Editing San Luis Valley (section)

====Western Interior Seaway====
In the middle and late [[Cretaceous]] [[Geologic time scale#Terminology|Period]], about 100&nbsp;million years ago, the San Luis Valley lay on the shore of or beneath the [[Western Interior Seaway]], a shallow sea which divided North America into two parts. As the sea deepened and expanded sand and gravel eroded from mountains in the west was deposited in the [[Dakota Formation]]; later, when the sea was deeper mud and sand were deposited in the [[Mancos Shale]]. The Lexam [[petroleum play|oil play]] near Crestone is based on the hypothesis that there is oil trapped in Dakota Formation sandstones lying beneath Mancos Shale to the west of the Sangre de Cristo fault at the western base of the Sangre de Cristo Range southwest of Crestone. Drilling at the base of the range during gold exploration in the vicinity showed small amounts of oil and evidence of the existence of those formations as well as the underlying [[Morrison Formation]]. Prior to this discovery it was assumed that these formations had eroded away during the [[Laramide orogeny]].<ref>{{cite web |title=Geology of the northeastern San Luis Basin, Saguache County, Colorado |url=http://geosurvey.state.co.us/education/Documents/Geology_of_the_Northeastern_San_Luis_Basin,Saguache_County,Colorado.PDF |archive-url=https://ghostarchive.org/archive/20221009/http://geosurvey.state.co.us/education/Documents/Geology_of_the_Northeastern_San_Luis_Basin,Saguache_County,Colorado.PDF |archive-date=2022-10-09 |url-status=live |publisher=Colorado Geological Survey |access-date=September 13, 2011 |first=Thomas A. |last=Watkins |df=dmy-all}}</ref>