Depositional settings and evolution of a fjord system during the Carboniferous glaciation in northwest Argentina. Guandacol Formation. Paganzo Basin.

ALONSO MURUAGA, P.J.; LIMARINO, C.O.; SPALLETTI, L.A. Y COLOMBO PIÑOL, F.,

SEDIMENTARY GEOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2018 vol. 369 p. 28 - 45

0037-0738

Fjord systems, represented by glacial diamictites and postglacial transgressive shales, formed in the basins of western Argentina during the late Carboniferous Gondwana glaciation. Well exposed fjord deposits of the Guandacol Formation were studied in the Loma de Los Piojos region (Protoprecordillera), where they fill a 2.9 km wide paleovalleywith steep side walls and a relatively flat floor. The valley cross-cuts Lower Devonian sandstones and Mississippianmudstones and sandstones, and provides evidence of glacial abrasion, including striatedpavements and glacial microtopography (grooves, ridges, and striae). Based on the analysis of seven sedimentary logs, eight sedimentary facies in the valley fill were recognized: (A) Massive diamictites; (B) Laminated mudstoneswith dropstones; (C) Stratified diamictites; (D) Clast-supported conglomerates and sandstones; (E) Deformed diamictites, conglomerates and sandstones; (F) Folded diamictites; (G) Mudstones interbedded with sandstones, and (H) Stacked and amalgamated sandstones. These sedimentary facies are grouped into two principal facies assemblages that represent different stages of the paleovalley fill. Assemblage 1 is composed of diamictites (Facies A, C and F), laminated mudstones with dropstones (Facies B), and conglomerates (Facies D and E), whichrepresent glacially influenced sedimentation in the paleovalley. Assemblage 2 represents the paleovalley fill when glacial influence ceased, and comprises laminated mudstones interbedded with sandstones (facies G) and stacked sandstone beds (facies H) thatmostly record deltaic sedimentation. Stratigraphic relationships, plant fossils found in the paleovalley walls and palynological assemblages recovered in mudstones of facies D help to establish an early Pennsylvanian age for both the incision and the filling of the paleovalley. The studied paleovalley records an exceptional example of the western Gondwanan glacial to postglacial transition. Due to the continuous stratigraphicsuccession within the paleovalley as well as palynological, megafloristic and radiometric data, this example provides a complete framework of the late Carboniferous postglacial evolution in western Gondwana.