Marked Depositional and Organic Facies Change across the Paleocene-Eocene in the Gippsland Basin, Australia.

Significant organic facies change occurred in the Latrobe Group during the Eocene to Oligocene transition (EOT) in the Gippsland Basin, controlled by climate, basin tectonics and sedimentary facies. The transition from more subdued progradational-transgressive tracts in the Paleocene-Early Eocene, to markedly westward backstepping transgressive systems tracts in Middle-Late Eocene, is documented by integrated sedimentary, palynology and organic petrology studies of recent extensive coring tied to seismic. The present-day nearshore area was dominated in the Palaeocene and Early Eocene by alluvial plain/upper delta plain facies, behind a barrier system located 50-70km further east near Nannygai-Luderick. Rapid transgression by the middle Eocene transformed the area into coastal plain/marginal marine facies behind a barrier system which had moved to near the Barracouta gas field, with the barrier system backstepping further west into the onshore area by the Oligocene. The organic-rich facies in Paleocene-Early Eocene upper delta plain facies mainly developed in abandoned channels or between channel belts in back-levee swamps and are thin, discontinuous and prone to splitting due to channel switching and splay avulsion. The coals are mainly durains with high telovitrinite:detrovitrinite, a gelified detrovitrinite groundmass, high inertinite from seasonal wet-dry conditions, clays and low pyrite. The middle-late Eocene organic facies include thick ombrogenous peats developed in lower coastal plains during the late transgressive/aggradational phases. They comprise upward drying clarain lithotypes with moderate telovitrinite:detrovitrinite and upwards increasing perhydrous liptinite, developed in extensive peat swamps adjacent to but isolated from the main distributary channel belts. More distal back-barrier marshes form thinner clarain and cannel peats with perhydrous high detrovitrinite:telovitrinite and liptinite (14-26%) and together with back barrier lagoon mudstones are very good source rocks. The abrupt change from inertinite rich to poor coals over the EOT results from the interaction of rapid and pronounced changes in climate, flora and depositional facies.