ammonites
Collection 18720-PC, Monroe Canyon Lmst., Cherty Lmst. Mbr., Chesterfield Range (Carboniferous to of the United States)

Where: Idaho (43.8° N, 111.5° W: paleocoordinates 1.6° N, 41.7° W)

• small collection-level geographic resolution

When: Spirifer brazerianus zone, Cherty Limestone Member (Monroe Canyon Formation), Meramecian to Meramecian (345.0 - 336.0 Ma)

• The Mississippian rocks in the Chesterfield Range consist of Lodgepole limestone overlain by a 1900ft. limestone-sandstone sequence here designated the Chesterfield Range Group. The lower terrigenous part, about 965 ft. thick is the Little Flat Fm., it has a basal siltstone member, a middle sandstone member, and an upper sandy limestone member. The upper part of the Chesterfield Range Group, here named the Monroe Canyon Limestone is about 925 ft. thick and contains a lower massive member and a medial medium-bedded member, and an upper cherty member. The range stretches northwest from Soda Springs, Idaho. It is an eastern dipping block of Paleozoic strata irregularly overlapped by the Tertiary Salt Lake Fm.

Environment/lithology: shelly/skeletal, cherty/siliceous limestone

• The main ridge-former in the Chesterfield Range is the massive bedded Monroe Canyon Limestone. This 924ft., predominatley carbonate rock sequence conformably overlies the Little Flat Fm., and like that Fm., contains three members. The lowest, massive limestone member is 408ft. thick and is composed of fossiliferous fine to coarse-grained, crinoidal limestone. Individual beds average about 3 ft. thick. Large horn corals are especially abundant in this member. The medium-bedded limestone member, 205 ft. thick, consists of mostly fine-grained limestone in beds about 1 ft. thick. This member is very fossiliferous, contains much bioclastic debris and forms most of the backslope on the east side of the main ridge of the Chesterfield Range. The uppermost member, about 311ft. of medium-bedded, cherty limestone, forms a subsidiary ridge along the east side of the Range. The cherty limestone member is composed of highly fossiliferous medium-grained limestone in beds 1-2ft. thick. Most of the chert is concentrated in the lower half of the member and grain size of the limestone tends to increase upward.

Primary reference: J. T. Dutro and W. J. Sando. 1963. New Mississippian Formations and Faunal Zones in Chesterfield Range, Portneuf Quadrangle Southeast Idaho. Bulletin of the American Association of Petroleum Geologists 47(11):1963-1986 [T. Olszewski/M. Gibson] more details

PaleoDB collection 12906: authorized by Tom Olszewski, entered by Melissa Gibson on 02.01.2002