A GEOMICROBIOLOGIGAL STUDY OF THE PRE-CAMBRIAN NONESUCH SHALE

Abstract

Summary The paper describes the organic content of specimens representing some part of the Nonesuch Shale of Upper Keweenawan age (1,075 ± 50 million years) forming part of the Copper Belt on the southern flanks of the Lake Superior Basin in Ontonagon County, Michigan. The remarkably well preserved organic matter has been studied optically in thin section, by acid maceration analysis, and separately, in ultrathin section of isolated organic matter, by electron microscopy. The amorphous organic groundmass consists of saprophytic microbiological degradation products which include spore-like bodies or “sporomorphs” (acritarchs) in various stages of degradation, with attacking microorganisms often in situ. The further microorganism content resembles morphologically bacteria, actinomycetes or fungi and the total organic content represents an ecosystem. The remarkable mattes of tissue composed of larger fungi are predominantly oriented with the bedding on the organic-rich elements of paired, laminated or varved deposits. The remains constitute an assemblage which recalls that of Moore (1964a) in which forms such as Anthracomyces cannellensis , Palynomorphites and “ Phellomyces ” were shown to be instrumental in organic degradation. This process now demonstrated from a Pre-Cambrian environment is one with an antiquity of at least a thousand million years and suggests that conditions with regard to physico-chemical-biological constitution and related environmental parameters have been comparable over this time period. The paper supports the general sedimentological conclusions of White and Wright (1954) concerning the depositional environment but deduces a tidal lagoon or mud flat environment of Pre-Cambrian age. The organic assemblage, the biological process of degradation and the characteristics of the sedimentary environment are compared with known geological and recent examples of laminated or varved, lacustrine; tidal lagoon and mud flat environments. Reference is made to the geochemical characteristics of some of these related sediments in an attempt to reach a more pertinent assessment of the Pre-Cambrian environment, and of the periodicity of events which the Nonesuch sediments display.