Shell beds from the Bardas Blancas Formation (Lower Toarcian-Lower Bajocian), Neuquén Basin, Argentina

Abstract

The Bardas Blancas Formation (lower Toarcian-lower Bajocian) crops out in northern NeuquénBasin (Argentina). This unit is composed mostly by thick packages of hummocky cross-stratified,analaminated and massive sandstones and laminated mudstones interpreted as shoreface/delta frontto offshore deposits settled in a storm dominated siliciclastic platform. Shell beds and trace fossilsare common in sandstone beds. This work makes reference to these shell beds, which have beenstudied in five localities considering stratigraphic, sedimentologic, taphonomic and paleoecologicdata. In general, these shell beds are dominated by disarticulated bivalve shells, fragmentationis variable and valves are highly altered by neomorphism (recrystallization and inversion) anddissolution making difficult its determination and the recognition of the processes that affectedthe specimens during their pre-depositional history (biostratinomic processes). Nevertheless, goodstratigraphic and sedimentologic data allowed recognizing shell beds produced mostly by stormprocesses, including proximal and distal tempestites and storm wave deposits, settled above stormwave base (foreshore, shoreface and transition zone). Alternatively, some sections of the BardasBlancas Formation have been interpreted as flood-dominated deltas based on sedimentologicfeatures. This interpretation seems acceptable for the lower half of the Arroyo Loncoche section(the northernmost section) where shell beds are extremely scarce, developed as well-preservedvery thin concentrations overlaid by thick sandstone beds. Whereas in other sections, shell bedsare highly reworked and exhibit evidence of long periods of permanence in the water-substrateinterface, associated with lower sedimentation rates and intense reworking related with stormevents. These reworked sections exhibit fully marine conditions, including the presence of crinoidossicles and brachiopods, not recognized in the Arroyo Loncoche section. It seems useful to use ataphonomic analysis of shell beds and their distribution as a tool to identify the processes involvedin the sedimentation in different sections of this unit. A second application, related to their utility inthe recognition of sequence stratigraphic significant surfaces, is not relevant in the Bardas BlancasFormation. The abundance of event and multievent beds associated with the base of the transgressivetracts and to the top of the regressive tracts satisfies the expectations, but the poor preservation ofthe shells prevents the distinction of lag and hiatal beds from event and multievent beds and thusprecludes a sequence stratigraphic analysis.