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History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, ⁴⁰Ar/³⁹Ar dating, and field studies: Implications for flow stratigraphy, timing of volcanic events, and lava dams

¹ Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
² New Mexico Bureau of Geology and Mineral Technology, Geochronology Laboratory, 801 Leroy Place, New Mexico Institute of Technology, Socorro, New Mexico 87801, USA

View larger version (120K): [in this window] [in a new window]   Figure 1. True color Landsat image of the Grand Canyon showing the location of the Uinkaret volcanic field, 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).

View larger version (31K): [in this window] [in a new window]   Figure 2. Hamblin's nomenclature for dams and volcanic stratigraphy and the inferred relative ages of each dam, based largely on inset relationships (Hamblin, 1994). The 40K/40Ar dates are in gray from Dalrymple and Hamblin (1998). New 40Ar/39Ar dates (see Table 1 for references) do not substantiate the proposed age sequence.

View larger version (41K): [in this window] [in a new window]   Figure 3. (A) Plot of 105 dates that we consider to be the most reliable ages on basalt remnants in western Grand Canyon. Age ranges reflect the 2 error on each date. The episodes of volcanism (based on available data) are color coded. (B) Cumulative probability plot of the reliable 40Ar/39Ar ages. (C) Cumulative probability plot of the reliable cosmogenic 3He ages; reliable cosmogenic 3He dates are younger than ca. 275 ka. This dating method is best applied to young features that have not been eroded or experienced burial. (D) Cumulative probability plot of the reliable 40K/40Ar ages. Reliable 40K/40Ar dates are older than ca. 200 ka. The 40K/40Ar dates are often too old due to undetected excess argon. The relative height of the peaks is a function of both the precision of the date and the number of samples within an age range, so a highly sampled flow will result in high peaks. Ages from Dalrymple and Hamblin (1998), Lucchitta et al. (2000), Fenton et al. (2001), Pederson et al. (2002), Fenton et al. (2002), Fenton et al. (2004), Raucci (2004), and Karlstrom et al. (2007) were included in the analysis.

View larger version (145K): [in this window] [in a new window]   Figure 4. Map of the southern Uinkaret volcanic field showing the location of all reliable dates between 800 and 475 ka. Lava flows of this age are interpreted to have entered the canyon at around river mile 179, near the Toroweap fault. Numerous dates on Black Ledge remnants at Granite Park and a single date from a remnant (Spencer Canyon) at river mile 246 indicate that this oldest stage of volcanism was the most voluminous and far-traveled. A 540 ± 30 ka 40Ar/39Ar date in Whitmore Canyon indicates that volcanism likely occurred along the Hurricane fault at this time as well but may not have resulted in intracanyon flows.

View larger version (162K): [in this window] [in a new window]   Figure 5. Photo looking south, up Prospect Canyon, showing thick tephra deposits under the Upper Prospect flow. Cinder-cone remnants, seen perched on the canyon's rim (right side of image), may indicate a possible source for this 518 ± 12 ka (n = 5) flow (Pederson et al., 2002).

View larger version (145K): [in this window] [in a new window]   Figure 6. Map of the southern Uinkaret volcanic field showing the location of all reliable dates between 425 and 250 ka. Circa 320 ka lava Whitmore flow(s) are interpreted to have entered the canyon at river mile 187.5, where they filled paleo–Whitmore Canyon. Similar 40Ar/39Ar ages have been obtained for a small remnant at river mile 177, which probably traveled upriver based on the dip of the flow's top and the lack of upstream sources. Two 40K/40Ar dates on Massive Diabase remnants are included in this group, even though their correlation is questionable.

View larger version (145K): [in this window] [in a new window]   Figure 7. Map of the southern Uinkaret volcanic field showing the location of all reliable dates between 225 and 160 ka. Lava flows of this age are interpreted to have entered the canyon near river mile 187.5 (Whitmore Wash) and possibly at river mile 182. Cosmogenic dates on a flow north of Vulcan's Throne and on Vulcan's Footrest indicate volcanism along the Toroweap fault during this time period; however, it is unclear if these flows entered the canyon.

