Philip A. Pearthree

Displacement rates on the Toroweap and Hurricane faults: Implications for Quaternary downcutting in the Grand Canyon, Arizona

The Toroweap and Hurricane faults, considered to be the most active in Arizona, cross the Uinkaret volcanic field in the western Grand Canyon. These normal faults are downthrown to the west, and the Colorado River crosses these faults as it flows west in the Grand Canyon. Cosmogenic 3 He ( 3 He c ) dates on basalt flows and related landforms are used to calculate vertical displacement rates for these faults. The two faults cross unruptured alluvial fans dated as 3 ka (Toroweap) and 8 ka (Hurricane), and 10 other landforms that range in age from 30 to 400 ka are displaced. Middle and late Quaternary displacement rates of the Toroweap and Hurricane faults are 70–180 and 70–170 m/m.y., respectively. On the basis of these rates, the combined displacement of 580 m on these faults could have occurred in the past 3 to 5 m.y. All 3 He c dates are younger than existing K- Ar dates and are consistent with new 40 Ar/ 39 Ar dates and existing thermoluminescence (TL) dates on basalt flows. These different dating techniques may be combined in an analysis of displacement rates. Downcutting rates for the Colorado River in the eastern Grand Canyon (400 m/m.y.) are at least double the downcutting rates west of the faults (70–160 m/m.y.). Faulting probably increased downcutting in the eastern Grand Canyon relative to downcutting in the western Grand Canyon during the late Quaternary.

An integrated approach to flood hazard assessment on alluvial fans using numerical modeling, field mapping, and remote sensing

Millions of people in the western United States live near the dynamic, distributary channel networks of alluvial fans where flood behavior is complex and poorly constrained. Here we test a new comprehensive approach to alluvial-fan flood hazard assessment that uses four complementary methods: two-dimensional raster-based hydraulic modeling, satellite-image change detection, field-based mapping of recent flood inundation, and surficial geologic mapping. Each of these methods provides spatial detail lacking in the standard method and each provides critical information for a comprehensive assessment. Our numerical model simultaneously solves the continuity equation and Mannings equation (Chow, 1959) using an implicit numerical method. It provides a robust numerical tool for predicting flood flows using the large, high-resolution Digital Elevation Models (DEMs) necessary to resolve the numerous small channels on the typical alluvial fan. Inundation extents and flow depths of historic floods can be reconstructed with the numerical model and validated against field- and satellite-based flood maps. A probabilistic flood hazard map can also be constructed by modeling multiple flood events with a range of specified discharges. This map can be used in conjunction with a surficial geologic map to further refine floodplain delineation on fans. To test the accuracy of the numerical model, we compared model predictions of flood inundation and flow depths against field- and satellite-based flood maps for two recent extreme events on the southern Tortolita and Harquahala piedmonts in Arizona. Model predictions match the field- and satellite-based maps closely. Probabilistic flood hazard maps based on the 10 yr, 100 yr, and maximum floods were also constructed for the study areas using stream gage records and paleoflood deposits. The resulting maps predict spatially complex flood hazards that strongly reflect small-scale topography and are consistent with surficial geology. In contrast, FEMA Flood Insurance Rate Maps (FIRMs) based on the FAN model predict uniformly high flood risk across the study areas without regard for small-scale topography and surficial geology.

A geomorphologic and hydrologic evaluation of an extraordinary flood discharge estimate: Bronco Creek, Arizona

The Bronco Creek, Arizona flood of August 19, 1971, was an extraordinary and rare flash flood. The published peak discharge estimate of 2080 m3 s−1 makes it virtually the worlds largest known rainfall-generated flood to come from a 50-km2 basin. Previous workers have suggested that the peak discharge was overestimated based on its unlikely hydrological and hydraulic implications. In this study we reevaluate the Bronco Creek flood discharge by modeling peak discharges using relict high-water marks in bedrock canyon reaches near the mouths of the three major subbasins of the watershed. The new estimated peak discharge of 750 to 850 m3 s−1 is based on summing the subbasin estimates and correcting for omitted drainage area. Qualitative and quantitative analyses of site-specific and regional hydrological, meteorological, and geomorphological information support a lower peak discharge. We assessed preflood and postflood channel characteristics at the site of the original estimate using historical aerial photographs. Our analysis indicates that much of the discrepancy between the estimates may be explained by significant morphologic changes in the channel caused by the flood. The original estimate was made in a wide, high-gradient alluvial channel where the flood changed the channel pattern from braided with vegetated bars to straight, wide, and devoid of vegetation. It is also likely that net vertical scour occurred in the channel. Although our new estimate is much lower than the original estimate, it is consistent with the dominant trend in regional flood magnitude-drainage area relationships. By integrating hydraulic modeling of peak discharges in stable channel reaches with assessment of ancillary hydrological, meteorological, and geomorphological information, this study presents a viable approach to assessing the accuracy of extreme flood estimates in general. It also highlights potential uncertainties in estimating extreme flood discharges in steep, alluvial channels.

Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA

The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fine sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. We briefly review 87 Sr/ 86 Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. We also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identified as the 4.83 Ma Lawlor Tuff derived from the San Francisco Bay region. We conclude, as have some others, that the Bouse Formation was deposited in lakes produced by first-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that first arrival of river water occurred ca. 4.9 Ma. If this interpretation is correct, addition of Bristol Basin to the Blythe Basin inundation area means that river discharge was sufficient to fill and spill a lake with an area of ∼10,000 km 2 . For spillover to occur, evaporation rates must have been significantly less in early Pliocene time than modern rates of ∼2–4 m/yr, and/or Colorado River discharge was significantly greater than the current ∼15 km 3 /yr. In this lacustrine interpretation, evaporation rates were sufficient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.

Late Holocene geomorphic record of fire in ponderosa pine and mixed-conifer forests, Kendrick Mountain, northern Arizona, USA

Long-term fire history reconstructions enhance our understanding of fire behaviour and associated geomorphic hazards in forested ecosystems. We used 14C ages on charcoal from fire-induced debris-flow deposits to date prehistoric fires on Kendrick Mountain, northern Arizona, USA. Fire-related debris-flow sedimentation dominates Holocene fan deposition in the study area. Radiocarbon ages indicate that stand-replacing fire has been an important phenomenon in late Holocene ponderosa pine (Pinus ponderosa) and ponderosa pine–mixed conifer forests on steep slopes. Fires have occurred on centennial scales during this period, although temporal hiatuses between recorded fires vary widely and appear to have decreased during the past 2000 years. Steep slopes and complex terrain may be responsible for localised crown fire behaviour through preheating by vertical fuel arrangement and accumulation of excessive fuels. Holocene wildfire-induced debris flow events occurred without a clear relationship to regional climatic shifts (decadal to millennial), suggesting that interannual moisture variability may determine fire year. Fire-debris flow sequences are recorded when (1) sufficient time has passed (centuries) to accumulate fuels; and (2) stored sediment is available to support debris flows. The frequency of reconstructed debris flows should be considered a minimum for severe events in the study area, as fuel production may outpace sediment storage.

A late Miocene–early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf of California

We report strontium isotopic results for the late Miocene Hualapai Limestone of the Lake Mead area (Arizona-Nevada) and the latest Miocene to early Pliocene Bouse Formation and related units of the lower Colorado River trough (Arizona-California-Nevada), together with parallel oxygen and carbon isotopic analyses of Bouse samples, to constrain the lake-overflow model for integration of the Colorado River. Sr isotopic analyses on the basal 1–5 cm of marl, in particular along a transect over a range of altitude in the lowest-altitude basin that contains freshwater, brackish, and marine fossils, document the 87 Sr/ 86 Sr of first-arriving Bouse waters. Results reinforce the similarity between the 87 Sr/ 86 Sr of Bouse Formation carbonates and present-day Colorado River water, and the systematic distinction of these values from Neogene marine Sr. Basal Bouse samples show that 87 Sr/ 86 Sr decreased from 0.7111 to values in the range 0.7107–0.7109 during early basin filling. 87 Sr/ 86 Sr values from a recently identified marl in the Las Vegas area are within the range of Bouse Sr ratios. 87 Sr/ 86 Sr values from the Hualapai Limestone decrease upsection from 0.7195 to 0.7137, in the approach to a time soon after 6 Ma when Hualapai deposition ceased and the Colorado River became established through the Lake Mead area. Bouse Formation δ 18 O values range from −12.9‰ to +1.0‰ Vienna Pee Dee belemnite (VPDB), and δ 13 C between −6.5∐ and +3.4‰ VPDB. Negative δ 18 O values appear to require a continental origin for waters, and the trend to higher δ 18 O suggests evaporation in lake waters. Sr and stable isotopic results for sectioned barnacle shells and from bedding planes of the marine fish fossil Colpichthys regis demonstrate that these animals lived in saline freshwater, and that there is no evidence for incursions of marine water, either long-lived or brief in duration. Lack of correlation of Sr and O isotopic variations in the same samples also argue strongly against systematic replacement of Sr in Bouse carbonates after deposition. Our results reinforce the conclusion that the Bouse Formation was deposited in a descending series of basins connected by overflow of Colorado River water. The Hualapai Limestone records a separate and earlier lake that may have been progressively influenced by Colorado River water as the time of river integration approached.

Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions

New detrital zircon U-Pb age distributions from 49 late Cenozoic sandstones and Holocene sands (49 samples, n = 3922) record the arrival of extra-regional early Pliocene Colorado River sediment at Grand Wash (western USA) and downstream locations ca. 5.3 Ma and the subsequent evolution of the river’s provenance signature. We define reference age distributions for the early Pliocene Colorado River (n = 559) and Holocene Colorado River (n = 601). The early Pliocene river is distinguished from the Holocene river by (1) a higher proportion of Yavapai-Mazatzal zircon derived from Rocky Mountain basement uplifts relative to Grenville zircon from Mesozoic supra crustal rocks, and (2) distinctive (∼6%) late Eocene–Oligocene (40–23 Ma) zircon reworked from Cenozoic basins and volcanic fields in the southern Rocky Mountains and/or the eastern Green River catchment. Geologic relationships and interpretation of 135 published detrital zircon age distributions throughout the Colorado River catchment provide the interpretative basis for modeling evolution of the provenance signature. Mixture modeling based upon a modified formulation of the Kolmogorov-Smirnov statistic indicate a subtle yet robust change in Colorado River provenance signature over the past 5 m.y. During this interval the contribution from Cenozoic strata decreased from ∼75% to 50% while pre-Cretaceous strata increased from ∼25% to 50%. We interpret this change to reflect progressive erosional incision into plateau cover strata. Our finding is consistent with geologic and thermochronologic studies that indicate that maximum post–10 Ma erosion of the Colorado River catchment was concentrated across the eastern Utah–western Colorado region.

Paleogeomorphology and evolution of the early Colorado River inferred from relationships in Mohave and Cottonwood valleys, Arizona, California, and Nevada

Geologic investigations of late Miocene–early Pliocene deposits in Mohave and Cottonwood valleys provide important insights into the early evolution of the lower Colorado River system. In the latest Miocene these valleys were separate depocenters; the floor of Cottonwood Valley was ∼200 m higher than the floor of Mohave Valley. When Colorado River water arrived from the north after 5.6 Ma, a shallow lake in Cottonwood Valley spilled into Mohave Valley, and the river then filled both valleys to ∼560 m above sea level (asl) and overtopped the bedrock divide at the southern end of Mohave Valley. Sediment-starved water spilling to the south gradually eroded the outlet as siliciclastic Bouse deposits filled the lake upstream. When sediment accumulation reached the elevation of the lowering outlet, continued erosion of the outlet resulted in recycling of stored lacustrine sediment into downstream basins; depth of erosion of the outlet and upstream basins was limited by the water levels in downstream basins. The water level in the southern Bouse basin was ∼300 m asl (modern elevation) at 4.8 Ma. It must have drained and been eroded to a level

Paleoseismology and Neotectonics of the Shivwits Section of the Hurricane Fault, Northwestern Arizona

The Shivwits section of the Hurricane Fault in northwestern Arizona has been largely ignored in evaluating the seismic hazard posed to the rapidly growing populations of southwestern Utah. To assess this hazard, we conducted studies along the Shivwits section using field observations and geomorphic modeling to understand the Quaternary tectonism of this portion of the Hurricane Fault. We have found evidence that it ruptured with up to 2 to 3 m of vertical displacement per event and likely produced ∼ M 7 earthquakes. Our results suggest that the slip rate along the southern Hurricane Fault has not decreased during the Quaternary. The Moriah Knoll basalt, offset 150 to 200 m along the Hurricane Fault, yielded a maximum long-term slip rate of 0.15 to 0.25 mm/yr, estimated using a new 40Ar/39Ar age of 0.85 ± 0.06 Ma. The late Quaternary slip rates on alluvial fan surfaces offset 2 to 7.4 m were estimated using pedogenic carbonate rind thickness as a calibrated proxy for age. These observations yielded slip rates of ∼0.05 to 0.3 mm/yr. Paleoseismic trench investigations showed that two surface-rupturing events occurred in the past 15-78 k.y. A radiocarbon sample from a most-recent event (MRE) fissure yielded a calibrated age of 8900-10,400 years B.P.; the penultimate event was likely ≥10 k.y. before the most recent event. Slip-rate estimates using the Moriah Knoll basalt (∼0.15-0.24 mm/yr; 850 ka), surface offset (∼0.05-0.3 mm/yr; <100 ka), morphologic modeling (∼0.06-0.21 mm/yr; <100 ka), and observations from the trench (∼0.06-0.34 mm/yr; 15-75 ka) suggest that there has been no detectable change in slip rate in the past 1 million years or so. This implies a constant deformation rate for this portion of the Colorado Plateau Margin; therefore, Basin and Range extension is actively encroaching. Manuscript received 24 November 2003.

Comment on "Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table–Type Speleothems"

Polyak et al. (Reports, 7 March 2008, p. 1377) reported that development of the western Grand Canyon began about 17 million years ago. However, their conclusion is based on an inappropriate conflation of Plio-Quaternary incision rates and longer-term rates derived from sites outside the Grand Canyon. Water-table declines at these sites were more likely related to local base-level changes and Miocene regional extensional tectonics.