Evaluating Exploration Potential of Suture Zones or Encased Minibasins Using an Outcrop Example From the Neoproterozoic Patawarta Salt Canopy, Central Flinders Ranges, South Australia

Abstract

Intrasalt clasts variously termed “chips, rafts, stringers, sutures, and encased minibasins” are intrasalt stratal packages that pose serious drilling hazards because of unknown fluid presence (tar) and pressure, disrupt homogenous salt velocity models, and potentially contain large quantities of hydrocarbons. Offshore petroleum exploration in deepwater salt basins is particularly risky due to the current difficulty in imaging clasts within salt canopies. In order to better understand the origin, distribution and characteristics of intrasalt clasts we use detailed outcrop analog studies in the Central Flinders Ranges of South Australia, where silicified and dolomitized carbonate, siliciclastic and igneous clasts ranging from 0.5-3km2 size have been mapped with in the Patawarta salt canopy. The exceptional exposures at Patawarta provide a unique opportunity to test two opposing models for the origin of the silicified and dolomitized carbonate clasts: 1) non-evaporite stringers from the autochthonous salt of the Lower Neoproterozoic Callana Formation, 2) suture zone or encased minibasin stratigraphy from the younger Wilpena Group. Silicified and dolomitized carbonate and clastic intrasalt clasts range in thickness from 10's to 280m thick and show a lithostratigraphy of green calcareous shale overlain by dark grey stromatolitic limestone followed by arkosic sandstone which matches the lithostratigraphy of the Wonoka Formation and overlying Bonney Sandstone. Regionally the Wonoka Formation contains the most negative carbon isotope excursion (-12‰) in Earth history, known as the “Shuram” excursion. Carbon isotope chemostratigraphy will be used to confirm the presence of the Wonoka Formation in the Patawarta salt canopy. Abundant calcite, quartz, hematite, azurite and malachite filled veins cross cut the hypothesized Wonoka Formation clasts and their distribution, deformation and vein-fill geochemistry will be used to constrain the timing of fluid flow and evolution of salt canopy formation. Additionally, halokinetic deformation in the clasts indicate the presence of a more porous and permeable encased minibasin as oppose to a stratigraphically condensed suture zone. Understanding the origin and paleofluid system structure of the clasts will enable us to evaluate whether encased minibasins are an attractive hydrocarbon play type to explore in deepwater salt basins.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017