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CRevolution 2: Origin and Evolution of the River System II themed issue

Introduction: CRevolution 2: Origin and Evolution of the System II

Karl E. Karlstrom1, L. Sue Beard2, Kyle House2, Richard A. Young3, Andres Aslan4, George Billingsley2, and Joel Pederson5 1Department of Earth and Planetary Sciences, University of New , Albuquerque, 87106, USA 2U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, 86001, USA 3Department of Geological Sciences, State University of New York at Geneseo, New York 14454, USA 4Department of Geology, Colorado Mesa University, Grand Junction, Colorado 81501, USA 5Department of Geology, State University, Logan, Utah 84322, USA

ABSTRACT BACKGROUND the evolution of the entire since Creta- ceous time, and his Colorado River synthesis A 2010 Colorado River symposium held Studies of the origin and evolution of the (Hunt, 1969) involved discussions of interacting in Flagstaff, Arizona, in May 2010, had 70 Colorado River System are central to under- geomorphic and structural controls on Colorado participants who engaged in intense debate standing the Cenozoic tectonic and geomor- Plateau drainages through time. about the origin and evolution of the Colo- phic evolution of the western U.S. orogenic In summary of the continuing debate, Hunt rado River system. This symposium, built plateau. This region was uplifted from sea level (1969, p. 63) stated, “The view that the Colo- on two previous decadal scientifi c meetings, in the late , to present elevations rado River is an ancient river considers the river focused on forging scientifi c consensus where that exceed 4 km in the Rocky Mountains and as a whole from the time of fi rst uplift of the possible, while also articulating continued 1.5 km over large of the Colorado Pla- present Rocky Mountains; the view that the controversies regarding the Cenozoic evolu- teau. The Colorado River is the trunk river of the river is young is based on particular segments.” tion of the Colorado River System and the single river system that drains the western slope This was a glimpse of subsequent controversies landscapes of the –Rocky of the Rockies and the entire Colorado Plateau, that attempt to reconstruct the regional picture Mountain region that it drains. New devel- and hence is central to understanding the uplift by study of both regional uplift history and indi- opments involved hypotheses that and history of the region. vidual segments of the river system. Additional mantle fl ow is driving plateau tilting and dif- The timing of the initial development of the advances in our understanding of the complexi- ferential uplift, with consensus that multidis- Colorado River, and its evolution into the drain- ties of the river system have been punctuated ciplinary studies involving differential inci- age network seen today, have been the focus of by three collaborative meetings in northern Ari- sion studies and additional geochronology over a century of research, since the early sci- zona, in 1964, 2000, and 2010. This paper pro- and thermochronology are needed to test entifi c trips of J.W. Powell down the Green and vides brief refl ections on the fi rst two meetings the relative importance of tectonic and geo- Colorado river systems. This fi eld laboratory, and a summary of the 2010 meeting. Our goal morphic forcings in shaping the spectacular because of its spectacular exposure, has been is to foster continued research on western U.S. landscapes of the Colorado Plateau region. at the forefront of scientifi c breakthroughs in landscape evolution at all scales. In addition to the scientifi c goals, the meet- geomorphology, stratigraphy, paleontology, and ing participants emphasized the iconic status tectonics for over a century (Dutton, 1882). 1964 MEETING: MUSEUM OF of Grand for geosciences, and the In early syntheses (Powell, 1875, 1879; NORTHERN ARIZONA COLORADO importance of good communication between Dutton, 1882), the Colorado River system was RIVER SYMPOSIUM the research community, the geoscience edu- presumed to be ancient and antecedent, follow- cation/interpretation community, the public, ing the path of today’s west-fl owing river system The fi rst meeting had 21 participants. It and the media. Building on a century-long that carries snowmelt from the Rocky Moun- was an outgrowth of discussions between tradition, this region still provides a globally tains to the Pacifi c. Longwell (1928, p. 143) Eddie McKee and Dick Young during visits to important natural laboratory for studies of noted the problem (“Muddy Creek problem”) McKee’s U.S. Geological Survey offi ce in Den- the interactions of erosion and tectonism in that the Colorado River did not exit the western ver, Colorado, related to Young’s PhD (Young, the shaping landscape of elevated plateaus. edge of the Colorado Plateau during the Plio- 1966), funded in part by the Museum of North- cene, the lower boundary of which was placed ern Arizona (MNA awards were $1000 each for at ~11 Ma until the early 1970s. Blackwelder the summers of 1962–1965). Young’s fi eldwork *Emails: Karlstrom: [email protected]; Beard: (1934) proposed that the regional river and can- on the Hualapai Reservation and Ivo Lucchitta ’s [email protected]; House: [email protected]; Young: [email protected]; Aslan: aaslan@ yon system did not exist until the , work in the region (Lucchitta, 1966, coloradomesa.edu; Billingsley: gbillingsley@usgs before which time there was a general lack of 1972) evolved with close interaction. Contrary .gov; Pederson: [email protected]. integrated river systems. Hunt (1956) outlined to the account in Ranney (2005), the new data

Geosphere; December 2012; v. 8; no. 6; p. 1170–1176; doi:10.1130/GES00716.1; 1 table. Received 28 April 2011 ♦ Accepted 15 August 2012 ♦ Published online 16 November 2012

1170 For permission to copy, contact [email protected] © 2012 Geological Society of America

