GSWA Report 111:
Sedimentology of the Miocene Nullarbor Limestone; Southern Australia
Author: Laura G. O’Connell
Publisher: Geological Survey of Western Australia
Year: 2011
Summary
This report represents a significant contribution to understanding the Miocene carbonate depositional systems of southern Australia and provides important insights into the paleogeography and paleoclimate of the region during this time period.
Key Findings
The early to middle Miocene Nullarbor Limestone forms the vast, karsted Nullarbor Plain in southern Australia, and may be the most extensive Miocene carbonate deposit described to date.
Depositional Environment: These carbonates were deposited at southern paleolatitudes of approximately 40°S and are interpreted to be subtropical to warm-temperate in character because of the presence of certain genera of tropical coralline algae (rhodoliths and articulated types), large benthic foraminifera, tropical molluscs, zooxanthellate corals, and micrite envelopes.
Three Main Paleoenvironments: Facies are dominated by skeletal grainstones and floatstones that accumulated in three interpreted paleoenvironments: (1) seagrass banks (2) rhodolith pavements (lower, and (3) open seafloors.
Platform Characteristics: A small slope angle (approximately 0.02°) over a wide shelf (approximately 300,000 km²) implies deposition on an epeiric platform or epeiric ramp. The inboard portion of the Nullarbor Limestone can be considered part of an extensive back-reef lagoon system on a rimmed epeiric platform, perhaps attaining a size similar to the modern Great Barrier Reef system.
Climate Influence: A decrease of tropical components from west to east across the platform implies that warm oceanic currents (possibly related to a proto-Leeuwin Current), as well as a period of warm climate (Miocene Climatic Optimum), resulted in subtropical deposition at southern latitudes.
Strengths as a Background Document
Formation and Origin The report provides the definitive account of how the Nullarbor Plain formed – explaining that the limestone was deposited in a vast shallow sea during the Miocene epoch (approximately 14-23 million years ago). This is fundamental to understanding why the Plain exists at all.
Geological Character It explains why the Nullarbor is composed of limestone and what this means for the landscape – establishing the foundation for understanding the Plain’s karst features (caves, sinkholes, underground drainage) that developed later.
Scale and Extent The report documents that this may be one of the most extensive Miocene carbonate deposits globally, helping contextualise the Plain’s remarkable size and uniformity.
Paleoenvironmental Context Understanding the subtropical to warm-temperate marine environment that created the limestone helps explain the rock’s composition and characteristics.