The California River and Its Role in Carving Grand Canyon
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Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30274.1 The California River and its role in carving Grand Canyon Brian Wernicke† Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA ABSTRACT the plateau. By Oligocene time, the lakes had basins may contain evidence of the time, place, largely dried up and were replaced by ergs. By and even rate of erosion, but impose few con- Recently published thermochronological mid-Miocene time, a pulse of unroofi ng had straints on the evolution of topographic form. and paleoelevation studies in the Grand Can- lowered the erosion level of eastern Grand A promising avenue of research in this other- yon region, combined with sedimentary prov- Canyon to within a few hundred meters of its wise discouraging endeavor stems from the fact enance data in both the coastal and interior present level, and the Arizona River drainage that isothermal surfaces in the uppermost crust portions of the North American Cordillera, below modern Grand Canyon was deranged more-or-less assume the geometry of ancient place important new constraints on the paleo- by extensional tectonism, cutting off the sup- topog raphy, leaving behind a sort of palimp- hydrological evolution of the southwestern ply of interior detritus to the coast. Increas- sest of the ancient landscape, especially in the United States. Review and synthesis of these ing moisture in the Rocky Mountains in late case of wide, deep canyons. After a period of data lead to an interpretation where incision Miocene time reinvigorated fl uvio lacustrine kilometer-scale erosion, the most direct expres- of a large canyon from a plain of low elevation aggradation NE of the asymmetrical divide, sion of the ancient topographic form is its thermal and relief to a canyon of roughly the length which was fi nally overtopped between 6 imprint. By using thermal structure to reconstruct and depth of modern Grand Canyon occurred and 5 Ma, lowering base level in the interior ancient relief and comparing it to modern relief primarily in Campanian time (80–70 Ma). of the plateau by 1500 m. This event reinte- in the same mountainous region, fundamental Incision was accomplished by a main-stem, grated the former Arizona drainage system questions about landscape evolution may be ad- NE-fl owing antecedent river with headwaters through a cascade of spillover events through dressed. For example, does relief generally de- on the NE slope of the North American Cor- Basin and Range valleys, for the fi rst time crease as a function of time in a “geographical dillera in California, referred to herein after connecting sediment sources in Colorado cycle” of youth, maturity, and old age (Davis, its source region as the California River. At with the coast. This event, combined with the 1899), or does topographic form quickly attain a this time, the river had cut to within a few intensifi cation of summer rainfall as the Gulf “dynamic equilibrium” that changes little as ero- hundred meters of its modern erosion level of California opened, increased the sediment sion proceeds (Hack, 1960)? Although both end in western Grand Canyon, and to the level of yield through Grand Canyon by perhaps two members have been extensively discussed and ap- Lower Mesozoic strata in eastern Grand Can- orders of magnitude from its Miocene nadir, plied to the time and length scales of late Quater- yon. Subsequent collapse of the head waters giving birth to the modern subcontinental- nary erosion (e.g., Heimsath et al., 1999), we are region into a continental border land and co- scale Colorado River drainage system. The only just beginning to address whether ex trapo- eval uplift of the Cordilleran foreland during Colorado River has thus played a major role la tion of these results applies to kilometer-scale the Laramide orogeny reversed the river’s in unroofi ng the interior of the Colorado Pla- erosion acting over time scales of 10–100 m.y. course by Paleogene time. After reversal, its teau, but was not an important factor in the (e.g., Reiners and Shuster, 2009). terminus lay near its former source regions in excavation of Grand Canyon. Relief on isothermal surfaces created by what is now the Western Transverse Ranges topog raphy decreases exponentially downward and Salinian terrane. Its headwaters lay in the INTRODUCTION (e.g., Section 4–12 in Turcotte and Schubert, ancient Mojave/Mogollon Highlands region of 1982), so thermochronometric measurements at Arizona and eastern California, apparently How do landscapes evolve through signifi - depths within 1–2 times the amplitude