CONICET | Buscador de Institutos y Recursos Humanos

Punta del Barco Formation is a classical Early Cretaceous fossiliferous unit of the southern Patagonia; it is the upper unit of the Baquero Group, and crops out in the Deseado Massif area. It constitutes a volcaniclastic sequence where sin- and inter-eruptive periods are recognized. Several fossiliferous levels bearing remains of the Gleichenites flora with preserved cuticles and abundant palynological assemblages. In this contribution we analyze the effect of the volcanism on the development and preservation of the mentioned flora. About fifty percent of the sequence was deposited under sin-eruptive conditions that led to deposition of tuffs, wedded tuffs and tuffaceous sandstones. During inter-eruptive periods breccias reworked the previously deposited pyroclastic levels of cross-bedded sandstones and fine grained conglomerate dominated. Architectural reconstructions allow recognizing: 1) small sin-eruptive channels, 2) large inter-eruptive channels, 3) inter-eruptive floodplains, 4) sin-eruptive floodplains and ash-fall deposits. Floras are well preserved in inter-eruptive floodplains that evolved to topographic terraces during the incision of the inter-eruptive channels. In this way, terraces become in time condensation horizons where neither fluvial action nor pyroclastic activity disturbed the plant growth during long time. It is the fact that controls the concentration of vegetal remains in specific stratigraphic intervals. However, some fossiliferous levels appear at the base of the inter-eruptive channels. In this case, the development of vegetated areas seems to have been favored by occasional abandon of channel belts. Permineralized trunks in life position have been found associated to the abandoned channels.