Dynamic environmental conditions recorded by the trace fossil Teichichnus and event beds during deposition of the basal Vaca Muerta Formation in the central Neuquén Basin (Argentina)

Abstract

The mud-dominated part of the basal Vaca Muerta Formation in the central Neuquén Basin accumulated mainly under dysoxic conditions as indicated by the absence of both fine lamination and large burrows, typical of anoxic and oxic conditions, respectively. Event beds and large, strata-bound concretions preserving a nearly uncompacted sediment fabric document rather discontinuous deposition, in particular as concretions must reside within the zone of carbonate supersaturation for a prolonged period of time to form. The mudrock hosts numerous ash layers. Among them is a bed up to 40 cm-thick that was deposited by storm-induced currents and later became calcified. It contains abundant, winding Teichichnus up to 220 cm-long. The trace producers partly reworked their own spreite and probably applied a gardening and sequestrichnial nutritional strategy (that is collecting and stowing of nutritional material in the burrow for later utilization). The spreite fill documents depositional processes that delivered argillaceous sediment and provided oxygen. The ash originally constituting the Teichichnus bed originated from the volcanic arc west of the basin. In contrast, the erosive surface limiting the Teichichnus bed is covered by mud, implying sediment delivery from the east or south where rivers entered the basin while the climate was (at least seasonally) humid, whereas aridity increased towards the north. Consequently, an estuarine circulation likely developed and prevailed during the initial transgression of the lower Vaca Muerta Formation while cold Palaeo-Pacific water flowed into the subsiding and expanding basin. Later, during prevailing dry climate, an anti-estuarine circulation was probably established.