View larger version (144K): [in this window] [in a new window]   Figure 8. Map of the southern Uinkaret volcanic field showing the location of all reliable dates younger than 140 ka. Ages younger than 51 ka are shown in yellow. Circa 100 ka lava flows are interpreted to have entered the canyon between river miles 182 and 185 and flowed downriver to at least river mile 192. Cosmogenic dates on the Toroweap Terrace and Vulcan's Throne indicate volcanism along the Toroweap fault during this time period; however, it is unclear as to what distance (if any) these flows traveled downriver.

View larger version (119K): [in this window] [in a new window]   Figure 9. Light detection and ranging (lidar) interpretation method: example from three inset flows at river mile 188. The Colorado River is flowing to the south. (A) Top and bottom of each flow is digitized based on 0.3 m resolution near-infrared aerial photographs. The digitized top and bottom are shown as black lines. (B) Grid of slope (derived from lidar) is overlaid over the aerial photographs. Pink areas denote cliffs where the slope is over 75°. Lidar data are then used to calculate top and bottom elevations for each remnant along its length. The data from Figure 9 are shown as three inset flow remnants of different ages on Figure 10 at river mile 188: Whitmore (green), Black Ledge (Orange), and Gray Ledge (blue). Shown in purple is the Whitmore Cascade, which is interpreted to overlie the Whitmore flow locally.

View larger version (51K): [in this window] [in a new window]   Figure 10. Longitudinal profile of the Colorado River from river mile 176 to 200 (A) and 200 to 224 (B). The Spencer Canyon flow at river mile 246 is shown as an inset in part B. Schematic synthesis showing the extent of different-aged intracanyon basalt flows from river mile 176 to 246 is shown on top of part A. Remnants are projected onto the profile based on their lidar heights and color coded based on their age (solid fills show remnants with 40Ar/39Ar dates). Note that the ca. 100 ka (blue) and ca. 200 ka flows (purple) are interpreted to have entered the canyon at similar places. These flows exhibit some of the best-defined flow tops. The ca. 320 ka Whitmore flows (green) are interpreted to have originated from Whitmore Canyon and to have a high surface that existed from river mile 187 to 192. The bold lower orange line between river mile 179 and 191 shows where the orientation of the 725–475 ka maximum strath surface indicates fault block rotation (see text). Figure is vertically exaggerated 13 times.

View larger version (20K): [in this window] [in a new window]   Figure 10 (continued).

View larger version (9K): [in this window] [in a new window]   Figure 11. Light detection and ranging (lidar) and field measurements obtained with a Jacob Staff generally agree within <4 m. Increasing error with height may indicate that compounding error associated with Jacob Staff measurements may be the largest source of error. Other sources of error include: river-level variations, spatial resolution of the lidar data, and inaccurate placement of remnant contacts on aerial photography.

View larger version (39K): [in this window] [in a new window]   Figure 12. Conceptual model for the various types of dams and edifices that may have formed in western Grand Canyon. The incorporation of volcanic debris into dams would add instabilities that would lead to more rapid destruction of that part of the dam but not necessarily cause catastrophic failure before overtopping. The presence of lava tubes would allow for infiltration of the edifices and cause rapid destruction. In this depiction, the stability of dams is shown by the degree to which they are preserved.

View larger version (79K): [in this window] [in a new window]   Figure 13. Hyaloclastite dam of Fenton et al. (2004, 2006) may alternatively represent slumped ca. 320 ka Whitmore flows. Both remnants have a very similar sequence of stacked thin flows.

View larger version (71K): [in this window] [in a new window]   Figure 14. At river mile 246, the top of the Spencer flow exhibits abundant evidence that the free-flowing Colorado River was once established on top of this distal Black Ledge flow. Large, 5-m-diameter potholes and channel forms are preserved at this location. The river level here is now controlled by Lake Mead. Predam river levels were 30 m lower (Karlstrom et al., 2007). (Photo by Laura Crossey)

View larger version (42K): [in this window] [in a new window]   Figure 15. New testable intracanyon flow stratigraphy and model for composite volcanic edifices. Note that the flows from the Prospect flow stack are correlated with downstream Black Ledge remnants. Whitmore remnants are correlated to downstream remnants originally mapped as Layered Diabase (and Massive Diabase). Absolute ages from Karlstrom et al. (2007) are given for the closest remnant dated by the 40Ar/39Ar method. Inset relationships are depicted to show relative ages. Figure is not to scale.