Abstract

Photogeological mapping, using color photography and Landstat images, delineated major elements of the structural framework of the buried Paleozoic section of the southern portion of the Mississippi embayment within the Helena 1° × 2° quadrangle. Geomorphological aspects of the surface noted while mapping the area included incised streams, deflected drainage channels, alluvial ponding, compressed meanders, anomalous drainage patterns, and color/tonal changes. A comparison of a small portion of this study with gravity and seismic data revealed a high degree of correlation between surface features and buried Paleozoic structure. These areas of correspondence included the surface reflections of two normal faults and a probable paleotopographic high developed on an anticline at the leading edge of a shallow thrust. The normal faults defined the southeastern flank of a large horst which is also reflected in gravity. The thrust mentioned is one of several noted in the seismic section having an apparent northwestward component of displacement. Although fault timing is not certain, it is suggested that the normal faults occurred first and that their presence introduced enough anisotropy to the section that formation of a ramp was induced during the later thrusting episode. Further study of regional aeromagnetic and Bouguer Gravity mapping and well-data suggests that the normal faults were inherited from the Precambrian-Cambrian rifting episode and that the shallow thrusts and deeper foreland-type thrust features were the products of right-lateral transpressional movement between the North American plate and a converging South American plate complex during the Appalachian-Ouachita orogeny.