--> Origin of the "Rim Dolomite" at the Salt-Sediment Interface of Patawarta Allochthonous Salt Sheet, Flinders Ranges, South Australia

AAPG ACE 2018

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Origin of the "Rim Dolomite" at the Salt-Sediment Interface of Patawarta Allochthonous Salt Sheet, Flinders Ranges, South Australia

Abstract

Patawarta Diapir is a Neoproterozoic allochthonous salt sheet exposed in the Flinders Ranges of South Australia. The margin of the diapir contains a cryptic carbonate unit mapped as the ‘Rim Dolomite’ (RD). The RD has been previously interpreted as either a metamorphic differentiation of dolomitic material from carbonate breccia with apparent bedding interpreted as a flow structure from the moving diapir or as a diapir-centric sedimentary facies within the adjacent mudstones of the Bunyeroo Fm. Using new detailed mapping, stratigraphic analysis, and petrographic studies we reinterpret the RD as lateral caprock, and thus provide the first outcrop description of caprock developed on allochthonous salt.

The RD is thickest (100m) along the NW margin and is thinned by erosion (3m) along the SE margin, where it is truncated beneath the composite halokinetic sequence boundary between the Bunyeroo and Wonoka fms. It contains the following dolomite caprock fabrics: 1) Massive: micro-to-coarse-crystalline dolomite with 15-25% porosity; 2) Banded (Parallel, Convolute, Clotted): microcrystalline dolomite with bands of coarser crystalline dolomite and chert with 0% porosity; and 3) Brecciated: microcrystalline dolomite with oil-stained rhombohedra of baroque dolomite, silica, and late-stage calcite as cements with 5-15% porosity. All fabrics contain various amounts of anhydrite, quartz silt, and non-evaporite clasts that represent the insoluble residue derived from the autochthonous Callanna Fm layered evaporite sequence during halite dissolution and caprock formation. The caprock layers parallel the strike and dip of the adjacent Bunyeroo strata, do not interfinger with the Bunyeroo, and do not contain sedimentary structures. The Bunyeroo Fm contains clasts derived from the RD amongst reworked Bunyeroo material within channel and debris flow facies. This relationship shows that the RD was formed prior to deposition of the Bunyeroo in a diapir crestal position and was subsequently rotated to a flanking and subsalt position along with the Bunyeroo during halokinetic drape folding.

The presence of carbonate caprock is a direct hydrocarbon indicator as the sulfate reducing bacteria that alter anhydrite caprock to carbonate caprock require hydrocarbons. Caprock is often a very porous lithology present at the salt-sediment interface that may serve as reservoirs or high flux conduits for fluids within salt basins.