Lisa Peters

40Ar/39Ar and field studies of Quaternary basalts in Grand Canyon and model for carving Grand Canyon: Quantifying the interaction of river incision and normal faulting across the western edge of the Colorado Plateau

40Ar/39Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40K/40Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40Ar/39Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and fault-slip rates and indicates that basalts flowed into and preserved a record of a progressively deepening bedrock canyon. The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150–175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50–75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97–106 m/Ma on the Toroweap fault (last 500–600 ka) and 70–100 m/Ma on the Hurricane fault (last 200–300 ka). As the river crosses each fault, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is sub-equal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ∼100 m/Ma of cumulative relative vertical lowering of the western Grand Canyon block relative to the eastern block and 70–100 m of slip accommodated by formation of a hanging-wall anticline. Data from the Lake Mead region indicate that our refined fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20–30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately one-third of western Grand Canyon incision. This “incision discrepancy” for carving Grand Canyon is best explained by higher rates during early (5- to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon. Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25–50 m/Ma (150–300 m) relative to the western Grand Canyon block and 125–150 m/Ma (750–900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5- to 6-Ma Colorado River paleoprofile, and other geologic data, is that most of the 750–900 m of relative vertical block motion that accompanied canyon incision was due to Neogene surface uplift of the Colorado Plateau.

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

A synthesis of the geochronology on basalt flows from the southern Uinkaret volcanic field indicates that basalts erupted within and flowed into Grand Canyon during four major episodes: 725–475 ka, 400–275 ka, 225–150 ka, and 150–75 ka. To extend the usefulness of these dates for understanding volcanic stratigraphy and lava dams in western Grand Canyon, we analyzed light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt-flow remnants along the river corridor. When projected onto a longitudinal river profile, these data show the original extent of now-dissected intracanyon flows and aid in correlation of flow remnants. Systematic variations in the elevation of flow bottoms across the Uinkaret fault block can be used to infer the geometry of a hanging-wall anticline that formed adjacent to the listric Toroweap fault. The 725–475 ka volcanism was most voluminous in the area of the Toroweap fault and produced dike-cored cinder cones on both rims and within the canyon itself. Mapping suggests that a composite volcanic edifice was created by numerous flows and cinder-cone fragments that intermittently filled the canyon. Reliable 40Ar/39Ar dates were obtained from flows associated with this period of volcanism, including Lower Prospect, Upper Prospect, D-Dam, Black Ledge, and Toroweap. Large-volume eruptions helped to drive the far-traveled basalt flows (Black Ledge), which flowed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, was most voluminous along the Hurricane fault at river mile 187.5. This episode produced flow stacks that filled Whitmore Canyon and produced the 215-m-high Whitmore Dam, which may have also had a composite history. Basaltic river gravels on top of the Whitmore remnants have been interpreted as “outburst-flood deposit” but may alternatively represent periods when the river established itself atop the flows. Remnants near river level at miles 192 and 195, previously designated as Layered Diabase and Massive Diabase, have been shown by 40Ar/39Ar dating to be correlative with dated Whitmore flow remnants, and they help document the downriver stepped geometry of the Whitmore Dam. The ca. 200 and 100 ka flows (previously mapped as Gray Ledge) were smaller flows that entered the canyon from the north rim between river mile 181 and Whitmore Canyon (river mile 187.5); they are concordant with dates on the Whitmore Cascade as well as other cascades found along this reach. The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifices, that were generally leaky in proximal areas, were built from 725 to 475 ka near Toroweap fault and around 320 ka near Whitmore Canyon. New data on these and other episodes present a refined model for complex interactions of volcanism and fluvial processes in this classic locality. Available data suggest that the demise of these volcanic edifices may have involved either large outburst-flood events or normal fluvial deposition at times when the river was established on top of basalt flows.

U-Pb and 40Ar/39Ar Ages for a Tephra Lens in the Middle Jurassic Page Sandstone: First Direct Isotopic Dating of a Mesozoic Eolianite on the Colorado Plateau

Joint U‐Pb and 40Ar/39Ar geochronology for pyroclastic zircon and biotite crystals from tephra lenses intercalated within eolianite of the Middle Jurassic Page Sandstone on the Colorado Plateau provides the first direct isotopic age for a Jurassic eolianite on the Colorado Plateau and indicates a chronostratigraphic age of 171.5–169.5 Ma for the Page Sandstone. With stated uncertainties in the age boundaries of global stratigraphic stages taken into account, the isotopic age of lower Bajocian is coordinate with the inferred biostratigaphic age of upper Bajocian from lateral correlation of Page Sandstone with equivalent marine strata. Comparative isotopic ages imply that the Page Sandstone was deposited during the same time frame as the Temple Cap Formation, inferred conventionally to be older, and that the J1 and J2 surfaces of the Colorado Plateau are therefore not reliable chronostratigraphic marker horizons.

A new model for Quaternary lava dams in Grand Canyon based on 40Ar/39Ar dating, basalt geochemistry, and field mapping

The geomorphic response to volcanic incursions is spectacularly documented in western Grand Canyon, where numerous Quaternary lava flows dammed the Colorado River. This paper uses new 40 Ar/ 39 Ar ages, geochemistry, paleomagnetism, and field relationships to suggest 17 damming events, requiring major revision to previously published intracanyon flow sequences. From ca. 850 to 400 ka and at ca. 320 ka, numerous lava dams formed near the modern-day Lava Falls area. Starting around 250 ka, major volcanism shifted to the Whitmore Wash area, where additional dams formed. From ca. 200 to 100 ka, cascades flowed over the north rim in areas between Lava Falls and Whitmore Wash to form the youngest set of lava dams. Field observations and new dam reconstructions require a new model for how the Colorado River interacted with ephemeral lava dams in Grand Canyon. Specifically, the structure of lava dams, the position, character, and provenance of basaltic gravels within and above dams, and cooling structures in intracanyon flows suggest that unstable upstream dam portions failed quickly, while stable downstream dam segments were dismantled by the Colorado River more slowly. Time scales of dam removal are hard to assess, but we infer that lava dams that are overlain by monomictic basalt gravels were removed by the river in tens of years to centuries. In contrast, dams overlain by far-traveled gravel may have persisted for millennia.