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6.2.10 Tertiary Volcanics <br />Intrusives: <br />Minor Tertiary -aged volcanics are present in the Basin and predate the Ogallala <br />Formation, which truncates some of the related volcanic dikes. The Two Buttes intrusive <br />is just north of the Baca County/District boundary in T. 27 N., R. 46 W. This intrusion has <br />lifted and exposed some of the deeper sediments and helps define the drainage divide. <br />Extrusives: <br />Several mesas in the southwestern part of the Basin are capped by basaltic lava flows. <br />The flow rocks range from 20 to over 100 feet thick and were deposited on top of the <br />Ogallala Formation. Thus, these extrusives are late Tertiary to Pliestocene in age and <br />chronologically should be listed after the Ogallala, but are noted here for continuity of <br />rock type. <br />6.2.11 Ogallala Formation <br />The Ogallala is a heterogeneous alluvial/colluvial deposit of unconsolidated sand, gravel, <br />silt and clay. It is locally bound by caliche or cemented with calcite or silica and contains <br />some freshwater limestones. The Ogallala once covered the entire Basin, but has been <br />eroded in major drainages to expose Cretaceous bedrock. The Ogallala ranges from 0 <br />to 310 feet thick in the Basin and is the dominant member of the High Plains Aquifer. It <br />thickens to the east and southeast into Kansas. <br />6.2.12 Quaternary Alluvium and Eolian Sediments <br />The alluvium includes stream deposits of silt, sand and gravel in and adjacent to major <br />stream valleys. The most significant alluvium is present in the Cimarron River valley in <br />the southeastern part of Baca County. Here alluvial deposits are reportedly up to 80 feet <br />thick. Some lesser alluvial sand and gravel occurs in the canyon bottoms and major <br />drainages in the southwestern part of the Basin. <br />Much of the south half of Baca County is covered with loess and dune sand that veneers <br />the uplands and masks the valleys. These eolian deposits are mostly wind blown silt <br />and fine sand, which range from 0 to 100 feet thick. <br />VI 5 <br />99-028.003\Phase 2 Report\Hydrogeology <br />