The preservation potential of some recent stromatolites

Stromatolites are laminated organo-sedimentary structures, generally compared to present day blue-green algal mats. Their morphology, species composition and overall extent are largely governed by the amount of wetting, although other factors such as competition, predation and desiccation, also contribute. The Trucial Coast mats are essentially intertidal. Stromatolite accretion rates in this area are of the order of 0·2 mm p.a. but lamina growth is far from regular. The area is also characterized by the development of evaporites, especially gypsum which proves to be an important agent of mat destruction. The growth of crystals causes disruption within the upper portions of the stromatolite section with the result that none of the upper intertidal mat forms are preserved. Other agencies of destruction include bacterial decay, desiccation and dehydration, and compaction under burial which may depress and deform the original mat relief. Decay results in the almost total loss of cellular contents, only a few empty sheaths and the pigment surviving into the fossil record. Preservation may be effected via (a) burial or (b) lithification. However, few modern algal mat structures bear any resemblance to fossil stromatolite heads with the exception of those from Shark Bay. From this, one might infer that pene-contemporaneous lithification is a prerequisite for their preservation.