Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia

Abstract

Concepts of genetic sequence stratigraphy evolved in marginal marine basin settings where widespread episodic marine flooding events provide readily recognizable correlative surfaces that bound successive progradational clastic wedges. However, application of these concepts to nonmarine aggradational basins has been limited. In cases where marine flooding surfaces are absent, different criteria for sequence recognition are necessary. This paper addresses the utility of regionally extensive coal seams as one type of sequence boundary. Peat accumulation and preservation as coal can only occur in the absence of significant clastic deposition. Formation of coal seams of regional extent requires processes capable of interruption in sediment supply at a basin-wide scale and can define times of major reorganization in basin tectonics or climate. Regionally extensive coals can exhibit the essential attributes of sequence boundaries. In many cases, they are readily recognized and correlated by their unique seam signature, or profile, which is a function of the original peat-forming plants and the physical and chemical conditions imposed during peat swamp evolution. The unique seam profile also records numerous isochronous events, further emphasizing their time equivalence and potential for accurate dating. The concept that regionally extensive coals can serve as genetic stratigraphic sequence boundaries is well illustrated by coal seams from the Gunnedah basin, Australia. The Hoskissons Seam marks the initial change to nonmarine conditions in the basin from underlying deltaic and shallow marine depositional systems. The stratigraphically higher Breeza Seam divides the nonmarine sequence into an underlying quartzose, lacustrine, bed load fluvial complex and an overlying volcanic-lithic, mixed load fluvial, alluvial fan complex.