WSP05862

• File Number: 8220.101.10
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powel-Glen Canyon Adaptive Management
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/2005
• Author: DOI-USGS
• Title: Sedimentology and Stratigraphy of the Palisades Lower Comanche and Arroyo Grande Areas of the Colorado River Corridor Grand Canyon Arizona
• Water Supply Pro - Doc Type: Report/Study

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various depositional environments doe) 'not show a clear trend
with distance from the river (fig. 20) because much of the total
stratigraphic thickness studied had no diagnostic sedimentary
strucWres. Well-defined Colorado River flood deposits (those
that contain fluvial sedimentary stmctures) \..-ere observed
only in sections 5 and 6. the two closest to the river. Locally
derived sediment, predominantly Dox sandstone and shale
clasts. is present in all of the sections described (fig. 20).
Numerous channel-fill s!nlctures are apparent in arroyo wall
exposures (sections I. 2, 4. and surrounding area). indicating
paleogully development similar to thai in the modem arroyo
network. Data from section 3 indicate that repeated wetting
episodes have oecLlrred on the larg~ flat area west of the main
dune field, interpreted as ponding due to precipitation runolY
that carrie.':l locally derived sediment.
The majority of archaeological sites at Lower Coman-
che are affected to a great degree by aeolian processes in the
main dune ficld. Fivc of the six cultural sit~s in this area are
located within this dune ficld IC: 13:274, C: 13:333, C: 13:335.
C: 13:337, and C: 13:373). Four of those five were originally
situaled on aeolian sediment (C: 13274, C: 13:333, C: 13:335,
and C: 13:373), with the fitih (C: 13:337) built on an inter-
dune/playa surface within the dune field. Four or the six sites
(C: 13:274, C: 13:333. C: 13:335. and C: 13:373) show evidence
of at least partial burial by aeolian sediment Iv.'o of those tour
IC: 13:274 and C: 13:373) have only minor sediment cover.
Three of the five sites within the dune field are atTected by
aeolian deflation IC:13:274. C:13:333, and C:13:335). and
one of those (C: 13:333) i~ exposed as a result of dun~ migra-
tion. One of the sites in the dune field, C: 13:3.13, has begun
to be atTected by gully incision that reaches into the upper
elevations ofa dune -10 m high. Dune migration and aeolian
sediment transport are expected to continue to atTect these
five sites: it is likely that new artifacts \vill become exposed
as dune migration continues, while others arc reburied by
wind-blown sand. The on~ site not located in the dune field
(C: 13:273) is affected significantly by an.oyo ineision and
local nlllotT(Gellis, 1994: National Park Service. 2004).
Arroyo-related erosion is ~evere at Feature _~ of that site and is
predicted to undercut more of Features I nnd ,) as backwast~
ing of the gully walls progresses. The site is also impacted to a
lesser degree by visitor use of the Beamer hiking trail.
Much of the fine-grained (silt to fine sand) material in
stratigraphic sections at LO\'..'cr Comanche does Tlot contain
preserved diagnostic sedimentary structures. Degradation of
original stnJcnlre:) is attributed primarily to bioturbation, in
the roml of animal burrowing and the growth of plant roots.
In the absence of lithic grains that imply a local sediment
source, such fine-grained deposits are generally interpreted as
having been originally derived from Colorado River deposits
(and later having b~en subject to aeolian and tributary-flood
r~working). Without sedim~ntary structures, it is not possible
to confidently ascertain whether such deposits remain in their
original position as fluvial layers or have been transported
and reuepo:)ited by wind. Although grain-size analysis may
in some cases distinguish between these options, the range of

Arroyo Grande Area 21

grain sizes present within verifiable fluvial deposits is great
enough that caution should be used in assigning depositional
environment solely on this basis (see dicussion in section titled
"Methods").
Ii is worthwhile to consider the possibility thai debris
flows on the large Tanner debris fan (-2 km downstream)
could have affected river discharge levels at Lower Coman-
che (R. H. Webb, written commun. with H. C. Fairley. 2004).
While none of the strata identified during this work explicitly
suggest quiescent ponding or lacustrine sedimentation, it
is possible that elevations of observed fluvial strata may be
higher than comparable discharge would reach today. because
of prehistoric damming behind the Tanner fan. This possibility
could be addressed more fully if a suite of dates \-\,'ere available
from fluvial deposits at this site and from debris flow events at
Tanner.
The origin of the so-called white layer remains to be
detenllined. It would be advantageous to analyze a sample by
x-ray diffraction, which could provide more infonnation on its
composition, and consequently on its emplacement.

Arroyo Grande Area
Geomorphic Setting

The lefl bank of the Colorado River in the area known
as Arroyo Grande is part of the Area of Cooperation (AOC)
between the Hualapai Nation and the National Park Service in
\vestem Grand Canyon (Memorandum of Understanding by
and among the Hualapai Tribe, Grand Canyon National Park,
and Lake Mead National Recreation Area, Oclober 2000). The
T1\'~r in this reach is located along the trend of the major Hur-
ricane Fault system (Huntoon and others. 1981), which locally
offsets Precambrian schist and gneiss along river left. Schist
and amphibolite-grade gneiss dominate locally derived sedi-
ment in the study area.
The study sile discussed here (fig. 21) is approximately
I km north of the limit of mapping work done by Hereford
and others (2000) at Granite Park. Many or Ihc alluvial ter-
races and other geomorphic features identified at Granite Park
by Hereford and others (2000) are present al Arroyo Grande,
including designations of the upper mesquite terrace (map unit
'umt'), lower mcsquite t~TTae~ (map unit '1mI') and predam
alluvium (map unit 'pda') depo~its considered by Hereford
and others (2000) to be equivalent to those mapped by Her-
eford (1993) and Hereford and olhers (1996) at Palisades. In
the Arroyo Grande region, the oldest two alluvia\ terrace units
defined by Hereford (1993) and Hereford and others (1996,
2000), the 'pda' map unit and the local equivalent of what was
called the striped alluvium ('sa') unit at Palisades by Hereford
(1993), are both truncated and ovcrlain by the 'umf alluvium.
This younger 'umt' deposit typically occurs at lower elevation
than the older 'sa' and 'pda' in the eastern canyon; in this area
ofrhe western canyon, where 'umt' forms a cap over the older
alluvial deposits, the covering of Pueblo II age cultural sites