!'f \, w. Alan Stewart Colorado Exploration Company, Golden STRUCTURE OF THE FOOTHILLS AREA WEST OF DENVER, COLORADO. Introduction

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1 STRUCTURE OF THE FOOTHLLS AREA WEST OF DENVER, COLORADO w. Alan Stewart Colorado Exploration Company, Golden ntroduction i ; The dominant structural features west of Denver and along the mountain front are the foothills monocline and its associated thrust faults. The mountains. which rise abruptly from the plains area repre sent the metamorphic crystalline core of a large anticlinal str,~cture known as the Front Range Arch. Sedimentary rocks exposed in' ; the foothills belt along the mountain front dip eastward into the Denver\ Basin at values ranging from 30 to 60 degrees. Locally, the dips are steepened and overturned, sometimes strongly, by drag associated with thrust faulting. To the west of Denver. for a distance of at least 40 miles along the mountain front. the tilted -sediments are severed by major thrust faulting. The greatest displacement along this line of overthrusting occurs just northwest of the town of Golden. Colorado, where the fault has been named the Golden Fault. Thrust faulting has taken place along two imbricate fault surfaces at the town of Morrison. t is not known whether these two faults represent a splitting of the Golden Fault as it turns to the south or if they are two major thrust faults with an en echelon relationship. The trace of the Golden Fault and the associated imbricate thrust are shown on Plate 2. Foothills Monocline!'f \, The flank mutually shared by the Front Range Arch and the Denver Basin has structqral characteristics of a monocline. The Denver Basin to the east is a synclinal basin exhibiting pronounced.. asymmetry. Dips on its east flank are in the magnitude of a few tens of feet per mile. Those on its west flank in the foothills area are as high as 60 degrees at the surface and. as indicated by seismic information,.70 degrees in the subsurface. Bedding planes decrease in dip westward ~

2 /, / \' \ i' across the foothills belt, except where abnormally steepened or overturned adjacent to planes of thrust faulting. ' Thrust Faulting 11 ', j' \ As shown on Plate 2, the foothills monocline is believed to be disrupted by thrust faulting from Deer Creek on the south to a point north of Ralston Reservoir on the north. Nowhere in this area are the thrust fault planes or zones clearly exposed. The presence and location of the faulting must be inferred from stratigraphic throw at a given point and from associated' structural features. The most diagnostic features for 'locating the position of thrust fault traces in surface work are the abnormally steep dips or overturned beds produced by dragging of the beds along the fault plane or zone. '\ ' As shown on the geologic map (Plate 2), the trace of a fault roughly parallels the outcrop of the Montana group (Pierre shale and Fox Hills sandstone) north of Deer Creek. Seismic studies and test drilling in the penver area indicate that an unfaulted Pierre shale section is about 8,000 feet thick. - Reconstruction of a cross-section 'along Deer Creek indicates that the Pierre shale has been reduced in thickness by faulting to about 6,500 feet. About two miles south of Deer' Creek, the McElroy Ranch Company, City of Englewood No. 1 test well penetrated a major fault at about 4,000 feet. The traces of two thrust faults are shown on the geologic map between Bear Creek and Deer Creek. The thrust fault to the east is believed to be the Golden Fault which has been traced continuously for about 13 miles along the mountain front and may be inferred to extend at least 3 or 4 :niles further at either end. The trace of the Golden Fault is mapped as swinging to the east through, and offsetting, the Laramie outcrop in Secs. 21 and 22, T. 55, R.69W. From the latitude of Bear Creek northward, it is believed that the Golden Fault is the only major thrust fault pre sent. The Golden Fault is probably not, in most places, a' plane surface but rather a brecciated or compound fault zone. Locally it may split to include segments of for'mations dragged along the fault surface. West of Golden and about 1/3 mile south of Clear Creek, for example, a sliver of Timpas limestone is stratigraphically out of place with respect to both Fountain redbeds in the hanging wall and upper Pierre shales in the foot wall. Where the Golden Fault has relatively sharp changes in strike, horsetail structure is found with minor thrust faults !! r---':;;~~--'''--ii i

