Microbialite elongation by means of coalescence: an example from the middle Furongian (upper Cambrian) Notch Peak Formation of western Utah

Abstract

Strongly elongate microbialites having axial ratios greater than 4:1 and sometimes exceeding 10:1, are currently forming in modern, shallow-subtidal to intertidal environments. Construction of these elongate forms is greatly dependent on hydrodynamics, microbially influenced trapping and binding of sediment, and/or precipitation of peloidal cement, and intertidally generated abrasion and mechanical scour over sheet-like microbial mats. Some of these processes, however, are inadequate for explaining the construction of strongly elongate structures in deep-subtidal or wave-restricted environments. Since elongate morphogenesis is an important factor in paleoenvironmental reconstructions, ancient examples of elongate-related growth sequences should be documented and compared with modern analogues. This paper explores such a growth sequence from a 13-m-thick, middle Furongian (upper Cambrian) microbialite bed in western Utah that records a morphological succession of deep, subtidal microbialites in vertical section over a large geographical area. Microbialites change from round, decimeter-sized forms to large, elongate structures many meters in length, reverting back to round, centimeter-sized shapes at the top of the bed. We suggest that these elongate microbialites formed as a result of coalescence, a process known to produce compound microbialite structures in shallow water, but seldom explored as a key factor in the elongation of deep, subtidal forms that grew in ancient environments.