Origin and Characterization of Intrasalt Non-halite Lithologies of the Neoproterozoic Patawarta Diapir, Central Flinders Ranges, South Australia
Non-halite lithologies in salt diapirs are variously referred to as clasts, chips, rafts, stringers, sutures, and encased minibasins in the literature. Due to the complexities of gathering and processing seismic reflection data within salt bodies, nonhalite intrasalt lithologies, collectively referred to as inclusions here, are often first identified while drilling wells. These wells may penetrate unexpected lithologies of unknown fluid pressure, which can result in economic failure and pose serious drilling hazards and a threat to human life. Therefore, it is imperative to identify and determine the nature of inclusions prior to drilling, and ideally during the well planning process in order to better risk the likelihood of drilling success that ultimately leads to the discovery and safe production of economic hydrocarbon accumulations. Our current understanding of intrasalt inclusions are based primarily on seismic reflection data observations and interpretations, a few well penetrations, and physical and numerical modeling, which provide limited information about the detailed nature and possible origins of intrasalt inclusions. This type of information is best derived from outcrop studies. The research results presented here aim to provide the first targeted outcrop study of intrasalt inclusions. The exceptionally well exposed Patawarta Diapir, Flinders Ranges, South Australia contains a wide variety of intrasalt inclusion lithologies of both sedimentary and meta-igneous in origin. Patawarta Diapir provides a unique opportunity to study two different types of carbonate intrasalt inclusions in detail allowing their attributes to be documented and their origin to be determined, thus increasing our ability to identify and predict the presence of these types of inclusions in other salt diapirs. The first is present at the margin of the diapir specifically at the salt-sediment interface, known as the Rim Dolomite and the second is a series of limestone-dominated inclusions surrounded by dolomicrite diapiric matrix and concentrated at the southern side of the diapir. The results of field and laboratory studies of the Rim Dolomite indicate that it represents lateral dolomite caprock. The caprock developed originally in a crestal position on the Patawarta salt sheet by dissolution of halite and corresponding concentration of anhydrite insoluble residue that was subsequently altered to dolomite through processes associated with sulfate-reducing microbes. Caprock formation occurred during deposition of the Marinoan Bunyeroo Formation. Shortly after formation the dolomite caprock was rotated to a diapir-flanking (lateral) position by halokinetic drape folding associated with passive diapiric flow of the Patawarta salt sheet. The results of field and laboratory studies of five limestone -dominated inclusions shows that the limestone was deposited as part of the post -salt Marinoan (Ediacaran) Wonoka and Bonney Sandstone formations. The inclusion strata were deposited on top of the Patawarta salt sheet as a carapace. The carapace strata were subsequently dismembered and encased in salt at an allosuture. The allosuture indicates that the mapped Patawarta Diapir actually represents two separate salt sheets that collided and superimposed one sheet over the other and incorporating the carapace of the underlying salt sheet at a suture zone as the overriding salt sheet rode over the top of the underlying salt sheet. The two inclusion case studies from Patawarta Diapir are used to develop an inclusion classification scheme that could be used for both surface and subsurface analysis of intrasalt inclusions. The classification system is based on the time stratigraphic age of the inclusion lithology in relation to the depositional age of the salt layered evaporite sequence: presalt, synsalt, or post salt in age. The classification scheme is used to explore the range possible types of non-halite lithologies that could be found in a salt diapir and provides an explanation of the mechanism of inclusion incorporation into the diapir. This classification system is the first of its kind and aims to provide a useful tool in attempting to identify the origin and nature of intrasalt inclusions.
Geology|Pacific Rim Studies
Kernen, Rachelle, "Origin and Characterization of Intrasalt Non-halite Lithologies of the Neoproterozoic Patawarta Diapir, Central Flinders Ranges, South Australia" (2019). ETD Collection for University of Texas, El Paso. AAI27666470.