Taphonomy and paleoecology of mollusk-dominated concentrations from the Monte León Formation, early Miocene of Santa Cruz, Patagonia

PARRAS, A.; GRIFFIN, M.; JULIÁN, M. J.

Córdoba

Congreso; 9º Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Asociación Paleontológica Argentina - Academia Nacional de Ciencias

Detailed analysis of the taxonomic composition and taphonomic features of fossil concentrations recorded in the Monte León Formation at the locality of Monte León (50º 21´25´´S, 68º 53´ 06´´W, southeastern Santa Cruz) reveal differences among them, suggesting a complex sedimentologic and taphonomic history for these concentrations. The section comprises bioturbated tuffaceous siltstone and fine sandstone, including 10 meters of intercalated lens and beds of mollusk-dominated concentrations. It was deposited in a marine environment, the lower beds in a subtidal setting, while the upper ones are intertidal (Parras et al., 2005), and represent sedimentation during the early Miocene Atlantic transgression in Patagonia. This study aims at assessing the differences in taxonomic structure and taphonomic features of the mollusk-dominated concentrations, and whether differences among these beds could point towards a mixture of faunas from different habitats or else towards differences in the taphonomic alteration of the fossil assemblage involved. For each fossil concentration we recorded stratigraphic, sedimentologic, paleoecologic, and taphonomic features. A standard volume of 1,000 cm3 was processed, i.e., disaggregated, washed, and sieved. Twenty two species of bivalves and 60 of gastropods were recorded. Taxonomic differences among the different concentrations were assessed performing Cluster Analysis using Euclidean distance and Simple linkage. The lowermost concentration (Concentration A) has a matrix of fine- to medium-grained sandstone, a bioturbated base, and planar top, and a bed-geometry. Small pods at the bottom that reach into the underlying sediment comprise Concentration B, which was produced by infilling of pre-existing burrows. Both concentrations are densely packed (70%) and well sorted. Mollusks are chaotically arranged, but some of them are in life-position. Disarticulation, fragmentation, abrasion, and bioerosion are very low. Incrustation is moderate. Age spectrum is complete and gastropods are dominant. Concentration C is a bed of medium-grained tuffaceus sandstone, bioturbated, with a wavy base and planar top. Close-packing is loose to dense (15- 55%) and size-sorting is bimodal. Mollusks show a chaotic orientation or else are in life position. Disarticulation is moderate and fragmentation, incrustation and bioerosion are low. No abrasion was observed and adult specimens predominate. Concentration D includes lens dominated by mollusks within a medium-grained, bioturbated sandstone. Contacts are wavy, close-packing dense (55%), and it is well-sorted. Orientation of fossils is chaotic, disarticulation and fragmentation are moderate. Incrustation, abrasion, and bioerosion are low. Concentration E is a bed of medium-grained bioclastic sandstone with a bioturbated base and a planar top. It is densely packed (70%) and poorly sorted. It carries a larger proportion of bivalves, which are disarticulated and concordant (most of them convex-up) at the base and fragmented and chaotic towards the top. Incrustation is high, abrasion low, and bioerosion moderate. Age spectrum is complete. Taxonomic and taphonomic differences among the five concentrations suggest the existence of two basic paleocommunities (concentrations A-B and C-D) modified by small-scale and short-term depositional events as well as by bioturbation. Cluster analysis suggests that biogenic-sedimentologic Concentrations A and B are not distinguishable by their taxonomic composition, and are dominated by gastropods of the families Turritellidae, Epitoniidae, Struthiolariidae, Nassariidae, Muricidae, Marginellidae, and Cancellariidae, and scarce bivalves of the families Malletiidae, Carditidae, Mactridae, and Veneridae. Concentrations C and D show similar taxonomic compositions, the latter being a sedimentologic concentration derived from biogenic-sedimentologic concentration C through the action of small-scale and short-term depositional events. They are dominated by bivalves of the families Arcidae, Lucinidae, Carditidae, Mactridae, and Veneridae, and gastropods of the families Acteonidae, Bullidae, Trochidae, Turritellidae, Columbellidae, Naticidae and Muricidae. Concentration E is a sedimentologic concentration clearly distinct from those below - both from taxonomic and taphonomic points of view. It includes taxa that were probably reworked from the underlying concentrations together with others that appear here for the first time and that were actually living in a paleocommunity different from the ones that originated concentrations A-D