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and participation of the two PhD students were time. There seemed to be no other place for the Beard, Keith Howard, and Ivo Lucchitta. Grand a major focus of the 1964 meeting. The sympo- ancestral river to go. Bidahochi ages have been Canyon National Park superintendent Robert L. sium began with a 3-day fi eld trip to the Lake revised since then (16–6 Ma), and an alternate Arnberger underwrote the major expenses for Mead country, then to Milkweed and Peach southern escape route through the ancestral Salt the meeting, with other fi nancial and informal Springs . This was followed by formal River Canyon has been resurrected by Potoch- donations of materials and personnel time pro- group discussions at the MNA for the better nik (this themed issue). Much of the uncertainty vided by the USGS Flagstaff, Arizona, offi ce part of a week. No formal talks, and very few concerning the timing of events as perceived in (George Billingsley, Sue Beard, Sue Priest), slides, were allowed. Most of the data provided 1964 needs to be put in the context of the rela- Association (Greer Price), by participants, other than Young and Lucchitta, tive lack of precise geochronology on key Ceno- SUNY Geneseo Department of Geological Sci- had been previously published, and all the infor- zoic units. ences (R.A. Young), NAU Departments of Geol- mation from the symposium was ultimately Nevertheless, the main accomplishment of ogy and Geography (Michael Ort, Lee Dexter), integrated into McKee et al. (1967). Gene Shoe- the 1964 symposium discussions was to more the Arizona Geological Survey (Jon Spencer ), maker attended sporadically due to his urgent clearly focus the state of knowledge for differ- and the Bureau of Mines (James Apollo Project commitments, but he was a ent parts of the plateau and to combine the ideas Faulds). The meeting was coordinated by Greer dynamic and infl uential force during the formal of major researchers, other than those of Charlie Price, with assistance from Tom Pittinger of discussions and the fi eld trips. Charlie Hunt, Hunt. McKee et al. (1967) re-articulated some the for accommodations, in spite of his infl uential works (Hunt, 1956, of the main questions about the Colorado River meals, and meeting facilities. 1969), did not attend the 1964 symposium . evolution: (1) time of initiation, (2) processes of Among other specifi c advances, many papers In retrospect, two things stand out about the integration, and (3) early paleodrainage courses. used relatively new dating techniques applied overall events at the 1964 symposium. First, There was continued emphasis on river seg- to high level terraces of the river system to some senior geologists (especially McKee, ments that may have had different earlier histo- infer incision rate data. The emerging picture Shoemaker, and Koons) refused to readily ries and been integrated into the Colorado River was a river with both spatially and temporally accept the idea that the paleodrainage chan- system that we see today in post–Muddy Creek varying incision rates along its course, with dif- nels on the Hualapai Plateau that converge on time. Then, as now, there was little consensus ferential incision rates related to both geomor- Peach Springs Canyon actually fl owed north- about pre–6 Ma river geometries, but the stage phic and structural controls. The existence of a east and exited to the north across the course of was set for continued debate. well-integrated ancestral upper Colorado River the modern Grand Canyon. This was in spite drainage system in Colorado and southern Utah, of the undeniable fi eld data from gravel imbrica- 2000 GRAND CANYON MEETING: as postulated by Hunt (1969), was not strongly tion and clast lithologies. The conceptual problem COLORADO RIVER, ORIGIN supported, with continued debate about where was that these deep Tertiary canyons seemed AND EVOLUTION ancestral upper Colorado River water and sedi- to them to be heading into a “deep hole” from ment loads would have been stored before inte- which there was no obvious outlet at appropriate This summary is modifi ed from Young and gration at the mouth of Grand Canyon between elevations on the other side of Grand Canyon. Spammer (2001, p. 1–3). This meeting had 73 4 and 5 Ma. The in east- The concept of NE tilting of the Plateau dur- formal registrants and was held at Grand Can- ern Arizona as evidence for such a Miocene lake ing Laramide uplift to solve this issue of gra- yon National Park in June 2000. By the time of seemed more acceptable from a chronologic dients (Young, 1982) was not strongly argued the 2000 meeting, the maturation and practical perspective, but not necessarily from a sedimen- until after McKee et al. (1967). There was application of plate tectonics concepts, much tological viewpoint. Timing of Colorado Pla- also hesitancy to accept the idea that the oldest more fi eldwork, and many more K-Ar ages teau uplift(s) remained controversial with both basal arkosic Rim gravels could be older than had improved the chronology and order of late Tertiary uplift and older Laramide uplift Miocene (now known to be or late events dramatically. Yet, the central problems proponents. Late Pleistocene incision rates were Cretaceous by Young and Hartman, this themed of where to send a postulated Miocene river, reported to be rapid enough to carve Grand issue). The subsequent McKee and McKee and how Colorado River integration occurred, Canyon within the last 10 Ma, but this raised (1972) article on “Pliocene Uplift” of the Colo- remained unresolved. the sig nifi cant issue of why rapid incision of the rado Plateau to explain the “Rim gravels” attests The meeting and resulting collection of entire basin did not begin and progress rapidly to the diffi culty of changing minds about that papers was an outgrowth of informal conversa- more immediately following Miocene Basin history, despite the 18.5 Ma age of the Peach tions among Colorado Plateau geologists dur- and Range extension, when appreciable relief Springs Tuff that caps the gravels in the Huala- ing the 1990s. The purpose of the symposium developed between the Colorado Plateau and pai sections (Young and Brennan, 1974) and was to update the status of current knowledge the extended terrane to the west. There was still even given that the Pliocene-Miocene bound- of the geologic issues, controversies, and prog- no consensus about mechanisms by which dif- ary at that time had recently been moved from ress surrounding the geologic evolution of the ferent river segments may have been integrated. 11 Ma to 5 Ma. Colorado Plateau and the Colorado River during Both lake spillover and headward erosion mod- Second, there were few individuals at the Cenozoic time. The meeting (5–11 June 2000) els were advanced again, and other controver- meeting who knew factual details of the little- was coordinated by R.A. Young, with signifi cant sies were aired: (1) when did canyon cutting fi rst studied Cenozoic history of the Little Colorado input from George Billingsley and fi eld trips led begin, (2) which way rivers were fl owing in the River Valley and environs. Therefore, despite by Michael Ort to view the Bidahochi Forma- early Tertiary, (3) how much rock and the perceived young age of the Bidahochi tion stratigraphy and Andre Potochnik to view Cenozoic sediment overlay the Kaibab surface deposits (~6–4 Ma age at the time; McKee et al., geology, and with a postmeet- in different areas and how fast was erosion 1967), it was decided to “send” the Colorado off ing fi eld trip to view the Tertiary geology of the denuding the landscape, and (4) how the 5–6 Ma to the south (by default), presumably accom- Hualapai and Lake Mead Bidahochi, Hualapai and Bouse lake systems panying ponding of the drainage in Bidahochi areas led by Richard Young, James Faulds, Sue were related to a through-going Colorado River.