of topog- reaching as far northeast as the eastern Grand cant amounts of geologic time? Because ero- raphy, or within the upper ~4 km of the crust Canyon region. This system is herein referred sion disaggregates rock masses (as opposed for most mountain belts, provide the best oppor- to after its source region as the Arizona River. to aggregating or modifying them), it presents tunity for reconstructing landscapes. Given that From Paleogene through late Miocene time, a special challenge for study. Most of what is temperatures at these depths are generally below the interior of the Colorado Plateau was a known about erosion concerns incremental 100 °C, the most effective thermochronometers closed basin separated from the Arizona River changes in modern landscapes. Unconformities for detecting this signal are fi ssion-track and drainage by an asymmetrical divide in the provide valuable records of the form of ancient (U-Th)/He dating of apatite (e.g., Stüwe et al., Lees Ferry–Glen Canyon area, with a steep erosion surfaces, but only provide a snapshot 1994; House et al., 1998, 2001). SW fl ank and gently sloping NE fl ank that of the transition from erosion to aggradation. Debate over the origin of Grand Canyon, the drained into large interior lakes, fed primar- When regionally developed, they are generally planet’s most vivid illustration of kilometer- ily by Cor dilleran/Rocky Mountain sources to cut on surfaces of very low relief. Kilometer- scale erosion, has been invigorated over the the north and west, and by recycled California scale topo graphic forms characteristic of moun- last two years by application of these and other River detritus shed from Laramide uplifts on tain belts, if preserved at all by unconformities, proxies for erosion and paleoelevation in the cover only a small fraction of eroding uplands. region (e.g., Flowers et al., 2008; Hill et al., †E-mail: [email protected] Studies of the eroded detritus in sedimentary 2008; Hill and Ranney, 2008; Karlstrom et al., GSA Bulletin; Month/Month 2010; v. 1xx; no. X/X; p. 1–29; doi: 10.1130/B30274.1; 14 fi gures. For permission to copy, contact [email protected] 1 © 2011 Geological Society of America Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30274.1 Wernicke 2008; Pearthree et al., 2008; Pederson et al., spired the classical concepts of antecedent and younger, developed on post-tectonic fi ll (e.g., 2008; Polyak et al., 2008a, 2008b; Young, superposed drainage (Powell, 1875). In ante- Emmons, 1897). 2008). Grand Canyon is a long, relatively wide cedence, the erosive power of a stream formed Over more recent decades, a contrary consen- canyon through which surface waters of a large in a region of low relief is suffi cient to maintain sus has emerged, holding that incision of Grand area of the southwestern U.S. interior pass be- its grade during tectonic distortion of the land- Canyon began in late Miocene time, when two fore ultimately reaching the Gulf of California. scape, whereas in superposition, the stream previously separate drainage basins became The high plateaus surrounding Grand Canyon originates in fl at-lying post-tectonic strata, and inte grated (e.g., Longwell, 1946; McKee et al., constitute the most imposing of a series of cuts downward across structure. The fi rst few 1967; Lucchitta, 1972, 2003). There is general kilometer-scale topographic obstacles (e.g., the decades of exploration of these canyons thus consensus that integration occurred between 6 Kaibab arch, Fig. 1) that the Colorado River and led to a general debate about whether these and 5 Ma, before which an older upper basin its primary northern tributary, the Green River, rivers were older than the Laramide structures and a younger lower basin were separated by a cut improbably across as “transverse drain- they cut through and therefore antecedent (e.g., drainage divide somewhere in the vicinity of the ages” (e.g., Douglass et al., 2009), which in- Powell, 1875; Dutton, 1882; Walcott, 1890), or Kaibab arch (e.g., Hill et al., 2008; Karlstrom 113°W 111°W NV UT Kaibab escarpment Kaiparowits Plateau E limit, major T extension B a s i n Plateau boundary a r c h UT Bidahochi Fm. (Miocene) Rim gravels, Claron Fm. Lees AZ (Paleogene) b Grand Upper K strata a Ferry Wash Mtns. b Trough i a BASAL UNCONFORMITIES FOR: C A N Y O K N Cretaceous C o c o n i n o Triassic Virgin Cambrian UG 36°N 36°N r K e LP G R A N D v LG i T e r r a c e R Figure 2 Tr-J NV a n d CA Plz o T r a n s i t i o n Z o n e Arizona homocline d a pC r R a n g e o l 34°N 34°N o C 100 km Ft. Apache 113°W 111°W Figure 1. Tectonic map showing selected geographical and geological features discussed in text.