Taphonomy of crinoid beds associated with large subaqueous dunes from the Lower Cretaceous of the Neuquén basin, west-central Argentina

Abstract

Exceptionally preserved stalked crinoids have been recorded from Valanginian beds of the Agrio Formation in the Neuquén Basin. The crinoid beds occurred within a 30-m thick succession consisting of cross-bedded sandstones and mixed sediments. Cross-bedded sets are up to 5 m thick and the crinoid beds are invariably associated with reactivation surfaces separating consecutive sets. The scale and architecture of the cross-bedded strata suggest that they represent the migration of large subaqueous dunes (or sand waves) in a tide-influenced subtidal setting. The best preserved specimens were recorded from the base of a key crinoid bed. Some of these specimens are almost completely articulated including stem with cirri, calyx and arms with pinnules, whereas other specimens are represented either by articulated arms with attached pinnules or isolated stem fragments with cirri. Complete articulated crinoids indicate that these specimens were suddenly buried, probably during life, and no subsequent reworking or bioturbation took place. Avalanching was the most likely burial mechanism of stalked crinoids that probably inhabited the dune toe and/or through on the leeward side of the bedforms. In contrast, disarticulated specimens suggest that these suffered in situ winnowing and significant exposure time prior to final burial in the dune through.