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Overall, the 2000 meeting marked renewed all aimed at understanding driving mechanisms, et al.; Beard and Faulds), whereas the post–6 Ma progress on all aspects of the Cenozoic evolu- timing, and magnitudes of differential river inci- Colorado River fl ows southwest. Debate contin- tion of the Colorado River system. At the same sion and landscape denudation and their tec- ues about the timing and drainage geometry of time that a plethora of models were discussed, tonic connections in this classic landscape. most of the pre–6 Ma paleorivers and the mecha- this meeting seemed to mark an attempt to Table 1 lists the abstracts presented at the nisms driving drainage reversals. compile all objective criteria for the timing of meeting; see Beard et al. (2010), for the text and Mid-Tertiary erosion. Middle Tertiary time, various surfaces and paleosurfaces, the tim- fi gures of these extended abstracts. The agenda after the Chuska erg (Cather), represented a time ing of cooling of rocks as they were unroofed of the meeting (Beard et al., 2010) preceded of regional deep erosion on parts of the Colorado toward today’s surface, and rates of river inci- from the , up the Lower Colorado Plateau that is documented by ~25 Ma cooling sion through time. The meeting and resulting River system, through Grand Canyon, across the based on thermochronology data in Grand Can- volume catalyzed renewed research and the Plateau, to the Rocky Moun- yon and the (Kelley et al.; integration of diverse scientifi c approaches all tain headwaters. Like the 2000 workshop, the Lee et al.). Tectonic infl uences on this denuda- aimed at resolving the landscape evolution of 2010 workshop reinvigorated research on the tion are debated. the Colorado Plateau/Grand Canyon region. It Colo rado River region in the context of regional Age of the upper Colorado River system. also spawned more popular treatments of long- and global questions about tectonic and geo- Gravels exist beneath 25 Ma basalts (Aslan standing controversies about evolution of Grand morphic processes that shape landscapes. et al.) and there are west-draining Oligocene Canyon (Ranney, 2005; Powell, 2005). Invitees submitted extended online abstracts paleocanyons in the Gunnison region (Sandoval Continued challenges and questions were to an Internet site (https://sites.google.com/site et al.). By 10 Ma, evidence of a paleo–Colorado identifi ed at the meeting: (1) What were the /crevolution2/home) so that all participants could River in the Colorado Rockies is seen in grav- causes and precise timing of plateau uplift(s)? access and read these informal contributions els beneath the 10–11 Ma Grand Mesa basalt (2) How much Mesozoic and Cenozoic sediment before the meeting. The format of the workshop (Aslan et al.; Cole) and several other ~10 Ma covered the Grand Canyon region and when did was designed to encourage discussion and data basalts (Lazear et al.). Onset of rapid incision it get stripped off? (3) Could a western Grand compilation in a format that differed from the for- and denudation in the upper Colorado River Canyon precursor stream have existed without mal talks presented at most professional meet- paleodrainages took place between 10 and leaving a preserved sedimentary record near the ings. Oral remarks were limited to 5 minutes and 6 Ma, as documented by thermochronology present mouth of Grand Canyon? (4) Did inte- were followed by extensive plenary discussion from the MWX well (Karlstrom et al.). gration across the Kaibab uplift take place by among the participants. Products of the work- Age of the Lower Colorado system. The headward erosion or “basin spillover”? (5) What shop include this summary report, developed, in 5–6 Ma age of integration of the Colorado River potential role did local or global climate change part, at the workshop. Electronic databases and system as we know it today, across the Kaibab play either in enhancing or delaying the incision resources on geochronology and incision data, as uplift to the Gulf of , continues to be and integration of the Colorado River system? well as useful maps and images of the Colorado supported based on data from the 5.3 Ma age of River system developed for this meeting (see the fi rst sediments arriving in the Gulf (Dorsey; 2010 FLAGSTAFF MEETING: website) will also be submitted as separate con- Kimbrough et al.), lack of Colorado River sedi- CREVOLUTION 2: ORIGIN AND tributions to this Geosphere themed issue. ments in the Grand Wash trough (Muddy Creek EVOLUTION OF THE COLORADO constraint; Lucchitta), and geometry of late RIVER SYSTEM II Toward Consensus Miocene alluvial fans that are now dissected by the Colorado River and Grand Canyon (Luchitta The 2010 meeting, reported on herein, fol- The meeting moved toward consensus on sev- et al.). Comparison of sedimentary budgets sug- lows in the footsteps of prior meetings in several eral topics. The references in this section refer to gests that the volume of sediment in sedimentary important respects. It represented an assembly abstracts listed in Table 1 (also in Beard et al., basins of southern California is roughly compat- of many of the key scientists and their stu- 2010), and many of these papers are elaborated ible with estimates for erosion of material off dents researching the evolution of the Colo- on and updated in this themed issue. the Colorado Plateau in the last 6 Ma (Dorsey), rado River system. There were 70 registered Multiple episodes of erosion and uplift. Punc- compatible with post–6 Ma integration. Debate participants. This meeting followed a compre- tuated episodes of erosion, and inferred uplift, continues about pre–6 Ma paleocanyons that hensive regional approach (e.g., Hunt, 1956) took place in the Laramide (Wernicke; Lee may have become reused and linked to evolve that involves detailed studies from the Gulf of et al.; Young and Hartman), in the middle Tertiary into the modern Grand Canyon (Young, 2008). California to the , including appli- (Cather; Lee et al.), and in the last 10 million Lake spillover along the lower Colorado. Lake cation of the latest geochronologic and analyti- (Karlstrom et al.; Hoffman et al.), as sup- spillover models for the lower Colorado River cal techniques to quantify rates and to model ported by regional geologic and