MIOCENE GRAND CANYON WITH BASE LEVEL IN PRECAMBRIAN STRATA? TESTING A SHINUMO SOURCE SCENARIO FOR SEEMINGLY SPECIAL SESPE CLASTS USING PALEOMAGNETISM

Field tests of remanence stability are the gold-standard for paleomagnetism, but they can be applied non-rigorously. An igneous contact test is considered only "half-baked" if altered host rock is variably remagnetized, yet no stable, distinct magnetization is recovered from distal, unaltered host rock. Similarly, a conglomerate test is only "half-scrambled" if clast remanences are randomly distributed in situ, but the identical lithology, where coherent, is not demonstrably stably magnetized. As an additional check, or in the absence of exposed coherent-clast lithology, paleomagnetists orient clasts and assess whether random magnetizations in situ restore to a common direction relative to clast fabric.

Debate over an ancient Grand Canyon, and a proposed solution in which remarkable cobble conglomerates in Miocene Sespe Formation distribute gravel of an "Arizona River" eroding ultramature Shinumo Quartzite at a low level in Grand Canyon, offers an opportunity to apply an "inverse" paleomagnetic conglomerate test in a novel way. Ultramature clasts in Sespe conglomerates can be approximated to paleo-horizontal using quartz grain sorting, heavy mineral laminae, pigment and/or magnetic susceptibility fabric. Feldspathic Sespe clasts are readily oriented similarly. While magnetic inclination in feldspathic Sespe clasts is consistent with low-paleolatitude magnetization common to Paleozoic deposits and remagnetized units throughout the Southwest, magnetic inclination in ultramature Sespe clasts permits mid-paleolatitude magnetization.

Among the few potential source units for ultramature Sespe clasts, only Shinumo Quartzite appears to match this magnetization. Although a number of caveats apply - principally low n, uncertain attitude of tectonized units during remagnetization, incompletely-reported Shinumo magnetization, and variable (Miocene?) weathering overprint on clast magnetizations - the paleomagnetic "inverse" conglomerate test easily could have rejected the Arizona River hypothesis. It fails to do so and sustains Miocene Grand Canyon as viably incising Shinumo Quartzite and terminating its drainage in the California Borderlands. This source-to-sink scenario for ancient Grand Canyon merits more extensive testing by these paleomagnetic and other independent methods.