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Proc. Natl. Acad. Sci. USA Vol. 95, pp. 9744–9749, August 1998 Geology K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona (radiometric datingyvolcanismybasaltypotassium-argon) G. BRENT DALRYMPLE*† AND W. K. HAMBLIN‡ *College of Oceanic and Atmospheric Sciences, 104 Ocean Administration Building, Oregon State University, Corvallis, OR 97331; and ‡Department of Geology, Brigham Young University, Provo, UT 84602 Contributed by G. Brent Dalrymple, June 17, 1998 ABSTRACT At least 13 times during the Pleistocene partial erosion between periods of lava extrusion and dam Epoch lava flowed into the inner gorge of the Grand Canyon reconstruction. The lakes behind the dams were formed in a and formed lava dams, as high as 600 m, that temporarily matter of years and probably filled with sediment in several blocked the flow of the Colorado River. K-Ar ages on these lava hundred years. Destruction of the dams was also rapid. The dams indicate that the seven youngest formed within a short overflow of the Colorado River at the downstream end of the period of time between about 0.6 and 0.4 mega-annum (Ma). dam undoubtedly formed rapids and waterfalls that quickly The physiography of the lava dam remnants within the canyon migrated upstream. After the lakes filled with sediment the shows that each dam was destroyed by erosion, the Colorado normal traction load transported over the dam must have River rapidly reaching its pre-existing grade level, before the resulted in vigorous downcutting. next dam was emplaced by new eruptions. The total time for The relative ages of most of the lava dams are clearly emplacement and destruction for an individual lava dam was expressed by juxtaposition (Fig. 2). Remnants of older basalt probably as little as 0.01–0.02 million years. The K-Ar ages of flows are adjacent to the canyon walls with succeeding younger the two oldest dams, the Lava Butte dam (0.577 6 0.054 Ma) units stacked in sequence side by side, with the youngest flows and the Prospect dam (0.684 6 0.051 Ma) are somewhat closest to the river channel. In addition, some expression of younger than the physiography of their remnants suggest. relative age is seen in the degree of erosion of remnants of dams that fill tributary valleys. Older remnants that fill major One of the most significant recent events in the history of the tributary valleys have been dissected by renewed side-canyon Grand Canyon was the formation of a series of basaltic lava cutting whereas younger remnants that fill similar side valleys dams in the area of Toroweap Valley, Prospect Canyon, and are only slightly modified. Whitmore Canyon approximately 200 km west of the national Despite the importance of these lava dams to the recent park headquarters (Fig. 1). More than 150 lava flows poured history of the Grand Canyon, there have been few attempts to into the canyon during the Pleistocene and formed a series of determine their age. McKee and others (10) obtained a K-Ar 6 s 13 major dams ranging from 60 to 600 m high. The lava dams age of 1.16 0.18 mega-annum (Ma) (1 ) for the basalt at the impounded the water from the Colorado River to form a series base of the Toroweap Dam. Their age was based on three Ar 40 of temporary lakes upstream. The shorelines of the deeper analyses whose radiogenic Ar content varied by 79% and 40 lakes were located near the base of the Redwall Limestone in whose atmospheric Ar content ranged from 0.7 to 3.1%. Paul the area of the park headquarters and extended far upstream Damon (personal communication, quoted in ref. 8) measured 6 into Utah beyond the present shores of Lake Powell. These K-Ar ages of 0.993 0.097 Ma on one of the middle flows in 6 lakes were larger in areal extent and impounded more water the Whitmore Dam, and one of 0.549 0.032 Ma on the flow than Lake Mead and Lake Powell combined. Sediment de- that formed the Black Ledge Dam. Some of our preliminary posited in these lakes still remain as isolated exposures, the results were referred to briefly by Hamblin (7, 8), but this is the largest being the silt deposits that fill Havasu Canyon. The lava first paper to document our radiometric ages for the lava dams. dams have been described by previous workers, including During field work in 1971 and 1992 we collected samples for Powell (1), Dutton (2), Moore (3), and McKee and Schenk (4), dating from more than 80 flows representing 13 of the major and discussed in detail by Hamblin (5–8); we include only a lava dams. The purpose of this paper is to present K-Ar ages summary description in this paper. for some of these samples and to relate these data to the Numerous remnants of the lava dams are preserved