Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia

Abstract

AbstractThis study integrates ichnology and sedimentology to refine the palaeoenvironmental and sequence stratigraphic interpretations of the Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia. This succession has been interpreted previously to reflect entirely inner to outer shelf and slope environments of deposition. Detailed analysis of the formation reveals ichnological and sedimentological characteristics that contradict a fully marine interpretation. Instead, the interval reflects the vertical superposition and lateral juxtaposition of brackish-water and fully marine units. Marine facies comprise: (1) thoroughly bioturbated muddy siltstone (lower offshore); (2) thoroughly bioturbated sandy siltstone (upper offshore); (3) interbedded bioturbated sandy siltstone and laminated sandstone (delta-influenced offshore transition); (4) thoroughly bioturbated muddy sandstone (distal lower shoreface); (5) interbedded laminated sandstone, bioturbated muddy sandstone and dark claystone (delta-influenced lower shoreface); and (6) bioturbated, laterally variable sandstones (transgressive sand sheets). Estuarine facies comprise: (1) channelized heterolithic sandstone-mudstone (active estuarine channels); (2) sheet-like heterolithic sandstone-mudstone (active estuarine basins); and (3) laminated mudstone (abandoned estuarine channels and basins). The interpreted fully marine deposits contain ichnological suites that exhibit moderate to intense bioturbation, high diversities (31 ichnospecies belonging to 20 ichnogenera), uniform distributions of ichnogenera, and significant numbers of structures reflecting specialized feeding/grazing behaviours. In marked contrast, interpreted estuarine (brackish-water) deposits contain impoverished ichnological suites (9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in ichnogenera distributions and the predominance of a few, simple forms representing simple feeding strategies of resilient trophic generalists. The new analysis allows the recognition of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and physical rise), which can be physically correlated over several kilometres. Sequence boundaries typically cut down through shoreface sandstones to directly overlie offshore facies, leading to an interface with little apparent lithological contrast. In the absence of laterally continuous exposure, these surfaces may be recognized by careful ichnofacies evaluation. Thus the re-evaluation presented herein has facilitated a more realistic sequence stratigraphic analysis of the Pebbley Beach Formation.