2021-10-11_REVISION - M1973021 (5)

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4 Aggregate Industries – Morrison Quarry GEI Consultants, Inc. AM-07 Geotechnical Stability Exhibit November 2020 2. Geology 2.1 Regional Geology Morrison Quarry is situated in the Colorado Front Range, within the Colorado Province of the southern Rocky Mountains. The Front Range forms the eastern margin of the southern Rocky Mountains, situated above the Denver Basin and High Plains to the east. The region generally marks the boundary between two major geological provinces of North America: The American Cordillera to the west and the stable North American Craton to the east. The Front Range is cored by metamorphic and crystalline Precambrian bedrock that was accreted to the southwest margin of the Wyoming Craton during the Proterozoic eon (about 2.5 billion to 541 million years ago). The chemistry and distribution of rock types suggests the Colorado Province is comprised of a juvenile island arc terrane (Selverstone and others, 1997). The oldest rocks in the Colorado Province are “supracrustal” metasedimentary and metavolcanic rocks. The supracrustal rocks are intruded by younger granitic units dated at about 1.7 billion years and 1.4 billion years ago. The Precambrian units are overprinted by north- to northeast-trending Proterozoic shear zones, several of which were reactivated during the Late Cretaceous Laramide Orogeny about 70 to 80 million years ago during the uplift that resulted in the modern-day Rocky Mountains. In many areas of the Front Range, the Proterozoic bedrock is overlain by a thick sequence of east-dipping Paleozoic and Mesozoic marine and non-marine sedimentary rocks. The Paleozoic and Mesozoic strata were deposited in shallow inland marine and deltaic environments and were later uplifted along with the Precambrian units during the Laramide Orogeny. The sedimentary rocks that overlie the Precambrian basement are exposed at the entrance to Morrison Quarry and at the Red Rock Amphitheater. Regional geologic mapping from the USGS of the Morrison and Indian Hills Quadrangles is shown in Figure 2. 2.2 Morrison Quarry Site Geology Aggregate Industries mines the Precambrian aged bedrock at Morrison Quarry for aggregate sales. The Precambrian units that are processed and sold are mostly granitic and biotitic gneiss. The mining also encounters localized zones of sillimanitic gneiss, which is a less- durable unit that is stockpiled on site and reclaimed. Foliations within the gneissic units undulate locally but generally strike in a northwest-southeast orientation and have a shallow, nearly horizontal dip. Foliations within the gneiss are cut by joints and inactive faults as well as mafic dikes and granitic intrusions (pegmatite) which can act as localized discontinuities in addition to the joints and fractures within the rock mass. In general, the rock blocks, boulders, and fragments at Morrison Quarry are very hard and clean, and the rock mass is highly fractured.