DISCERNING ALLOGENIC PARASEQUENCES IN THE ROCK RECORD: SILICICLASTIC EXAMPLES FROM THE LOWER PERMIAN OF THE SOUTHERN SYDNEY BASIN, SOUTHEAST AUSTRALIA

Abstract:

Exceptional coastal exposures of Lower Permian strata are examined along the SE coast of Australia. These siliciclastic intervals encompass the Wasp Head, Pebbley Beach, and Snapper Point formations, and record numerous stacked shallow-marine and paralic parasequences of both allogenic and autogenic origin. Parasequences, as used in this study, are reserved for stratal units that directly overlie surfaces reflecting the rate of accommodation development exceeding the rate of sediment supply (i.e., flooding surfaces), passing upwards into strata recording a period of time wherein the rate of sedimentation exceeded the rate of accommodation development (e.g., normal regression). As such, parasequences constitute building blocks that are predominantly encountered in the transgressive systems tract and the highstand systems tract. Most parasequences in the study area record progradational regression of non-deltaic (shoreface) and deltaic cycles and so conform well to the concept as originally defined. The Pebbley Beach Formation, however, also contains incised valleys, wherein the estuarine deposits were initiated by a base-level rise across an older subaerial unconformity, followed by a rate of sedimentation that exceeded the rate of accommodation development, during which the valley filled. These were top-truncated by ravinement during resumed transgression. The marine-influenced valley-fill deposits are regarded as estuarine parasequences.

Allogenic and autogenic parasequence boundaries may be erosional or depositional but are recognized by the juxtaposition of distal facies over proximal facies. Allogenic parasequence boundaries typically record a true base-level rise and are more regionally extensive. Autogenic parasequence boundaries record variations in sediment supply, are not associated with base-level changes, and are largely restricted to channel avulsion, lobe switching, and/or auto-compaction of abandoned lobes in the deltaic successions. The two boundary types can be differentiated ichnologically by the character of the overlying facies as well as the details of the bounding surfaces. Allogenic parasequence boundaries in the marine realm are generally overlain by distal facies that show elevated bioturbation intensities, homogeneously distributed burrowing, and unstressed fully marine trace fossil suites attributable to the Zoophycos, Cruziana or Skolithos ichnofacies. Such associations record a marked decrease in sedimentation associated with the landward shift of the shoreline. By contrast, allogenic parasequence boundaries within the incised valleys, show low bioturbation intensities and sporadically distributed bioturbation consisting of salinity stressed trace fossil suites attributable to the Teichichnus Ichnofacies. Where allogenic parasequence boundaries are erosional, trace fossil omission suites are commonly developed, reflecting a temporal gap between exhumation of the surface and its burial, during which the discontinuity was colonized. Omission suites in the study area correspond to palimpsest softgrounds or to firmgrounds attributable to the Glossifungites Ichnofacies.

Autogenic parasequence boundaries in the study area tend to be sharp and mainly depositional. In all cases, they lack trace fossil omission suites because, despite the shift of distal facies over proximal, the transition nonetheless marks continued regression (i.e., regionally, the rate of sedimentation exceeding the rate of accommodation). Facies overlying the autogenic parasequence boundaries retain a regressive character, marked by lower and variable bioturbation intensities, sporadically distributed burrowing, and physico-chemically stressed ichnological suites recording river-sediment influx and delta progradation that alternate with fully marine trace fossil suites. The trace fossil suites of these distal regressive facies are attributable to the Phycosiphon or Rosselia ichnofacies.