thermo chrono- (House et al.; Howard) are increasingly well processes of landscape evolution. New aspects logic data (Kelley et al.; Lee et al.). There is documented by mapping of Pliocene deposits for of this meeting were: (1) the examination of links continued debate regarding durations and nature Mojave Basin, and there was continued support between mantle processes and their potential of tectonic and/or climatic forcings, and which for a lacustrine origin for the Bouse Formation in surface effects, (2) discussion and quantifi ca- episode was dominant in a given region or reach the Mojave-Parker reaches (House et al.; Malmon tion of the isostatic response to denudation that of the river system. et al.). Contrary to some older models for marine affects landscapes, (3) increased emphasis and Drainage reversal(s). The concept of drain- origin, Sr, O, and C isotopes support a lacustrine emerging syntheses of low-T thermochronology age reversal seems well established. Rivers origin for the upper Bouse (Mojave Basin) and (apatite fi ssion-track and apatite helium stud- fl owed north (Davis et al.; Hill et al.), or north- Hualapai (Spencer et al.; Crossey ies), (4) discussion of groundwater sapping as an east and east (Wernicke; Potochnik), during the et al.; Lopez Pearce et al.). important river integration mechanism, and (5) late Cretaceous (Wernicke), and – Bullhead aggradation. Major aggradation in greater emphasis on process-oriented studies, (Davis et al.; Young and Hartman; Young the lower Colorado River at ~5.5–3.3 Ma is well

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TABLE 1. ABSTRACTS PRESENTED AT THE 2010 MEETING—ALL CAN BE REFERENCED AS, 2010, IN BEARD ET AL., 2010, CREVOLUTION 2— ORIGIN AND EVOLUTION OF THE COLORADO RIVER SYSTEM, WORKSHOP ABSTRACTS: USGS OPEN-FILE REPORT OF-2011-1210 Amoroso, L., Felger, T., and Wan, E., The Willow Beach beds—A pre–Colorado River axial-basin deposit. Aslan, A., and the CREST Working Group, Origin of the ancestral Colorado and Gunnison Rivers and post-10 Ma river incision rates in western Colorado. Beard, L.S., and Faulds, J.E., Kingman uplift, paleovalleys and extensional foundering in northwest Arizona. Blakey, R.C., Ranney, W., and Loseke, T., Oligocene–Early Miocene incision, strike-valley development, and aggradation, Mogollon rim, Verde Valley region, Arizona—A potential analogue for pre–Grand Canyon development. Cather, S.M., Late Oligocene–early Miocene deep erosion on the southern Colorado Plateau and the southern . Cole, R.D., Signifi cance of the Grand Mesa basalt fi eld in western Colorado for defining the early history of the upper Colorado River. Crossey, L.J., Karlstrom, K.E., Lopez-Pearce, J., and Dorsey, R., Geochemistry of springs, travertines and lacustrine carbonates of the Grand Canyon region over the past 12 million years: The importance of groundwater on the evolution of the Colorado River system. Crow, R., Karlstrom, K.E., and McIntosh, W., Incision history of Grand Canyon from dated Colorado River gravels. Darling, A., Karlstrom, K.E., Aslan, A., and Granger, D., Differential incision rates in the upper Colorado River system: Implications for knickpoint transience. Davis, S.J., Dickinson, W.R., Gehrels, G.E., Spencer, J.E., Lawton, T.F., and Carroll, A.R., The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons. Dickinson, W.R., Bidahochi paleogeography and incision of the Grand Canyon. Dorsey, R.J., A sediment budget for the Colorado River. Douglass, J., One Grand Canyon but four mechanisms: Was it antecedence, superimposition, overflow, or piracy? Embid, E.H., Crossey, L.J., and Karlstrom, K.E., Incision history of the based on K-Ar dating of basalts and U-series dating of travertine in the Springerville area. Felger, T.J., Fleck, R.J., and Beard, S.J., Miocene-Pliocene basalt flows on the east and west flanks of Wilson Ridge, Arizona, preserve multiple stages in the depositional history of adjacent Detrital Wash and Black Canyon basins, and may help constrain timing of incision by the Colorado River. Ferguson, C.A., Powder Rim gravel, deposit of a late Miocene, north-flowing river through the -Colorado-Utah borderland. Hanks, T., Blair, L., Cook, K., Davis, M., Davis, S., Finkel, B., Garvin, C., Heimsath, A., Lucchitta, I., Webb, B., Whipple, K., and Young, D., Incision rates of the Colorado River in . Hill, C., Ranney, W., and Buecher, B., A working model for the evolution of the Grand Canyon/Colorado Plateau Region: Laramide to present. Hoffman, M., Stockli, D., Kelley, S., Pederson, J., and Lee, J., Mio-Pliocene erosional exhumation of the central Colorado Plateau, eastern Utah: New insights from apatite (U-Th)/He thermochronometry. House, P.K., Pearthree, P.A., Brock, A.L., Bell, J.W., Ramelli, A.R., Faulds, J.E., and Howard, K.A., Robust geologic evidence for latest Miocene–earliest Pliocene river integration via lake spillover along the Lower Colorado River: Review and new data. Howard, K., Pliocene aggradational sequence of the lower Colorado River in longitudinal profi le. Howard, K.A., and Malmon, D.V., Boulders deposited by Pliocene and Pleistocene floods on the lower Colorado River. Howard, K., Malmon, D., McGeehin, J., and Martin, P., aggradation of the lower Colorado River in , California and Arizona. Karlstrom, K.E., Coblentz, D., Ouimet, W., Kirby, E., Van Wijk, J., Schmandt, B., Crossey, L.J., Crow, R., Kelley, S., Aslan, A., Darling, A., Dueker, K., Aster, R., MacCarthy, J., Lazear, G., and the CREST Working Group, Evidence from the Colorado River system for surface uplift of the Colorado Rockies and Western Colorado Plateau in the last 10 Ma driven by mantle flow and buoyancy. Kelley, S.A., Karlstrom, K.E., Stockli, D., McKeon, R., Hoffman, M., Lee, J., Pederson, J., Garcia, R., and Coblentz, D., A summary and evaluation of thermochronologic constraints on the exhumation history of the Colorado Plateau–Rocky Mountain region. Kimbrough, D., Grove, M., Gehrels, G.E., Mahoney, B., Dorsey, R.J., Howard, K.A., House, P.K., Peartree, P.A., and Flessa, K., Detrital zircon record of Colorado River integration into the Salton trough. Lazear, G.D., Karlstrom, K.E., Aslan, A., Schmandt, B., and the CREST Working Group, Denudational flexural isostacy of the Colorado Plateau: Implications for incision rates and tectonic uplift. Lee, J.P., Stockli, D.F., Kelley, S., and