Depositional environments and vegetation of Aptian sequences affected by volcanism in Patagonia

Abstract

The continental deposits of the Baqueró Group preserve one of the most abundant Aptian floras on the southern margins of Gondwana. These floras represent an extinct biome that was subject to increasing regional volcanism. In combination with the presence of shallow water bodies, this biome provided suitable environments for the preservation of imprints and compressions of foliage together with permineralized trunks and palynofloras. Eight facies associations were identified in nine sections, characterizing the evolution of lacustrine and fluvial environments under intense volcanic activity. In addition, three depositional sequences were defined, with the purpose of establishing a chronostratigraphic framework to which the positions of the fossiliferous levels identified in the different sections can be referred. A 206Pb/238U age of 114.67 ± 0.18 Ma from the upper depositional sequence constrains the succession as no younger than the late Aptian and probably represents an interval of 4?5 Myr, during which the vegetation was affected by increasing biotic stress conditions. A qualitative analysis of the palynological assemblages and plant remains allowed a reconstruction of the dynamics of the vegetation developed during the deposition of the Baqueró Group. Volcanic activity appears to have had relatively moderate effects on the biota, at least at the Order level. Nevertheless, low species richness is observed toward the top of the Baqueró Group, reflecting an increase in adverse environmental conditions. The possible interpretations of the assemblages identified as biozones or, in contrast, biofacies related to changeable environmental conditions are discussed herein.