A SEQUENCE STRATIGRAPHIC ANALYSIS OF THE LATE PERMIAN SUCCESSION IN THE SOUTHERN COALFIELD, SYDNEY BASIN, NEW-SOUTH-WALES

The stratigraphic succession of the Late Permian upper Shoalhaven Group and Illawarra Coal Measures in the southern Sydney Basin has been re-interpreted using a stratigraphic sequence model based on relative sea-level changes. The results of an integrated study using outcrops, drill cores, wireline logs and reflection seismic data indicate that six major Permian eustatic cycles have controlled sedimentation of this stratigraphic succession. The depositional sequences corresponding to the eustatic cycles are initiated and terminated by rapid eustatic falls of sea-level. The resulting unconformity surfaces which bound such sequences extend over significant areas and may be used in basin-wide correlations of stratal packages. Internally, these sequences comprise a basal incised fluvial valley fill overlain by a transgressive cycle and a progradational cycle. The base of the transgressive cycle may contain regionally extensive coal. The peak of the transgression is marked by a condensed interval; a widespread chronostratigraphic surface resulting from hemipelagic sedimentation on a starved surface. These condensed intervals are useful datums for section correlations. The upper part of each sequence is represented by a highstand progradational cycle, which may contain a thin dirty discontinuous coal. The stratigraphic sequences described from the southern Sydney Basin bear a strong similarity to the Pennsylvanian cyclothems in North America. The development of widespread coal deposits, such as the Bulli and Wongawilli Coals, is related to ponding along the coastal plain at the commencement of the trangressive phase of the eustatic cycle. These transgressive cycle can be correlated over large distances and have basin-wide significance. In contrast, the thin dirty discontinuous coal units associated with the highstand progradational phase of the eustatic cycle cannot be correlated over any significant distance.