Differentiating Juxtaposed Marine and Estuarine Mudstones Using Integrated Sedimentology and Ichnology: Permian Pebbley Beach Formation, Australia

Kerrie L. Bann¹, James A. MacEachern², and Christopher R. Fielding^3
1 University of Alberta, Edmonton, AB
² Simon Fraser University, Burnaby, BC
³ University of Nebraska-Lincoln, Lincoln, NE

Integrating sedimentological and ichnological analysis refines paleoenvironmental and sequence stratigraphic interpretations. The Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia was previously interpreted to reflect entirely inner to outer shelf and slope depositional environments. Detailed ichnofacies analysis reveals, however, that facies display characteristics incompatible with an exclusively open marine interpretation. On the contrary, the interval demonstrates the vertical superposition and lateral juxtaposition of brackish-water facies against fully marine units. Marine mudstones encompass thoroughly bioturbated siltstones (lower offshore), thoroughly bioturbated sandy siltstones (upper offshore) and interbedded bioturbated sandy siltstone and laminated sandstones (delta-influenced offshore transition). Estuarine deposits comprise channelized, heterolithic mudstones-sandstones (active estuarine channels), sheet-like heterolithic mudstones-sandstones (active estuarine basins), and laminated mudstones (abandoned estuarine channels and basins – i.e., billabong deposits).

The fully marine mudstones contain ichnological suites that exhibit moderate to intense levels of bioturbation (BI 3-6), high diversities (32 ichnospecies belonging to 21 ichnogenera), uniform distributions of ichnogenera, and an abundance of structures that reflect specialized feeding/grazing behaviors. In marked contrast, estuarine (brackish-water) deposits contain impoverished ichnological suites (up to 9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity (BI 0-3), pronounced variability in ichnogenera distributions, and the predominance of a few, simple forms that represent the simple opportunistic feeding strategies of resilient trophic generalists. This refined facies scheme has facilitated the identification and interpretation of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and rise), characteristic of low accommodation conditions, that can be correlated physically over several kilometres.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005