Shallow-marine, intertidal, and supratidal detrital and carbonate strata of late Precambrian (less than 850 m.y.) and early Paleozoic (more than 345 m.y.) age thicken from a few thousand feet in cratonic areas east of the Great Basin to 40,000 ft in the central Great Basin 200-300 mi west. Coeval rocks in the western Great Basin are deep-water strata characterized by chert and argillite associated with mafic pillow lavas. Strata deposited at moderate depths are present between the shallow- and deep-water facies, but have a limited distribution, suggesting a relatively abrupt transition from shelf to deep-ocean basin. The thick accumulation of shallow-water deposits in the Great Basin is similar to deposits along present-day stable continental margins. Such accumulations h ve been termed miogeoclines, rather that miogeosynclines, because they are open to the sea on one side and are not synclinal in form.

The continental margin along which the late Precambrian and early Paleozoic miogeocline was constructed apparently developed as the result of a late Precambrian (less than 850 m.y.) continental separation. Extensional faulting and flowage related to this separation extended well into the continent and may have produced major crustal thinning as far east as the "Wasatch line," across which the rate of westward thickening of upper Precambrian and Paleozoic strata increases markedly. A persistent positive belt, perhaps analogous to the buried ridge beneath the outer edge of the present-day Atlantic continental shelf, may account for regional thinning and local erosional truncation of lower Paleozoic strata along the western margin of the Cordilleran miogeocline.

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