Ignacio Formation

Ignacio Formation
Ignacio Formation
Stratigraphic range: upper Devonian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Formation
Sub-units	Spud Hill Member; Tamarron Hill Member
Underlies	Elbert Formation
Overlies	Precambrian basement
Thickness	32 m (105 ft)
Lithology
Primary	Sandstone, mudrock
Other	Carbonate rock, conglomerate
Location
Coordinates	37°34′59″N 107°47′56″W / 37.583°N 107.799°W
Region	Four Corners
Country	United States
Type section
Named for	Ignacio Lake
Named by	Cross and Spencer
Year defined	1899
	Ignacio Formation (the United States) Ignacio Formation (Colorado)

The Ignacio Formation (Ingacio Quartzite) is a

upper Devonian in age, based on detrital zircon geochronology

and other evidence.

Description

The Ignacio Formation is a complex and diverse formation, made up mostly of sandstone and mudrock with smaller amounts of carbonate rock, conglomerate, and evaporites that have been replaced by other minerals. The formation crops out in the San Juan Mountains of Colorado^[1] but may be present in the subsurface over a much wider region,^[2] including the Paradox Basin.^[3] The maximum thickness is about 32 meters (105 ft).^[4] The formation is overlain by the Elbert Formation.^[1]

The formation is divided into a lower Tamarron

Member and upper Spud Hill Member. The latter shows more shale and trace fossils than the former.^[4]

The age of the Ignacio Formation has been controversial. It was assigned to the

late Devonian in age, based on discovery of an Ordovician zircon grain in the formation and the presence well-dated placoderm fish plates.^[4] This conclusion has been shared by some subsequent researchers.^[6]^[1]

The formation is interpreted as

longshore current to the north.^[1] The area was at the western edge of the Transcontinental Arch.^[4]