STROMATOLITES IN THE LATE ORDOVICIAN EUREKA QUARTZITE: IMPLICATIONS FOR MICROBIAL GROWTH AND PRESERVATION IN SILICICLASTIC SETTINGS

Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1.5 m high, are found in a 10–15 m thick interval within the Late Ordovician Eureka Quartzite of southern Nevada and southeastern California. These stromatolites appear as either isolated forms or patchy clusters and contain more than 70 vol. % quartz grains. Their association with planar, trough, and herringbone cross-bedding indicates that stromatolites formed in an upper shoreface environment with high hydraulic energy. In this environment, sand bars/dunes may have provided localized shelter for initial microbial mat colonization. Biostabilization of microbial mats effectively prevented erosion, and the prevalence of translucent quartz sand grains permitted light penetration into the sediment, eventually leading to thick microbial mat accretion and the formation of domal stromatolites. Decimeter to meter scale stromatolite domes may have served as localized shelter for further microbial mat colonization, forming patchy stromatolite clusters. The presence of interstitial clay minerals preferentially within stromatolite laminae suggests fine sediment trapping by microbial mats, whereas clay minerals were winnowed from surrounding sands. In addition, enrichment of iron minerals including pyrite and hematite within dark internal laminae of the stromatolites suggest that anaerobic mineralization of mat-generated organic matter may be crucial for siliciclastic stromatolite preservation. Anaerobic mat mineralization may have created elevated carbonate alkalinity in pore spaces of sandy sediment, promoting early cementation/lithification of microbial mats and stromatolites. The occurrence of stromatolites in the Eureka Quartzite provides an example of microbial growth in highly stressed sedimentary environments and provides new insight into the preservation potential of microbial structures in siliciclastic settings.