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P. A. Pearthree

Publications and source records attributed to P. A. Pearthree.

10 recordsLinked to original sources
USGS2021

Redefining the age of the lower Colorado River, southwestern United States

Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto–Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6–6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level.

Arizona, California, Nevada↗
USGS2011

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 14 C 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.

Arizona↗
USGS2010

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.

Arizona, Baja California, California, Nevada, Sono↗
USGS2008

Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona

Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach of the river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to the north and Mohave Valley to the south. Lacustrine marls interfingered with and conformably overlying a sequence of post-5.6 Ma finegrained valley-fill deposits record an early phase of intermittent lacustrine inundation restricted to Cottonwood Valley. Limestone, mud, sand, and minor gravel of the Bouse Formation were subsequently deposited above an unconformity. At the north end of Mohave Valley, a coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan deposits from subaqueous deposits of the Bouse Formation. We interpret this key unit as evidence for overtopping and catastrophic breaching of the paleodivide immediately before deep lacustrine inundation of both valleys. Exposures in both valleys reveal a substantial erosional unconformity that records drainage of the lake and predates the arrival of sediment of the through-going Colorado River. Subsequent river aggradation culminated in the Pliocene between 4.1 and 3.3 Ma. The stratigraphic associations and timing of this drainage transition are consistent with geochemical evidence linking lacustrine conditions to the early Colorado River, the timings of drainage integration and canyon incision on the Colorado Plateau, the arrival of Colorado River sand at its terminus in the Salton Trough, and a downstream-directed mode of river integration common in areas of crustal extension. ?? 2008 The Geological Society of America.

Conference Paper↗
USGS2008

An evaluation of the evolution of the latest miocene to earliest pliocene bouse lake system in the lower Colorado river valley, southwestern USA

The upper Miocene to lower Pliocene Bouse Formation in the lower Colorado River trough of the American Southwest was deposited in three basins - from north to south, the Mohave, Havasu, and Blythe Basins - that were formed by extensional fault ing in the early to middle Miocene. Fossils of marine, brackish, and freshwater organ isms in the Bouse Formation have been interpreted to indicate an estuarine environment associated with early opening of the nearby Gulf of California. Regional uplift since 5 Ma is required to position the estuarine Bouse Formation at present elevations as high as 555 m, where greater uplift is required in the north. We present a compilation of Bouse Formation elevations that is consistent with Bouse deposition in lakes, with an abrupt 225 m northward increase in maximum Bouse elevations at Topock gorge north of Lake Havasu. Within Blythe and Havasu Basins, maximum Bouse elevations are 330 m above sea level in three widely spaced areas and reveal no evidence of regional tilting. To the north in Mohave Basin, numerous Bouse outcrops above 480 m elevation include three widely spaced sites where the Bouse Formation is exposed at 536-555 m. Numerical simulations of initial Colorado River inflow to a sequence of closed basins along the lower Colorado River corridor model a history of lake filling, spilling, evaporation and salt concentration, and outflow-channel incision. The simulations support the plausibility of evaporative concentration of Colorado River water to seawater-level salinities in Blythe Basin and indicate that such salinities could have remained stable for as long as 20-30 k.y. We infer that fossil marine organ isms in the Bouse Formation, restricted to the southern (Blythe) basin, reflect coloniza tion of a salty lake by a small number of species that were transported by birds.

Conference Paper↗
USGS2007

Impact of recent extreme Arizona storms

Heavy rainfall on 27–31 July 2006 led to record flooding and triggered an historically unprecedented number of debris flows in the Santa Catalina Mountains north of Tucson, Ariz. The U.S. Geological Survey (USGS) documented record floods along four watercourses in the Tucson basin, and at least 250 hillslope failures spawned damaging debris flows in an area where less than 10 small debris flows had been documented in the past 25 years. At least 18 debris flows destroyed infrastructure in the heavily used Sabino Canyon Recreation Area (http://wwwpaztcn.wr.usgs.gov/rsch_highlight/articles/20061 l.html). In four adjacent canyons, debris flows reached the heads of alluvial fans at the boundary of the Tucson metropolitan area. While landuse planners in southeastern Arizona evaluate the potential threat of this previously little recognized hazard to residents along the mountain front, an interdisciplinary group of scientists has collaborated to better understand this extreme event.

Arizona↗
USGS2005

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 Manning's 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.

Arizona↗
USGS2004

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.

Bulletin of the Seismological Society of America↗