Quantitative Assessment of Causal Mechanisms for Cyclicity of the Cisco Group, Eastern Shelf, Midland Basin

Wan Yang and Michelle A. Kominz

A regional analysis of the Late Pennsylvanian, Early Permian, Cisco Group suggests that while orbital controls dominated, other periodic and stochastic mechanisms were also recorded. Ninety stratigraphic sections were measured in the outcrop Cisco Group and correlated along 3 traverses. Three composite sections were derived from these data for analysis. Detailed correlation of cyclicity in the subsurface included interpretation of 71 stratigraphic sections in 3 dip and 3 strike sections. Twelve subsurface sections were selected for quantitative analysis.

Stratigraphic sections were subject to gamma analysis and spectral analysis. Gamma analysis forces facies-dependent sedimentation rates to be assigned which minimize the variation in cycle period, thus, tuning the time series. Gamma analysis was performed under two assumptions: 1) all cycles observed have essentially the same time period and 2) all high magnitude, medium-order cycles have essentially the same period. Spectral analysis requires an environmental interpretation of the facies and reveals the periodic components of the stratigraphic record (with and without tuning via the gamma results). Gamma results consistently revealed that nonmarine to marginal marine facies had higher sedimentation rates than did the deep and shallow-marine sediments with one exception in one outcrop composite section. The periodicities observed in the spectra were calibrated by best fit to an orbital signature. The short eccentricity peak (95-133 k.y.) was the most consistent and prominent, while long eccentricity (413 k.y.) and obliquity (34 and 43 k.y.) were also quite persistent. Precessional index peaks (17 and 21 k.y.) were less persistent. Non-orbital peaks of period 50-80 k.y. were persistent, and occasionally prominent. If the orbital calibration of the sections is true, then only 6 to 24% of time span of Cisco deposition is represented by rock, increasing from proximal (outcrop) to the more shelfal (subsurface) sections.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California