Named as basal member of Entrada Sandstone for Iyanbito, a Navajo village, McKinley Co, NM in San Juan basin. Type locality measured in NW1/4 sec 15, T15N, R16W, Church Rock 7 1/2 min quad. These rocks were erroneously assigned to Wingate Sandstone in earlier reports. Is composed dominantly of reddish-orange, medium- to fine-grained, well sorted, subrounded to well-rounded, friable, cross-bedded sandstone. Basal sandstone is also coarse grained with pebbles and granules 6 mm (about 1/4 in) across of white, black, clear, gray and pink quartz. Interbeds are dark reddish-brown, laterally discontinuous claystone and siltstone. Is 38.2 m (125.3 ft) thick at type; maximum 44.2 m (145 ft) thick; averages 40 m (130 ft). Unconformably overlies Chinle Formation; underlies middle siltstone member of Entrada gradationally. Is unfossiliferous; assignment to Late Jurassic based on stratal and depositional relations with adjacent units. Is a continental, mostly eolian deposit.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Age of the Iyanbito Member [names not used in report] as the basal member of the Entrada Sandstone changed from Late Jurassic to Middle Jurassic on the basis of the assignment of the Entrada Sandstone to the Middle Jurassic. Iyanbito occurs in the San Juan basin of NM.

Source: GNU records (USGS DDS-6; Denver GNULEX).