Mapping Subtle Regional Structures in Wyoming

The purpose of this study was to investigate the relationship of the deposition of ancient delta lobes that formed around subtle syntectonic structures within the Cretaceous Frontier Formation in the Powder River basin in Wyoming. This study focused on the larger scale tectonic features that control the position of the delta sandstone bodies. The structures have amplitudes of a few tens of meters but information about the spacing, wavelength and shape are unknown. These types of structures may relate to changes in tectonic intraplate stresses and may be an important factor in creating subtle unconformities in the stratigraphic record, as well as controlling the position of reservoir sandstone bodies in hydrocarbon reservoirs. GPS and reflectorless laser rangefinder readily allow rapid measurements in seconds to minutes of positions including elevations from a few centimeters to decimeters. Consequently, it is possible to map subtle variations in the elevations of beds that relate to the long wavelength, low amplitude structures. This study demonstrated the logistics, methodology and timetable for collecting data over 40 square km in detail and over 150 square km of this terrain more generally. Over three days personnel were trained to become familiar with the outcrop locations and the detailed allostratigraphy mapped earlier. An RTK GPS base station receiver and radio was established on a high topographic point, which allowed a few centimeter accuracy positions within almost a 10-km radius in real time. The bentonite beds to be mapped are extraordinarily visible as white layers and other beds such as pebble and massive sandstone beds are identifiable because of their association with these. Most of the contacts of interest are distinctive even at several hundred meters range especially when seen through the telescopes of the laser systems. Sites from which important features and surfaces were visible were picked and then positioned by RTK GPS before tracing by laser gun. The preliminary result of this project will be presented by Dr. Bhattacharya at AAPG New Orleans Meeting in April.

Birdeye regional view of Frewens Castle shows sandstone cliffs underlain by bentonite-bearing mudstones of Lower Belle Fourche member

Detailed view of white, reflective bentonites encased in gray mudstones, with mounted reflectorless laser rangefinder in operation.

3D perspective view of mapped the Clayspur bentonite, mapped in this pilot study using a laser and GPS. The 3D view clearly shows the present-day folding of the Clayspur bentonite. A total of 8181 data points were collected over 10 days in 1999 on the Clayspur alone.

Thirty meter resolution terrain map of the pilot study area with the Clayspur data points overlain (red dots).

Interpolation of the present day Clayspur bentonite surface. The contoured surface shows the NW fold axis, also obvious in the perspective view.