Great Divide Home Place
Mineral Remote Assessment

Local Geology — Surface geology consists of the Eocene Cathedral Bluffs Tongue of the Wasatch Formation; a unit consisting of light to medium-gray, red, pink, yellow, and maroon shale, claystone, and minor lenticular sandstone and oil shale (Miller, 1977; Scarbrough et al., 1999). At the subject parcel, the Wasatch Formation represents the uppermost local geologic unit. Beneath the Wasatch Formation lies the Fort Union, Lance, Williams Fork, and Iles Formations within the subsurface. As part of the Green River Coal Region of the Sand Wash Basin, coal layers are found in these formations.

Locatable Minerals — First discovered in 1891, placer gold deposits, with lesser placer silver, have been located along both the tributaries of the Yampa River on the southern slopes and tributaries of the Little Snake River on the northern slopes of Iron Springs Divide.  Productive placer gold deposits have been reported along Bighole Gulch, Scandinavian Gulch, Dry Gulch, Timberlake Creek, Fourmile Creek and their subtributaries on the north slopes of the divide; and Lay Creek, Big Gulch, Blue Gravel, and Fortification Creeks on the south slopes of the divide. 

The distribution of the placer deposits along Iron Springs Divide indicates that most of the gold was primarily derived from the headwaters area of the Timberlake and Lay Creeks.  Lesser amounts of gold have been derived from the headwaters of Blue Gravel Fortification, and Fourmile Creeks to the east; and the headwaters of Dry and Scandinavian Gulches to the west. 

Along each of these drainages, the apparent upstream termination of important placer deposits occurs at roughly 7,000 feet in elevation where bedrock consists of poorly consolidated sands and gravels within the lower part of the Cathedral Bluffs member of the Wasatch Formation. It has been suggested that the gold and lesser silver found in the Iron Springs Divide area are derived from paleo-placer deposits located within the lower Cathedral Bluffs member and possibly the underlying Tipton tongue of the Green River Formation (Parker, 1961). 

Within literature, deposits in this area are referred to as located within the Fourmile and Timberlake Mining Districts (Davis and Streufert, 2011; Dunn, 2015; Singel, 2018).  Placering operations included sluicing, hydraulic washing, and several bucket dredges along creek beds and adjacent terraces.  In some locations, placer gravels were mined from shallow trenches, pits, shafts, and tunnels. Production figures for operations are unavailable or incomplete.

Coal — The region contains substantial coal deposits, primarily in the Iles and Williams Fork formations of the Mesaverde Group, and in the Lance, Fort Union, and Wasatch formations (Carroll and Bauer, 2001; Carroll, 2006). The thickest and most continuous coal beds are found in the Fort Union and the Mesa Verde Group consisting of the Williams Fork and Iles Formations.

The Fort Union coals rank as subbituminous B and C, the lowest rank coals which have no recorded productive coalbed methane. Coals within the Mesa Verde group are ranked high-volatile C to high- volatile A bituminous, and have published gas content (Tremain, 1990). The depth of suitable coal resources in the Great Divide area is typically at and greater than depths of over 5,000 feet below the ground surface, making extraction by conventional underground mining generally uneconomical (Boreck et al., 1981).

Oil & Natural Gas — Within the area lies the Great Divide Gas Field within the Sand Wash Basin (Wray et al., 2002). Faults and stratigraphy provide reservoir trap control within the field, where the structural trap pinches to the east. The primary gas producing layer lies within the Middle Lewis Formation at depths between approximately 7,300 and 7,500 feet beneath the surface.

Locally, the Lewis Formation includes an alternating series of fine-grained discontinuous sandstones and thick impermeable shales which act as stratigraphic traps. Research indicates that the stratigraphic drainage areas are small; that the permeability is low; high rates of water production; and low gas recovery ranges between 2% to 12% have been cited as impediments to future development (Wray et al., 2002; Barkmann, 2011).

References —

Barkmann, P.E., 2011, Coalbed Methane Stream Depletion Assessment Study Sand Wash Basin, Colorado: Colorado Geological Survey WAT-2011-03, 117p. https://doi.org/10.58783/cgs.wat201103.xulc2550

Boreck, D.L., Tremain, C.M., Sitowitz, L., and Lorenson, T.D., 1981, The Coal Bed Methane Potential of the Sand Wash Basin, Green River Coal Region, Colorado: Colorado Geological Survey, 35 p., 4 plates. https://doi.org/10.58783/cgs.of8106.ywfz6710

Carroll, C. J., 2006, Coal Resource Maps of Colorado, Colorado Geological Survey Map Series 43, 4 maps. https://bit.ly/44OIIeB

Carroll, C. J., and Bauer, M.A., 2001, Historic Coal Mines of Colorado: Colorado Geological Survey Information Series 64, 55 maps, GIS database.  https://bit.ly/3q4uAyM

Crouch, M.C., 1982, Oil and Gas Fields of Colorado-Nebraska and Adjacent Areas: Rocky Mountain Association of Geologists, 804 pp. 180-183.  

Davis, M.W. and Streufert, R.K., 2011, Gold Occurrences of Colorado: Colorado Geological Survey Resource Series 28, 101 p., 2 plates. http://bit.ly/4olqUBZ 

Dunn, L.G., Colorado Mining Districts (revised), 2015, Arthur Lakes Library, Colorado School of Mines, p. 135.  http://dx.doi.org/10.25676/11124/170391

Miller, A.E., 1977, Geology of Moffat County Colorado: Colorado Geological Survey Map Series 3. https://coloradogeologicalsurvey.org/publications/geologic-map-moffat-colorado

Parker, B., 1974, Gold placers of Colorado: Colorado School of Mines Quart 69(4), p. 150-155, 159. 

Scarbrough, Jr., L.A., TerBest, H., and Hemborg, H.T., 1999, Evaluation of Mineral and Mineral Fuel Potential of NE Moffat County State Mineral Lands Administered by the Colorado State Land Board: Colorado Geological Survey – Open File Report 99-16. https://bit.ly/47DUook

Singel, K.A., 2018, Finding Gold in Colorado: Prospector’s Edition: A guide to Colorado’s casual gold prospecting, mining history and sightseeing, p. 231-236.  http://bit.ly/3UB1KBt

Tremain, C.M., 1990, Coalbed methane development in Colorado: Colorado Geological Survey – Information Series 32, 41 p.  https://doi.org/10.58783/cgs.is32.gfxy8481 

Wray, L.L., Apeland, A.D., Hemborg, H.T., and Brchan, C.A., 2002, Oil and Gas Fields Map of Colorado: Colorado Geological Survey Map Series 33, scale 1:500,000 https://bit.ly/4qYQJce