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Origin, Characteristics, and Distribution of Non-Halite Clasts Within Patawarta Diapir, Flinders Ranges, South Australia


The Neoproterozoic Patawarta diapir, in the Central Flinders Ranges, South Australia, contains silicified and dolomitized evaporite, carbonate, siliciclastic and basalt clasts ranging in size from 0.5-5 km2. Previous work interprets all the clasts as originating from the autochthonous layered evaporite sequence of the Callana Group (850-777m.y.). The clasts were carried with the salt during subsequent diapirism and allochthonous salt emplacement forming the Patawarta salt sheet. Our study of the clasts using new detailed mapping, stratigraphic sections, petrography and stable-isotope geochemistry demonstrates that although the clasts in the northern part of the diapir originated from the layered evaporite sequence, those in the southern part are primarily of postsalt origin and define a discontinuous suture zone.

One of the larger layered evaporite sequence clasts to the north forms a 5km long & 1.3km wide sheath fold with an interlimb angle of 25ᵒ and an X axis plunging ~60ᵒ toward the east-northeast. This clast is composed of dolomitized layered evaporites interbedded with siltstone, sandstones and basalt flows or shallow sills. The postsalt clasts to the south form disrupted 0.5-2km2 tight to isoclinal, recumbent folds with fold axes that trend east-west. These clasts are composed of silty carbonates overlain by green calcareous shales, dark gray stromatolitic limestones, and arkosic sandstones that match the lithostratigraphy of the Wonoka Formation and overlying Bonney Sandstone (Wilpena Group; 635-542 m.y.). The postsalt clasts are chemostratigraphically correlated to the adjacent minibasin stratigraphy by the presence of the Shuram carbon isotope excursion. The postsalt clasts are stratigraphically condensed (500 m thick) compared to equivalent suprasalt minibasin strata to the north (1,000 m) and subsalt minibasin strata to the south (1,300 m).

Because the postsalt clasts are stratigraphically condensed and contain a higher percentage of green calcareous shale, they are interpreted to represent salt sheet carapace strata enveloped by salt in a suture zone. Stratigraphic data and regional geometries suggest that two diapir feeders are present, one to the southwest and one to the northeast, and that two salt sheets amalgamated during the early stages of the latest-Neoproterozoic to Ordovician Delamerian Orogeny. Thus, Patawarta diapir is interpreted to be a canopy, with the postsalt clasts defining an allosuture.