The postglacial transgressive conglomerate in the Hirnantian of the Central Precordillera.

JESSICA GOMEZ; BERESI MATILDE; PERALTA SILVIO

NOVOSIBIRSK, RUSSIA

Congreso; 13TH INTERNATIONAL SYMPOSIUM ON THE ORDOVICIAN SYSTEM; 2019

In transgressive successions, the conglomerate is presented as transgressive lags or accumulates in tidal systems (Clifton, 2003). The rapid transgression, caused by the post-glacial event and the wave agitation, causes the erosion of the exposed facies of the San Juan Formation. The carbonates were reworked and redeposited in two important psephitic levels in the coarse beach configuration (Arche, 1992). The first level corresponding to the sediments of the berm- the Talacasto conglomerate subfacies- closest to the contact with the San Juan Formation. It consists of discoidal and ellipsoidal forms, subangular to subrounded, polymictic composition and matrix-support fabric (Poblete Sur section). The second level occurs at the beginning of the berm, with subfacies corresponding to the top of the psephitic deposit. The clasts exhibit greater mineralogical (reduction of the carbonate components) and textural (clasts sub-rounded to well rounded and high sphericity) maturity and clast-support fabric. The conglomerate deposit is the result of the transgressive post-glacial process of the Hirnantian, a register of the sea-level rise, and represents a continue mantle significant to regional correlations and to reconstruct the paleoenvironmental changes across the Ordovician-Silurian critical interval in the Precordillera. The increase of the available space during the final stage is represented by the oolitic sand facies, subjected to the action of the oscillatory processes of the wave, and which develop to ferruginous oolitic deposits, in a coastal land environment, associated with a regressive stage during the postglacial transgression.