Pictured Cliffs Formation

The formation consists of yellowish-gray to grayish-orange cliff-forming sandstone. The upper beds are massive and crossbedded, consisting of well-sorted, fine-grained, friable sandstone. The lower beds are alternating thin beds of sandstone and light- to dark-gray silty shale. The upper part of the shale beds contains up to 6.5 feet (2.0 m) of discontinuous ironstone beds. The total thickness is about 60 feet (18 m). The formation is transitional with both the underlying Lewis Shale and the overlying Fruitland Formation.[1]

The formation is interpreted as a marine littoral formation deposited during the final regression of the Western Interior Seaway from the San Juan Basin, with recognizable delta-front and barrier facies.[3] The regression was interrupted by at least three brief transgressions that produced tongues of the upper Pictured Cliff Sandstone in the northern San Juan Basin.[4]

The formation contains fossil marine invertebrates and trace fossil Ophiomorpha major, tyical of a nearshore marine environment.[1]

The formation serves as a reservoir rock for natural gas. Production by 1988 amounted to 87.7 x 10⁹ cubic meters with remaining reserves estimated at that time as 127.4 x 10⁹ cubic meters. The gas likely originated both in the underlying Lewis Shale and the coal beds of the overlying Fruitland Formation.[5]

The formation was first described as the Pictured Cliffs group or Pictured Cliffs sandstone by W.H. Holmes in 1877.[6]

• List of dinosaur-bearing rock formations
  • List of stratigraphic units with indeterminate dinosaur fossils

• Ambrose, William A.; Ayers, Walter B. (August 2007). "Geologic controls on transgressive-regressive cycles in the upper Pictured Cliffs Sandstone and coal geometry in the lower Fruitland Formation, northern San Juan Basin, New Mexico and Colorado". AAPG Bulletin. 91 (8): 1099–1122. doi:10.1306/03080706040.
• Flores, Romeo M.; Erpenbeck, Michael F. (April 1981). "Differentiation of delta-front and barrier lithofacies of the upper Cretaceous Pictured Cliffs Sandstone, southwest San Juan Basin, New Mexico". The Mountain Geologist. 18 (2): 23.34. Retrieved 16 September 2020.
• Holmes, W.H., 1877, Report [on the San Juan District, Colorado], IN Hayden, F.V., Ninth annual report of the United States Geological and Geographical Survey of the Territories, embracing Colorado and parts of adjacent territories, being a report of progress of the exploration for the year 1875; Part I, Geology: U.S. Geological and Geographical Survey of the Territories (Hayden), Annual Report, 9, p. 237-276, pls. 34-49., Reprint, 1873
• Scott, Andrew R.; Kaiser, W.R.; Ayers, Walter B., Jr. (1991). "Composition, distribution, and origin of Fruitland Formation and Pictured Cliffs Sandstone gases, San Juan Basin, New Mexico and Colorado". Coalbed methane of western North America. Rocky Mountain Association of Geologists. Retrieved 16 September 2020.
• Scott, G.R.; O'Sullivan, R.B.; Weide, D.L. (1984). "Geologic map of the Chaco Culture National Historical Park, northwestern New Mexico". U.S. Geological Survey Miscellaneous Investigations Series Map. I-1571. doi:10.3133/i1571.
• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.