Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.
Comment on "Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table–Type Speleothems"
Philip A. Pearthree,1*Jon E. Spencer,1James E. Faulds,2P. Kyle House2
Polyak et al. (Reports, 7 March 2008, p. 1377) reported thatdevelopment of the western Grand Canyon began about 17 millionyears ago. However, their conclusion is based on an inappropriateconflation of Plio-Quaternary incision rates and longer-termrates derived from sites outside the Grand Canyon. Water-tabledeclines at these sites were more likely related to local base-levelchanges and Miocene regional extensional tectonics.
1 Arizona Geological Survey, 416 West Congress Street, Suite 100, Tucson, AZ 85701, USA. 2 Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA.
* To whom correspondence should be addressed. E-mail: phil.pearthree{at}azgs.az.gov
In their geochronologic study of carbonate speleothems fromwithin and near the Grand Canyon, Polyak et al. (1) reported9 uranium-lead dates that record the approximate time of cavedewatering due to water-table decline. Their work provides valuableinsights into the usefulness of this methodology for estimatingriver incision rates in general and incision rates of the ColoradoRiver in the Grand Canyon during the past few million yearsin particular. However, the data they presented do not supporttheir interpretations about the age of initial canyon developmentand they did not appropriately consider the results of othergeologic studies that provide insight into the Neogene historyof the region.
Two sample sites used by Polyak et al., located within the westernGrand Canyon, yielded dates of less than 4 million years ago(Ma) (Fig. 1). These sites clearly are spatially related tocanyon development and imply relatively low incision rates thatare consistent with other recent findings (2). Incision ratesinferred from these sample sites have no clear bearing, however,on the age of initial development of the western Grand Canyon.Geologic evidence indicates that the Colorado River arrivedin the western Grand Canyon region 5 to 6 Ma (3–5) asa consequence of either upstream lake overflow (6, 7), drainagecapture by headward erosion (3, 8), or some combination of theseprocesses. The introduction of a major river into this arealikely resulted in high initial incision rates followed by exponentiallydecaying rates, perhaps even including intermittent aggradation.Relatively low post–4 Ma incision rates in the westernGrand Canyon are consistent with a pre–Colorado Rivercanyon, rapid incision after introduction of the Colorado River,or both, but it is not appropriate to extrapolate these ratesbackward in time to estimate the age of the Grand Canyon.
Fig. 1. Locations of caves studied by Polyak et al. (1) and hypothetical groundwater tables showing descent over time. Restoration of displacement on western faults recreates the highlands to the west, which were the inferred source of water that sustained an east-sloping water table in the western Grand Canyon region at 20 Ma. By 10 Ma, this highland no longer existed and the water table sloped westward. We infer that the 7.5 Ma dewatering of the Grand Wash Cliffs cave site occurred because of local base-level fall, due to erosional cliff retreat and/or to spillover of the lake that occupied the Grand Wash Trough.
[View Larger Version of this Image (49K GIF file)]
The two sample sites that yielded older ages [sites 1 and 4in (1)] are not in or directly connected with the western GrandCanyon and thus do not bear directly on Grand Canyon incisionrates or the age of initial canyon development. Site 1 (7.5Ma) is 40 km north of the river in the Grand Wash Cliffs, inthe footwall of the Grand Wash fault, a major normal fault thatwas active primarily between 16 and 10 Ma (9). From 11 to <7.5Ma, limestone was accumulating in a large lake in the GrandWash trough immediately west of the cliffs (10, 11), which impliesthat base level in this area was relatively stable or slowlyrising during that period. Given the location of site 1, water-tabledecline at 7.5 Ma may have been caused by local cliff erosionand retreat or base-level fall associated with spillover ofthe late Miocene lake and subsequent incision in Grand Washtrough, but any direct connection to Grand Canyon incision isunclear. Site 4 is about 90 km southeast of the mouth of theGrand Canyon and 30 km south of the Colorado River in the GrandCanyon. The 17-Ma date of water table decline is roughly coincidentwith inception of extension, surface lowering, and basin genesisin the Basin and Range province to the west (e.g., 9–14).For a proto-canyon related to displacement on the Grand Washfault to cause water-table decline at site 4, it would havehad to develop and rapidly propagate tens of kilometers upstreamthrough resistant strata. Furthermore, no evidence of clastic-sedimentinflux due to proto-Canyon excavation has been documented inGrand Wash trough; this has been the primary evidence againsta west-flowing proto–Grand Canyon (3). A more plausibleexplanation is that the slope of the water table changed fromeast-dipping to west-dipping and that the water table declinedthroughout the western Grand Canyon region in the middle andlate Miocene because of large-magnitude extension and regionalsubsidence in the Basin and Range province directly west ofthe Grand Wash fault (Fig. 1).
