Abstract

The colloquially named leopard rock of the Holder and Laborcita formations (Late Pennsylvanian-Early Permian) is a microbial boundstone that occurs within the cyclic carbonate-siliciclastic system in the Sacramento Mountains in southern New Mexico. Weathered outcrops characteristically exhibit dark, irregular spots (~2 cm in diameter) within a light matrix yielding a pattern that resembles leopard spots. Layers of leopard rock bound-stone often, but not habitually, occur near Holder and Laborcita formation phylloid algal bioherm complexes. Leopard rock occurs in transgressive and highstand intervals. Field observations, polished hand sample and thin section analyses indicate leopard rock is microbially influenced if not wholly microbial in origin, likely representing a gradational organic suite. The lithology’s most dramatic expression is revealed in distinct bulbous, domed mounds (0.3 to 0.9 m thick and 0.6 to 1.2 m wide) that occur in outcrop stratigraphically below the algal bioherms to the west of U.S. Highway 82, east of Alamogordo, New Mexico and are composed of irregular, centimeter thick layers that circumscribe the mounds in a composite growth structure that defies gravity. The lithology also presents a more subtle, shallow domed or undulating expression in the Laborcita on the upper surface or section of two horizontal beds, stratigraphically parallel, approximately 30 to 120 cm thick and that occur immediately northeast of Tularosa, New Mexico. Variability in expression between the two extremes of large dome formation (Holder and Laborcita) and upper outcrop surface undulations is evident in lateral outcrop trace. Hand samples from both formations reveal significant variability in the size (1-18 cm), character, and concentration of the dark spots. Millimeter-scale, asymmetric, concentric layering has also been observed in fine-grained, dark patches for one unique Holder Formation locality. The heterogeneous spot distribution dominates outcrop appearance and often occurs 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. Gravity-defying geopetal structures support a microbial role in the lithology’s genesis. SEM images reveal striking log-jam clusters of cylindrical, segmented curvilinear cylinders over 1000 nm long and 80 nm in diameter that are 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 bacteria, contrast sharply with, and are distinct from, the underlying substrate of smooth and dissolution pitted planar crystal surfaces. The variability in leopard rock sample structures observed under SEM complements the variety of expressions observed in outcrop.