Pederson, J., Unroofing and incision of the Grand Canyon region as constrained through low-temperature thermochronology. Lopez Pearce, J., Crossey, L.J., Karlstrom, K.E., Gehrels, G., Pecha, M., Beard, S., and Wan, E., Syntectonic deposition and paleohydrology of the spring-fed Hualapai limestone and implications for the 6–5 Ma integration of the Colorado River system through the Grand Canyon. Lucchitta, I., The Muddy Creek Formation at the mouth of the Grand Canyon: Constraint or chimera? Lucchitta, I., Holm, R.F., and Lucchitta, B.K., Crooked Ridge of Northern Arizona: A precursor drainage of the Colorado River system. Malmon, D.V., Howard, K., and Hillhouse, J.W., New observations of the Bouse Formation in Chemehuevi and Parker Valleys. Marchetti, D.W., Bailey, C.M., Hynek, S.A., and Cerling, T.E., Quaternary geology and geomorphology of the Fremont River , south-central Utah. Martin, M.E., and Reynolds, S.J., Geologic evolution of the mid-Tertiary Ash Creek Paleovalley, Black Hills, central Arizona. Matmon, A., Stock, G.M., Granger, D.E., and Howard, K.A., Cosmogenic burial dating of Pliocene Colorado River sediments. McDougall, K., Update on microfossil studies in the northern Gulf of California, Salton Trough, and lower Colorado River. Pederson, J., Drainage integration through Grand Canyon and the Uintas—Hunt and Hansen’s groundwater-driven piracy via paleocanyons. Pederson, J., Tressler, C., Cragun, S., Mackey, R., and Rittenour, T., The Colorado Plateau bullseye of erosion and uplift—Linking patterns of quantified rates, amounts, and rock strength. Potochnik, A., Ancestral Colorado River exit from the plateau province; Salt River hypothesis. Resor, P.G., and Seixas, G., A tale of two monoclines. Robert, X., Moucha, R., Whipple, K., Forte, A., and Reiners, P., Cenozoic evolution of the Grand Canyon and the Colorado Plateau driven by mantle dynamics? Sandoval, M.M., Karlstrom, K.E., Darling, A., Aslan, A., Granger, D., Wan, E., Noe, D., and Dickinson, R., Quaternary incision history of the Black Canyon of the Gunnison, Colorado. Spencer, J.E., Patchett, P.J., Roskowski. J.A., Pearthree, P.A., Faulds, J.E., and House, P.K., A brief review of Sr isotopic evidence for the setting and evolution of the Miocene-Pliocene Hualapai-Bouse Lake system. Tressler, C., Pederson, J., and Macley, R., The hunt for knickzones and their meaning along the Colorado—Signatures of transience after integration, bed resistance, or differential uplift? Umhoefer, P., Lamb, M., and Beard, S., Updates on the tectonics and paleogeography of the Lake Mead region from ~25 to ~8 Ma: Lakes and local drainages within an extending orogen, but no through-going river? Wernicke, B., The California River and its role in carving Grand Canyon. Young, R.A., and Hartman, Early Cenozoic “Rim gravel” of Arizona: Age, distribution and geologic significance. Young, R.A., Crow, R., and Peters, L., Oligocene tuff corroborates older Paleocene-Eocene age of Hualapai Plateau basal Tertiary section.

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documented by the Bullhead and related gravels western U.S., based on new geophysical images south-fl owing river (Pederson) and became inte- (House et al.; Howard et al.), although explana- that show large contrasts in mantle velocity grated with drainage from the Colorado Rockies, tions for this event and for the steep profi le at (and inferentially, temperature and rheology) and whether the different gradients refl ect differ- the top of the aggradational sequence (Howard) over <100 km spatial scales. In addition, petrol- ential uplift of the Colorado Rockies relative to are debated. ogy of volcanic rocks shows both astheno- the central Colorado Plateau (Karlstrom et al.). Integration processes. In general, river inte- spheric and lithospheric sources for Neogene However, if the Green River switched its fl ow gration processes of lake spillover, headward basalts (Karlstrom et al.). Geodynamic models direction from north- to south-fl owing in the latest erosion, and groundwater sapping (Crossey suggest that observed mantle velocity varia- Miocene (Ferguson), that may allow the uplift et al.; Pederson; Hill et al.) were all consid- tion should drive surface uplift and subsidence rates to be roughly equivalent. ered viable processes (Douglass), and may have (Karlstrom et al.; Robert et al.), but there is con- Where to “send” Miocene paleorivers. The operated in combination. Although the 5–6 Ma tinued debate about timing and nature of mantle longstanding question of where Miocene upper timing is generally agreed upon, the dominant fl ow. Several geodynamic models suggest that Colorado paleorivers may have exited, or termi- mechanisms for integration of the Colorado the magnitude of predicted effects on surface nated within, the Colorado Plateau is unresolved. River system remain controversial. topography are on the order of 400–800 m of One model is for internal drainage in the west- Differential incision. There is increasing evi- surface uplift. ern Rockies until 6 Ma, separated by the Kaibab dence for different incision rates through time uplift from west-fl owing Miocene paleorivers in and space in the river system and patterns of dif- Continued Controversies western Grand Canyon (Wernicke; Hill et al.; ferential incision are becoming better resolved but cf. Lopez Pearce et al.). In contrast, evidence by combined geochronologic and geomorphic Many of the same controversies raised during from gravels in Wyoming suggests a possible data (Crow et al.; Marchetti et al.). Where dif- the 1964 and 2000 symposia persist. north-fl owing system in the Miocene (Ferguson). ferential incision can be shown to be related Opening of the Gulf of California. While Alternatively, a south exit, along the Salt River to fault dampening of incision, as in western most published data support a latest Miocene age system, was revived (Potochnik). Grand Canyon (Crow et al.), this is indicative (~6.5 Ma) for initial marine incursion (Dorsey), Pre–6 Ma paleocanyons and paleorivers. of a dynamically changing river system that paleontological data support marine conditions Numerous workers have proposed models by is adjusting to tectonic forcings. The relative starting in middle Miocene time (McDougall). which pre–6 Ma paleocanyons on the southern importance of geomorphic, climatic, and