Laramide structure of the Black Hills uplift, South Dakota-Wyoming-Montana

Abstract

The 290-km-long Black Hills uplift extends from the South Dakota-Nebraska border to southeast Montana. A structurally higher east block of northward trend and a northwest-trending west block are separated by the west-facing Fanny Peak monocline. Paleozoic, Mesozoic, and Paleocene sedimentary rocks 2,220 to 3,000 m thick overlaid the anisotropic Precambrian igneous, metasedimentary, and metaigneous basement during Laramide uplift. Draping of the sedimentary section over basement fault blocks produced monoclines with, opposed to, and parallel to regional dip and ramps, terraces, and anticlines as subsidiary structures. Individual monoclines as much as 170 km long consist of linear segments commonly joined in gentle arcs. The dominant fold trend is northward, although one major segment of the Black Hills monocline trends northwest. The folds terminate at intersections with other monoclines, by decreasing stratigraphic offset along strike, or by splaying into several folds of lesser structural relief.Gravity data suggest single basement faults beneath steep, narrow folds and multiple faults beneath wider structures. In a single locality the basement is exposed in fault contact with Paleozoic rocks that show a narrow zone of rotation in both fault blocks. Normal and reverse faults locally extend as high as the massive Mississippian and Pennsylvanian carbonate rocks, but elsewhere these units are folded into five linear segments. Mesozoic siltstone and shale are believed to be more rounded in section view due to flexural flow. In the inclined limb conjugate fault sets and horizontal tension cracks affect the brittle rocks, and ductile layers are thinned by flexural flow.The uplift is separated from the Powder River Basin on the west by the Black Hills and the Fanny Peak monoclines which have structural relief as great as 1,670 m and dips as great as 90°. Gently inclined planar sedimentary units, inferred to parallel the surface of the basement, show changes of a few degrees dip across the folds. Such differences may result from rotation of rigid blocks of the anisotropic 165 on May 31, 2015 memoirs.gsapubs.org Downloaded from 166 ALVIS L. LISENBEE basement along curved faults in the style outlined by Stearns (1975).The arcuate eastern boundary with the Interior Lowlands province forms a partial dome. Strain was distributed across at least 32 km, possibly by shear along the Precambrian schistosity. North-and east-trending folds, believed to reflect longitudinal and transverse faults in the arched basement, locally parallel the basement fabric. At this easternmost margin of the Wyoming province, the uplift and adjoining province were both elevated, although unequally, whereas the basin to the west of the uplift was an area of subsidence. This difference in absolute movement patterns is believed responsible for the asymmetry and composite structural style of the Black Hills uplift.

Gravity and magnetic features and their relationship to the geothermal system in southwestern South Dakota

“…The Cottonwood Creek anticline in western Fall River County is structurally different than these three folds in that it appears symmetrical and plunges to the southwest (Lisenbee, 1978).…”

“…The Black Hills uplift is considered the easternmost and least deformed of the Laramide uplifts associated with Rocky Mountain tectonism (Lisenbee, 1978 Discussions pertaining to lithologies and structures relevant to the present geophysical study are given below, although the reader is referred to Redden (1975), Kleinkopf and Redden (1975), Darton and O'Harra (1925), Gott and others (1974), and Lisenbee (1975Lisenbee ( , 1978 for more detailed discussions.…”

Gravity and magnetic features and their relationship to the geothermal system in southwestern South Dakota

“…I f drape folding of sedimentary u n i t s , over f a u l t e d basement blocks is commonplace within t h e southern Black Hills region as suggested by Lisenbee (1978), then e i t h e r of these a n t i c l i n e s may be a s s o c i a t e d with deep-seated s t r waters. The e a s t e r n limb of t h e r e f l e c t f a u l t i n g a t depth because it is impressively s t e e p ( f i g .…”

“…Hydrological and geothermal d a t a (Head and o t h e r s , 1978;oon and McGregor, 1974) i n d i c a t e t h a t t h e Black H i l l s function as a coo ter source f o r a q u i f e r s trending ward t o the east and w e s t One would then a n t i c i p a t e t h a t t h e regions flanking t h e Black Hills are a l s o characterized by low geothermal gradients.…”

“…The Black Hills uplift is considered' the easternmost and least deformed of the Laramide uplifts associated with Rocky Nountain tectonism (Lisenbee, 1978 (Lisenbee, 1978).…”