Year

1998

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

The Early Permian sequences of the Pebbley Beach and Snapper Point Formations in the southern Sydney Basin are interpreted as a complex of wave-dominated, aggradational, siliclastic shoreline parasequences deposited during a period of thermal basin sag. An ichnotaxonomic list containing 43 species from 29 genera is provided for these strata and four new ichnospecies are defined; Cylindrichnus eccentricus, Teichichnus sinuosus, Taenidium synyphes and Rosselia motivus. A previously unsubstantiated link between Asterosoma, Rosselia and Cylindrichnus, which also contains Teichichnus as an intergradational end member, is clearly established.

Roughly one third of the Pebbley Beach Formation consists of a wave-dominated, microtidal barrier island complex. Alternating changes in trace fossil assemblage diversity and ichnofabric complexity thoughout the sequence provide evidence for variation in bathymetry and energy levels. Stratification of a low diversity, opportunistic, restricted Cruziana ichnofacies with a higher diversity opportunistic Arenicolites ichnofacies, reflects the storm-washover of marine sands into a brackish backbarrier environment.

Sedimentary facies ranging from lower offshore to foreshore occur in the remainder of the Pebbley Beach and Snapper Point Formations; these facies can be readily defined through the integration of ichnofacies, ichnofabric and sedimentological characters. Analysis of ichnofabrics and ichnofacies has allowed subdivision of the offshore and lower shoreface facies into sub-facies on the basis of degree of storm influence. Each sedimentary facies contains characteristic ichnofabrics and ichnofacies.

Four distinct ichnofacies are recognized in the study area, the Skolithos, Cruziana, Arenicolites ichnofacies and Glossifungites ichnofacies. The Cruziana ichnofacies is subdivided here into four categories, proximal, diverse, and distal open marine subfacies and restricted brackish water subfacies. The Glossifungites ichnofacies occurs along transgressive surfaces of erosion in the Pebbley Beach Formation and at the base of the Snapper Point Formation.

Lower offshore facies in the Pebbley Beach and Snapper Point Formations are storm influenced to strongly storm-dominated and characteristically contain complex ichnofabrics comprising a fairweather distal Cruziana ichnofacies assemblage with an associated opportunistic Arenicolites ichnofacies assemblage in storm-deposited units.

Upper offshore and offshore transition facies in the study area are storm influenced to strongly storm-dominated and tempestites comprise a high proportion of the facies. Consequently ichnofabrics are composite and comprise complex mixtures of proximal or diverse Cruziana ichnofacies, reflecting fairweather conditions, and Arenicolites ichnofacies reflecting opportunistic colonization of storm beds.

Lower shoreface facies are characteristically storm-dominated in the Pebbley Beach and Snapper Point Formations. They contain composite ichnofabrics produced by an Arenicolites ichnofacies, which reflects storm deposition, and a proximal Cruziana ichnofacies reflecting fairweather conditions. A glacially influenced deposit in the Pebbley Beach Formation containing a mixed distal Cruziana / Glossifungites ichnofacies is also interpreted as lower shoreface facies affected by alternating perennial and seasonal ice cover.

Upper and middle shoreface facies are wave-dominated and characteristically contain a Skolithos ichnofacies with rare resilient proximal Cruziana ichnofacies. Tide-dominated foreshore facies occur at the top of the Snapper Point Formation and are exclusively inhabited by a Skolithos ichnofacies reflecting ongoing high levels of hydrodynamic energy.

The Pebbley Beach and Snapper Point Formations consist of parasequences combined to form parasequence sets. The Pebbley Beach Formation contains four parasequences which display an overall basinward facies shift and occur as regressive half sequences separated by thin transgressive deposits. The lower Snapper Point Formation contains nine parasequences that have thick transgressive half sequences reflecting rapid creation of accommodation space balanced by a very high rate of sediment supply. The upper Snapper Point Formation marks an environmental change to a tide-dominated constricted seaway and parasequences become progradational. This latter change resulted from the emergence or near emergence above sea level of the developing Currarong orogen to the east. The overall aggradational pattern of the Pebbley Beach and Snapper Point Formations reflects a sediment supply that balanced the creation rate of accommodation space.

Fourth and fifth order Milankovitch cyclicity recognised in this succession is attributed to eustatic (commonly glacio-eustatic) change. The Pebbley Beach and Snapper Point Fomations are a part of a larger third order cycle, which was controlled by combined tectonic and glacial eustacy.

The overall transgression represented by the Pebbley Beach and Snapper Point Formations is marked by the Eurydesma fauna. This transgression corresponds to the Tastubian Transgression that affected most other Permian Gondwanan Basins. The maximum flooding surface in the overlying Wandrawandian Siltstone marks the onset of the Sterlitamakian regressive phase which is attributed here, and throughout the Permian Gondwanan Basins, to tectonically induced eustacy.

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