Palinspastic restoration of the eastern flank of the Colorado Front Range at Coal Creek, constrained by outcrop geology, wells and a pre-stack depth migration of the Rocky Flats deep seismic line, reveals a system of stacked triangle zones

Abstract

A new pre-stack depth migration of the deep seismic line across the former Rocky Flats plant at Coal Creek provides the best image yet of the eastern flank of the Colorado Front Range. The new processing reveals a detailed picture of stacked triangle zones detached at the X-Bentonite of the Cretaceous Benton Group and within the lower part of the Cretaceous Pierre Shale. The seismic line also images back thrusts at several levels along which nonstandard, younger-over-older juxtapositions are evident. Two wells confirm the tip and detachment at X-Bentonite level, and piercement thrusts cutting to the higher triangle tip in the lower Pierre. The palinspastic restoration relies on several key constraints. First, the seismic line fixes the positions of the floor thrusts and footwall ramps of the principal forethrusts. Second, offset of the east-west trending axis of a syncline in the basement cored by Precambrian Coal Creek quartzite helps quantify significant slip along the East Rogers back thrust – the restoration only succeeds by recognizing this offset and its trajectory. Finally, regional structural mapping of the 67 Ma isochron surface based on normalized low-temperature thermochronology data allows the position of the Great Unconformity and its overlying sedimentary cover to be estimated across the denuded basement within the range. The restoration requires relatively minor strain within the basement blocks and indicates approximately 4 miles (6 km) of translation within the thrusts bounding the eastern flank of the range at this latitude. The kinematic model shows a breakback sequence with an initial East Rogers back thrust cut by two blind forethrusts at progressively higher levels and their associated back thrusts. The new pre-stack depth migration provides an excellent snapshot of geometries at one spot within a complex system of blind thrusts utilizing detachments and forming triangle zones that change stratigraphic level along strike to build the eastern flank of the Colorado Front Range.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019