Geology and Geochemistry of the Toolebuc Formation, an Organic-Rich Chalk from the Lower Cretaceous of Queensland, Australia
David R. Pevear, George J. Grabowski, Jr., 1985. "Geology and Geochemistry of the Toolebuc Formation, an Organic-Rich Chalk from the Lower Cretaceous of Queensland, Australia", Deep-Water Carbonates: Buildups, Turbidites, Debris Flows and Chalks—A Core Workshop, Paul D. Crevello, Paul M. Harris
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The Upper Albian Toolebuc Formation is a thin, laterally persistent chalk which occurs within a thick section of fine-grained clastics in the Eromanga Basin, Queensland. In the Esso Australia JCD-6 core from the Julia Creek area, the Toolebuc consists of 10 m of laminated, pelletal, kerogenous chalk overlain by 10 m of interbedded chalk and Inoceramus-rich coquinite. Thin bone-rich lag deposits occur at the bases of each of these units. The upper contact is gradational with the overlying Allaru Mudstone.
The Toolebuc shows little evidence of diagenesis other than physical compaction. The chalk consists of well preserved, low-magnesium calcite coccoliths and coccospheres and immature amorphous kerogen. The chalk from the lower laminated half of the Toolebuc averages 50% calcite, 10% quartz, 10% clay minerals, 7% pyrite, and 11-21 %Corg. The Inoceramus-rich chalk in the upper Toolebuc is composi-tionally similar to the chalk in the lower Toolebuc but contains slightly more calcite and clay and less organic matter (4-10 %Corg). The Toolebuc is enriched by two orders of magnitude in heavy metals compared with average shale and limestone. Abundance of these elements correlates well with abundance of organic matter, except for high values of uranium in lag deposits associated with apatite.
The Toolebuc Formation is a condensed section formed during a relative rise in sea level. The depositional environment was a shallow sea (50-200 m deep) with a lush growth of planktic algae in the photic zone. The Toolebuc accumulated below wave base and below the photic zone, in anoxic bottom waters. Coccoliths and organic matter were rapidly transported to the bottom in fecal pellets from organisms feeding in the surface waters. The upward transition to a benthic fauna and to fine-grained siliciclastics reflects better oxygenated bottom waters and encroachment of shorelines during a relative fall in sea level.
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Deep-water carbonates represent on the few frontiers remaining for carbonate exploration and research. The last decade has experienced a rapid evolution in concepts of depositional models and diagenesis which underscores the importance of these deposits as significant reservoirs and source rocks. This workshop displayed cores selected to provide subsurface geologic examples of deepwater carbonates from a variety of depositional settings. Several papers discuss depositional models, platform-to-basin reconstructions, and diagenetic sequences that are important in the development and exploration of Paleozoic carbonate debris flow and turbidite reservoirs of the Palo Duro, Delaware and Midland Basins. Many other examples are included from several different regions.