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Oklahoma and are rarely penetrated by water wells in Colorado. They are shown as the basal unit in the cross-sections. The top of the Permian is generally based upon the electric logs or from American Stratigraphic Log picks (oil field lithologic logs), and forms the base of the Dockum Formation. 6.2.3 Dockum Group The Dockum Group outcrops only in the deep canyons of Black Mesa in southwestern Baca County and southeastern Las Animas County; at Two Buttes Reservoir; and in limited exposures adjacent to the Cimarron River in southeastern Baca County. These Triassic red beds are mostly interbedded red siltstones, shales and limestones with red to buff, fine- to coarse-grained sandstones. The lithology of the group varies significantly at each of these outcrops. The Dockum is correlative with the Lykins Formation to the north in central Colorado and may be confused with the previously described Taloga near Two Buttes. The Group is rarely penetrated completely by wells or erosion, and is estimated to be between 400 and 570 feet thick in Baca County. Although discounted as an effective aquifer in much of the literature and rarely described, the Dockum has become an important aquifer in southeastern Colorado. Many high volume irrigation wells in southeastern Baca County are completed open hole in the upper Dockum. As shown specifically in cross-section C — C', the Cretaceous and Jurassic formations unconformably onlap the Dockum, and thus the aquifers effectively recharge the Dockum in a downdip and southeasterly direction. Recognizing the unknown character of the lower Dockum and its unknown lateral continuity, this report has only attributed water resources to the Dockum in the upper portions that correspond to known well completions. Thus there may exist significant additional resources of water in the lower part of the formation. The quality and quantity of this resource is unknown and the greater depth of the formation to the north and west will necessitate the drilling of several relatively expensive "exploratory" water wells before ascribing water reserves to it. 6.2.4. Jurassic Undifferentiated Rocks of Jurassic age underlying the Basin include the Entrada Sandstone and Morrison Formation. These outcrop only in the canyons in the southwestern part of the Basin and adjacent to Two Buttes and are undifferentiated in the cross-sections of this study. Both formations are missing in eastern Baca County where they were truncated prior to deposition of Cretaceous rocks. The Entrada unconformably overlies the Dockum and ranges in thickness from approximately 380 feet in the north near Two Buttes to non- existent in the east where it pinches out on top of the Dockum. The Entrada is VI 2 99-028.003\Phase 2 Report\Hydrogeology