3 ---.-,, _.J _4_ ! _ splitting off the major fault. Examples may be noted on the geologic map just south of Golden where the Golden Fault trace parallels U. 5. Highway 6. A second example is found north of Golden, just north of the road up Golden Gate Canyon and west of the Golden Fire Brick plant. j i The trace of the Golden Fault is arcuate. The point of maximum s,tratigraphic throw occurs about one-half mile north of Clear Creek, northwest of Golden. At this point the Fox Hills formation of the Montana group is in f~ult contact with the lower beds of the Fountain formation. North and south of this point, the fault trace swings eastward away from the mountain front and formations normally found in the sedimentary section between the above formations appear in the hanging wall of the fault in pr oper sequence. At the point of maximum displacement, 11,000 feet of strata are cut out by thrust faulting.! ~ '; ii ;l 'j,j ~l The Golden Fault displays many characteristics typical of row angle, overthrust faulting. The fault trace is quite _ sinuous, with a close coincidence locally between topographic contours and fault line. n Secs. 9 and 10, T. 35., R.70W. an overthrust sheet of the Montana group and Laramie formations,overlies Arapahoe sediments; Sandstones identified as Laramie are overturned to the west, with dips as low as 38 degrees. Between this outcrop and the linear belt of exposed Niobrara formation. a full section of the Montana group could be accommodafed. There is abundant direct and indirect evidence that the Golden Thrust is a strongly warped surface. Dips vary locally from gentle east dips to shallow, westerly dips to steeper, we sterly dips as great as 35 to 40 degrees. Just south of Gold~n, in Sec. 3, T. 4S., R.70 W., at the edge of the overthrust sheet, core drilling has revealed low angle easterly dips in the fault zone. Refraction seismic investigations in the same area indicate that average dips may be as little as 4 degrees to the west for a distance of about feet. Other evidence of a very gently inclined fault zone are: the correspondence between the fal,l1t line and topographic contour s, and the very nearly horizontal attitude of formations exposed adjacent to the fault, interpreted to be the result of drag. n Sec. 10, T. 3S., R. 70W., for example, the sinuous fault trace is very nearly parallel to topographic contour s. An example of extreme drag is exposed in an excavation just south of U. S. Highway 40 in Sec. 10, T. 4&., R.70W. The Benton shale here is close to the Golden Fa~t trace, and displays overturned, very nearly horizontal, bedding surfaces. n general, the inclination of the fault plane appears to decrease rather sharply to the west toward the roots of the mountains. For example, near the mouth of Clear Creek Canyon, close to the sedimentary-crystalline contact, a well defined h :f. i' : ( i, '. -.: 27

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5 fault contact between Fountain redbeds and Pierr'e shale dips as high ' as 35 degrees to the west. n other areas, such as south of Bear Creek in the vicinity of the S. D. Johnson Lillie Pallaoro No. well, beds may dip steeply on the east,' becoming gentler toward the crystallines. Folding Small folds are present in the foothills belt and their axes bear both diagonal and parallel relationships to the general trend of the foothills monocline and of the Front Range Arch. A small anticline and associated syncline' are found in the Fountain fortnation north and south of Deer Creek. The axes of these folds are roughly parallel to the mountain front. To the east, concealed beneath the overthrust block of the major fault in that area, is a much larger subsurface anticline... with a parallel trend. This larger anticline has no surface expression and was detected by a seismic survey. As reconstructed from the seismic data, this larger fold may have a structural relief on the top of the Dakota of as much as 1,200 feet, relative to the subsurface syncline to the west, also concealed beneath the olerthrust fault. An interpretation of the structure of Deer Creek from surface tnapping and seismic data is shown in the accompanying cross-section. f Northwest of Ralston Reservoir, formations are exposed in sigmoidal belts. The curvature of the belts outlines two en echelon folds, an anticline and a syncline. These folds have been broken by a scissors fault, along the flank shared mutually by the folds, close to the axis of the syncline. These tnay be knuckle folds formed by the draping of sediments around a diagonal scissors fault in the basement complex. Further north, along the mountain front in the vicinity of Loveland and Ft. Collins, are a series of much larger plll1.ging str,uctures quite similar to these smaller folds. The westernmo'st edge of the Denver Basin, in the underthrust block of the Golden Fault, contains a number of interesting structural irregularities. Surface geological and subsurface seismic evidence indicate that the margin of the basin is undulatory or warped, with a nutnber of broad folds of comparatively shallow amplitude plunging toward the Basin. Attention is called to the curvature of formational outcrops shown on Plate 2, just south of Turkey Creek, west of Harritnan Lake, and between Bergen Reservoir and the Soda Lakes. To the north, at the mouth of Clear Creek Canyon, immediately west of Golden, the Laramie fortnation in the footwall of the fault swings, sharply to the west and reflects a synclinal warp on the Basin flank ! /, 1

6 Close scrutiny of the lava caps on North Table Mountain reveals that the flows there lie along the axis of a syncline. whose trend is diagonal to the mountain front. This syncline is parallel to a subsurface. plunging anticline. whose axis is northeast of North Table Mountain. This fold has no surface expression and was located by reflection seismic work. The S. D. Johnson. Farmers Highline No.1. is testing this structure. 1. ;, t.. ~!,). :i: -,..!.; ;,,': -,'- j i