Internal Fabrics and Microbial Precipitation in the Stromatolites of Hamelin Pool, Western Australia

Stromatolites are lithified organosedimentary structures that dominate the fossil record during the Precambrian and are present throughout the Phanerozoic. Modern examples of active stromatolites are rare. The hypersaline waters of Hamelin Pool in Shark Bay, Western Australia provide a habitat suitable for an abundance of stromatolite building bacteria. This embayment hosts the largest and most diverse known assemblage of modern stromatolites. Stromatolite accretion occurs through two primary processes: 1) trapping and binding of sediment by microbial mats, and 2) microbial precipitation of calcium carbonate. This thesis focuses on the identification and distribution of internal microfabrics of the stromatolites of Hamelin Pool and microbial precipitation within these structures. A diverse assortment of microfabrics was identified in the stromatolites, and microbial precipitation was found to be exceptionally abundant and critical to stromatolite accretion and lithification. Thirty-nine stromatolite heads were collected from various locations around the shoreline of Hamelin Pool. The heads were cut into slabs and hand sample scale mesofabrics were described by observing the slabs. Over 250 thin sections were prepared and studied under petrographic microscope in order to describe the fabrics present within the stromatolites. Sixteen distinct microfabrics were identified and all of the samples were found to be composed of biologically precipitated microcrystalline carbonate (micrite) and sediment. Distribution of fabrics was studied geographically and by plotting the fabrics vertically, based on real-time kinematic (RTK) absolute elevation data, collected for 25 of the stromatolites. Patterns of geographic fabric distribution were noted around the Pool, and a strong correlation of microfabric to water depth was apparent. Two types of micrite (Micrite A and Micrite B) were identified in this study and subsequently compared by distribution, both geographically and vertically. Stable isotopes of oxygen and scanning electron microscopy (SEM) were also employed to compare the micrites. Geographic plots of the micrite types did not yield clear patterns of distribution; however, when plotted vertically by RTK data it became apparent that water depth was an important control of micrite type. Stable isotopes of oxygen were examined using Secondary Ion Mass Spectrometry (SIMS). This study produced a wide range of values within similar features, but indicated that the micrite was precipitated in a highly evaporative environment. High resolution SEM imagery demonstrated that, although each micrite type had its own assemblage of associated features, both micrites appeared nearly identical at very high magnification