REVISED AGE OF A LATE PLEISTOCENE LAVA DAM ACROSS THE LITTLE COLORADO RIVER AT GRAND FALLS, ARIZONA

Grand Falls, AZ, is the site of a massive basalt pour-over that filled the canyon of the Little Colorado River (LCR) in late Pleistocene time. No consensus exists as to which of several nearby cones is the specific source vent, but basalt lava clearly flowed at least 10 km eastward from its vent area and cascaded into a 65-m-deep canyon. Lava completely filled the canyon, spilled over onto the northeastern canyon rim, and advanced a kilometer beyond the rim. From the point where it entered the canyon, the lava flowed ~25 km downstream. Subsequently, sediment has filled the reservoir behind the lava dam. The river flows across this sediment plain, follows the margin of overflow tongue of lava, and plunges back into the pre-eruption canyon at Grand Falls.

Early geochronologic work in the 1970s by whole-rock K-Ar provided a date of 150±30 ka for the lava dam. The nearly pristine condition of fragile surface features of the lava upslope from the dam and the near absence of physical and chemical weathering on the intracanyon flow, however, strongly suggest a younger age. Significantly, the elevation of the buried fossil LCR channel coincides with the present channel, indicating that no system-wide post-eruptive downcutting has occurred.

Multiple dating techniques better define the age of the lava flow that makes up the dam. A sample of silty mudstone intensely heated at the base of the lava yielded an optical-luminescence age of 19.6 ±1.2 ka. Three samples from the apparently non-eroded surface of the lava yielded cosmogenic ³He ages of 15.1 ±1 ka, 17 ±1 ka, and 19 ±1 ka. Four samples of dense holocrystalline basalt collected near the core of the lava dam yielded a weighted mean age of 8 ±19 ka by the ⁴⁰Ar/³⁹Ar step-heating technique. In addition, the paleomagnetic direction of the dam-forming lava matches the curve of magnetic secular variation for ~19 ka, 23 ka, 28 ka, and 34 ka. We conclude that the dam formed at about 19 ka.

It is widely understood that accurate age determination by the K-Ar method is difficult for late Pleistocene materials that contain less than about 1% K₂O. Our experience at Grand Falls illustrates the potential for application of other techniques to circumvent this limitation, and helps to add confidence in the stand-alone results of the individual techniques in situations where application of multiple techniques is not possible.