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Stratigraphy and Geochemistry of the Organic-Rich Toolebuc Formation, Upper Albian, Eromanga and Carpenteria Basins, Australia


The Toolebuc Formation is a widespread, organic-rich rock consisting of 1-3 basinally restricted units. It unconformably overlies the Wallumbilla Formation, thinning over paleo-topographic highs and lapping out on the depositional basin margin that runs E-W across the southern part of the Eromanga Basin. The top of the Toolebuc is gradational with silty mudstones of the Allaru Formation. An organic-rich claystone occurs at the base of the Toolebuc in 4 cores. The claystone is laminated and contains an average of 18% CaCO3 as lime mud in the matrix. It is present where the Toolebuc is thickest and reaches a maximum thickness of 14.1 meters in BMR Croydon 1. The average TOC is 6.6% and the average HI is 491 mg HC/g TOC, and both are constant or slightly increase upwards. The underlying Wallumbilla mudstones have an average TOC of 1.8%, average HI of 82 mg HC/g TOC, and 5% CaCO3. Overlying the basal claystone is laminated organic-rich chalk with an average content of 57% CaCO3. Thickness varies greatly, from 2.7-14.4 meters, due to onlap of lower beds onto the underlying claystone or older Wallumbilla, and it is absent in the southeast Eromanga Basin. TOC and HI are uniform across the basins, averaging 10.6% and 591 mg HC/g TOC. TOC slightly decreases upwards from the base in several cores, but HI remains fairly constant. The upper Toolebuc sequence is the most widespread. This upper sequence is 4.0-13.3 meters thick. In most cores it consists of interlayered bivalve-shell limestone and organic-rich chalky mudstone with an average of 56% CaCO3. It changes facies to claystones that have an average of 28% CaCO3 in south and southeast Eromanga Basin, where older sequences are absent. TOC and HI are uniform across the basins, averaging 7.4% and 534 mg HC/g TOC. Both TOC and HI decrease upwards in most cores, especially at the top 2-5 meters of the formation, due to an increase in terriginous organic matter and detrital sediments transitional with the Allaru mudstones, which have an average TOC of 1.6%, average HI of 73, and 8% CaCO3. The base of each sequence of the Toolebuc is unconformable and laps onto the underlying strata. The Toolebuc was deposited during a sea-level low and is underlain and overlain by highstand siliciclastics. There are no systematic variations in organic content across the basin. The high content of marine organic matter was formed by increased productivity and preservation in a wide, shallow and somewhat restricted sea.