Stratigraphy, taphonomy and sequence stratigraphic significance of oyster-dominated concentrations from the San Julián Formation, Oligocene of Patagonia, Argentina

PARRAS, ANA; CASADIO, SILVIO

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

Elsevier

Año: 2005 vol. 217 p. 47 - 66

0031-0182

Oligocene siliciclastic shallow marine sediments of the San Julia´n Formation in southeastern Patagonia (Argentina) are interpreted as a deepening–shallowing cycle that represents a depositional sequence in which lowstand system tract deposits are not present. There are no significant compositional differences in the assemblages, all are dominated by the large oyster Crassostrea? hatcheri. However, four types of shell concentrations can be distinguished by differences in stratigraphic, sedimentologic, palaeoecological, and taphonomic features. These four types of shell concentrations are found in distinct positions within the depositional sequence: as a transgressive lag at the early phase of the transgressive system tract (TST), at the top of the TST, at the early phase of the high stand system tract (HST), and at the top of the HST. Oyster-dominated concentration in the early TST has a distinct basal erosional surface (ravinement surface), and the shells are completely disarticulated, fragmented and abraded with a chaotic orientation, suggesting reworking of previous deposited sediments, and deposition in a shoreface environment. Concentration at the end of the TST is a hiatal concentration formed from continued sealevel rise in the context of backlap deposition. It is characterized by high percentages of articulated oysters in life position, with high grades of bioerosion and encrustation. This concentration was accumulated during times of high rates of production of biogenic hard parts and low sedimentation rates, below fair weather wave base in an offshore environment. Concentrations over the maximum flooding surface in the early HST are in situ event concentrations (Census assemblages), characterized by high percentages of individual articulated adult specimens and clusters of juvenile oysters with well-developed endolithic bioerosion and encrustation, formed by rapid sedimentation in an offshore to lower shoreface environment. Concentrations at the top of the HST are multiple-event concentrations formed by densely fossiliferous deposits with completely disarticulated, convex-up oysters with low grades of abrasion and high grades of bioerosion and encrustation, intercalated with repeating beds with benthic colonizers. They were originated below fair weather wave base, in an offshore to lower shoreface environment, by sporadic high-energy events. The basic pattern of condensed deposits formed in the context of onlap, backlap, downlap, and toplap can be applied to the studied deposits. However, some differences are recorded, and these are attributed mostly to differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters. differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters.? hatcheri. However, four types of shell concentrations can be distinguished by differences in stratigraphic, sedimentologic, palaeoecological, and taphonomic features. These four types of shell concentrations are found in distinct positions within the depositional sequence: as a transgressive lag at the early phase of the transgressive system tract (TST), at the top of the TST, at the early phase of the high stand system tract (HST), and at the top of the HST. Oyster-dominated concentration in the early TST has a distinct basal erosional surface (ravinement surface), and the shells are completely disarticulated, fragmented and abraded with a chaotic orientation, suggesting reworking of previous deposited sediments, and deposition in a shoreface environment. Concentration at the end of the TST is a hiatal concentration formed from continued sealevel rise in the context of backlap deposition. It is characterized by high percentages of articulated oysters in life position, with high grades of bioerosion and encrustation. This concentration was accumulated during times of high rates of production of biogenic hard parts and low sedimentation rates, below fair weather wave base in an offshore environment. Concentrations over the maximum flooding surface in the early HST are in situ event concentrations (Census assemblages), characterized by high percentages of individual articulated adult specimens and clusters of juvenile oysters with well-developed endolithic bioerosion and encrustation, formed by rapid sedimentation in an offshore to lower shoreface environment. Concentrations at the top of the HST are multiple-event concentrations formed by densely fossiliferous deposits with completely disarticulated, convex-up oysters with low grades of abrasion and high grades of bioerosion and encrustation, intercalated with repeating beds with benthic colonizers. They were originated below fair weather wave base, in an offshore to lower shoreface environment, by sporadic high-energy events. The basic pattern of condensed deposits formed in the context of onlap, backlap, downlap, and toplap can be applied to the studied deposits. However, some differences are recorded, and these are attributed mostly to differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters. differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters.