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Using Seismic and Physical Modeling to Characterize the Origin and Deformation of Sediment Clasts Within Patawarta Diapir, Flinders Ranges, South Australia

Abstract

A field study of the Neoproterozoic Patawarta allochthonous salt canopy, Flinders Ranges, South Australia, investigated the origin, deformation, & distribution of silicified and dolomitized evaporite, carbonate, siliciclastic and basalt clasts that range in size from 0.5-5 km2. With new detailed mapping, clasts originating from the layered evaporite sequence (Callanna Group) are identified to the north forming a 5km long & 1.3km wide sheath fold with an interlimb angle of 25ᵒ and an X axis plunging ~60ᵒ toward the east-northeast. Additionally, 0.5-2km2 clasts from a postsalt origin (Wilpena Group) are identified to the south forming a zone of disrupted isoclinal recumbent folds that trend east-west.

Typically, clasts forming sheath or recumbent folds inside a diapir or allochthonous salt sheet are interpreted to be a part of the autochthonous layered evaporite sequence. However, the Wilpena Group clasts are much younger (150-300 m.y. younger) and, by analogy with other salt-tectonic provinces and published physical models, are interpreted to represent encasement of a postsalt carapace in a suture zone. Analogous fold styles in interpreted postsalt sedimentary clasts are seen in a seismic dataset from the Sigsbee salt canopy, Gulf of Mexico. The bulk of these clasts are concentrated along, and above, base-salt highs that trend SE roughly parallel to the bulk salt-transport direction. Sheath folds within these clasts have orientations also indicative of southeast-directed flow. We interpret these sedimentary clasts as locally sourced, representing an early stage of lateral allosuturing from local radial spreading of a series of small salt sheets, concentrating clasts along SE-oriented basal suture lines (base-salt highs). Subsequent override by a larger salt sheet with SE-directed flow folded these clasts into the observed geometries. Physical models support our hypothesis, with extreme deformation of sedimentary clasts resulting from flow perturbations associated with multidirectional flow during the canopy amalgamation process. A similar process likely deformed the clasts at Patawarta diapir, as field relationships suggest the suturing of two salt sheets, one sourced from the southwest and the other from the northeast.