History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, 40Ar/39Ar dating, and fi eld studies: Implications for fl ow stratigraphy, timing of volcanic events, and lava dams Ryan Crow* Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA William McIntosh Lisa Peters Nelia Dunbar New Mexico Bureau of Geology and Mineral Technology, Geochronology Laboratory, 801 Leroy Place, New Mexico Institute of Technology, Socorro, New Mexico 87801, USA ABSTRACT down-canyon over 120 km. A second episode of normal fl uvial deposition at times when the volcanism, from 400 to 275 ka, was most volu- river was established on top of basalt fl ows. A synthesis of the geochronology on basalt minous along the Hurricane fault at river mile fl ows from the southern Uinkaret volcanic 187.5. This episode produced fl ow stacks that Keywords: Grand Canyon region, Uinkaret, ba- fi eld indicates that basalts erupted within and fi lled Whitmore Canyon and produced the salt fl ows, lava dams, volcanic history. fl owed into Grand Canyon during four major 215-m-high Whitmore Dam, which may have episodes: 725–475 ka, 400–275 ka, 225–150 also had a composite history. Basaltic river INTRODUCTION ka, and 150–75 ka. To extend the usefulness of gravels on top of the Whitmore remnants have these dates for understanding volcanic stra- been interpreted as “outburst-fl ood deposit” The Uinkaret volcanic fi eld in northwestern tigraphy and lava dams in western Grand but may alternatively represent periods when Arizona is a north-south–trending fi eld of cinder Canyon, we analyzed light detection and the river established itself atop the fl ows. Rem- cones and basalt fl ows that is situated between ranging (lidar) data to establish the eleva- nants near river level at miles 192 and 195, the Hurricane and Toroweap faults on the north tions of the tops and bottoms of basalt-fl ow previously designated as Layered Diabase and edge of western Grand Canyon (Fig. 1). This remnants along the river corridor. When pro- Massive Diabase, have been shown by 40Ar/39Ar study focuses on basalts that erupted within and jected onto a longitudinal river profi le, these dating to be correlative with dated Whitmore fl owed into western Grand Canyon. Although data show the original extent of now-dissected fl ow remnants, and they help document the fl ows on the Uinkaret Plateau are as old as 3.7– intracanyon fl ows and aid in correlation of downriver stepped geometry of the Whitmore 3.4 Ma at higher elevation on Mount Trumble fl ow remnants. Systematic variations in the Dam. The ca. 200 and 100 ka fl ows (previously (40K/40Ar; Best et al., 1980), dated fl ows within elevation of fl ow bottoms across the Uinkaret mapped as Gray Ledge) were smaller fl ows Grand Canyon are Quaternary in age (Hamblin, fault block can be used to infer the geometry that entered the canyon from the north rim 1994; Dalrymple and Hamblin, 1998; Fenton et of a hanging-wall anticline that formed adja- between river mile 181 and Whitmore Can- al., 2001; Pederson et al., 2002; Fenton et al., cent to the listric Toroweap fault. yon (river mile 187.5); they are concordant 2002, 2004). Very recent volcanic activity on the The 725–475 ka volcanism was most volu- with dates on the Whitmore Cascade as well as north rim (ca. 1 ka) is indicated by the presence minous in the area of the Toroweap fault and other cascades found along this reach. of pottery fragments in welded basaltic spatter produced dike-cored cinder cones on both The combined results suggest a new model (Ort et al., 2008) and by cosmogenic 3He dating rims and within the canyon itself. Mapping for the spatial and temporal distribution of (Fenton et al., 2001). suggests that a composite volcanic edifi ce was volcanism in Grand Canyon in which com- New 40Ar/39Ar results indicate that Quater- created by numerous fl ows and cinder-cone posite lava dams and edifi ces, that were gen- nary basalts fl owed into western Grand Canyon fragments that intermittently fi lled the canyon. erally leaky in proximal areas, were built between ca. 725 and 100 ka (Karlstrom et al., Reliable 40Ar/39Ar dates were obtained from from 725 to 475 ka near Toroweap fault and 2007) and profoundly affected the erosional and fl ows associated with this period of volcanism, around 320 ka near Whitmore Canyon. New geomorphic processes within Grand Canyon by including Lower Prospect, Upper Prospect, data on these and other episodes present a forming lava dams and armoring terraces and D-Dam, Black Ledge, and Toroweap. Large- refi ned model for complex