New Mexico Geological Society Annual Spring Meeting — Abstracts

The Middle Proterozoic Lanoria Formation, preserved in the Franklin Mountains of El Paso County, Texas, is at least 750 m thick. It consists of metamorphosed sandstone, siltstone and mudstone. These strata were deposited in an array of shallow marine, nearshore and tidal flat paleoenvironments on a gently sloping shelf in the foreland basin of the Grenville orogeny. Geochronology of detrital zircons, cross-cutting granites, and overlying rhyolites constrain the age for the Lanoria Formation to the Stenian period. The fourth member of the Lanoria Formation at NMMNH locality 10809 contains abundant Microbially Induced Sedimentary Structures (MISS). These include the following: wrinkle structures (aff. Rugalichnus), multidirectional ripple marks, several types of syneresis cracks (elongate, spindle, and polygonal, and sinusoidal aff. Manchuriophycus), mat roll-ups (and possible “cigar rolls”), gas domes, discoidal microbial colonies and associated merged gas domes, elongate microbial surface trails with levees, lobate radial projections (or tool marks), and impressions of filamentous extracellular polymeric substances. Sinusoidal traces (similar to Cochlichnus) are found on some bedding planes, crossing ripples, but appear to be surface traces and are probably sinusoidal microbial trails. Similar sinusoidal traces from mid-Proterozoic rocks have been reported as metazoan burrows, but these are disputed, as is the molecular evidence for a mid-Proterozoic origin of metazoans. Thus, we take a conservative approach and describe the Lanoria structures that resemble metazoan burrows as sinusoidal wrinkle structures or microbial traces. This is the first report of MISS from the Proterozoic of the Franklin Mountains.