tec- A middle Miocene age for opening of the Gulf Colorado Plateau may have become re-occupied tonic controls on drainage evolution are impor- would suggest that it did not play a major role and linked to evolve into the modern Grand tant issues being debated. in the integration of the Colorado River at ca. Canyon. A possible west-fl owing Miocene river Rapid onset of denudation 6–10 Ma. There 5.5–6 Ma. Top-down (e.g., lake spillover) inte- is preserved along Crooked Ridge (Lucchitta is also improved evidence from apatite helium gration models also do not rely on opening of et al.), and it may have fed into a system occu- measurements as well as geologic studies for the Gulf of California to lower base level and pying the present location of eastern Grand Can- regional acceleration of exhumation and incision directly facilitate integration, although Gulf yon (Lee et al.; Pederson) or the entire Grand in the Neogene. This occurred after ~5–7 Ma opening may have intensifi ed summer mon- Canyon (Wernicke), but the latter model, espe- in the upper Grand Canyon (Lee et al.), Little soons and erosion rates (Wernicke). cially, is in confl ict with the Muddy Creek con- Colorado River (Embid et al.), Monument The Bouse Formation. A marine versus non- straint (Lucchitta). Geologic evidence argues Uplift, Canyonlands, and Roan Cliffs (Hoffman marine origin for the “lowermost” Bouse For- against the existence of at least some of the et al.). Incision accelerated starting 6–10 Ma in mation along the Colorado River in the southern proposed paleocanyons; for example, a precur- the Grand Mesa area (Aslan et al.; Cole; Karl- Yuma and Blythe Basins was debated again in sor western Grand Canyon drainage (Hill et al.; strom et al.). Debates continue about the extent 2010 (McDougall). Some workers suggested Wernicke; Young, 2008) seems to be negated by to which this was driven by tectonic uplift (Karl- that the Bouse Formation records a change the absence of detritus in detrital zir- strom et al.) or combinations of drainage inte- from a marine environment in the Yuma Basin con populations in 13–6 Ma rocks of the Muddy gration (Pederson), enhanced Pleistocene runoff, to nonmarine conditions in the northern Mojave Creek Formation near Pearce Ferry, suggesting the southwest monsoon climate, and the opening paleolake. Sr, O, and C isotopes from “lower” these deposits could not have had detrital input of the Gulf of California (Hoffman et al.). Bouse carbonates are consistent with mixing from the Paleozoic strata of western Grand Can- Isostatic response to denudation. Isostatic trends between river, marine, and deep bedrock yon to the east (Lopez Pearce et al.). consequences of erosion involve rebound sources of water for parts of the Yuma Basin An old Grand Canyon. The possibility of a of buried rocks to balance the load removed such that Sr isotopes alone do not provide suf- Late Cretaceous (70 Ma) paleocanyon coinci- (Pederson; Lazear et al.). Faulting and differ- fi cient evidence for nonmarine origin (Crossey dent with both the eastern and westernmost seg- ential erosion during possible tilting also have et al.). In contrast, other workers point toward ments of the modern Grand Canyon, and cut to isostatic responses. Quantifi cation of this com- the similar character of the basal Bouse lime- within 400 m of its present depth, was supported ponent of landscape evolution is important and stone in all areas, and nonmarine isotopic sig- by a new interpretation of published thermo- is being studied by several groups. natures (Spencer et al.), to support a nonmarine chronology data (Wernicke). This was hotly Paleogeography reconstructions. Because of origin in all of the sub-basins. debated by both thermochronologists (Kelley any tectonic and isostatic adjustments to surface Comparisons between the Green and Colo- et al.; Lee et al.) and geologists (Karlstrom elevation, we cannot rely solely on modern ele- rado Rivers. Two great rivers converge in et al.), and provides a hypothesis that challenges vations to reconstruct past elevations and geom- Canyon lands to form the Colorado River system. other existing models and needs to be tested by etries of paleolake shorelines, spillover points, The Colorado River is steeper and has higher more comprehensive thermochronologic and and paleoriver gradients. incision rates over the last few million years than geologic datasets. Mantle-driven uplift. There is strong evidence the Green River (Aslan et al.). Controversies Integration mechanisms. Possible mecha- for Neogene mantle fl ow and tectonism in the involve when the Green became established as a nisms of integration of the upper and lower

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Colorado River basins across the Kaibab uplift anism of drainage reversal as well as a possible whole mantle fl ow (Robert et al.). Evidence for include lake spillover (Douglass), piracy, driving force for river integration and propaga- Neogene and ongoing mantle fl ow and resulting headward erosion, and groundwater sapping tion of knickpoints (Darling et al.). uplift can be paired with young canyon models involving karst connections. Different types of Nature of knickpoints. The cause and sig- (Karlstrom et al.). If onset of rapid denudation on proposed groundwater and karst connections nifi cance of knickpoints and convexities seen in the Colorado Plateau predated Colorado River included: (1) ~6 Ma karst-piping of river waters the longitudinal profi les of the Colorado River integration at ~6 Ma, recent surface uplift and from the upper basin, under the Kaibab uplift and its tributaries were discussed. (1) These fea- tilting seem required (Karlstrom et al.). Alterna- (Hill et al.); (2) upper basin drainage through tures may be relatively fi xed and pinned at less tively, if rapid onset postdated integration, then paleocanyons to a seepage-integration point in erodible rock layers or reaches, as documented tectonic uplift would not be required to create a central-western Grand Canyon region (Peder- by studies of bedrock strength properties along young Grand Canyon (Hoffman et al.). son); and (3) groundwater sapping from locally the river profi le (Tressler et al.). (2) Alterna- sourced groundwater (not upper basin river tively (or in addition), they may be incision New Developments and Future water) where