The authors' interpretation that their data support middle Miocenedevelopment of the western Grand Canyon is based on the broadsimilarity of Plio-Quaternary incision rates with longer-term"incision rates." Unfortunately, the two samples indicatingmiddle to late Miocene water-table decline probably have nodirect bearing on Grand Canyon incision. Other interpretationsfor water-table decline before the arrival of the Colorado Riverare much more compatible with regional geologic relations.
3. I. Lucchitta, Geol. Soc. Am. Centennial Field Guide – Rocky Mountain Section, 2 (1987), pp. 365–370. [CrossRef]
4. J. E. Faulds et al., in Colorado River: Origin and Evolution, R. A. Young, E. E. Spamer, Eds. (Grand Canyon Association, Grand Canyon, AZ, 2001), pp. 81–87.
5. J. E. Spencer et al., in Colorado River: Origin and Evolution, R. A. Young, E. E. Spamer, Eds. (Grand Canyon Association, Grand Canyon, AZ, 2001), pp. 89–91.
6. N. Meek, J. Douglass, in Colorado River: Origin and Evolution, R. A. Young, E. E. Spamer, Eds. (Grand Canyon Association, Grand Canyon, AZ, 2001), pp. 199–206.
7. J. E. Spencer, P. A. Pearthree, in Colorado River: Origin and Evolution, R. A. Young, E. E. Spamer, Eds. (Grand Canyon Association, Grand Canyon, AZ, 2001), pp. 215–219.
8. J. Pederson, GSA Today18, 4 (2008).
9. J. E. Faulds et al., in Colorado River: Origin and Evolution, R. A. Young, E. E. Spamer, Eds. (Grand Canyon Association, Grand Canyon, AZ, 2001), pp. 93–99.
10. M. W. Wallace et al., Geologic Map of the Meadview North Quadrangle, Arizona and Nevada: Nev. Bur. Mines and Geol. Map 154, scale 1: 24,000 (2005).
11. J. E. Faulds et al., Geol. Soc. Am. Field Guide11, 119 (2008).
14. R. G. Bohannon, U.S. Geol. Survey Prof. Pap.1259, 1 sheet, scale 1: 750,000 (1984).
Received for publication 8 April 2008. Accepted for publication 25 August 2008.
The editors suggest the following Related Resources on Science sites:
In Science Magazine
TECHNICAL COMMENTS
Joel Pederson, Richard Young, Ivo Lucchitta, L. Sue Beard, and George Billingsley (19 September 2008) Science321 (5896), 1634b.
[DOI: 10.1126/science.1158019] |Abstract »|Full Text »|PDF »
TECHNICAL COMMENTS
Victor Polyak, Carol Hill, and Yemane Asmerom (19 September 2008) Science321 (5896), 1634d.
[DOI: 10.1126/science.1159762] |Abstract »|Full Text »|PDF »
40 km north of the river in the Grand Wash Cliffs, in the footwall of the Grand Wash fault, a major normal fault that was active primarily between 16 and 10 Ma (
