Bidirectional fabric evolution in Hamelin Pool microbialites, Shark Bay, Western Australia

Abstract

Hamelin Pool, Shark Bay, Western Australia hosts the world's largest and most extensive assemblages of living marine microbialites, comparable in size and shape to ancient structures found throughout the fossil record. Here, we document the internal fabrics of modern microbialites collected throughout Hamelin Pool. Meso‐ and microscale observations of microbialite polished slabs and thin section scans, optical microscopy, and scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy formed the basis for a fabric classification system that combines accretionary mat type with microfabric. Accretionary mat types included pustular, smooth, colloform, as well as ‘transitional’ mats that are a cross between pustular and smooth mats. Mapping of fabrics in 45 microbialite heads indicated bi‐directional evolution. An upward progression of fabrics corresponded to changes in mat type as the head grew upward into shallower water. A downward evolution of microfabrics occurred as surface mats transitioned into the subsurface of the microbialite structure. Downward microfabric evolution occurred as a result of early taphonomic processes, and involved a progression from the original depositional architecture to subsequent stages of micritic thickening, and finally, cement infilling. The observed bi‐directional evolution of microbialite microfabrics within Hamelin Pool offers a conceptual framework for the study of modern microbialites, not simply as the sole product of accretionary mat types but rather as the combined result of the activity of surface mats and their taphonomic evolution. Early taphonomic processes induce further lithification of the microbialites which may enhance preservation potential in the geological record.

Microbialite Accretion and Growth: Lessons from Shark Bay and the Bahamas

“…The early taphonomy of discrete microbial buildups in Hamelin Pool is characterized by microfabric evolution as pustular, smooth, and colloform mats are buried by subsequent generations of accreting mats (Vitek et al 2023) (Figure 4c; Supplemental Figure 1c,d ). Thickening of micritic clots and laminae in the initial architecture is associated with progressive decay of organic matter.…”