Depositional settings and evolution of a fjord system during the carboniferous glaciation in Northwest Argentina

ALONSO-MURUAGA, PABLO J.; COLOMBO PIÑOL, FERRÁN; SPALLETTI, LUIS A.; LIMARINO, CARLOS O.

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 ofwestern Argentina during the late Carboniferous Gondwana glaciation. Well exposed fjord deposits of theGuandacol Formation were studied in the Loma de Los Piojos region (Protoprecordillera), where they fill a 2.9km wide paleovalley with steep side walls and a relatively flat floor. The valley cross-cuts Lower Devonian sand-stones and Mississippian mudstones and sandstones, and provides evidence of glacial abrasion, including striatedpavements and glacial microtopography (grooves, ridges, and striae). Based on the analysis of seven sedimentarylogs, eight sedimentary facies in the valley fill were recognized: (A) Massive diamictites; (B) Laminated mud-stones with dropstones; (C) Stratified diamictites; (D) Clast-supported conglomerates and sandstones; (E) De-formed diamictites, conglomerates and sandstones; (F) Folded diamictites; (G) Mudstones interbedded withsandstones, and (H) Stacked and amalgamated sandstones. These sedimentary facies are grouped into two principalfacies 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 fillwhen glacial influence ceased, and comprises laminated mudstones interbedded with sandstones (facies G) andstacked sandstone beds (facies H) that mostly record deltaic sedimentation. Stratigraphic relationships, plant fossilsfound in the paleovalley walls and palynological assemblages recovered in mudstones of facies D help to establishan early Pennsylvanian age for both the incision and the filling of the paleovalley. The studied paleovalley records anexceptional 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 pro-vides a complete framework of the late Carboniferous postglacial evolution in western Gondwana