The Summerville Formation is a geological formation in New Mexico, Colorado, and Utah of the Southwestern United States. It dates back to the Oxfordian stage of the Late Jurassic.Wilcox 2007
Description The formation consists of up to of red mudstone, with thin interbeds of green and red sandstone. The lower portion of the formation shows polygonal desiccation cracks and localized salt-hopper casts while the upper portion contains considerable gypsum, consistent with deposition in a sabkha on the margin of the Sundance Sea.It is exposed in the San Rafael Reef, the Waterpocket Fold, in the Henry Mountains, with additional exposures scattered across the region from the San Rafael Reef to the Paradox Basin, and in north-central New Mexico. The thin bedding is characteristic throughout the formation, but gypsum is not found in the San Juan Basin and some conglomerate is found on the south and southwestern margins of the formation.Lucas and Anderson 1992, p.81 The correlation of late Jurassic beds in northwestern New Mexico with the Summerville Formation in Utah has been questioned, and it has been suggested that they be assigned to the Beclabito Formation instead.
The Summerville Formation rests conformably on the underlying Curtis Formation (Utah and western Colorado) or Todilto Formation (southwest Colorado and New Mexico) but is separated from the overlying Morrison Formation by the regional J5 unconformity. It thins significantly in the Moab-La Sal area, the likely area of the divide between the marine Curtis basin to the northwest and the salina lake Todilto basin to the southeast. Here the formation is just thick and rests directly on Entrada Sandstone. In many locations the Summerville is separated from the Morrison by eolian sandstones, such as the Bluff Sandstone, variously assigned to the Morrison Formation or the San Rafael Group. The Morrison Formation represents a return to more humid conditions with increased clastic input.Lucas and Anderson 1992, p.80
The Bell Ranch Formation of northeastern New Mexico is correlative with the Summerville Formation.Lucas and Anderson 1992
The Summerville Formation is interpreted as recording a regression of the Sundance Sea to the north, with simultaneous infilling of both the Curtis and Todilto basins. This produced a depositional environment of very low relief, in which occasional encroachments of eolian sand during times of drought were subsequently worked into thin strata by wave action in ephemeral saline lakes.
Fossil content The formation is almost completely lacking in body fossils.Lucas and Anderson 1992, p.84 However, theropod tracks were found in the formation in Utah, New Mexico, and Colorado.Weishampel 2004
Avemetatarsalians Avemetatarsalians of the Summerville Formation Genus Species Location Stratigraphic position Material Notes Images PteraichnusLockley et al. 2008 P. saltwashensis Arizona Pterosaur trackways. Includes the P. saltwashensis type locale. Specimens housed at the University of Utah. These tracks may actually be from the Morrison Formation. Indeterminate Arizona, near Summerville Also present in Colorado (Cactus Park, Furnish Canyon, Colorado National Monument) and Utah (Delmonte Mines, Starr Springs, Flaming Gorge, Ferron, Moab, Duchesne County). Summerville, Furnish Canyon, Cactus Park, Delmonte Mines, Starr Springs, Ferron, and Moab specimens are housed in the University of Colorado's Dinosaur Tracks Museum in Denver. The Flaming Gorge and Colorado National Monument tracks remains in the field. Ferron specimens are also housed at the University of Utah.
History of investigation
The formation was first defined in 1928 for exposures in the San Rafael Reef.Gilluly and Reeside, 1927 Similar beds in northeastern Arizona and the San Juan Basin are assigned to the Wanakah Formation,Condon et al. 1988 but continue to be assigned to the Summerville Formation in north-central New Mexico.Koning and Read 2010
See also
List of dinosaur-bearing rock formations Geologic formations of the United States
Footnotes
References
Lockley, M.; Harris, J.D.; and Mitchell, L. 2008. "A global overview of pterosaur ichnology: tracksite distribution in space and time." Zitteliana. B28. p. 187-198. .
Weishampel, David B; et al. (2004). "Dinosaur distribution (Middle Jurassic, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 537–538. .