Year

1994

Degree Name

Doctor of Philosophy

Department

Department of Geology

Abstract

The Late Permian to Middle Triassic Narrabeen Group succession is mostly composed of fluviatile deposits and was deposited in the retro-arc foreland Sydney Basin, Australia. Two major sources were present: the New England Magmatic Belt towards the north and the Lachlan Fold Belt in the west. Detailed outcrop, borehole and laboratory studies, using the concepts of sequence stratigraphy and genetic relationships between the strata, were used to study the lithofacies in detail and delineate depositional sequences, depositional complexes and depositional systems.

Three depositional sequences are recognised. The major part of the studied succession belongs to the Narrabeen depositional sequence. However, the lowermost and uppermost depositional systems belong to the Illawarra depositional sequence and Hawkesbury depositional sequence respectively. The basin-wide erosional surface at the base of the Scarborough depositional system is considered to be the lower boundary of the Narrabeen depositional sequence and upper boundary of the Illawarra depositional sequence. The hiatal surface at the top of the Bald Hill Claystone marks upper boundary of the Narrabeen depositional sequence and lower boundary of the Hawkesbury depositional sequence.

Based on detailed lithofacies and bounding surfaces analysis, architectural element studies and palaeohydrology, one depositional complex and six depositional systems are recognised. In an ascending stratigraphic order, they are: the Wombarra depositional system including mixed-load fluvial deposit A (Coal Cliff Sandstone Member), bed-load fluvial deposit A (Otford Sandstone Member) and floodplain deposit A (Wombarra Shale); Scarborough depositional system including bed-load fluvial deposit B (Scarborough Sandstone) and floodplain deposit B (Stanwell Park Claystone ); Bulgo depositional complex including bed-load fluvial deposit C (lower Bulgo Sandstone), braidplain fluvial deposit (middle Bulgo Sandstone), mixed-load fluvial deposit B (upper Bulgo Sandstone) and floodplain deposit C (Bald Hill Claystone ); and Newport depositional system including Garie Claystone Member (a tuffaceous unit) and lagoonal to lacustrine deposits (Newport Formation).

Based on detailed sedimentological studies and considering the genetic relationships between the strata, eight modifications to the currently-accepted stratigraphy are recommended. The W ombarra depositional system has a strong affinity with the Illawarra Coal Measures and should be excluded from the Narrabeen Group and included in the former. The Coal Cliff Sandstone is degraded to member status and together with the Otford Sandstone, form two sandstone members in the newly defined Wombarra Formation. The Narrabeen Group, as defined in this study starts with the Scarborough Sandstone and terminates with the Bald Hill Claystone. The Newport Formation rests on a hiatal surface at the top of the Bald Hill Claystone and has a strong genetic relationship with the Hawkesbury Sandstone. It is recommended that the Newport Formation be excluded from the Narrabeen Group. The Garie Formation is down-graded to member status and termed Garie Claystone Member of the newly defined Newport Formation. The Gosford Subgroup is recommended to be upgraded to a group status to accommodate the genetically-related Newport Formation (including Garie Claystone Member), Terrigal Formation, Hawkesbury Sandstone and Mittagong Formation.

In terms of sequence stratigraphy, four new terms are defined based on two controlling factors: major sedimentation processes (aggradational versus progradational) and tectonism (syntectonic versus post-tectonic). These new terms are: Syntectonic Aggradational Systems Tract (SAST); Syntectonic Progradational Systems Tract (SPST); Post-tectonic Progradational Systems Tract (PPST); and Post-tectonic Aggradational Systems Tract (PAST). These new terms are successfully applied to interpret the Sydney Basin development (a foreland basin), but they have potential to be applied on other types of basins as well.

In contrast to previous studies, the lower fine-grained sequences in the studied succession are considered to be syn tectonic (excluding the uppermost fine-grained units) and the major coarse-grained sequences are considered to be post-tectonic. These deposits have been laid down in two tectonic cycles. The mostly fine-grained deposits of the Wombarra depositional system (uppermost part of the Illawarra Coal Measures) was deposited during extensive basin subsidence resulting from compaction of the underlying coal-bearing sequence and basin floor flexure related to the thrust-front loading in the northern part of the basin. The Wombarra depositional system represents the SAST of the first tectonic cycle. Alluvial fan deposits of the Munmorah Conglomerate close to the thrust belt are considered to be a SPST of the same cycle. Rebounding of the basin floor, due to the erosion of the thrust belt, caused uplifting of the proximal parts of the basin, and resulted in progradation of the coarse-grained materials farther into the basin. The PPST of the Scarborough Sandstone was formed at this time and marks the end of the first tectonic cycle. The second tectonic cycle constitutes the SAST of the Stanwell Park Claystone, SPST of coarse-grained units close to the thrust belt and PPST of the Bulgo Sandstone. The Bald Hill Claystone and Newport Formation are considered to be PAST in nature.

This study revealed that the new concepts in foreland basin development are quite applicable to the Sydney Basin. It also was revealed that sequence stratigraphic concepts can be implemented on fluviatile deposits. This study also emphasised the importance of genetic relationships between strata in a succession for solving stratigraphic problems.

Share

COinS