PALEOSEISMITE DEVELOPMENT IN DISTAL FORELANDS: A RECORD OF INITIAL TECTONISM IN OROGENIC SYSTEMS

Soft-sediment deformational structures, especially in the form of upwardly injected sandstone clastic dikes, provide an opportunity to understand paleo-seismicity in pre-Neogene tectonic settings. However, these features have been underutilized in distal foreland studies because establishing a link between soft-sediment deformational structures and paleo-seismicity often requires identification of a concealed basement fault(s) and corroborating stratigraphic-structural data. This presentation provides an overview of the required processes for clastic dike generation, identification of paleoseismite development in distal foreland regions, and potential interpretations for orogenic systems based on three case studies.

Late Cretaceous paleoseismites in the northern Bighorn Basin are interpreted to represent a surficial expression of an end load stress generated at the plate margin from the arrival and collision of a thick, buoyant oceanic plateau, the conjugate Shatsky Rise. A summary of previously published and recently identified soft-sediment deformational structures in the Laramide foreland from central Wyoming to northern Arizona is presented to further evaluate an end load hypothesis.

Mississippian paleoseismites in the northern Black Warrior foreland, at the juncture of the southern Appalachian-Ouachita thrust belts, coincide with local normal faulting associated with basement offsets and initial Ouachita tectonism. These features exhibit similar structural-stratigraphic relationships to paleoseismites reported in the southern Appalachian foreland of southwest Virginia-West Virginia and may represent a temporally short-lived, Laramide-like phase of tectonism.

Pennsylvanian paleoseismites in the Taos Trough of northern New Mexico record initial Ancestral Rocky Mountain tectonism. Shallow marine facies of the Sandia Formation contain clastic dikes that are laterally and spatially associated with an intraformation angular unconformity. Recumbent folds, resulting from slumping, are also found in a similar stratigraphic interval over a 15 km range away from clastic dikes. These features presumably form from basement deformation driven by far-field stress from southern Laurentian plate margin(s).