Stratigraphic and Tectonic Framework of Upper Proterozoic and Cambrian Rocks in the Western United States
Published Online: 17 MAR 2013
Copyright 1989 by the American Geophysical Union.
Late Proterozoic and Cambrian Tectonics, Sedimentation, and Record of Metazoan Radiation in the Western United States: Pocatello, Idaho, to Reno, Nevada 20-29 July, 1989
How to Cite
Christie-Blick, N., Levy, M., Mount, J. F., Signor, P. W. and Link, P. K. (1989) Stratigraphic and Tectonic Framework of Upper Proterozoic and Cambrian Rocks in the Western United States, in Late Proterozoic and Cambrian Tectonics, Sedimentation, and Record of Metazoan Radiation in the Western United States: Pocatello, Idaho, to Reno, Nevada 20-29 July, 1989, American Geophysical Union, Washington, D. C.. doi: 10.1029/FT331p0007
- Published Online: 17 MAR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875906577
Online ISBN: 9781118667415
- Cambrian Rocks;
- Middle and Late Proterozoic;
- Phanerozoic deformanon;
- Precambrian crust
Sedimentary and volcanic rocks of Late Proterozoic to Cambrian age from the lower part of a miogeoclinal wedge that thickens generally towards the west, and extends for about 4500 km along the inner edge of the North American Cordillera between eastern Alaska and northern Mexico. The rocks overlie crystalline basement of Archean to early Middle Proterozoic age and in places as much as several thousand metres of relatively unmetamorphosed sedimentary rocks of Middle to earliest Late Proterozoic age. The contact with the overlying miogeocline ranges from one of angular unconfonnity or nonconfonnity to an ill-defined transition within a relatively conformable succession. The age of this transition is not well constrained, but from dating in western Canada and northeastern Washington it appears to be younger than 770 Ma and older than 720 Ma. An especially distinctive feature of the lower part of the miogeocline throughout the Cordillera is the presence of diamictite, in part of glacial origin. Overlying rocks of Late Proterozoic to Cambrian age are predominantly siliciclastic, although carbonate rocks are abundant locally, especially in the Mackenzie Mountains of northwestern Canada and in the southern part of the Cordillera between eastern California and Sonora, Mexico. The upper part of the miogeocline consists largely of platformal carbonate rocks and mudstones of Cambrian to Devonian age. The transition from siliciclasticdominated to carbonate-dominated rocks is typically in the Lower to Middle Cambrian part of the section, with the age of the transition younging toward the east.
It is generally recognized that as a whole the Upper Proterozoic and Lower Paleozoic strata record a transition from intracontinental rifting to the development of a passive continental margin, but the precise location of the transition within the stratigraphy is controversial. Quantitative analysis of tectonic subsidence mainly for strata of Cambrian and Ordovician age between British Columbia and eastern California indicates that thermally driven subsidence of the passive margin began after about 590 Ma, that is, in latest Proterozoic or Early Cambrian time, and this is corroborated by limited stratigraphic evidence for rifting through much of the lower part of the miogeoclinal wedge. An alternative view is that the transition from rift to post-rift phases is well within the lower part of the miogeocline, stratigraphically as much as several kilometres below the base of the Cambrian. This view stems mainly from the fact that most of the geological evidence for rifting is concentrated near the base of the miogeocline, which is some 100–200 m.y. older than the estimated time of onset of the rapid thennal subsidence characterizing the early part of the Paleozoic. The results of subsidence analysis and the geological data may be reconciled by supposing that the region experienced at least two extensional events in LateProterozoic and Early Cambrian time, and that for the later event either 1) all of the areas studied are continentward of the hinge zone between stretched and relatively unstretched lithosphere, or 2) the lithosphere extended in a heterogeneous manner, perhaps in association with regional detachment faults. It is not known whether the region was internal to a supercontinent prior to about 800 Ma, or near a passive continental margin of Middle Proterozoic age, leaving open the possibility that the Late Proterozoic counterpart may consist of one or more relatively narrow slivers rather than a large continental fragment.
The Upper Proterozoic and Paleozoic miogeoclinal rocks were affected by several orogenic events beginning in late Devonian time, and in places the rocks were buried by thick successions of younger Paleozoic strata. Much of the defonnation, metamorphism and igneous activity observed within the miogeoclinal belt is related to collision in Mesozoic and Cenozoic time of North America with exotic or suspect terranes of varying affmity, to associated largescale strike-slip deformation, and especially in the western United States to mid- to late Cenozoic lithospheric extension.