Relative sea level control of deposition in the Late Permian Newcastle Coal Measures of the Sydney Basin, Australia

Abstract

Accumulation of the 400 m-thick Late Permian Newcastle Coal Measures of the Sydney Basin was controlled by changes in relative sea level. Three 3rd-order sequences, which constitute the coal measures, consist of 4th-order depositional sequences of fluvial conglomerate, coal, and small paralic/lacustrine deltas/crevasse splays, deposited on a coastal plain landward of a marine shoreline. Each 4th-order sequence was deposited during a single 4th-order relative sea level cycle. Following falls in relative sea level alluvial conglomerates derived from the New England Orogen filled incised valleys above sequence boundaries forming lowstand systems tracts. Sigmoidal conglomerates with ‘giant crossbeds’ were deposited as alluvial fill in compactional moats formed at the toes of abandoned paralic deltas. Alluvial sediments passed through the coastal plain directly to the marine shoreline causing the shoreface to prograde. Rising relative sea level, caused siliciclastic sedimentation to wane. During these hiatuses, in the transgressive systems tract, a rising water table stimulated peat mire growth blanketing the entire non-marine area. When the vertical accumulation of peat was outpaced by increasing rates of rising relative sea level, transgressing lagoons, interdistributary bays and lakes inundated the mires above maximum flooding surfaces. Continuing relative sea level rise in the highstand systems tracts caused paralic/lacustrine crevasse splays, crevasse subdeltas, and small deltas to prograde westwards into the Newcastle half-graben from a volcanic source on the Offshore Uplift. At this time of rising base-levels sediments were trapped on the coastal plain while the marine shoreface was starved. Falling relative sea level terminated paralic/lacustrine delta progradation and initiated exposure and erosion to repeat the cycle and initiate another 4th-order sequence. Two source areas supplied sediment into the Newcastle half-graben. An easterly source on the Offshore Uplift shed volcanic detritus into the Newcastle Coalfield via paralic/lacustrine deltas, and the New England Orogen shed volcano-lithic detritus via braided streams. The supply from the New England Orogen was switched on or increased by a fall in relative sea level while supply from the Offshore Uplift was switched off, reduced, or diverted, and vice versa during a rise in relative sea level. Increasing rates of 2nd-order falling relative sea level resulted in an upward change from predominantly marine shoreface and coastal plain sedimentation to predominantly fluvial sedimentation in the upper Newcastle Coal Measures.