Sedimentology and seismic geomorphology of Paleocene coastal peatlands from the offshore Gippsland Basin, SE Australia

Abstract

The Eocene–Miocene succession of the onshore Gippsland Basin is host to some of the world's thickest coal seams. Here, we document well-preserved Paleocene coals from the offshore Gippsland Basin. The Paleocene sediments in the basin consist of three stacked coastal units, known informally here as T8, T7 and T6, in order of decreasing age. Each unit consists of, from seaward to landward, a sediment starved marine facies, a sand-rich, wave-dominated shoreface facies, lower coastal plain tidal and peatland facies, and upper coastal plain sandy, fluvial facies. Seismic amplitude extractions reveal unprecedented details of the peatland palaeogeography. The early Paleocene T8 palaeogeography reveals a wave-dominated, sandy strandplain seaward of a >1000 km2 lower coastal plain peatland. The peats extend inland to ~35 km from the paleocoastline and are dissected by a series of meandering and anastomosing channel systems. This geometry is remarkably similar to the modern tropical peatlands of the Rajang Delta in Sarawak, where forested domed peats have formed over tidal and shoreline sediments. The Paleocene coals of the Gippsland Basin have likely formed in a very similar stratigraphic setting and provide a large-scale ancient example of lower coastal plain coal deposition.