Bluff Formation

The Bluff Formation is a geological formation found in the Four Corners area. It was deposited in the late Jurassic Period.

Bluff Formation
Stratigraphic range: late Jurassic PreꞒ Ꞓ O S D C P T J K Pg N
Bluff Formation at its type area, Bluff, Utah, US
Type	Formation
Unit of	San Rafael Group
Underlies	Morrison Formation
Overlies	Summerville Formation
Thickness	350 ft (110 m)
Lithology
Primary	Sandstone
Other	Mudstone
Location
Coordinates	37.2893°N 109.5518°W / 37.2893; -109.5518
Region	Four Corners
Country	United States
Type section
Named for	Bluff, Utah
Named by	Baker, Dane, and Reeside
Year defined	1936
Bluff Formation (the United States) Show map of the United States Bluff Formation (Utah) Show map of Utah

Description

The Bluff Formation consists of massive wind-deposited (eolian) sandstone beds at its type location at Bluff, Utah. These cap the cliffs north of town and are 100–350 feet (30–107 m) thick.^[1] Further to the southeast, the sandstone beds lack high-angle cross beds, being dominated instead by horizontal bed forms, and are overlain by mixed sandstone and shale beds of the Recapture Member. The Recapture Member has been variously assigned either to the overlying Morrison Formation^[1] or to the Bluff Formation.^[2][3] The Recapture Member is separated from the Salt Wash Member of the Morrison Formation by the regional J-5 unconformity.^[4]

History of investigation

The formation was first named as the Bluff Sandstone by A.A. Baker, C.H. Dane, and J.B. Reeside, Jr., in 1938. They assigned it to the lower Morrison Formation.^[5][1] J.W. Harshbarger, Charles Repenning, and J.H. Irwin promoted it to formation rank in 1957,^[6] but this was not universally accepted.^[7][8] Spencer G. Lucas advocated both for formation rank and for assigning the Recapture Member to the Bluff Sandstone.^[3] Steven M. Cather agreed with promotion of the Bluff Sandstone to formation rank but advocated leaving the Recapture Member with the Morrison Formation.^[9]