Genetic Stratigraphic Sequence Analysis in Terrestrial Aggrading Basin: Sequence Stratigraphy of Narrabeen Group, Sydney Basin, Australia

Shawn A. Reynolds, William E. Galloway

Development of event/sequence stratigraphy concepts over the past 15 years has been one of the most significant contributions to systematic stratigraphic analyses of sedimentary basins. Most of these approaches, however, have not been applied to terrestrial aggradational settings but rather were developed in marginal basin settings where progradational and transgressive units are important components of the sequence framework. The principal objective of sequence definition is the identification of genetic stratigraphic units that share a common paleogeographic, climatic, and tectonic origin. Thus, the basic premise of sequence stratigraphy should be useful in the delineation of the depositional framework for any basin.

Criteria were devised to identify three genetic stratigraphic sequences within the Triassic Narrabeen Group of the Sydney basin, an aggrading basin with a terrestrial fill. Individual sequences were chosen to emphasize significant changes in tectonic and climatic conditions, as reflected in the distinguishing characteristics of depositional style, sediment composition, and paleodrainage trends.

The Munmorah sequence is characterized by lithic sandstone deposits of an extensive alluvial plain sourced by the New England foldbelt to the northeast. The overlying Bulgo sequence is distinguished by changes in sediment transport direction, lithology, and depositional style. The western Lachlan foldbelt emerged as a source of a quartz-rich braid-plain system. The Bald Hill sequence lies above the Bulgo and consists of sandstones and kaolinitic claystones that were deposited in extensive, highly oxidized alluvial plains that were dissected by numerous small streams. The distinctive mineralogy and depositional style of the Bald Hill deposits suggest climatic and tectonic changes in the Lachlan foldbelt source area.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.