hydrologic head facilitated inci- transients propagating upstream in response to Research Directions sion and integration, while geochemical signals downstream tectonic and/or geomorphic (e.g., of local groundwater were preserved (Crossey piracy) events (Darling et al.). A process of Directions for productive further research et al.). Simple headward erosion as a domi- diffuse knickpoint migration to bypass a bed- involve application of new methodologies and nant integration mechanism was supported by rock obstruction (Cook et al., 2009) may help better integration of diverse datasets. some (Hill et al.) as was piracy and integration explain high incision rates above Detrital zircons. Further detrital zircon studies of existing drainage systems by a top-down (Hanks et al.; Marchetti et al.; Pederson). of all tributaries, and of paleo–Colorado River process (Douglass). The striking similarity of Mantle tomographic images suggest the possi- deposits in different places along the mainstem detrital zircon populations in the modern river bility that the Lees Ferry knickpoint is caused and tributaries may help resolve processes of delta to 5.3 and 4.4 Ma Colorado River depos- by dynamic forcings due to mantle fl ow asso- integration and evolution of the Colorado River its (Kimbrough et al.) suggests a top-down ciated with a pronounced mantle velocity gra- system. integration because headward erosion would dient (Karlstrom et al.; Karlsrom et al., 2012) Paleoaltimetry. There is continued need to be predicted to show progressive changes that may help explain differential incision rates develop and test paleobarometers to estimate in detrital populations through time in the above and below Lees Ferry (Darling et al.). absolute elevation changes through time and river’s lower reaches and delta, which are not Isostatic response to denudation. The rela tive thus demonstrate any links between topographic observed. In addition, thermochronology data roles of tectonic uplift and isostatic responses to changes and surface uplift events. for rapid onset of denudation at about the same denudation to drive rock uplift were discussed Drainage reversals. Geologic work is needed time in several places across the region (Hoff- to explain differences in incision rates. By one to evaluate the timing and locations of drainage man et al.) are hard to reconcile with headward model (Pederson et al.), calculated magnitudes reversals by studies of ages of terrace gravels erosion models. of isostatic response to erosion were correlated and lake deposits. Lake Bidahochi. The size and signifi cance of with the pattern of faster Pleistocene incision Dates on old, high terraces. Additional a paleo “Hopi Lake,” or “Lake Bidahochi” and rates in the central Colorado Plateau, suggesting work on the highest terraces of the Green and the depositional setting for the Bidahochi For- that the isostatic feedback accounts for much of Colorado Rivers, southern Wyoming, and the mation were debated and several models were those amplifi ed rates. A second model (Lazear Browns Park Formation offer potential to bet- presented. (1) This Miocene basin was a termi- et al.) suggested that the difference between ter document how and when the Colorado and nal, internally drained depression for southward river incision rates and calculated isostatic Green Rivers became integrated and whether fl owing river waters from the Rockies. (2) This response to denudation over the last 10 Ma the steeper gradients in the Colorado River are lake system may have been a headwater lake for in the Grand Canyon and Rocky Mountains due to rock uplift of the Rocky Mountains. a regional northward fl owing river that carried requires Neogene tectonic uplift components at Thermochronology. Additional studies are Rocky Mountain drainage into Wyoming (Fer- both ends of the river system. needed to reconcile apatite fi ssion-track ages guson), or drained into other hypothetical lakes Timing and mechanisms of uplift. The timing and U-Th-He ages with each other and with near Lees Ferry (Hill et al.). Models for inte- and process of uplift of the surface of the Colo- other geologic constraints. Application of both gration of the Colorado River driven by spill- rado Plateau and , techniques to the same samples needs to be done over from Lake Bidahochi were not strongly from sea level in the late Cretaceous to modern routinely. Additional discussion and cross-lab supported by facies analysis of the Bidahochi high elevations, and the interactions of uplift, comparisons should be done to try to produce Formation, which suggests low sediment accu- drainage development, and erosion (incision/ protocols and reduce uncertainty in how to inter- mulation rates in a small lake (Pederson) where denudation), remain the underlying questions. pret variable-age apatites from the same sam- fl uvial beds aggraded across a more limited Paleoelevation data from clumped isotopes sug- ple. It is important to apply thermal models to lacustrine facies (Dickinson). gest that most uplift in the southwestern pla- reduce uncertainty in estimating timing of onset Drainage reversal. The concept of drainage teau was accomplished in the Laramide, and of rapid denudation (the kink) in diffuse kinked reversal from the Paleocene to Eocene N- to this model is paired with “old canyon” models age-elevation plots. For example, it is critical E-fl owing systems (including the paleo Salt (Wernicke). Thermochronology indicates Mio- to resolve whether onset of rapid denudation River), to the post–6 Ma SW-fl owing Colorado cene cooling in eastern Grand Canyon (Lee in the Colorado Plateau areas was pre–6 Ma, in River system is now better constrained (Young et al.) about the same time as broad denudation which case river integration is not the causative and Hartman), but there was much discussion across the southern plateau (Cather). This is explanation, versus syn– to post–6 Ma, in which on the timing and mechanisms for this reversal. consistent with various proposed mechanisms, case a river integration explanation predicts an Tilting due to mantle-driven epeirogeny (Robert including lithosphere delamination, conduc- upstream younging of onset of rapid denudation et al.; Karlstrom et al.) was discussed as a mech- tive mantle heating, Farallon slab removal, and (so far not observed).

