Abstract

The Lower-Middle Pennsylvanian Gobbler Formation is a 250 m-thick, cyclic limestone sequence deposited in the Orogrande Basin in south-central New Mexico. The basin originated during the Late Paleozoic as a north-trending pull-apart basin within a broad zone of dextral strike-slip deformation transecting the southwestern margin of the North American craton. Basin asymmetry resulted from detachment faulting along the eastern basin margin, producing a narrow eastern shelf with a sharply-defined margin and a broad western ramp. Tectonic stability of the western ramp is reflected in greater lateral continuity of cycles and facies belts relative to the eastern shelf, which exhibits rapid depth-related facies changes, greater variability in cycle thicknesses, synsedimentary slumping and debris flows, and a fault-bounded, clastic-filled intrashelf trough. High sediment production rates on the eastern shelf resulted in basinward progradation of the shelf margin through the Middle Pennsylvanian at a rate of 2.5-5.0 Km/My.

Gobbler Formation cycle tops were subaerially exposed and subjected to meteoric diagenesis, as evidenced by karstic solution features, oxidation surfaces, lag deposits, and negative carbon isotopic excursions. Average cycle periods were 330-370 Ka (based on a Desmoinesian Epoch duration of 10 Ma), reflecting episodic sea level changes associated with the Gondwanan Ice Age. As the fundamental control on cyclicity was basin-wide, differences in cyclostratigraphy between localities reflect local tectonic and sedimentational controls. Modal cycle thicknesses differ between the eastern (6.0 m) and western basin margins (4.5 m) and the basin center (3.0 m), recording systematic regional variation in subsidence rate. A mid-Desmoinesian deepening event on the eastern shelf, evidenced by changes in lithology and carbon cycle amplitude, is not observed on the western ramp. Recurrence of thick, shaly units at intervals of four to six cycles on the western basin margin records episodic clay influx, possibly the result of episodic uplift of source areas. Cycles generated by such synchronous, basin-wide mechanisms hold great potential in unraveling events in basin evolution.