Precambrian Basement Control on ‘Plains-Type Folds’ (Compacttonal Features) in the Midcontinent Region, USA

Abstract

‘Plains-type folds’ were described in the early 1930s by petroleum geologists working in the Midcontinent, USA. The folds are characterized by small size, increased sharpness with depth, asymmetry, and association with faulting. These folds are ascribed to the compaction of sediments over crystalline, rigid basement, in this instance tilted Precambrian fault blocks. Initially, the ‘plains-type folds’ were formed when the present major structural features of the region were formed and adjustments continue today as recognized by neotectonics in the region. Development of these features in sediment veneers over the basement in the many cratonic environments of the world is of interest because locally they serve as traps for petroleum. Timing of development of the ‘plains-type folds’ determined when and if they were available for entrapment of petroleum as fluids moved through the system. This study indicates that there are two types of ‘plains-type folds’ in the Midcontinent: (1) the normal type developed mainly in the early Middle Pennsylvanian as a result of intraplate stresses caused by collision of the North and South American plates; and (2) those formed later as a result of other influences. By using a structural-depth gradient, which expresses the change in thickness of stratigraphic units on and off structure, it is possible to determine at which time(s) these features were affected by local or regional structural adjustments in the basement. By plotting the structural gradient, which is the change in structural closure with depth, the nature of the ‘plains-type fold’ can be recognized as being either (1) or (2). We propose that features exhibiting a low or no structural gradient (2), that is a similar structural closure with depth, and that are small and nearly circular in nature can be attributed to the emplacement of late Mesozoic or early Tertiary intrusive igneous plugs.