THE LONGMONT DETACHMENT EAST OF BOULDER, COLORADO

The 225 mi.² Longmont Detachment formed during the Laramide Orogeny on the northwest limb of the Denver sub-Basin following the down dip movement of a 45 cubic mile block of upper Cretaceous sedimentary rocks approximately 0.75 mile. Its detachment areas were the 11.5 mile-long eastern boundary fault of the Boulder Oil Field and the 12 mile-long Longmont Fault. The decollement movement of this feature terminated in a series of ramp anticline reverse faults with offsets up to 1,350'. Proximal to the detachment areas, pull-apart extensional forces developed a series of subparallel northeast southwest-oriented horst-graben blocks up to 0.5 mile wide.

The structural geology of the Longmont Detachment was mapped utilizing a large volume of subsurface data. Its faults were identified during mining of upper Laramide formation coal from the Boulder-Weld coal field from 1859 through 1979. The detachment surface, identified as the Kp2 horizon, was mapped after correlating nearly 1,350 oil and gas geophysical well logs. This marker bed is the top of a 40'-65' sandstone whose high interstitial fluid pore pressures are interpreted to be the glide plane for the decollement movement of the overlying upper Cretaceous sedimentary rocks. The areal dimensions of the Longmont Detachment were identified by noting that geophysical well log correlations above the Kp2 horizon within this feature are imperfect, while below this marker, they are excellent. This is a product of the sedimentary rock movement above this glide plane. Outside of the Longmont Detachment, correlations both above and below this marker bed are excellent. Geophysical well log correlations were also utilized to map repeated upper Cretaceous sedimentary rock sections within the ramp anticline reverse faults.

The Longmont Detachment is analogous to the intensely studied 1,225 mi.² Heart Mountain Detachment of northwest Wyoming. Both features have well defined detachment zones, a singular glide plane horizon for sedimentary rock movement and ramp anticline reverse faults marking the terminus of detachment movement. The Heart Mountain Detachment has been primarily characterized utilizing outcrop information, while the Longmont Detachment is a subsurface feature.