SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environmentsmall earthquakes with generally undetectable effects (CH2MHill 2005). According to the ColoradoGeological Survey (CGS) and USGS, faults in the FTC area could have a low to moderate potentialfor causing damaging earthquakes (CGS 1999, USGS 2005). It is estimated that several thousandfaults within the state have not been extensively mapped or studied; therefore, an accurate estimationof timing or location of potentially dangerous earthquakes is not possible (CGS 1999).Since 1973, most earthquakes within 60 miles of FTC registered at a magnitude of less than 4.0 onthe Richter Scale. The largest earthquake in the area was recorded at a magnitude 4.0 approximately75 miles from the center of FTC (USGS 2005). The Oil Creek fault, located northeast of FTC, is potentiallyassociated with the Divide earthquake that occurred in the vicinity in 1979. The Oil Creekfault may also be associated with other nearby faults based on its present-day northeast-southwestdirectedstress regime orientation (CGS 1997).No landslides are known to have occurred on FTC within the past few years (USACE 2007a), althoughlocalized slides resulting in hillwash have occurred in the past given the colluvium nature ofsoil deposits.3.3.1.2 Pinon Canyon Maneuver SitePhysiographyThe PCMS is located within the Raton basin, along the western margin of the Great Plains PhysiographicProvince. Topographic features, such as mesas, cuestas, dissected plateaus, deep canyons,and volcanic formations, are the typical landscape within this section. The basin gradually slopesdownward, to the east, with elevations ranging from 5,500 feet amsl in the west to 2,500 feet amsl inthe east (USACE 2002a). The topography of PCMS is divided into four general regions, as shown onFigure 3-1 of the PCMS Master Final Transformation EIS (USACE 2007a). Woodlands made up ofprimarily piñon pine and juniper cover limestone highlands in the north and northwest. The Hogback,which consists of a basalt dike of volcanic origin, runs east-to-west along the southern boundary ofthe PCMS. Grassy plains cover the area between the Purgatoire River and the woodlands. The fourthregion along the eastern boundary of PCMS consists of canyons that drain to the Purgatoire River.Elevations on the PCMS range from 4,262 to more than 5,576 feet amsl (DECAM 2002a).GeologyThe Raton basin is one of a series of intermontane basins that developed during the late Cretaceousand early Tertiary (approximately 66 million years ago) along the eastern margin of the Rocky Mountainforeland, because of compression associated with the Laramide Orogeny. Numerous volcanoesintruded the Raton basin, forming lone mountain peaks. Volcanic vents, cinder cones, and lava fieldstypify the geology of the area. Geologic structures at the PCMS are generally associated with the ApishapaUplift, which is oriented southeast to northeast across the southern portion of the PCMS.Sedimentary rocks associated with the uplift typically dip northeast ranging from 1 to 3 degrees up to36 degrees (DECAM 2002a). The Black Hills (5,365 feet amsl), Sheep Canyon, and Muddy CreekMonoclines (strata inclined in the same direction) are major smaller structures within the PCMS. Severalsmaller synclines and anticlines are also associated with these monoclines, including the ModelAnticline in the western portion of the PCMS (Nakata Planning Group, LLC 2000).SoilsSoil types commonly occurring in the Raton section are aridisol and entisol soils. These soil types arecharacterized by moderate to severe soil erodibility, landslides, and unstable clay formation movementattributable to variations in moisture content and temperature (USACE 2002a). NRCS recog-February 2008 3–141 2/25th SBCT Final EIS