the GH1, related to the middle megacycle and the GH2, located in the contact between the Sauce Grande and Piedra Azul formations.