Economic coals of the Sydney Basin lie in high-frequency sequences mostly bounded by calcretes, indicative of base-level lowering on the coastal plain. Most coals represent blanket coastal peats that accumulated just prior to maximum transgression, which is commonly marked by dark limestone and shale with a restricted marine fauna. Thin coals are also present in highstand deposits. Coal and sequence thickness show a general correlation, confirming a link between potential accommodation and peat accumulation. Composite sequences commence with sustained coastal progradation and base-level lowering, and coals are especially prominent in a transgressive sequence set with a slightly retrogradational to aggradational style. Sequence architecture was controlled by high-accommodation events of relative sea level rise followed by relative falls, the expression of glacioeustatic events in a cratonic basin with moderate subsidence rate. Although thin (average 19 m [62 ft]) and thick (average 55 m [180 ft]) sequences show apparently similar architecture, thick sequences contain thick alluvium with cryptic transgressive units, and their detailed architecture probably reflects channel switching, climatic, and/or tectonic effects during prolonged, low-accommodation periods. Thick sequences represent the most landward transgressions and probably pass basinward into composite sequences.

The coal-bearing interval or coal window in the basin fill is about 1.5 km (0.9 mi) thick and reflects long-term accommodation driven by subsidence, as well as climatic control. Economic coals formed while the outcrop belt lay within range of high-accommodation, relative sea level events, and they show little petrographic change through the coal window. Upward loss of coals reflects regional progradation of the alluvial plain, coupled with climatic change as Pangea drifted northward.