Sedimentology, Palaeoecology and Taphonomy of Valanginian Shellbeds of the Neuquén Basin (Argentina): Palaeoenvironmental and Sequence Stratigraphic Implications

SCHWARZ, ERNESTO; ARCHUBY, FERNANDO M; SIMANAUSKAS, TRISTÁN

Davos, Switzerland

Congreso; 21st. I.A.S. Meeting of Sedimentology, 3-5 September 2001, Davos, Switzerland; 2001

I.A.S. (International Association of Sedimentologists)

This work leads with a Lower Cretaceous carbonate sequence (Middle Mulichinco Formation, 70 m thick) developed towards the eastern passive margin of an interior sea (Neuquén Basin), with an incipient (?) magmatic arc to the west, forming an archipielago between this interior sea and the proto-Pacific ocean. We carried out a combined analysis in order to clarify environmental and sequence stratigraphic aspects. Based on macrofauna and its taphonomy, stratigraphy and sedimentology we identified 8 facies. Among them only one has scarce fauna (marl facies), while the others are bioclastic carbonate bodies. These fossiliferous facies are grouped into three major associations that share similar macrofaunal attributes. They are: reclined epifaunal/thick shelled shallow infaunal-dominated associations (A1), thin-shelled infaunal-dominated association (A2) and cemented epifaunal-dominated association (A3). Bivalves largely dominate the three associations and major components are: big oysters, astartids, ptychomids and echinoids un A1; pinnids, astartids and Pleuromya in A2; aggregated serpulids and small oyters in A3. Associations 1 and 2ad the marl facies occupied wide belts along a low relief marine margin. They represent a progressive faunal replacement from close (proximal) to well below fair-weather wave base (distal) settings. We also envisage shallow marine conditions for A3, but under relatively low sedimentation rate and basin isolation. Relative sea-level fluctuations have driven salinity changes, but also affected sedimentation rates and consistency of the substrate. We recognised five different types of shellbeds according to their sequence stratigraphic position. They are found in lowstand, transgressive and highstand deposits. Lowstand deposits contain serpulid reefs followed by widespread oyster build-ups of A3, which have formed during strong physical stress conditions around the sequence boundary. This situation implies minimum connection of the interior sea with the ocean and consequent reduction of water salinity by even small fluvial discharge. A1 belongs to transgressive shellbeds with proximal and relatively higher-diversity facies, accumulated during expansion of the basin, that means the closest to normal-marine conditions. An oyster pavement facies marks the transgressive flooding surface, which is immediately overlain by the closely packed pelecypod facies of A2. This dense biofabric is related with rapid deposition of eroded carbonate material. Maximum transgression is thought to be located somewhere along the marls facies. Finally, these marly deposits pass upward into the highstand-typed shellbed, characterized by the loosely packed bioclastic facies of A2, formed during fairly constant and relatively high sedimentation rate. We conclude that macrofauna adapts to environmental changes, which are mainly controlled by relative sea-level changes.