ABSTRACT

Carbonate rocks from three sections of Missourian strata lying between the Pennsylvanian-age Zuni-Defiance and Pedernal landmasses were investigated in the field and laboratory. The high percentages of clastic rocks within these Missourian sequences suggest that the nearby Joyita axis was an important positive element during Missourian time.

The clay fraction of the insoluble residues derived from the Zuni-Defiance, Pedernal, and Joyita landmasses is similar in the northern two sections (Ojo de Amado and Mesa Sarca), but different in the northern Oscura Mountains. Montmorillonite is present in all three sections, but kaolinite occurs only in the central and northern sections. High percentages of unaltered montmorillonite have been reported infrequently in rocks of this age. Their common occurrence in these sections suggests that the potassium: magnesium ratio within the basin of deposition was unfavorable for alteration of montmorillonite to illite. Kaolinite found in the carbonate rocks indicates that the source areas for the northern two sections were rich in this mineral.

Microlithologies and faunas indicate that both epineritic and infraneritic conditions existed throughout the area during Missourian time. Sea depths probably varied within the limits of 50 and 150 feet, and marine waters were warm and generally calm. Small algal bioherms developed east of Socorro not far from the Joyita axis, and were situated on a shallow-marine platform that was occasionally exposed to subaerial conditions.