Coal and conglomerate in the Newcastle Coal Measures—coeval facies or temporally unrelated?

Abstract

In the Central Coast region of the Sydney Basin, the Vales Point, Wallarah and Great Northern Coals coalesce and comprise the topmost unit of the Late Permian Newcastle Coal Measures. These coals reach a combined thickness of 8 m and, in places, are separated by 50-m-thick conglomeratic, incised valley fills at several stratigraphic horizons. Early Triassic, terrestrial sedimentary rocks unconformably onlap onto the eroded coal. Although the coalesced coals are remarkably free of siliciclastic interbeds, coal plies (subsections) have been correlated for more than 20 km using brightness profiles (lithotypes) and gamma-ray logs. Plies average about 0.5 m thick and indicate that the original peat was deposited episodically. The concept that coal originated from low-lying peat mires on flood basins adjacent to, and coeval with, active river systems is not supported here. Instead, correlations show that gravel was deposited at a time that equates to the hiatus between coal plies. Gravel-filled incised valleys were eroded into peat mires, removing all peat in the deepest places. Where the coals are coalesced, the only evidence for adjacent, thick conglomerate is a gamma-ray spike and a thin clay pellet layer of altered volcanogenic fragments. It is proposed that the coal is composed of individual plies originating from raised peat mires that grew during cyclical base-level rises. Afterwards, the mires became dormant during base-level falls, creating a hiatal surface above the degrading peat. During base-level fall, the increasing gradient initiated erosion of an incised valley into the dormant peat mire. Subsequently, as base level rose again, fluvial gravel aggraded in the valley. When the water table rose to the top of the incised valley, the flanking dormant peat mire was reactivated, initiating the accumulation of another ply above the hiatal surface. As peat grew and accumulated in raised mires, the sediment-filled incised valley may have resembled a muddy, estuarine body of water, no longer capable of transporting the coarse clastic bed load it did at times of lower base level. Repetition of base-level rise and fall is considered the main controlling factor in producing vertically stacked coal plies separated by hiatal and erosional surfaces, split in places, by thick conglomerate.