Siliciclastic Stromatolites and Other Microbially Induced Sedimentary Structures in an Early Devonian Barrier-Island Environment (Muth Formation, NW Himalayas)

摘要

Microbially induced sedimentary structures, including spectacular siliciclastic domal stromatolites, up to 80 cm wide and 30 cm high, have been found in coastal quartzites of the Lower Devonian Muth Formation (Pin Valley, NW Himalayas). The microbial structures occur in intervals of the formation associated with physical sedimentary structures that indicate, at least temporally, emergent conditions. These observations support the interpretation of a peritidal setting for the microbial structures within the wave-dominated, barrier-island depositional environment of the Muth Formation. The siliciclastic domal stromatolites probably formed in a shallow subtidal to intertidal environment with high hydraulic energy, where periods of high sedimentation rates are interrupted by periods of low or zero sedimentation. Microbial gas pits and microbial gas domes were produced by ascending gas from degrading buried organic material in a lower supratidal zone, influenced by tidal flushing. Multidirected ripple marks document a series of erosion events interfering with microbial stabilization in lower supratidal settings. Polygonal shrinkage cracks from supratidal environments indicate a semiarid paleoclimate. The replacement textures of the microquartz matrix in the stromatolites indicates the former existence of syngenetic carbonate cements, which may explain the preservation of the domal stromatolites in the high-energy, siliciclastic environments of the Muth Formation. The Muth stromatolites, commonly containing more than 60 vol. % siliciclastic grains, prove the existence of microbial activity in environments of higher sedimentary stress than generally thought and extend our knowledge of the capability of benthic microbial organisms to settle in siliciclastic shallow marine environments.