New Mexico Geological Society Annual Spring Meeting — Abstracts

Outcrops of the Owl Rock and Rock Point Members around Petaca Pinta (SW1/4 T6N, R6W and SE1/4 T6N, R7W, Cibola County) are the southeasternmost outcrops of these two members of the Chinle Formation. An excellent exposure south of Petaca Pinta (SW1/4 SW1/4 NW1/4, sec. 31, T6N, R6W) documents the presence of 36.5 m of Owl Rock Member overlying dark reddish brown mUdstone of the Petrified Forest Member. The Owl Rock is dominated by grayish red and very pale green, calcareous mUdstone (76% of the section) and grayish red-purple, bioturbated sandy siltstone (19%) with a minor amount (5%) of silty sandstone. The overlying 69.8 m of Rock Point Member are disconformably overlain by the Middle Jurassic Entrada Sandstone. The Rock Point consists of laterally extensive beds of massive, moderate reddish brown sandy siltstone (71%) and laminar to trough crossbedded, moderate reddish brown, very fine grained, mature, quartzose sandstone (29%).

Previously, the Owl Rock Member was thought not to be present east of Thoreau, 100 km northwest of Petaca Pinta. Maxwell (1988, USGS Maps MF-2033, MF-2049) mapped outcrops of the Rock Point Member in the Petaca Pinta area, but they are much more extensive than his maps indicate. The relatively thick, southeasternmost outcrops of the owl Rock and Rock Point Members do not suggest pinchout of either unit near Petaca Pinta. The broad distribution of the Owl Rock and Rock Point Members does not fit well with recent suggestions that their deposition was controlled by persistent subsidence along the Zuni lineament.