Named as lower member of Abo Formation for Oso Ridge in Zuni Mountains, Cibola Co, NM, San Juan basin. Type section (Sawyer section) designated in NW/4 sec 19, T11N, R13W where unit is best exposed. Contact with Proterozoic metamorphic rocks is exposed for many miles as shown in geologic maps of Smith (1958, NMBMMR Geologic Map 9); Smith (1959, NMBMMR Geologic Map 4) and Goddard (1966, USGS Map I-454). Consists of about 30 ft of conglomerate and arkose with thin, fossiliferous limestone lenses interbedded with arkose. At type (measured section, fig. 2) consists of a basal 8 ft of coarse clastics composed of weathered Proterozoic maroon to brownish-red pebble conglomerates, sandstones, and siltstones; limestones occur at 8-14 ft, 21-24 ft, and a thin bed at 30 ft consist of fossiliferous arenaceous packstones to grainstones; brownish red siltstones and sandstones separate the limestones. Nonconformably overlies Proterozoic rocks; conformably underlies unnamed part of Abo. Fossils present in limestones include abundant calcareous phylloid alga EUGONOPHYLLUM associated with a rich molluscan fauna of gastropods, pelecypods, and nautiloids. Limestones were deposited in estuarine or lagoonal environments. Fossils including brachiopods, mollusks, foraminifers, algae, and conodonts all suggest Late Pennsylvanian (Virgilian) and Early Permian (Wolfcampian) age.

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