New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cambro-Ordovician Bliss Formation is the oldest Phanerozoic sedimentary unit in New Mexico. It crops out in the mountain ranges of Sierra County where it rests nonconformably on Proterozoic basement and is conformably overlain by the Lower Ordovician Sierrite Formation of the El Paso Group or, at its northernmost outcrops, disconformably by the Middle Pennsylvanian Red House Formation. In Sierra County, the Bliss Formation is up to 55 m thick and can be divided into two members: (1) a lower member dominated by crossbedded and bioturbated quartz-rich sandstone; and (2) an upper member with numerous beds of ripple-laminated glaucarenite and some coarse carbonate beds. The lower member contains brachiopods, trilobites and a Skolithos-dominated ichnoassemblage. The upper member yields numerous graptolites and a Palaeophycus-dominated ichnoassemblage. Previously published conodont and trilobite biostratigraphy in the Caballo Mountains indicates that most or all of the lower member is late Cambrian (Sunwaptan) in age, whereas most or all of the upper member is Early Ordovician (Skullrockean) in age. The Cambrian-Ordovician boundary is stratigraphically high in the lower member. The upper member graptolite assemblage is dominated by Rhabdinopora flabelliformis (Eichwald), which confirms its Early Ordovician age. Deposition of the lower member was in a shallow marine siliciclastic setting, whereas the upper member indicates a mixture of shallow tidal flat sedimentation intercalated with deeper, subtidal marine deposition. In the iron oolite beds of the lower member in the Caballo Mountains (Sierrite Mine), two types of ooids are present: (1) tangential concentric ooids composed of hematite and (2) structureless ooids composed of hematite, berthierine and chamosite. We interpret the iron ooids to have formed during diagenesis from carbonate ooids. Glauconite is abundant in the upper member and occurs as rounded grains, rarely also as cement. It is cryptocrystalline and most probably formed by glauconitization of fecal pellets. Both iron ooids and glauconite indicate that the Bliss Formation is the result of an extensive regional transgression (“Sauk Transgression”) coupled with low rates of sedimentation.