THE LONGMONT DETACHMENT MEGALANDSLIDE EAST OF BOULDER, COLORADO

The 730 square kilometer (281 square miles) Longmont Detachment Megalandslide (LDM) is a gravity-slide detachment of almost 188 cubic kilometers (45.1 cubic miles) of upper Cretaceous bedrock above a singular bedding plane horizon. The initial interpretative LDM framework was presented by the author at the 2013 Geological Society of America Annual Convention with subsequent recognition as only one of several models interpreting area fault development.

The author's talk presents new major discoveries proving LDM existence and genesis. These include a volcanic mechanism for its development utilizing new area core data and a new map of its faulted boundaries, including its division into an eastern portion composed of reverse faults, that developed in a compressional structural environment and a western area, composed of both reverse and normal faults that developed in both compressional and extensional environments. New well data indicates the eastern area developed first, up to 25 million years ago (mya), followed by the western. This is much later than the Laramide Orogeny (70-50 mya) historically interpreted for the LDM area faulting.

The tremendous volume of LDM subsurface data provides supporting evidence for the existence and genesis of two other major western United States bedrock mega landslides sharing LDM characteristics: the 3,400 square kilometers (1,313 square miles) Heart Mountain Detachment (HMD) and the 3,496 square kilometers (1,350 square miles) Markagunt Detachment (MD). The latter two features have been considered very interpretative by scientists in the geological community since large portions of both are defined by outcrop data. The tremendous volume of LDM subsurface data presented in the author's investigation has allowed supporting validation for both HMD and MD existence and genesis. The author's information can also be utilized to re-evaluate worldwide structural features, defined by limited subsurface data, to determine if they are products of a bedrock gravity-slide detachment.