SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentGeologyFRA is covered by Quaternary age glacial, glacio-marine (estuarine), and glacio-alluvial sedimentarydeposits, with bedrock outcrops occurring in the south and east along the Chugach Mountains. Thedeposits form a westward-thickening wedge beginning at the base of the Chugach Mountains to athickness of approximately 656 feet locally (Hunter et al. 2000). Based on well logs, the thickness ofsediments below the cantonment ranges from 230 to 322 feet (Cederstrom et al., 1964). Because theglacial sediments underlying FRA were deposited during multiple ice advances, the stratigraphy iscomplex, particularly under the cantonment, where sediments deposited along the south margin ofElmendorf Moraine interfinger with alluvial fan sands and gravels (Hunter et al. 2000). The mostcommon and spatially extensive deposits are end, lateral, and ground moraines; glacio-alluvial, alluvial,and alluvial fan deposits; and estuarine and lacustrine deposits. Loess (wind-deposited silt), colluvium(poorly sorted and uncompacted sand and gravel), and rock glaciers are less abundant in thehigh mountain valleys (Hunter et al. 2000).SoilsParent material that formed the various soil types on FRA varies widely and includes glacial moraines,glacial outwash, tidal flats, and peat bogs. These soils are shallow, recently formed, nutrientdeficient, and have low water-holding capacity, which are all factors that limit vegetative growth duringdry periods and limit the potential to reclaim land after surface disturbance. Surface soil horizonsmay be covered with peat in areas containing depressions, wetlands, and other saturated areas. TheNRCS soil survey of the Anchorage area (Moore 2002) identifies two distinct climate zones and associatedsoil types for FRA: the lowlands surrounding Anchorage, and the neighboring Chugach Mountains.(USARAK 2004)Along the coast and tidal plains, the Cook Inlet sediments are silty and clayey, with broad depressionsin the area occupied by poorly drained bogs and fens. The soils on the lowland plains inland from thecoast have less developed horizons because of lower precipitation, mid-winter thaws, and strong localizedwinds, with the exception of wind-protected forested areas (Moore 2002). The uplands at thebase of the Chugach Mountains are covered by a layer of silty loess, which is formed by deposition offine glacial sediments from the floodplains and volcanic ash. The portion of the FRA that encompassesthe Chugach Mountains contains soils that were formed because of weathering and leaching ofminerals, which was influenced by high annual precipitation, deep snowfall, strong localized winds,and deep annual frost (USARAK 2004).Permafrost is present on less than 1 percent of FRA, occurring primarily in patches of forested bogsalong Muldoon Road as well as in the higher elevations of the areas within the Chugach Mountains.The effects of thermokarst, described above as irregular subsidence of permafrost that causes mounds,hummocks, water-filled depressions, flooded forests, and mudflows on steeper slopes, have been lessthan 0.1 percent in the last 200 to 300 years in the FRA.Erosion ManagementU.S. Army Garrison Alaska (USAGAK) conserves and manages soil resources as the foundation ofother natural resources, through planning level soil and topographical surveys, soil resource monitoring,and soil resources rehabilitation and management strategies. The Draft Natural Resources Guidancefrom Army Chief of Staff for Installation Management (USARAK 2007) requires the installationto identify and map soils, correlate soils to permafrost areas, and establish relationships among componentsof terrain. The data from these efforts are required for input into the military training andscheduling process. AR 200–3 requires 10-year updates of topographical planning level surveys toimplement the INRMP, as mandated by the Sikes Act (USARAK 2007).February 2008 3–96 2/25th SBCT Final EIS