Lower and Middle Pennsylvanian Gobbler Formation: Using Carbonate Shelf Cyclicity to Unravel Basin History in Transtensional Regime

Thomas J. Algeo, James L. Wilson

The Lower and Middle Pennsylvanian Gobbler Formation of southern New Mexico is a carbonate shelf sequence consisting of 25 to 30 limestone cycles. Cycles comprise 3-10 m-thick subtidal wackestone units capped by thin cross-bedded encrinites and pebbly lag deposits. Subaerial exposure of cycle tops, evidenced by karstic dissolution pits, brecciation, reddish oxidation crusts, and negative carbon isotope excursions, indicates episodic sea level fall. Average cycle periods of 330,000 to 370,000 years (based on a Desmoinesian Series thickness of 160 to 180 m and lasting 10 m.y.) are roughly in accord with previous estimates of Carboniferous cyclothem periods and support glacioeustatic control of sedimentation modulated by the long-period orbital eccentricity cycle. Because th fundamental control on cyclicity was basin wide, differences in cyclostratigraphy between localities reflect local tectonic and sedimentologic controls.

The Gobbler Formation was deposited during the initial phase of subsidence of the Orogrande basin, a north-trending pull-apart basin resulting from cratonward overstep of a broad dextral strike-slip zone transecting the southwestern margin of North America. Basin asymmetry is reflected in greater lateral continuity of cycles and facies belts on the broad, stable western shelf relative to the narrow eastern shelf, which exhibits rapid depth-related facies changes, variable cycle thicknesses, synsedimentary slumping, and a fault-bounded clastic-filled intrashelf trough. Differences in cycle thicknesses between the eastern and western shelves (4.5 m vs. 6.0 m mode) record differential subsidence of opposite basin margins. Recurrence of thick, shaly units at intervals of four to six cycle indicates the existence of a longer period (2 m.y.) oscillation that may relate to pulses of basin subsidence.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.