Lower-Middle Pennsylvanian Gobbler Formation: Isotopic Constraints on Temperature, Water-Rock Ratio, and Timing of Burial Diagenesis: ABSTRACT

Abstract

The Lower-Middle Pennsylvanian Gobbler Formation is a 250 m-thick, cyclic limestone sequence deposited in the Orogrande basin of south-central New Mexico. meteoric diagenesis during episodic sub-aerial exposure of cycle tops produced meteoric calcite trends within individual cycles (constant {delta}{sup 18}O, variable {delta}{sup 13}C). Subsequent burial diagenesis proceeded in a closed system (W/R = 1-3), as shown by preservation of high-frequency variation in {delta}{sup 13}C and limited isotopic variation between host rocks and coexisting cements. As a consequence of variable burial depths (0.8 km on shelves to 1.6 km in basin center), some locales were subjected to higher burial temperatures, increasing the {delta}{sup 18}O depletion of burial precipitates. Inter- and intrasectional trends in {delta}{sup 18}O with respect to burial depth permit estimation of ambient geothermal gradients at the time of burial stabilization. Such isotopic trends demonstrate elevated geothermal gradients (90{degree}-120{degree}C/km), while a strong correlation between section-average {delta}{sup 18}O values and end-Virgilian overburden thicknesses implies completion of burial stabilization around this time. The timing of geothermal gradient elevation and burial stabilization was not coincidental but probably was linked to rapid subsidence of the Orogrande basin during the Virgilian Epoch. Such isotopic trends offer considerable potential for studying basinal thermal regimes during burialmore » diagenesis.« less