--> Deducing Processes of Shale Deposition, Erosion, and Transport From Textural Attributes and Insights From Experimental Studies: Observations From the Middle Velkerri Member of the Beetaloo Basin, Northern Australia
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AAPG ACE 2018

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Deducing Processes of Shale Deposition, Erosion, and Transport From Textural Attributes and Insights From Experimental Studies: Observations From the Middle Velkerri Member of the Beetaloo Basin, Northern Australia

Abstract

The Velkerri Formation of northern Australia is a laterally extensive Meso-Proterozoic succession that contains hydrocarbons and is shale dominated in its middle member. The characteristic “striped” appearance of these shales reflects alternating layers (mm-cm) of Previous HitvariableNext Hit TOC content (>4% vs <2%). TOC-rich layers show a wavy-lenticular internal texture of anastomosing C-org and pyrite-rich streaks and laminae (5-15 micron thickness) that mingle with lighter colored lenticular features composed largely of clay and silt. Such textural attributes may indicate benthic microbial mats, an interpretation that potentially applies to a subset with high continuity of carbonaceous-pyritic laminae.

Other features, however, suggest that the majority of 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. Because of high water contents (70-80%), post-depositional compaction flattens these aggregates into lens shaped features that vary in size from 0.1 to 1 mm. Average aggregate size varies systematically between layers, and we observe imbrication and inclined layering suggestive of x-lamination and bedload transport. Layers consisting of “coarse” lenses are interbedded with layers of “fine” lenses, and successive layers are marked by changes in dominant aggregate type, Previous HitvariableNext Hit content of dark carbon-rich lenses, and general TOC content. These features are consistent with current events of varying strength eroding portions of the seabed and distributing clasts across the basin via bottom current transport. Lenticular fabrics like these are reproduced in flume experiments under conditions as presumed above.

Bedload transport is also indicated by interspersed parallel laminae of coarse silt, and homogenous clay-silt layers with systematically inclined fabric elements (micas, carbonaceous rip-ups from microbial mats). Flume studies suggest that these may result from migration of silt ripples at low sedimentation rates and bedload transport (as ripples) of flocculated muds, respectively.

Textural studies of shale formations can provide significant insights into conditions of sediment transport and deposition. Because of linkage to experimental work, quantitative parameters, such as likely current Previous HitvelocityTop and duration of transport events, can potentially be introduced and enhance the ability to develop predictive models of shale accumulation.