Sample descriptions and geophysical logs for cored well BP-3-USGS, Great Sand Dunes National Park and Preserve, Alamosa County, Colorado

Grauch, V.J.S.; Skipp, Gary L.; Thomas, Jonathan V.; Davis, J. K.; Benson, Mary Ellen

doi:10.3133/ds918

Cited by 2 publications

(3 citation statements)

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“…Faulted sediments and/or sedimentary rocks are an obvious possibility. However, known density contrasts present across lithologic boundaries within the sedimentary section are small (e.g., Grauch et al, 2015), so the specific geologic origin of those density contrasts is unknown. Alternate possibilities are locally inadequate terrain corrections, and unrecognized problems with the AGG data processing.…”

Section: Discussionmentioning

confidence: 99%

“…The density of unsaturated dune sand was estimated at 1800 kg/m 3 by Nettleton profiling (Nettleton, 1942;Drenth, 2013). The estimated density of saturated dune sand, 2080 kg/m 3 , is from a shallow density log near the western portion of the dunefield (Grauch et al, 2015). This density value is only an estimate because the sand sampled by the density log may be mainly part of the Alamosa Formation and not directly representative of sand in the dunefield (Madole et al, 2008(Madole et al, , 2013Grauch et al, 2015).…”

Section: Forward Modelingmentioning

confidence: 99%

“…The estimated density of saturated dune sand, 2080 kg/m 3 , is from a shallow density log near the western portion of the dunefield (Grauch et al, 2015). This density value is only an estimate because the sand sampled by the density log may be mainly part of the Alamosa Formation and not directly representative of sand in the dunefield (Madole et al, 2008(Madole et al, , 2013Grauch et al, 2015). The water table surface comes from a groundwater model of the GRSA region (HRS Water Consultants, 2007) (Fig.…”

Section: Forward Modelingmentioning

confidence: 99%

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Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

et al. 2017

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Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre de Cristo fault system that are part of the northern Rio Grande rift. GRSA lies within a prominent embayment in the range front where two separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, underlies most of the high dunefield at depths of 500 m to 1 km. The bench is truncated on its southwest margin by a northwest-trending, southwest-dipping normal fault. A northeast-trending, northwest-dipping normal fault with ~600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest-and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre-and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand.

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Section: Discussionmentioning

confidence: 99%