New Mexico Geological Society Annual Spring Meeting — Abstracts

The Middle Pennsylvanian (Desmoinesian) Gray Mesa Member (240 m) of the Madera Group is well exposed in the Lucero uplift region in central New Mexico along Mesa Sarca. Facies present include: 1) terrigenous mudstones, 2) dark, thin-bedded, finegrained wackestones, 3) medium-to thick-bedded skeletal wackestones to wackestone/packstones, 4) medium-to thick-bedded phylloidal wackestone/packstones, 5) skeletal packstones to grainstones, and 6) rare pebbly sandstones. These facies represent deposition in deeper subtidal (facies 1&2) to shallower subtidal waters (facies 3-6). Facies stack into ~60 meter-scale upward-shallowing cycles. Cycle tops commonly display early diagenetic features (irregular black calcite infilling vertical and horizontal cracks, oxidized burrow mottling, and brecciated fabrics) suggestive of subaerial exposure.

Two major cycle types are present: Type 1 cycles (4-7 m thick) are characterized by basal mudstones overlain by either skeletal or phylloidal wackestone/packstone or skeletal packstone caps. Type II cycles (3-5 m thick) are composed of basal fine-grained wackestones overlain by skeletal or phylloidal wackestone/packstone or skeletal packstone caps. These cycle types are interpreted to record <100 ky relative sea-level fluctuations.

The two cycle types stack systematically into seven larger-scale (25-50 m) depositional sequences. Type cycles, which contain basal mudstone facies, are found predominantly in the lower parts of sequences, whereas, limestone-dominated type II cycles (with skeletal packstone to grainstone caps), are dominant in the upper parts of sequences. Stratigraphic distribution of specific meter-scale cycle types within sequences suggests that longer-term (0.5-1 my) relative sea-level fluctuations were superimposed upon the high-frequency oscillations recorded by cycles.