Possible indicators of microbial mat deposits in shales and sandstones: examples from the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.
It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (a) domal buildups resembling stromatolites in carbonates; (b) cohesive behaviour of laminae during soft-sediment deformation, erosion, and transport; (c) wavy-crinkly character of laminae; (d) bed surfaces with pustular-wrinkled appearance; (e) rippled patches on otherwise smooth surfaces; (f) laminae with mica enrichment and/or randomly oriented micas; (g) irregular, curved-wrinkled impressions on bedding planes; (h) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (i) lamina-specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences.