Rifting and the Passive Margin
by Paul Link & Laura DeGrey, Idaho State University
Neoproterozoic and Paleozoic Passive Margin
Field Trip Guides
PDF Slideshows: Paleozoic rocks north of Snake River Plain and South of Snake River Plain
Vocabulary Words to Know
Neoproterozoic and Paleozoic Passive Margin
The rocks involved in the Idaho-Wyoming Thrust Belt are generally thick packages of sedimentary strata, deposited along the passive Continental margin of western North America that started to form in late Neoproterozoic time, about 700 Ma. The figure below shows a cartoon of the geometry of these strata. This tectonic unit is also known as the Cordilleran miogeocline.
After rifting in the late Neoproterozoic, and thermal subsidence in the early
Paleozoic (Cambrian through Devonian time), the passive margin was compressed
in latest Devonian time by the Antler orogeny, and modified by areas of major
subsidence during the Pennsylvanian to Permian Ancestral Rockies orogeny.
This story is summarized in Historical Geology textbooks (Prothero and Dott,
2004, and elegantly by W.R. Dickinson (2004) in Earth Science Reviews. In
Mesozoic time the passive margin rocks were progressively compressed and thrusted
eastward as part of the Cordilleran orogeny, forming the Idaho-Wyoming thrust
See Link and Janecke, 1999 for a summary of the Paleozoic development of Idaho north of the Snake River Plain.
Passive Margin Sediments in Idaho
Fortsch and Link, 1999 provides a field trip description to see the rocks in the stratigraphic column below east of Pocatello.
Stratigraphic columns from Rocks, Rails, and Trails (Link and Phoenix, 1996). Click on figure above for a larger version.
Late Proterozoic Sedimentary Rocks
The following text is revised from Link and Phoenix (1996). The oldest rocks in the southeast Idaho thrust belt belong to the Pocatello Formation (720 to 660 million years old), which is exposed in the Bannock Range from the Pocatello area south through Oxford Mountain and almost to the Utah border. The Pocatello Formation contains two unique rock types, metamorphosed basaltic lava or greenstone, and dark-brown or gray rock known as diamictite, which contains clasts of older rocks, ranging from pebbles to boulders in size, floating in a matrix of sand and clay. Some of this diamictite was deposited as glacial-marine till during the Sturtian glaciation, one of the worldwide Neoproterozoic ice ages (Lorentz and Corsetti, 2007). Uranium-Lead (U-Pb) ages of 709 to 670 Ma are reported from the Pocatello Formation by Fanning and Link (2004).
The Brigham Group overlies the Pocatello Formation, and contains mainly sandstone or quartzite. The Brigham Group is exposed in much of the Portneuf, Bannock, and Bear River Ranges. The Wilbert Formation in the southern Beaverhead and Lemhi Ranges is correlative. The sediments which now make up the Brigham Group were deposited from about 650 to about 530 million years ago, during the time when complex life forms were developing. However, the sandstones of the Brigham Group were inhospitable places for latest Proterozoic and early Cambrian life to leave fossilized remains, and so the Brigham is largely devoid of fossils. The Brigham Group was deposited in shallow oceans and wide alluvial plains near the shorelines of those oceans.
Since the Brigham Group was deposited during Ediacaran (latest Precambrian) time, it contains records of sea-level changes during the Marinoan and Gaskiers glaciations (Levy et al., 1994), unusual excursions in the carbon-isotope compositions of sea water (Smith et al., 1994; Lorentz et al., 2004), and also could contain soft-body fossils of Ediacaran organisms. These have not yet been found, however.
Some of the micaceous quartzite or "Oakley stone" found on the flanks of the Albion Range south of Burley is probably correlative to parts of the Brigham Group. The distinctive bright green Elba Quartzite, now quarried at Park Valley, Utah, is older, of Middle Proterozoic age. It has been metamorphosed so that the chromium rich clay minerals have recrystallized to green, shiny plates of mica.
Paleozoic Limestones and Sandstones
A thick sequence of carbonate rocks (limestone and dolomite), of early Paleozoic age (530 to 300 million years) lies above the Brigham Group and is present in most of the mountains of eastern Idaho. These rocks were deposited on broad carbonate platforms in warm seas off the western edge of the paleo-North American continent, settings not dissimilar to today's Bahama Banks or Great Barrier Reef. In many places the Paleozoic limestones contain invertebrate fossils, including trilobites, bryozoans, brachiopods, corals, and molluscs, which allow determination of their age and the precise conditions under which they were deposited (Isaacson and Dorobek, 1988; Isaacson et al., 1988).
Surface streams are rare in the limestone country. The limestone cliffs are highly porous and most of the water sinks into the subsurface. In places extensive cavern systems exist, as at Minnetonka Cave west of St. Charles. Large springs form where the subsurface water (groundwater) intersects the land surface. Such springs supply important parts of the municipal water supplies of southeast Idaho. They are easily contaminated and their purity must be guarded carefully.
Thin units of fine-grained sandstones or quartzite are present within this stack of Paleozoic limestones in the Swan Peak Quartzite of Ordovician age (James and Oaks, 1977; Oaks et al., 1977) and the Wells Formation of Pennsylvanian age. These sandstones represent times when wedges of quartz sand, eroded from distant mountain ranges, prograded over the carbonate platforms.
Also present in some limestone units are gray and black chert beds and nodules, which weather to distinctive pebbles of hard flint. This chert formed from dissolved skeletons of silica-secreting invertebrates, which also existed in the warm shallow seas.
Some of the limestone has been altered to the crystalline carbonate rock dolomite. In this process original fossils are generally destroyed and the result is a light or dark gray rock that preserves bedding structures but little of the fine detail found in limestone.
In southeast Idaho passive margin conditions persisted through Permian time. In the Permian unique phosphate-bearing black shales of the Phosphoria Formation were deposited in a restricted circulation basin that had high primary surface productivity (Piper and Link, 2002).
Copper Basin Group-Antler Orogeny
However, in central Idaho, sedimentary relations were modified starting in the latest Devonian time, with the Antler orogeny, and again in Pennsylvanian and Permian time, when the Wood River basin developed, in response to the Ancestral Rockies orogeny so the southeast in Colorado and new Mexico (Dickinson, 2004).
The Antler orogeny produced a thrust fault in central and northern Nevada, the Roberts Mountains thrust. This fault placed deep water, western siliceous facies continental rise strata over eastern carbonate facies carbonate rocks, and uplifted the Antler highland, which then shed sediment into a subsiding foreland basin to the east. In central Idaho the Antler event produced the thick conglomeratic Copper Basin Group (Wilson et al., 1994; Link et al., 1996) which contains evidence of rapid subsidence, and coeval normal faulting, perhaps in an oblique normal (transtensional) structural setting.
The Copper Basin Group contains up to 3 km of strata now exposed in the Pioneer Mountains between Ketchum and Mackay. Clasts in the Copper Basin Group include Ordovician quartzites of the Kinnikinic formation, and black cherts, likely sourced from the Devonian Milligen Formation from the area west of Ketchum.
Sun Valley Group-Wood River Basin
The Milligen Formation of the Antler highland subsided in Pennsylvanian time, and the Wood River Basin formed above it. The Sun Valley Group, which contains mixed carbonate and siliciclastic rocks that underlie the Sun Valley ski area on Bald Mountain and are exposed in the Boulder and White Cloud Mountains (Mahoney et al., 1991; Link et al., 1995) was deposited into this deeply subsiding Wood River basin that was part of the larger Oquirrh Basin system of northern Utah (Geslin, 1998).
Subsidence of the Oquirrh-Wood River basin may have been controlled by interaction between compression of the Ancestral Rockies orogeny to the south and renewed uplift and isostatic loading of the Antler highland to the west.
Continue to Module 4 - Accreted Terranes
Field Trip Guides
Southeast Idaho thrust belt
Fortsch, D. E., and Link, P. K., 1999, Regional Geology and Fossil Sites from Pocatello to Montpelier, Freedom, and Wayan, Southeastern Idaho and Western Wyoming, in Hughes, S. S., and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, Pocatello, p. 281-294.
Chidsey, T.C., Jr., Crook, S.R., and Link, P.K., 1985. Overthrusts and stratigraphy in the Wasatch, Bear River, Crawford Ranges and Bear Lake Plateau, north-central Utah. Field Conference Road Log, Day 1, in Kerns, G., and Kerns, R., eds., Orogenic Patterns and Stratigraphy of North-Central Utah and Southeastern Idaho, Utah Geological Association Publication 14, p. 269-290.