from Pleistocene history of the Grand Canyon. Vulcan’s Throne (mile 179) downstream to mile 263, a distance of 135 km. (Locations along the Colorado River in the Grand Analytical Methods Canyon traditionally are measured in miles downstream from Lee’s Ferry and nearly all of the relevant literature uses this Samples consisted of both hand samples and one-inch cores. location system. We also use the mile system for location.) All samples were examined in thin section and many were Remnants range from a few kilometers long to small patches rejected as unsuitable for K-Ar dating, a few because of a few meters thick clinging precariously to the near vertical cliff alteration but most because of large amounts of glass in the of the inner gorge. Some dams were formed from a single lava ground mass. Those samples that were selected for dating were flow, others were composed of multiple flows, some of which free of alteration and were either holocrystalline or contained are interbedded with river gravel. relatively small amounts of isotropic ground-mass glass. Hydrologic data from Lake Mead surveys suggest that both Two methods of sample preparation were used: small blocks the formation and destruction of the lava dams occurred in a cut with a diamond saw and 0.5–1.0 mm crushed material. For remarkably short period of time (9). The small single-flow the former, small chips were sawed from each end for the K2O dams were formed in a matter of days. Larger complex dams analyses whereas for the latter an aliquant was taken with a involving multiple flows may have required several thousand sample splitter. The material taken for the K2O analyses was years to form and undoubtedly included several cycles of Abbreviations: Ma, mega-annum; MSWD, mean square of the weighted deviates. © 1998 by The National Academy of Sciences 0027-8424y98y959744-6$2.00y0 †To whom reprint requests should be addressed. e-mail: gbd@ PNAS is available online at www.pnas.org. oce.orst.edu. 9744 Downloaded by guest on September 29, 2021 Geology: Dalrymple and Hamblin Proc. Natl. Acad. Sci. USA 95 (1998) 9745 FIG. 1. Map showing general locations of Pleistocene lava dams in the Grand Canyon in Arizona, and the locations of places mentioned in the text. Mile markers, measured downstream from Lee’s Ferry, are the traditional way to locate features along the Colorado River in the Grand Canyon. After Hamblin (8). further crushed to 275 mm before analysis. Many of the basalts avoid incorporating either of these into the sample analyzed. contain xenoliths (primarily dunite) and carbonate and as Both the carbonate, which interferes with the clean-up process much care as possible was taken during sample preparation to during Ar extraction and whose results are unpredictable, and the older inclusions, which contribute unknown and varying amounts of inherited 40Ar, would be expected to result in calculated ages that are inconsistent and, where inherited 40Ar is present, too old. Despite our precautions, the inconsistency of the results for several of the samples suggests that we were not entirely successful for several of the flows. Potassium analyses were by flame photometry using a lithium metaborate flux (11). Argon analyses were by isotope dilution using 38Ar tracers calibrated with our intralaboratory standard biotite SB-3 (162.9 Ma) and methods described previously (12). Errors assigned to the individual ages are SDs of precision unless specified otherwise. Weighted means and s weighted SDs ( best), were calculated following Taylor (13). For some samples the dispersion of the measured ages is greater than expected from random analytical errors, i.e., the mean square of the weighted deviates (MSWD) is greater than one. This finding indicates that ‘‘geological’’ error, perhaps because of carbonate, xenoliths, or other factors, is present. For these samples we have adjusted the uncertainties by multiplying each individual error by the square root of MSWD s before calculating best (14, 15). In general, our results are not sufficiently precise to distinguish differences in age between flows within a single dam, and so we treat all of the ages from such flows as a single population and use the weighted mean of the ages to represent the age of the dam. FIG. 2. Relative age of major dams as indicated by juxtaposition insofar as known. Paleozoic sedimentary units that form the walls of Description of the Lava Dams and Their K-Ar Ages the Grand Canyon in the vicinity of the lava dams are shown on the left. This is a composite diagram constructed from numerous expo- The Prospect Dam. A single remnant of the Prospect Dam sures between miles 179 and 207.

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