Pleistocene to Holocene carbonate sedimentation on the Northwest Shelf of Australia

Hallenberger, Maximilian; Back, Stefan (Thesis advisor); Iryu, Yasufumi (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

The tropical to subtropical continental shelf of Northwest Australia (NWS) stretches between 13° to 21°S and hosts an extensive carbonate ramp system comparable in size to the carbonate systems of the Bahamas or the Persian Gulf. It forms an important, yet poorly understood, template for the interpretation of comparable ramp systems occurring within the sedimentary record. The sediment distribution on the seafloor is well documented, but information on the Neogene to Pleistocene sedimentary section of the NWS has until recently been limited to cuttings from industry wells and geophysical data. In 2015, the International Ocean Discovery Program (IODP) Expedition 356 cored the continental margin sequence of the southern and central part of the NWS to investigate its depositional history starting from the middle Miocene. Data collected during this expedition represents an exceptional opportunity to investigate the development of a tropical open marine distally steepened ramp on an orbital resolution. This thesis presents a combination of sedimentological, geochemical, petrophysical, and climate proxy datasets derived from the mid-Pleistocene to Holocene section of three IODP Sites (U1461 - U1463) positioned on the mid- to outer ramp of the NWS. At Site U1461, core data is further integrated with 2D seismic-reflection data, allowing regional and detailed studies of the carbonate system on a tens-of-meters scale. During arid glacials, sedimentation across the studied part of the NWS is characterized by inorganically precipitated carbonates, including aragonite-needle mud and ooids. Ooids developed under shallow marine conditions on small-scale flat-topped platforms. Seismic and sedimentological evidence indicates that these platforms formed locally on top of the present-day mid-ramp and were typically active during a single glacial period. Aragonite-needle mud precipitated (inorganically) in shallow-water areas. Much of these fine sediments were subsequently exported into deeper water where they mixed with pelagic carbonates. By comparison, humid interglacial periods are generally characterized by reduced sedimentation, resulting in the amalgamation of glacial lowstand deposits. However, considerable amounts of skeletal carbonates were deposited across extended parts of the mid ramp northwest of Barrow Island during the Holocene and Marine Isotope Stage 11. These sediment accumulations are interpreted as a spatially limited feature caused by a local decline in current energy. A comprehensive analysis of the last glacial to interglacial transition at IODP Site U1461 revealed a rapid transition from carbonates rich in inorganic aragonite, including small needles (<5 μm) and ooids, into sediments composed of skeletal calcite and siliciclastics. This switch occurred at 10.1 ka and is synchronous to a pronounced change towards a more humid climate and elevated fluvial runoff. These changes are associated with a gradual southward shift of the Intertropical Convergence Zone and the onset of the Australian Summer Monsoon. It further coincides with a decrease in carbonate ion concentration measured on the shells of the planktonic pteropod species Heliconoides inflatus derived from IODP Site U1461 and U1463.It can be concluded that an elevated carbonate ion concentration and ocean water alkalinity caused by an arid climate are a prerequisite for the formation of (inorganic) shallow water aragonite-rich sediments on the NWS. These conditions are not necessarily synchronous to glacial-interglacial transitions but are rather linked to the regional hydrological cycle. This thesis therefore highlights a ramp system, where climate is as important as sea-level in controlling carbonate deposition. It thereby contributes to the understanding of the Quaternary NWS and ancient carbonate ramp systems observed on seismic-reflection data and in outcrops.

Institutions

• Division of Earth Sciences and Geography [530000]
• Chair of Geology and Paleontology and Geological Institute [531110]