Abstract: Surface to Subsurface Correlation of the Claiborne and Jackson Groups, Colorado River to Trinity River
Thomas E. Ewing
Detailed correlation of log sections using flooding surfaces and facies analysis clarifies the stratigraphic framework of the middle and upper Eocene. Continuing sections updip to the outcrop allow detailed surface studies to be placed within a comprehensive stratigraphic framework.
Maximum flooding surfaces (MFS) at the top of thick transgressive deposits divide the succession into four major stratigraphic units. The two lowest, the Queen City and Sparta units, are internally simple. Above each MFS, shale units coarsen upward into one or two cycles of sand deposits near the outcrop. Both units conclude with thick transgressive members (Weches Formation and the Stone City Beds of outcrop). Both Queen City and Sparta cycles show little evidence of internal sequence-bounding unconformities.
The Yegua unit is internally complex, consisting of eight regional progradational cycles. The lowest cycle (Yegua 90) includes the entire outcropping Cook Mountain Formation. The overlying cycles merge landward into nonmarine sandstone and lignitic shale (Yegua Formation). Five cycles above Yegua 70 contain evidence of sequence boundaries. Topping the Yegua are backstepping sandstones and a thin transgressive deposit (Moodys Branch marl), which is not recognized in Texas surface exposures.
The Jackson unit shows intermediate complexity. Near the outcrop, basal marine shales (Caddell Formation) grade upward into marginal marine sands (Wellborn sand), then into lignite-bearing nonmarine deposits (Manning Formation); all units grade basinward into marine shale. One basinward-shifted sandstone suggests a minor sequence boundary, possibly correlative to the Cocoa Sand of Alabama. Marine Jackson overlies the prograded Jackson wedge in the subsurface, but is truncated by the sub-Vicksburg and sub-Frio/Catahoula unconformities before reaching outcrop.
AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994