New Mexico Geological Society Annual Spring Meeting — Abstracts

The dominant lithology of the Morrison Formation in eastern New Mexico is variegated shale with minor sandstone. The lower half of the unit is dominated by illite and montmorillonite and the upper by mixed-layer montmorillonite-illite. Sandstones are subarkosic to arkosic. Minor lithologies include nodular chert, nodular limestone, bedded limestone and bentonite. In San Miguel, Quay, Guadalupe and Harding Counties the Morrison can be divided into three mappable members. The lower member is characterized by brick-red mudstone and minor discontinuous sandstones. The middle member consists of fine-grained sandstone lenses, locally with conglomeratic bases, and minor green mudstones. The upper member is dominated by green and variegated shales and minor lenticular sandstones. In portions of Guadalupe, Harding and Union Counties a further sandstone member is present, consisting of a laterally continuous coarse-grained sheet at the top of the Morrison. This sequence is homotaxial with the Morrison section in central New Mexico. In the Hagan Basin, the Morrison consists, in ascending order, of the Recapture, Westwater Canyon, Brushy Basin and Jackpile Members, respectively, consisting of dominantly shale, dominantly lenticular sandstone, dominantly shale, and a sheet sandstone. The "agate bed" lies near the base of each section as it does in the Morrison from Kansas to Wyoming. The Morrison of eastern New Mexico was deposited in a separate basin from that in the part of the state.