A focus on the sedimentology of transgressions in interior seaways: utilising modern and outcrop analogues to interpret the subsurface Cretaceous Murta Formation, Eromanga Basin, Australia

Abstract

Comprehension of the character and stratigraphic architecture of sedimentary rocks in the subsurface is derived from the observation of modern depositional processes on the surface of the Earth and field-based studies of similar depositional systems exposed in outcrop. In Australia’s Eromanga Basin, the Murta Formation is a substantial hydrocarbon reservoir; however it does not outcrop, data from wells are sparse and the depositional setting has previously been interpreted to be both continental lacustrine and marginal marine. Through extensive field and laboratory work, both for the Murta Formation and depositional analogues, this study investigates and discusses the sedimentology, stratigraphy and provenance of the Murta Formation in the Eromanga Basin. Murta Formation sediments were deposited in the Lower Cretaceous during the Berriasian to Valanginian, a key time of increased variation in global eustacy, continental breakup, climate change and just after a catastrophic mass extinction event at the Jurassic-Cretaceous boundary. Core investigations reveal that the Murta Formation is primarily composed of fine sands and muds, often arranged in coarsening up parasequences that become increasingly sand-dominated up section. The Murta Formation thickens over the Patchawarra and Poolowanna troughs, suggesting a basin depocentre in this area. Two basin-wide transgressive-regressive events are interpreted to have occurred and these most likely correspond with marine incursion in the Upper Murta Formation as the Eromanga Basin transitioned from continental lacustrine to marginal marine conditions. Zircon data indicate that sediments are sourced from mature cratons and younger volcanic provinces. This implies that the potential for unexplored reservoir presence on the western side of the basin is substantial, as mature, clean sands in were most likely deposited there in proximal deltaic environments. This project was motivated by research questions arising from the discovery of the Cuisinier Field, which unexpectedly yielded hydrocarbons in a new facies type within the Murta Formation. Sands within the Cuisinier system most likely represent a delta system deposited during a basin-wide marine regression and transgression event. As data for the Murta Formation are sparse, fluvial terminations in low accommodation basins are not widely-studied and marine transgressions in epicontinental seaways tend to be complex, analogue studies were also conducted. Lake Yamma Yamma in central Australia was investigated as it includes a substantial area of fluvial termination deposits at the main lake inlet, and has a similarly low-gradient basin setting to that of the Murta Formation. The geomorphology and sedimentology of deposits at the Lake Yamma Yamma site were described in detail, and controls on deposition interpreted. Based on this analogue study and literature review, ideas around the theme of fluvial termination deposits in a low gradient basin setting were applied to interpretation of the Murta Formation. The Dakota Formation, deposited at the initiation of the Cretaceous Western Interior Seaway, in Colorado, USA, was considered in detail at a specific outcrop locality and used as an analogue for the Murta Formation because it comprises a net transgressive system preserving internal transgressive and regressive cycles. Overall, the transgression was complex and piecewise. The size and shape of the deltaic features are similar to those observed in core in the Murta Formation, so thus provide a useful indicator for likely facies arrangements, as well as reservoir connectivity and geometries in the Murta Formation. These new studies of deposits in modern and outcrop localities, in combination with published literature, allowed an improved facies model to be developed for the Murta Formation. They also provide new insights into previously unstudied deposits, and contribute to aspects of research focus that are presently understudied. Lake Yamma Yamma has not previously been the focus of any papers despite being the largest playa lake in Queensland, Australia, and containing a substantial dryland terminal fluvial deposit, features often interpreted in the ancient record but not well studied in modern environments. As a part of this research, a new classification scheme to aid in the description and interpretation of dryland fluvial termination deposits is proposed. Although the Dakota Formation has been the focus of previous studies, the particular locality studied in this thesis has not been described in detail or assigned a comprehensive stratigraphic framework. As well as providing a detailed description of the sedimentology and a stratigraphic framework for the study area, this study also contributes new detrital zircon ages, which enabled an improved understanding of regional paleogeography. Furthermore, deposits preserved as a result of transgressions of epicontinental seas are not well understood and with no observable modern analogues, the detailed process-based understanding contributed by this study is very important in understanding similar deposits in the subsurface. In addition to contributing new perspectives on the Murta Formation of the Eromanga Basin, dryland fluvial termination deposits and the Dakota Formation of the Western Interior Seaway, the results of this thesis will provide a useful resource for the interpretation of similar systems in the geologic record.