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Sediment budgets. Studies of sediment bud- sive databases. This will lead to “movie(s)” of Karlstrom, K.E., Coblentz, D., Dueker, K., Ouimet, W., gets should continue to be integrated with the evolving landscape tied to evolving litho- Kirby, E., Van Wijk, J., Schmandt, B., Kelley, S., Lazear, G., Crossey, L.J., Crow, R., Aslan, A., Darling, thermochronology to establish links between spheric structure. A., Aster, R., MacCarthy, J., Hansen, S.M., Stachnik, upland denudation events and downstream sedi- J., Stockli, D.F., Hoffman, M., McKeon, R., Feldman, Outreach J., Heizler, M., Donahue, M.S., and the CREST work- ment volumes and major aggradation events. ing group, 2012, Surface response to mantle convec- Differential incision rates through time. tion beneath the Colorado Rocky Mountains and Colo- Incision rate studies at all temporal and spa- In addition to the scientifi c goals, the meet- rado Plateau: Lithosphere, v. 4, p. 3–22. ing participants emphasized the iconic status of Longwell, C.R., 1928, Geology of the Muddy Mountains, tial scales are needed to evaluate evolution of Nevada: U.S. Geological Survey Bulletin 798, 152 p. river profi les through time. Precise dates, fi rm Grand Canyon for geosciences, and the impor- Lucchitta, I., 1966, Cenozoic geology of the Lake Mead strath heights, and, ideally, depth to bedrock tance of good communication between the area adjacent to the Grand Wash Cliffs, Arizona [PhD dissertation]: University Park, Pennsylvania State Uni- are needed to calculate reliable bedrock inci- research community, the geoscience education/ versity, 218 p. sion points. Strath-to-strath comparisons for a interpretation community, the public, and the Lucchitta, I., 1972, Early history of the Colorado River in media. About 5 million visitors come to Grand the Basin and Range Province: Geological Society of given reach are especially valuable to calculate America Bulletin, v. 83, p. 1933–1948, doi:10.1130 bedrock incision rates independent of depth to Canyon each , and most want to know how /0016-7606(1972)83[1933:EHOTCR]2.0.CO;2. bedrock in the river channel. Strath-to-strath old it is and the processes that shaped it. The McKee, E.D., Wilson, R.F., Breed, W.J., and Breed, C.S., meeting promoted the awareness of the impor- eds., 1967, Evolution of the Colorado River in Arizona: age data are also needed to test changes in rates Museum of Northern Arizona Bulletin 44, 67 p. through time versus steady incision models. tance of our evolving research understanding McKee, E.D., and McKee, E.H., 1972, Pliocene uplift of Tectonic infl uences. Improved structural in the eyes of the world. There is an important the Grand Canyon region: Time of drainage adjust- obligation to convey new research advances and ment: Geological Society of America Bulletin, v. 83, studies and models (e.g., Resor) are needed to p. 1923–1932, doi:10.1130/0016-7606(1972)83[1923 provide better understanding of fault slip his- educational resources involving the spectacular :PUOTGC]2.0.CO;2. tory, monoclinal fold formation, and possible landscapes and fi eld laboratories of the Colorado Powell, J.L., 2005, Grand Canyon: Solving Earth’s grandest Plateau–Rocky Mountain region. We conclude puzzle: New York, Pi Press, 308 p. eperiogenic doming or tilting. Structural studies Powell, J.W., 1875, Exploration of the Colorado River of the need to be better integrated with incision and that informal research meetings of the type con- West and its tributaries: D.C., U.S. Gov- cooling/denudation (thermochronology) inter- ducted here provide exceptional research and ernment Printing Offi ce, 291 p. Powell, J.W., 1879, Report on the arid regions of the United pretations. Locations need to be sought (e.g., public relations value for the geosciences. States with a more detailed account of the lands of Lees Ferry and Grand Mesa regions) to integrate Utah, U.S. Geographical and Geological Survey of the ACKNOWLEDGMENTS Rocky Mountain Region: U.S. Government Printing long-term incision rate data with age-elevation Offi ce, 195 p. low T thermochronology data to merge denuda- The 2010 meeting benefi ted from logistical help Ranney, W., 2005, Carving Grand Canyon: Evidence, theories, from the U.S. Geological Survey, including use of tion and incision rate data (in m/Ma). and mystery: Grand Canyon, Arizona, Grand Canyon their conference room and logistical support. We espe- Association, 160 p. Geodynamic models. Geodynamic models of cially thank Sue Priest, as well as Joe Manning, Zach Young, R.A., 1966, Cenozoic geology along the edge of the mantle fl ow need to be tested against improved Anderson, Margot Truini, Darlene Ryan, and Tracey Colorado Plateau in northwestern Arizona [PhD dis- differential incision and differential denudation Felger for organizational support. This summary sertation]: St. Louis, Missouri, Washington University, 167 p., 4 maps, scale 1:31,680. models. bene fi ted from reviews by Andre Potochnik, Carol Hill, and Wayne Ranney. Young, R.A., 1982, Paleogeomorphologic evidence for the Knickpoint migration. Geomorphic models structural history of the Colorado Plateau margin in of knickpoint migration also need to be tested REFERENCES CITED western Arizona, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado against improved differential incision and dif- Beard, L.S., Karlstrom, K.E., Young, R.A., and Billingsley, River region, California, Arizona and Nevada (Ander- ferential denudation data. G.H., 2010, CR_Evol_ 2: Origin and Evolution of the son-Hamilton volume): San Diego, California, Cor- Community database. A community effort Colorado River System II Workshop Abstract Volume: dilleran Publishers, p. 29–39. U.S. Geological Survey Open-File Report OF 2011- Young, R.A., and Brennan, W.J., 1974, The Peach Spring is developing to produce improved databases 1210, 300 p. Tuff: Its bearing on structural evolution of the Colo- on geochronologic, incision rate, and thermo- Blackwelder, E., 1934, Origin of the Colorado River: Geo- rado Plateau and development of Cenozoic drainage logical Society of America Bulletin, v. 45, p. 551–566. in Mohave , Arizona: Geological Society of chrono logic constraints for evolution of the Cook, K.L., Whipple, K.X., Heimsath, A.M., and Hanks, America Bulletin, v. 85, p. 83–90, doi:10.1130/0016 Colorado River system. These need to be con- T.C., 2009, Rapid incision of the Colorado River in Glen -7606(1974)85<83:PSTIBO>2.0.CO;2. tinually updated from new research. Canyon—Insights from channel profi les, local incision Young, R.A., and Spamer, E.E., 2001, Colorado River: Ori- rates, and modeling of lithologic controls: Earth Surface gin and evolution: Grand Canyon, Arizona, Grand Can- 3-D visualizations and animations. For Processes and Landforms, v. 34, no. 7, p. 994–1010. yon Association, 280 p. visualization, building on graphic methods Dutton, C.E., 1882, Tertiary history of the Grand Cañon Young, R.A., 2008, Pre–Colorado River drainage in west- that evolved between the 1964, 2000, and district: U.S. Geological Survey Monograph 2, 264 p. ern Grand Canyon: Potential infl uence on Miocene and atlas. stratigraphy in Grand Wash Trough, in Reheis, M.C., 2010 meetings, portrayal of models should Hunt, C.B., 1956, Cenozoic geology of the Colorado Plateau: Hershler, R., and Miller, D.M., eds., Late Cenozoic involve improved GIS-based paleogeographic U.S. Geological Survey Professional Paper 279, 99 p. drainage history of the southwestern and Hunt, C.B., 1969, Geologic history of the Colorado River, in Lower Colorado River region: Geologic and Biotic maps, be tied to a detailed timeline, and be The Colorado River region and : U.S. Perspectives: Geological Society of America Special spatially referenced to updated comprehen- Geological Survey Professional Paper 669-C, p. 59–130. 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