Evaluation of Reported Late Tertiary Thrusting in Northeastern Nevada: ABSTRACT

Abstract

In 1950, Wheeler and McNair (Bull. Geol. Soc. America, Vol. 61, No. 12, Pt. 2 (December, 1950), p. 1513) reported low-angle thrust faulting involving late Tertiary Humboldt lacustrine sediments in two areas in eastern Elko County, Nevada. Since this concept has an important bearing on petroleum exploration, an examination of the areas mentioned was made. SPRUCE MOUNTAIN AREA (a) Along U. S. Highway 93 in T. 31 N., R. 62 E., about 40 miles south of Wells, a low escarpment paralleling the highway on the east is capped by a 50-75-foot limestone breccia which was apparently End_Page 2627------------------------------ believed the basal part of a thrust plate. Study of the lithologic and structural features of the breccia and of its distribution and regional topographic relations shows that it is an old alluvial fan deposit related in origin to the present fan at the mouth of a large canyon draining the western slope of Spruce Mountain. This breccia rests upon the eroded surface of gently tilted Humboldt beds, with complete absence of structural features indicative of low-angle thrust faulting. A dissected piedmont fault scarp about two miles east of the highway probably accounts for the slight eastern tilt of the lower portion of the fan and the exposure by erosion of what is undoubtedly its basal breccia. (b) About 10 miles to the south, a thrust was reported at the western base of Phalen Mountain in T. 29 N., R. 63 E. Excellent exposures of the contact between the Humboldt beds and the Paleozoic rocks occur along the southwestern side of the mountain. Here the basal maroon conglomeratic silt unit of the Humboldt is in depositional contact with the Paleozoic. Minor post-Humboldt normal faulting has occurred. Again there is complete absence of structural features indicative of major low-angle faulting. End_Page 2628------------------------------ End_Page 2629------------------------------ THOUSAND SPRINGS VALLEY AREA No specific localities are given by Wheeler and McNair for this area but the observations made throw grave doubt on the possibility of low-angle thrusts which involve the Humboldt beds. (a) East of Thousand Springs Creek along the western base of Nine Mile Ridge, in Secs. 27 and 34, T. 42 N., R. 66 E., the Paleozoic-Humboldt contact is well exposed. For approximately 3/4 mile along the range front, steeply west-dipping Humboldt beds rest in depositional contact on the Paleozoic. The basal depositional breccia and conglomerate show no evidence of low-angle deformation. (b) West of Thousand Spring Creek along the western edge of T. 42 and 43 N., R. 66 E., ridges of Paleozoic rocks stand above the gravel-covered pediment which cuts across gently to steeply dipping Humboldt beds. The Paleozoic-Humboldt contact was studied at two localities. In Sec. 29, T. 43 N., R. 66 E., Paleozoic limestone occurs as an up-faulted wedge between two intersecting normal faults, one of which continues into the Humboldt sediments. In Sec. 6, T. 42 N., R. 66 E., a high-standing area of Paleozoic rocks has Humboldt beds in normal depositional contact dipping away from it on at least three sides. The Paleozoic is anticlinally folded, and relations indicate that these rocks and the Humboldt beds were folded at the same time. Immediately south of the anticlinal area, the lake eds are faulted against the older rocks. No features indicative of low-angle faulting were found. End_of_Article - Last_Page 2630------------