UPPER MISSISSIPPIAN STRATIGRAPHY AND SYNDEPOSITIONAL FAULTING REFLECT LIKELY OUACHITA FLEXURAL FOREBULGE EFFECTS, EAST-CENTRAL KENTUCKY, U.S.A

Flexural models for foreland-basin evolution nearly always entail the presence of a low-amplitude, long-wavelength forebulge on the distal basin margin. However, evidence for such structures in the past is not easily discerned, because low-amplitude flexural waves like this have little preservation potential. Yet, a passing bulge might well reactivate basement structures that generate stratigraphic anomalies in otherwise stable cratonic areas, and in this way leave a record of its passage; this is what we report from the Mississippian of east-central Kentucky. Stratigraphic analysis of late Middle to early Late Mississippian (late Meramecian–early Chesterian) carbonates (Slade Fm.) in east-central Kentucky shows six unconformity-bound sequences with many stratigraphic anomalies, such as disjunct distribution, abrupt facies changes, seismites, and unexpected erosional thinning, in the tectonically stable foreland of the western Appalachian Basin. Anomalies are all proximal to basement structures, suggesting the possibility of bulge involvement. The earliest, late Meramecian, St. Louis sequence shows relatively uniform distribution and a typical, eastward Appalachian thickening gradient. In contrast, overlying early Chesterian sequences show mosaic-like distributions, related to WNW-oriented basement structures, and a thickening gradient to the southwest. Change in regional dip to the southwest and preferential reactivation of WNW-oriented structures suggest regional compressional stress from the southwest, implicating the coeval Ouachita orogeny. These early Chesterian events coincided with the major pulse of Ouachita orogeny on the southern margin of Laurussia and concomitant forebulge moveout to the ENE due to orogenic loading. Modeling of regional-stress distribution, as well as spatio-temporal factors during bulge moveout, strongly indicates that a migrating Ouachita bulge most likely reactivated basement structures and generated stratigraphic anomalies in Kentucky and nearby areas. Hence, the likelihood of low-amplitude flexural waves, like the Ouachita bulge, migrating across areas where they were not previously considered, has major implications for understanding structural, stratigraphic, and eustatic anomalies in supposedly stable cratonic areas.