The Arequita Formation (Lower Cretaceous): Petrographic features of the volcanic facies In the Laguna Merín Basin, east Uruguay

Abstract

The southeastern border of the Laguna Merín Basin (Rocha Department, East
Uruguay) consists of Mesozoic volcanic deposits of the Puerto Gómez and Arequita
formations, overlain by a sedimentary cover of Cenozoic and recent deposits. Wellpreserved
lava flow and pyroclastic deposits have been identified in the Arequita Formation
after detailed mapping (1.50.000) in Sierra de los Ajos region. The present study focuses
on the petrographic characterization of this felsic volcanism, with special emphasis on
the pyroclastic units. Differences in the structural and petrographic distribution of these
volcanic units are related to the N20ºE trending India Muerta Lineament. East of this
lineament, SiO2-rich rhyolites and pyroclastic deposits (ignimbrites) are present whereas
to the west, the volcanic rocks are mainly represented by basic to felsic lavas. Based
on petrographic features and regional distribution, explosive mechanisms have been
assigned to some of the volcanic units, interpreted as pyroclastic flow and pyroclastic
surge deposits.