Algal Mats, Cryptalgal Fabrics, and Structures, Hamelin Pool, Western Australia1
Brian W. Logan, Paul Hoffman, Conrad D. Gebelein, 1974. "Algal Mats, Cryptalgal Fabrics, and Structures, Hamelin Pool, Western Australia", Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia, Brian W. Logan, James F. Read, Gregory M. Hagan, Paul Hoffman, Raymond G. Brown, Peter J. Woods, Conrad D. Gebelein
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Cryptalgal sediments and structures in Hamelin Pool, Western Australia, are formed by interactions between blue-green algae, which trap and bind sediment particles, and a variety of mechanical and diagenetic processes. The algae form a cohesive mat that tends to cover intertidal, supratidal, and some shallow subtidal surfaces. The mat is differentiated into seven intergradational types as an expression of variations in algal species present, ratio of filamentous to unicellular forms, quantity of mucilaginous matrix, life habits, and quantity and nature of the host sediment. The distribution of mat types is controlled by environmental factors such as elevation of substrate, drainage, depth and nature of interstitial groundwater, and sediment influx. In tidal flats with gentle gradients, there is a broad zonation of mat types, whereas the mat is highly differentiated and has a condensed, patchy development of types on headlands and locations with irregular topography.
The sediments trapped and/or bound by the algalmat communities are imprinted with distinctive fabrics. These fabrics, which can be related to specific mat types, reflect a complex interaction between the algae and processes of sedimentation and diagenesis. Important factors in the development of fabric are surface texture and internal structure of the mat, rate and frequency of sediment influx, and processes such as oxidation, cementation, and lithification. Changes in mat type with changes in environmental conditions (e.g., shoaling and sediment influx) lead to the development of successions of fabrics in the sediment pile.
The mat-sediment complex is shaped by physical factors into a variety of structures: (1) extensive flat-lying sheets, (2) ridge and rill structure, (3) rings and crescents, (4) linked ellipsoids and columns, (5) discrete ellipsoidal and circular columns, and (6) calyx and tiered calyx structures.
The size range of structures varies from a few centimeters to several meters; confluent and branched structures also are common. The gross morphology of the structures is largely independent of the mat type (or types) involved in the primary trapping and/or binding processes. Major environmental factors involved in shaping of structures are waves, currents, substrate gradient, and long-term sea-level change; minor factors include burial, exhumation, growth of epiphytes, activity of browsing organisms, gas evolution, corrosion, precipitation, desiccation, and variation in sediment type. These factors also influence the external surface texture of structures.
Figures & Tables
Following on the research presented in AAPG Memoir 13, which focused on environment and Quaternary history of Shark Bay, this publication examines the same area again, but with a strong stratigraphic emphasis running as a common thread through all 7 papers in this volume.