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~ / <br />4.2 Geolo¢v <br />The Uravan site is located in the northcastem part of the Colorado Plateau. The Colorado Plateau <br />is a stable, intracontinental subplatc having a greater crusta] thickness than the adjoining regions. <br />Its mazgins exhibit seismic activity and paleovolcanic features similar to the more disturbed <br />adjacent provinces, while the central portion exhibits the more subdued characteristics of a <br />continental plate interior. <br />Uravan is near the deeply incised San Miguel and Dolores Rivers and [heir nibutary drainages. <br />Both main trunk streams art unde~t in their valleys which were shaped by higher average flows <br />during the Pleistocene deglaciation of the region. The braided channel patterns and wide valleys <br />of the trunk streams contrast with the steeper and narrower valleys of their principal tributaries <br />such as Hieroglyphic Canyon and the dendritic, youthful drainages of the headwaters of those <br />tributaries. <br />Bedrock in the site region consists of a thick sequence of marine and continental sedimentary <br />rocks, representing the Paleozoic and Mesozoic Systems. Tertiary age sediments, if any existed, <br />have been eroded. Mesozoic sedimentary rocks are well exposed in the canyon walls surrounding <br />Uravan. The sedimentary rocks range from Triassic to Cretaceous in age and include the Chinle, <br />Wingate, Kayenta, Navajo, Carmel, Entrada, and Surttmetville Formations. In addition, the Salt <br />Wash and Brushy $asin Members of the Momson Formation and the Dakota /Burro Canyon <br />Formation crop out in the Uravan area. The geology is described in further detail in Appendix A <br />of the Final Plans and Specifications for Remedial Activities the B-Plant Repository (Umetco <br />August, 1998). <br />The rocks at the site dip gently eastward, about 1 to 3 degrees, homoclinally toward the axis of the <br />northwest trending Nucla Syncline which lies to the northeast near the site, between the Salt <br />Anticline region of the Paradox Valley and the southwestern flank of the Uncompahgre Uplift. <br />The outcrops are regular and the apparent dips and apparent thicknesses of each rock unit are <br />consistent and even on the steep slopes of the canyons all the way around Club Mesa. No abrupt <br />changes in stratigraphic thicknesses, faults or folds are observed or indicated by the geologic <br />mapping of the nearly continuously exposed rocks in the Uravan area. <br />4.3 Geoloeir, Stability <br />The site repositories are Situated on the erosionally resistant Salt Wash Sandstone within a <br />geomorphically stable area on Club Mesa. There is 120 feet of Salt Wash Sandstone underlying <br />the bottom of the site repositories. The Salt Wash Sandstone is the caprock of Club Mesa. The <br />principal geomorphic process that could affect the stability of the repositories is lateral erosion of <br />the mesa rims. This process is known as rim retreat. The rate of rim retreat in the Uravan Arca <br />has bem estimated using rwo independent methods as 1 to 4 feet per one thousand years. Studies <br />based on packrat midden geochronology establish site area rates of cliff retreat of 1 to 4 feet per <br />thousand years. The second method, produced independently of the packrat midden study <br />involved an estimate valley widening based upon the regional rate of downcutting. The <br />mechanisttt for cliff retreat involves the failure of the sandstone cliff rims along fractures that <br />parallel the valley. The fractures are spaced about 4 to 8 feet apart. At some point in time, a <br />portion of the tim fails, resulting in some small rock falls. The most significant concerns are <br />rockfalls that would occur from the rim on which the repositories reside. Rockfalls that occur <br />Umetco-Uravan DA February I I, 2000 t1 <br />FT/80 39dd CIDfIN daW ?J3110U LBZLb980L6 9L~TT L00b/5t /90 <br />