Shale facies in the Mesoproterozoic Beetaloo Sub-basin, Northern Australia—Experiment-based assessment of depositional conditions for the world’s oldest gas shale

Interpreting depositional settings of ancient shales relies on careful facies analysis, recognition of primary sedimentary structures and textures, and the “readout” they provide with regard to physical, chemical, and biological processes at the seabed. For Phanerozoic shales, modern analogues facilitate such analysis, but for Precambrian examples, nonactualistic scenarios need to be seriously considered, given that the biosphere was fundamentally dominated by prokaryotic microbes. Fortunately, in spite of their antiquity, many Precambrian shales show an abundance of sedimentary features that can be calibrated to process variables via creative use of experimental studies. The Mesoproterozoic Velkerri Formation of northern Australia contains the shale-dominated carbonaceous Amungee Member, known for abundant hydrocarbon shows and producible shale gas. The dark-light “striped” appearance of these shales reflects alternating layers (millimeters to centimeters) of variable total organic carbon (TOC) content (>4% versus <2%, respectively). Some TOC-rich layers show a wavy-lenticular internal texture of anastomosing C-org and pyrite-rich streaks and laminae, textural attributes suggestive of benthic microbial mats. The majority of TOC-rich layers shows lenticular fabric, composed of sand-sized (0.1–1 mm) elongate fine-grained aggregates of variable composition. Aggregate size varies between layers; imbrication and inclined layering is suggestive of bedload transport. Flume studies of lenticular fabric suggest that these layers reflect seafloor erosion of soft muds in the form of sand-sized aggregates that were transported across the basin floor by bottom currents. In the context of the Amungee Member, the observed features imply that current events of varying strength eroded parts of the muddy seabed and distributed soft “clasts” across the basin via bottom current transport. Interspersed parallel laminae of coarse silt, and homogenous clay-silt layers with inclined fabric elements suggest bottom current transport of silt and flocculated muds, an interpretation that is consistent with flume investigations. In shales, textural studies at the bed- and lamina scale provide insights into conditions of sediment transport and deposition. Through linkage to experimental work, parameters like current velocity and duration of transport events, can be estimated and support predictive models of shale accumulation even for “deep-time” strata.