Abstract

The Boulder-Weld Fault Zone is a 120 mi² structural feature that has been progressively mapped since the discovery of coal in 1859 along its western boundary near Marshall, Colorado. Several compelling and often conflicting historic interpretations have been presented to explain its origin. For example, the western portion was mapped by the coal miners as primarily dip-slip normal faults. A later interpretation determined the eastern portion was composed of a number of reverse faults (Kittleson, 1992). Weimer (1996) proposed a wrench fault origin for the Boulder-Weld Fault Zone to reconcile this reversal in structural styles.

The correlation of nearly 1,450 geophysical well logs by the author has indicated the Boulder-Weld Fault Zone is not an autonomous structural feature, but part of a much larger 205 square mi allochthonous decollement feature. As a dimension perspective, Denver County is nearly 155 mi² in size. This structural feature formed by its detachment and subsequent movement down-to-the basin during the Laramide Orogeny along a bedding plane fault located approximately 250 ft below the top of the Pierre Shale. Since this detachment occurred as a result of movement along the Longmont Fault, identified by Weimer (1996), this feature will be referred to as the Longmont Detachment. The Longmont Detachment has many analogous features with the Heart Mountain Detachment that formed on the western side of the Big Horn Basin. Both features are down-to-the basin gravity slides with a defined detachment zone, a well defined low dipping (1 to 5°) stratigraphic horizon along which the detachment moved and a footwall ramp containing reverse faults where movement ceased.