Matground structures and redox facies

A characteristic succession of sedimentary structures was observed in elastic Silurian tempestites of the Murzuk Basin, of southwest Libya. The structures ("Manchuriophycus" syneresis cracks, "Astropolithon" load cracks, palimpsest ripples, "Kinneyia" microfault sets, and millimeter ripples) occur in sandstones and siltstones that show evidence of unusual cohesive behavior (cracks resulting from shrinking or degassing; resistance to erosion). In a sequence ranging from fully aerobic sandstones to anoxic graptolite shales, these structures characterize several zones of increasing redox stress, which correspond to ichnofacies zonation. It is postulated that microbial mats develop on the sediment surface whenever grazing and burrowing are suppressed. Depending on local conditions, biomats were leathery, felt-like, or gel-like. The observed matground structures resulted from the interaction of storm waves with microbial stabilization of progressively finer-grained sediments. Whereas such structures were ubiquitous in Precambrian clastics, they became restricted to hostile environments in the Phanerozoic, where they can be used to indicate environmental (mostly redox) stress, and possibly hydrocarbon potential, in sandstones and siltstones lacking other indicators.