INTRAFORMATIONAL DOMAL FEATURES IN THE MIDDLE JURASSIC TODILTO FORMATION, GRANTS DISTRICT, NEW MEXICO

Intraformational domal features in the Luciano Mesa Member of the Todilto Formation near Dos Lomas in sec. 23, T13N, R10W in the Grants district, New Mexico, are not known from elsewhere in the Todilto. The Todilto at Dos Lomas is 4.2 m thick and overlies eolian, crossbedded sandstone of the Entrada Formation. The lower 2.1 m are gray micritic limestone, intercalated with gray, micaceous and bituminous siltstone. Some limestone beds include abundant small angular quartz grains near the base. Above are 1.3 m of evenly bedded, thinly laminated gray lime mudstone containing a few small angular quartz grains, a few large micritic intraclasts and many ostracods. The uppermost 80 cm consist of laminated or stromatolitic and partly vuggy limestone beds (microbial crusts), some of which contain ostracods. The uppermost stromatolitic bed is about 30 cm thick, includes the domal structures, and is overlain by greenish, gypsiferous siltstone and shale of the Summerville Formation. The domal structures are up to 2.2 m high and 1.8 m wide (n=16), and in many respects resemble tepee structures. The base is horizontal beds of evenly laminated or more commonly stromatolitic limestone. The core is up to 50 cm thick and 1 m wide at the base and consists of intensely folded or brecciated limestone and evaporites, overlain by a dcm-thick crust of stromatolitic limestone, which may be vuggy and contain thin beds of gray micritic limestone. Although the domal features are in some ways fold-like, they differ from the hundreds of intraformational folds that have been observed throughout the Grants district. These intraformational folds vary in appearance and geometry but typically have mappable fold axes that in some cases can be traced for well over 100 meters. The domal structures are overlain and sealed by stromatolitic layers that are thinner above the crest of the features, thickening downward from the crests. These beds thus are domal, tepee-like structures but are not typical bioherms. The basal Luciano Mesa Member of the Todilto Formation, predominantly limestone, formed in a paralic salina that developed above the Entrada eolian sandstone. Stromatolitic limestone and tepee-like domal structures developed in an environment of increasing salinity and are overlain by coastal sabkha sediments of the basal Summerville Formation.