A geologic formation in New Mexico
| Gilman Conglomerate | |
|---|---|
| Stratigraphic range: Oligocene PreꞒ Ꞓ O S D C P T J K Pg N | |
 Gilman Conglomerate at Gilman, New Mexico | |
| Type | Formation |
| Underlies | Abiquiu Formation |
| Overlies | Abo Formation |
| Thickness | 59 m (194 ft) |
| Lithology | |
| Primary | Conglomerate |
| Location | |
| Coordinates | 35°43′03″N 106°46′01″W / 35.7175607°N 106.7669774°W / 35.7175607; -106.7669774 |
| Region | New Mexico |
| Country | United States |
| Type section | |
| Named for | Gilman (settlement) |
| Named by | Kelley et al. |
| Year defined | 2009 |
  Gilman Conglomerate (the United States)Show map of the United States Gilman Conglomerate (New Mexico)Show map of New Mexico | |
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 (194 feet).
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.
Footnotes
References
- DuChene, H.R. (1973). Structure and stratigraphy of Guadalupe Box and vicinity, Sandoval County, New Mexico . Albuquerque: University of New Mexico.
- Kelley, S.A.; Kempter, K.A.; McIntosh, W.C.; Maldonado, F.; Smith, G.A.; Connell, S.D.; Koning, D.J. (2009). "Stratigraphy and paleogeography of Oligocene to early Miocene sedimentation in the Jemez Mountains region, north-central New Mexico". New Perspectives on the Rio Grande Rift: from Tectonics to Groundwater. Geological Society of America. ISBN 9780813724942.
- Maldonado, Florian; Kelley, Shari A. (February 2009). "Revisions to the stratigraphic nomenclature of the Abiquiu Formation, Abiquiu and contiguous areas, north-central New Mexico" (PDF). New Mexico Geology. 31 (1). Retrieved 9 June 2020.
- Osburn, G.R.; Kelley, Shari; Rampey, Michael; Ferguson, Charles; Frankel, Kurt; Pazzaglia, Frank (May 2002). "Preliminary geologic map of the Ponderosa 7.5-minute quarangle, Sandoval County, New Mexico" (PDF). New Mexico Bureau of Geology and Mineral Resources Open Map Series. DM-57b. Retrieved 9 June 2020.