Depositional and sequence stratigraphic framework of an exposed Neoproterozoic suprasalt minibasin at Patawarta Diapir, Flinders Ranges, South Australia
Outcrops of mixed carbonate/siliciclastic strata comprise the Neoproterozoic (Marinoan) Wilpena Group at Patawarta diapir in the Flinders Ranges, South Australia. Patawarta diapir is a ramping allochthonous salt sheet flanked by suprasalt and subsalt minibasin strata. The stratal and structural attributes of the coeval minibasin fills afford a unique perspective on lateral and vertical salt migration and the impacts of salt-modified bathymetry in shallow marine depositional environments. This study documents the depositional and diagenetic facies, stratal thickness trends, and structural style and stratal geometries of the suprasalt minibasin fill and correlates it to the previously studied subsalt minibasin fill. The goal is to develop a predictive model utilizes stratigraphic and structural relationships of suprasalt strata to effectively identify potential characteristics of correlative subsalt strata. Enhanced understanding of potential relationships adjacent to allochthonous salt will allow for improved pre-drill prediction of minibasin reservoir attributes, thereby limiting the risks involved with subsalt petroleum exploration. The stratigraphic units of interest for this study include the Bunyeroo Formation, Wonoka Formation, and Patsy Hill Member of the Bonney Sandstone, which form parts of two 3rd-order depositional sequences. The Bunyeroo Formation records deposition in a deep, offshore basin to outer shelf environment during the Transgressive Systems Tract of the lower depositional sequence. The formational contact between the Bunyeroo Formation and overlying Wonoka Formation is interpreted as the maximum flooding surface. The Wonoka Formation represents a regional regressive sequence deposited in a storm-dominated carbonate shelf environment, whereby the lower, middle, and upper limestone, and green mudstone members reflect upward-shallowing outer shelf to shoreface, foreshore, lagoonal, and subtidal to intertidal settings. The Patsy Hill Member, which includes the lower dolomite, sandstone, and upper dolomite beds, records deposition in a barrier bar-lagoonal complex. The lower dolomite beds of the Patsy Hill Member reflect ongoing normal regression from the underlying Wonoka Formation to an intertidal setting, whereas the sandstone and upper dolomite beds document a non-Waltherian shift to more basinward deposition in the barrier bar-lagoonal complex. Therefore, the Wonoka Formation and Patsy Hill lower dolomite beds comprise the Highstand Systems Tract of the lower depositional sequence, while the Patsy Hill sandstone and upper dolomite beds form the Lowstand Systems Tract of the upper depositional sequence. An erosional unconformity between the Patsy Hill lower dolomite and sandstone beds is interpreted as the sequence boundary separating the lower and upper depositional sequences. Suprasalt Bunyeroo, Wonoka, and Patsy Hill strata thin gradually, but dramatically towards Patawarta diapir from ~3500 to 20 m over distances of 3-6 km, primarily by low-angle onlap and depositional thinning with local angular stratal truncations. Flanking suprasalt Wilpena Group strata form a broad, open fold above the allochthonous salt body with significant thinning, but only minor halokinetic upturn, which represents minibasin-scale folding and thinning. The local angular stratal truncations documented at the basal contact of the Wonoka middle limestone member and at the sequence boundary within the Patsy Hill Member are interpreted as tapered composite halokinetic sequence boundaries adjacent to the suprasalt margin. Correlation of suprasalt strata to the previously studied subsalt strata utilizes the depositional sequence stratigraphy to evaluate characteristic stratigraphic, geometric, and halokinetic relationships in coeval minibasin fills. Wonoka strata reflect consistent trends in depositional facies and interpreted depositional settings between the suprasalt and subsalt minibasins with the exception of the uppermost green mudstone member. Patsy Hill strata vary significantly in regards to depositional facies and interpreted depositional settings in the suprasalt and subsalt minibasins. Subsalt Bunyeroo, Wonoka, and Patsy Hill strata form two stacked tapered composite halokinetic sequences, whereby significant folding and thinning occurred within 560 m of the subsalt-sediment interface. Because halokinetic sequence boundaries developed in different stratigraphic levels in the coeval minibasins despite presumably analogous sedimentation rates, it is concluded that halokinetic deformation cannot be correlated from one side of an allochthonous salt body to another. The stratigraphic and structural analysis of suprasalt strata flanking Patawarta diapir and correlation to coeval subsalt strata reveal that, despite many similarities in the shallow marine environment, the depositional and halokinetic history of coeval minibasin fills do not display characteristic, predictable relationships. Although this does not allow for a predictive model of the characteristics of subsalt minibasin fill based on correlative suprasalt strata, it does contribute to a growing catalogue of petroleum system possibilities that ideally reduce exploration risks.
Geology|Petroleum Geology|Sedimentary Geology
Gannaway, Cora Evelyn, "Depositional and sequence stratigraphic framework of an exposed Neoproterozoic suprasalt minibasin at Patawarta Diapir, Flinders Ranges, South Australia" (2014). ETD Collection for University of Texas, El Paso. AAI1583911.