Abstract: Casper Mountain Area (Wyoming)--Structural Model of Laramide Deformation

W. G. Brown

The structural style of the Laramide orogeny (Late Cretaceous-early Tertiary) is typified by Casper Mountain area. Laramie Range is a large east-facing asymmetric uplift; its northwestern continuation is the west-facing Casper arch. Casper Mountain area is at the junction of these two features.

Compartmental deformation is the term applied to the structural style of Casper Mountain area and is characterized by: (1) reversals of asymmetry, (2) abrupt terminations of local structures (e.g., Emigrant Gap anticline), and (3) large changes in structural elevation across faults (e.g., Casper Mountain fault) and panels of steep dip. The major faults in the area are transverse to the regional northwest Laramide trends of central Wyoming. It can be shown that these easterly trending faults are parallel with Precambrian trends in the Laramie Range.

A synthesis of the geology of Casper Mountain area and Laramie Range results in the theory that compartmental deformation is controlled by Precambrian zones of weakness which are aligned obliquely to the northeast-southwest direction of Laramide compression. These zones of weakness then were reactivated in the Laramide orogeny as compartmental faults.

Subsidiary structures on the major uplifts are of two types: (1) Precambrian-cored folds which are thrust toward the crests of the uplifts, and (2) anticlines which are shown by surface and subsurface data to be "rootless." These rootless features result from bedding-plane slippage which is a natural consequence of flexure-slip folding. Both types of structures support the interpretation that Laramide orogenic forces were compressional.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico