LATERALLY-EXTENSIVE DOMAL STROMATOLITES NEAR THE BASE OF THE LOWER TRIASSIC VIRGIN LIMESTONE (MOENKOPI FORMATION) NEAR BLUE DIAMOND, NV: ENVIRONMENTAL AND PALEOECOLOGICAL SIGNIFICANCE

Numerous studies have documented the emergence of unusual sedimentary features in the aftermath of the Permian-Triassic biotic crisis (e.g., Schubert and Bottjer 1992; Woods et al. 1999; Pruss and Bottjer 2004; Pruss et al. 2006). Study of the lower portion of the Lower Triassic Virgin Limestone near Blue Diamond NV, reveals the presence of laterally-extensive, domal stromatolites. The stromatolites occur within a stratigraphic section comprised of micritic limestone, shale, and lesser amounts of siliceous sandstone. The stromatolites are underlain by numerous horizons of bivalve shell debris that are interpreted as storm deposits, and suggest deposition in a shelf environment. A distinctive marker bed of mudstone with 5-cm cross-bedding and an exposed top surface of ripple marks crops out approximately 1 m below the stromatolites. Beds overlying the stromatolites include bioturbated mudstone with abundant planolites, poorly preserved shale, and resistant meter-thick units of packstone including abundant crinoid columnals. The stromatolites are laterally extensive, as they persist over 1 km across the study area, although they pinch out locally. The stromatolites occur in a bed of grey micritic limestone, and consist of laterally-linked hemispheroids up to 0.75 m thick, as well as 5 to 15 cm tall isolated stacked hemispheroids. Lamination is planar to wavy, and is often very prominent. The form and size of the domes varies; symmetrical hemispherical domes range in diameter from 10 – 20 cm, but there are instances of elongate, oblate forms that measure up to 50 cm in length. Laterally-linked hemispheroids are more common, with aggregates of smaller, 10-15 cm diameter domes forming continuous layers. In both the aggregates and isolated domes, lamination appears to favor a constant basal radius (Mode “C” after Logan et al., 1964). The domes show little or no evidence of erosion prior to deposition (i.e. flattened or truncated surfaces), and very fine-grained carbonate mud has infilled topographic lows. Lower Triassic stromatolites from the Virgin Limestone have been reported elsewhere (e.g., Schubert and Bottjer 1992, Pruss and Bottjer 2004), however the Blue Diamond stromatolites appear to be the earliest example of stromatolites from the Virgin Limestone based on stratigraphic position.