Abstract

A structural disturbance that ended deposition of Chuaria-bearing marine shale of middle Proterozoic age in northern Arizona is recorded by strata within the Sixtymile Formation at the top of the Grand Canyon Supergroup. The disturbance involved marine emergence and uplift, accompanied by block faulting with as much as 3.2 km of structural displacement. It was the most severe episode of structural deformation to affect either the Proterozoic or Phanerozoic strata of the Grand Canyon, but it was markedly less severe than an earlier deformation which resulted in meta-morphism and intrusion of the underlying 1,700-m.y.-old crystalline basement. Two groups of disturbed K-Ar ages obtained from the Proterozoic rocks, 930 ± 25 m.y. and 823 ± 26 m.y., appear to broadly reflect the time of uplift and faulting of the Grand Canyon disturbance. The older age was obtained from mineral analyses of ∼ 1,150-m.y.-old sills and 1,700-m.y.-old crystalline basement buried at a depth of about 4 km at the time, of disturbance. The younger age was obtained from whole-rock analyses of ∼1,100-m.y.-old lava flows buried only half as deeply at the time of the disturbance. A high consistency of ages (values within about 10%) was obtained from both mineral and whole-rock samples having widely varying K2O contents, suggesting that little differential loss of Ar has occurred since a time of general resetting. Moreover, scatter on an 40Ar/39Ar isochron and a disturbed pattern of incremental heating ages indicate that the Ar clock in minerals from the diabase sills was not completely reset. This implies that the Ar clock in the lava flows was more nearly if not completely reset, presumably because of a lower Ar retentivity of the cryptocrystalline matrix of the flows. The 823 m.y. age from the lavas thus is believed to generally reflect a time of cooling and Ar retention accompanying the Grand Canyon disturbance.

Strata of the upper Grand Canyon Supergroup correlate paleontologically and paleomagnetically with sedimentary rocks and isotopically dated intrusive rocks of the Little Dal Group of the Mackenzie Mountains Supergroup (northwest Canada), and with sedimentary rocks of the Uinta Mountain Group (Utah and Colorado). An apparent correlation also exists with some poles reported from crystalline rocks of the Grenville Province of eastern North America, and this correlation accords with the broad range of disturbed K-Ar ages from the Grand Canyon (950 to 800 m.y.). Intrusions emplaced after deposition of the Chuaria-bearing Little Dal Group, very near the top of the Mackenzie Mountains Supergroup of Canada, are geologically well controlled and have an internal Rb-Sr isochron age of 770 ± 20 m.y. Intrusion in the Mackenzie Mountains preceded or accompanied deposition of basaltic lava flows and sedimentary strata that accumulated during a time of block faulting, and these strata are in turn overlain by glacio-genic deposits assigned to the late Proterozoic Windermere Supergroup. On isotopic-age, paleontologic, and paleomagnetic grounds, the structural disturbance in the Grand Canyon thus appears to correlate, with post–Little Dal to early Windermere faulting in northwest Canada, a faulting that in turn has been correlated with the disturbance called the “East Kootenay orogeny” in the southern cordillera of Canada.

The disturbance in the Grand Canyon is assigned a nominal age of 823 m.y. from an average of the reset K-Ar dates for the Cardenas Lavas. This age would seem to imply that the onset of structural activity occurred somewhat earlier in Arizona than in northwest Canada. If such is the case, data from the Grand Canyon and northwest Canada provide an age range of about 820 to 770 m.y. for a structural disturbance in western North America that separates the deposition of middle Proterozoic from late Proterozoic strata. This range brackets the nominal 800 m.y. age assigned to the boundary between Proterozoic Y and Z rocks on a geologic time scale adopted by the U.S. Geological Survey in 1980. Deposition of the Sixtymile Formation in the Grand Canyon, which accompanied and which also appears to have postdated the disturbance, is assigned to the late Proterozoic.

The structural disturbance in the western United States, here called the “Grand Canyon–Mackenzie Mountains disturbance,” was marked by a westward shift in the apparent polar wandering path. A somewhat similar westerly shift is seen in some of the reset poles reported from rocks of the Grenville Province, but a strong southerly shift also is seen, which has given rise to an apparent southerly track (and a postulated loop) in the Grenville polar path. The southerly shift is not seen in the stratigraphically controlled polar path from the western cordillera, which temporally overlaps the age range assigned to the Grenville poles. Although the apparent southerly shift unquestionably is present in the Grenville paleomagnetic data, and also is seen in poles from Grenville-age rocks from Fennoscandia, we suggest that the shift may be an artifact recording the uncorrected effects of structure and structural rotation. The Grenville terrane very likely was subjected to at least some structural deformation at the time of a wide-ranging disturbance beginning about 820 m.y. ago. It is a disturbance that is reported to have affected adjacent rocks in the Appalachian region of eastern North America. Along the western margin of the craton, the. disturbance gave rise to block-fault mountains and to the Cordilleran miogeocline. We suggest that this mountain-making event was the terminal event of the Grenville orogeny.

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