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Microbial Origin of Bioherm-Associated Leopard Rock, Holder Formation (Pennsylvanian, Virgilian), Sacramento Mountains, New Mexico


Enigmatic, colloquially named leopard rock of the Holder and Laborcita formations (Late Pennsylvanian-Early Permian) occurs within a cyclic, interfingered, carbonate siliciclastic system in the Sacramento Mountains in southeastern New Mexico. Weathered outcrops exhibit dark, irregular spots (∼2 cm in diameter) within a tan matrix yielding a pattern that resembles leopard spots. Layers of leopard rock boundstone occur stratigraphically just above and below phylloid algal bioherm complexes of the Holder and Laborcita formations and have been interpreted as occurring in transgressive and highstand systems tracts. Field observations, polished hand sample and thin section analyses indicate leopard rock is microbial in origin. Locally abundant and lithologically distinct bulbous, domed mounds occur in outcrop stratigraphically below the algal bioherms to the west of U.S. Highway 82, east of Alamogordo, New Mexico. The mounds are composed of irregular, centimeter thick layers that circumscribe them in a composite growth structure that defies gravity. The mounds are approximately 0.3 to 0.9 m (1-3 ft.) thick and 0.6 to 1.2 m (2-4 ft.) wide. Hand samples from both formations reveal significant variability in the size (1-18 cm) and nature of the dark spots. Millimeter scale, asymmetric, concentric layering is occasionally visible in the fine-grained, dark patches. These ‘spots’ dominate outcrop appearance, are heterogeneous in distribution, and often occur as aggregate clumps or clusters >24 cm in diameter. Optical microscopy on Holder Formation samples reveals thrombolytic patches of micrite, upward-oriented concentric gradational laminae of peloidal micrite, and occasionally micrite-enveloped foraminifera. Geopetal structures indicate gravity-defying fabric structure and support a microbial origin. SEM images reveal distinct log-jam clusters of cylindrical, segmented curvilinear strings over 1000 nm long and 80 nm or less in diameter interpreted as fossilized bacteria. SEM images also show nanometer-scale kidney bean and dome-shaped structures composed of numerous intertwined threads (100-750 nm long) as well as unique, densely packed (100-300 nm long) squat cylinders that resemble foam packing peanuts. These sporadically distributed fields of rounded nanometer-scale structures, interpreted as relict organic matter, contrast sharply with, and are distinct from, the underlying substrate of smooth and dissolution pitted planar crystal surfaces.