Gilman Conglomerate

The Gilman Conglomerate is a geologic formation in northern New Mexico dating to the Oligocene epoch.

Description
The formation is composed of poorly sorted, weakly bedded, greenish-gray volcaniclastic conglomerate. Its lowermost and uppermost sections are a pinkish color from their content of Proterozoic pebbles, but the bulk of the formation is dominated by a mixture of crystal-rich porphyritic dacite and crystal-poor porphyritic andesite. Radiometric dating yields ages of 28.55 to 29.38 Ma for the clasts. Paleocurrents are to the north, suggesting a source in an Oligocene volcanic field now buried under the southern Jemez Mountains or in the northern Albuquerque Basin.

The formation crops out in a limited area of the southwestern Jemez Mountains. Maximum thickness is 59 meters.

The formation rests unconformably on the Abo Formation and has a gradational contact with the overlying Abiquiu Formation. It occupies the same stratigraphic position as the Ritito Conglomerate, and was originally included in that formation, but its composition and paleocurrent directions point to a quite different source provenance.

History of investigation
The unit was originally included in the Ritito Conglomerate by H.R. DuChene in 1973. It was broken out as a separate formation by Shari Kelley and her coinvestigators in 2009.