Crook, S.R., Link, P.K., and Chidsey, T.C., Jr., 1985. Structure and stratigraphy of the Paris and Meade thrust plates and transition to the Basin and Range Province: Bear River, Preuss and Bannock Ranges, southeastern Idaho. Field Conference Road Log, Day 2, in Kerns, G., and Kerns, R., eds., Orogenic Patterns and Stratigraphy of North-Central Utah and Southeastern Idaho, Utah Geological Association Publication 14, p. 291-314.
Central Idaho thrust belt
Link, P. K., and Janeke, S. U., 1999, Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys, in Hughes, S. S., and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, p. 295-334.
Link, P.K., Skipp, B.A., Hait, M.H., Jr., Janecke, S. and Burton B.R., 1988, Structural and stratigraphic transect of east-central Idaho: Lost River, Pioneer, and Smoky Mountains: in Link, P.K., and Hackett, W.R., Guidebook to the geology of central and southern Idaho: Idaho Geological Survey Bulletin 27, p. 5-42.
Pocatello Formation and Brigham Group
Christie-Blick, N. Mount, J.F., Levy, M, Signor, P.W., and Link, P.K., 1989, Late Proterozoic and Cambrian tectonics, sedimentation, and record of Metazoan radiation in the western United States, Field Trip Guidebook T331, 28th International Geological Congress, Washington D.C., American Geophysical Union, 113 p.
Link, P.K., 1987, The Late Proterozoic Pocatello Formation; A record of continental rifting and glacial marine sedimentation, Portneuf Narrows, southeastern Idaho: in Beus, S.S. ed., Centennial Field Guide Volume 2, Rocky Mountain Section of the Geological Society of America, p. 139-142.
Link, P.K., and LeFebre, G.B., 1983, Upper Proterozoic diamictites and volcanic rocks of the Pocatello Formation and correlative units, southeastern Idaho and northern Utah: Utah Geological and Mineral Survey Special Studies 60, p. 1-32.
Link, P.K., Crook, S.R., and Chidsey, T.C., Jr., 1985. Hinterland structure, Paleozoic stratigraphy and duplexes of the Willard thrust system: Bannock, Wellsville and Wasatch Ranges, southeastern Idaho and northern Utah. Field Conference Road Log, Day 3, in Kerns, G., and Kerns, R., eds., Orogenic Patterns and Stratigraphy of North-Central Utah and Southeastern Idaho, Utah Geological Association Publication 14, p. 315-328.
Dickinson, W.R., 2004, Evolution of the North American Cordillera: Annual Review of Earth and Planetary Sciences: v. 32, p. 13-45, doi: 10:1145/annurev.earth.32.101802.120257.
Fanning, C.M., and Link, P.K., 2004, U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho: Geology, v. 32, no. 10, p. 881-884.
Geslin, J.K., 1998, Distal Ancestral Rocky Mountains tectonism: Evolution of the Pennsylvanian-Permian Oquirrh-Wood River basin, southern Idaho: Geological Society of America Bulletin, v. 110, no 5, p. 644-663.
Isaacson, P.E., and Dorobek, S.L., 1988, Regional significance and interpretation of a coral-stromatoporoid carbonate buildup succession, Jefferson Formation (Upper Devonian), east-central Idaho, in McMillan, N.J., Embry, A.F., and Glass, D.J., eds., 1988, Devonian of the World, Canadian Society of Petroleum Geologists, v. ii, p. 581-590.
Isaacson, P.E., Bachtel, S.L., and McFaddan, M.D., 1983, Stratigraphic correlation of the Paleozoic and Mesozoic rocks of Idaho: Idaho Bureau of Mines and Geology Information Circular No. 37.
Isaacson, P.E., McFaddan, M.D., Measures, E.A., and Dorobek, S.L., 1988, Coral-stromatoporoid carbonate buildup succession, Jefferson Formation (Late Devonian), central Idaho: U.S.A., in Geldsetzer, H.H.J, James, N.P., and Tebbutt, G.E., eds., Reefs: Canada and adjacent area: Canadian Society of Petroleum Geologists Memoir 13, p. 471-477.
James, W.C., and Oaks, R.Q., Jr., 1977, Petrology of the Kinnikinic Quartzite (Middle Ordovician), east-central Idaho: Journal of Sedimentary Petrology, v. 47, p. 1491-1511.
Levy, M, Christie-Blick, N., and Link, P.K., 1994, Neoproterozoic incised valleys of the eastern Great Basin, Utah and Idaho: Fluvial response to changes in depositional base level: in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., editors, Incised Valley Systems: Origin and Sedimentary Sequences, Tulsa, OK., SEPM (Society for Sedimentary Geology) Special Publication 51, p. 369-382.
Link, P. K., and Phoenix, E. C., 1996, Rocks, Rails, and Trails: Idaho Museum of Natural History, Pocatello, 194 p.
Link, P.K., Mahoney, J.B., Batatian, L.D., Bruner, D.J., and Williams, F., 1995, Stratigraphic setting of sediment-hosted mineral deposits in the eastern part of the Hailey 1o x 2o quadrangle, and part of the southern part of the Challis 1o x 2o quadrangle, South-Central Idaho: U.S. Geological Survey Bulletin 2064-C, p. C1-C33.
Link, P.K., Warren, Ian, Preacher, J.M., and Skipp, Betty, 1996, Stratigraphic analysis and interpretation of the Copper Basin Group, McGowan Creek Formation and White Knob Limestone, south-central Idaho: in Longman, M. W. and Sonnenfeld, M. D eds., Paleozoic Systems of the Rocky Mountain Region, Rocky Mountain Section, SEPM (Society for Sedimentary Geology), p. 117-144.
Link, P.K., Christie-Blick, N., Devlin, W.J., Elston, D.P., Horodyski, R.J., Levy, M., Miller, J.M.G., Pearson, R.C., Prave, A., Stewart, J.H., Winston, D., Wright, L.A., and Wrucke C.T., 1993, Middle and Late Proterozoic stratified rocks of the western United States Cordillera, Colorado Plateau, and Basin and Range Province: in Reed., J., Sims, P., Houston, R.S., Rankin, D.W., Link, P.K., Van Schmus, W.R., and Bickford, M.E., editors, Precambrian: Conterminous United States: Geological Society of America Decade of North American Geology Series, v. c-3, p. 474-690.
Lorentz, N.J., and Corsetti, F.A., 2007, Another test for snowball Earth: Geology, v. 35, no. 4: p. 383-384: doi: 10.1130/Focus042007:1.
Lorentz, N.J., Corsetti, F.A., Link, P.K., 2004. Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic Earth System behavior. Precambrian Research, v. 130, p. 57-70.
Mahoney, J.B., Link, P.K., Burton, B.R., Geslin, J.K., and O’Brien, J.P., 1991, Pennsylvanian and Permian Sun Valley Group: Wood River basin, south-central Idaho, in Cooper, J.D., and Stevens, C.H., eds., Paleozoic paleogeography of the western United States – II: Pacific Section, SEPM (Society for Sedimentary Geology), v. 67, p. 551-580.
Oaks, R.J.,Jr., James, W.C., Francis. G.G., and Schulingkamp, W.J., II, 1977, Summary of middle Ordovician stratigraphy and tectonics, northern Utah, southern and central, Idaho, Wyoming Geological Association Guidebook: 29th Annual Field Conference, p. 101-118.
Piper, D.Z., and Link, P.K., 2002, An upwelling model for the Phosphoria sea: A Permian, ocean-margin sea in the northwest United States: AAPG Bulletin, v. 86, no. 7, p. 1217-1235.
Prothero, D. R., and Dott, Jr., R. H., 2004, Evolution of the Earth, 7th edition: McGraw Hill, New York, 524 p.
Smith, L.H., Kaufman, A.J., Knoll, A.H., and Link, P.K., 1994, Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions: a case study of the Pocatello Formation and Lower Brigham Group, Idaho, USA: Geological Magazine, v. 131, no. 3, p. 301-314.
Wilson, E., Preacher, J.M., and Link, P.K., 1994, New constraints on the nature of the Early Mississippian Antler sedimentary basin in Idaho: in Embry, A.F., Beauchamps, B. and , Glass, D.J., eds., Pangea: Global Environments and Resources: Canadian Society of Petroleum Geologists Memoir 17, p. 155-174.