BIOTIC AND ABIOTIC PROCESSES IN THE FORMATION AND DIAGENESIS OF PERMIAN DOLOMITIC STROMATOLITES (ZECHSTEIN GROUP, NE ENGLAND)

The Crinkly Bed of the Upper Permian Zechstein Group succession (Roker Formation = Hauptdolomit, Z2C) in northeast England is a distinctive 1.4-meter-thick unit that has been interpreted as a stromatolite of microbial origin, or, alternatively, as the result of purely abiotic precipitation. Close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes, and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the Zechstein basin. The internal structure of the Crinkly Bed consists of fine, millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt-size peloids, with rare hollow spheroids. The lamination is extremely regular in thickness and laterally persistent for up to 4 meters. A microbial community composed of coccoid and filamentous microorganisms, inferred from fossil evidence, colonized the original sediment surface. It is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae; the peloids are consequently interpreted as autochthonous. This interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite, which constitutes both the continuous micritic laminae and the peloids themselves. Primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters, mediated by bacterial metabolism, similar to the process that operates in modern intertidal microbial mats. The Crinkly Bed shows macro-structures that vary along a continuum from symmetrical to asymmetrical, centimeter scale, ripple-like structures, some showing interference patterns, through to domical and conical structures, more typical of stromatolites. The more ripple-like structures have a lamination of microbial origin identical to that within the domal-conical stromatolites; there is no internal cross lamination. Their origin is equivocal: they could be the result of physical (current- or wave-induced) deformation of a surficial biofilm, or their form could be inherited from true mechanically deposited ripples (not observed) that provided a template for biofilm growth. Changes in the nature and strength of the wave and/or current regime, and possibly in the community structure of the microbial mat, produced this array of microbially induced macrostructures (MISS). The latter, plus the absence of desiccation features indicative of intertidal to supratidal deposition, suggest subtidal sedimentation at a depth of several meters. Intergranular gypsum cement precipitated during early diagenesis, and ferroan sparry dolomite precipitated during burial. Tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre-existing dolomite, probably via the biological mediation of heterotrophic bacteria. The interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record.