interactions of hillslopes (Hamblin, 1994). As fi rst reported by volume eruptions helped to drive the far-trav- volcanism and fl uvial processes in this clas- Powell (1895), it is clear that canyon incision, eled basalt fl ows (Black Ledge), which fl owed sic locality. Available data suggest that the basaltic volcanism, and extensional faulting have demise of these volcanic edifi ces may have all interacted here with interesting feedbacks *[email protected]. involved either large outburst-fl ood events or between these processes (Hamblin et al., 1981; Geosphere; February 2008; v. 4; no. 1; p. 183–206; doi: 10.1130/GES00133.1; 15 fi gures; 1 table; 6 supplementary fi les. For permission to copy, contact [email protected] 183 © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/1/183/3340294/i1553-040X-4-1-183.pdf by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/1/183/3340294/i1553-040X-4-1-183.pdf 184 Geosphere, February2008 Crow etal. Crow Figure 1. True color Landsat image of the Grand Canyon showing the location of the Uinkaret volcanic fi eld, Toroweap fault, Hurricane fault, and the Grand Wash Cliffs. Basalt remnants are present from river mile 177 to 254 (measured downstream from Lee’s Ferry). Spencer Canyon is located at river mile 246. Major Quaternary faults, including the Toroweap and Hurricane faults, are shown in red (from U.S. Geological Survey and Arizona Geological Survey, 2006). History of Quaternary volcanism and lava dams in western Grand Canyon Jackson, 1990; Stenner et al., 1999; Fenton et al., Previous Work on Basalt Stratigraphy on a combination of superposition (e.g., Lower 2001, 2002, 2004; Pederson et al., 2002). Earlier and Upper Prospect fl ows) and inset relationships studies have suggested that these fl ows are over a Powell (1895, p. 275) was the fi rst to report: (Fig. 2). This stratigraphy and nomenclature were million years old (based on 40K/40Ar dates; McKee “What a confl ict of water and fi re there must have developed in the context of a model for forma- et al., 1968; Hamblin, 1994), but new 40Ar/39Ar been here!” with “a river of molten rock running tion and destruction of lava dams, so names were dates indicate that all dated intracanyon fl ows are down into a river of melted snow.” He was also the presented for different “dams.” Here, we retain younger than 723 ka (Karlstrom et al., 2007). fi rst to postulate that high lava dams and extensive these names in part, but we modify the nomen- Flows from the Uinkaret volcanic fi eld traveled lakes may have resulted. Hamblin (1994) formal- clature where necessitated by new dates (shown from vents in the Uinkaret Mountains: eastward ized an elegant model for lava dams and hypoth- in black on Fig. 2). Problems with Hamblin’s fl ow to fi ll the Toroweap paleocanyon, westward to fi ll esized a series of 13 major dams that were gener- chronology were apparent even with the 40K/40Ar the Whitmore paleocanyon, and southward to par- ally stable and relatively long-lived for dams on dates of Dalrymple and Hamblin (1998). Dams tially fi ll the Grand Canyon (Hamblin, 1994). The a major river (<20,000 yr). He envisioned dams, that were inferred to be quite young stratigraphi- fl ows that cascaded into Grand Canyon poured up to 700 m high, that backed up lake water as far cally (Layered and Massive Diabase) gave similar into a canyon that had reached ~90% of its cur- as Moab, Utah, and deposited lake sediments in 40K/40Ar dates to dams inferred to be much older rent depth, and they are hypothesized, according several localities. Much of the lacustrine evidence (Fig. 2). New 40Ar/39Ar dates (Karlstrom et al., to the prevailing models (Hamblin, 1994; Fenton for large reservoir-sized lakes, cited by Hamblin 2007) show that named fl ows, such as those iden- et al., 2004), to have built a series of major dams (1994), has been questioned by Kaufman et al. tifi ed by Hamblin as “Black Ledge,” give dispa- and related lakes. Hamblin (1994) proposed stable (2002), who suggested that lakes in Grand Can- rate dates due to miscorrelation of remnants (see dams and large lake systems that lasted ~20 k.y., yon may have been less extensive both spatially following). This paper presents a new synthesis of and Fenton et al. (2004) suggested that catastrophic and temporally than previously thought. Fenton all existing geochronology on the basalts and adds dam failure may have occurred before dam over- et al. (2002, 2004, 2006) revised Hamblin’s stable new lidar and fi eld data on the geometry of basalt topping in some cases.