Cryptozoon and Associate Stromatolites from the Recent, Shark Bay, Western Australia

Abstract

Stromatolitic sediments and structures resembling Cryptozoon and Collenia are presently forming in the bay-head environment at Shark Bay, Western Australia. Algal-laminated sediments are due to binding of mechanically deposited sediment by an algal-mat coating which covers moist intertidal zone surfaces. The formation of high relief, laminated stromatolitic structures, also in the intertidal zone, is dependent on the interaction of the sediment, the binding algal mat and environmental factors. The structures are composed of detrital carbonate sediment which is laminated. The detrital debris is cemented by aragonite to form hard, lithified and semilithified structures, which may attain a maximum height of 2.5 feet above the intertidal surface. There are three structural groups in the stromatolitic sediments of Shark Bay; these are 1. 1. Flat-lying, algal-laminated sediments underlying a terrain of continuous algal mat. 2. 2. Low, sinuous domes and discs of laminated, detrital sediment underlying relatively continuous algal mat. These forms, which are best equated with Collenia-type stromatolites, are associated with flat, algal-laminated sediment in the intertidal mudflat environment and occur in other locations where mechanical energy of waves is normally slight. 3. Discrete club-shaped and columnar structures ("heads") of laminated detrital sediment with the algal mat limited to the top and sides of the heads and absent in the interareas between structures. The discrete, club-shaped structure is the classical Cryptozoon-type stromatolite. Discrete, club-shaped, and columnar structures are clustered into reef-like aggregations in the intertidal zone of headlands, where the stromatolites are subject to mechanical energy expended by waves and currents. Within the reefs there is a form zonation evident which is related to the low-water and high-water tidal levels. The tops of mature Cryptozoon structures reach a common level at a few inches above high water so that height at maturity is determined by tidal range and the position of the structure in the intertidal zone.