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CORRELATION OF ALLUVIAL DEPOSITS
AT THE NEVADA TEST SITE
ABSTRACT
Because characteristics of rock layers and problems in drilling must be studied before
radioactive waste can be safely contained, an evaluation was made of methods for
correlating alluvial deposits at Yucca Flat of the Nevada Test Site (NTS). Although correla-
tion of Tertiary volcanic tuff beds at the NTS has been successfully achieved, correlation of
stratigraphic zones in the overlying alluvium has posed technical difficulties. We have
evaluated several methods for correlating alluvial deposits from drillholes, including electric
resistivity logs (E logs), visual examination of sidewall samples and comparison of their car-
bonate (CO 2) content, downhole stereo photography for identifying debris flow deposits,
caliche age-dating, and specific yield and permeability measurements of deposits. For
predicting the thickness of zones having similar physical properties in the alluvium, E log
measurements were found to be the most useful of these methods.
INTRODUCTIONThe late Tertiary and Quaternary colluvial and
alluvial deposits at the Nevada Test Site (NTS) con-
sist predominately of waterlaid sands and gravels
with occasional debris-flow deposits. The alluvial
deposits in this area are comprised of mainly Ter-
tiary volcanic tuffs and Paleozoic and Precambrian
clastics and carbonates.
A major task of persons responsible for the con-
tainment of radioactivity from underground nuclear
explosions at the NTS has been to correlate
stratigraphic zones in the alluvium that possess
similar physical properties (e.g., CO 2 content). This
is done so that the physical-chemical characteristics
of rock layers can be predicted. In turn, more ac-
curate forecasting of potential drilling problems will
be possible at proposed sites. A knowledge of the
alluvial stratigraphy and its correlation will alsohelp us to better understand the geologic history of
Yucca Flat.
Correlation of the overlying alluvium in the
northern part of Yucca Flat has posed considerable
technical difficulties. In this study, we have
reviewed the applicability of resistivity log data for
correlating zones of alluvial deposits, and explored
the potential of other methods of correlation, some
of which have been successfully used by others
elsewhere. For this project, we studied Area 2 of the
NTS (Fig. 1). It lies in the north-central portion of
Yucca Flat, and is bordered on the east by Yucca
fault and on the west by the Carpetbag fault. West
of the Carpetbag fault is a north-south subsurface
ridge of Paleozoic rocks covered by less than 250 ft
of alluvium in some places (Fig. 2). ' We obtained
our data from the deposits in holes drilled east of
this ridge where the alluvium abruptly thickens.METHODS
A wide variety of data were used for correlating
alluvial deposits: electric resistivity logs (E logs),
gamma-gamma density logs, lithologic descriptionsfrom sidewall samples and downhole photos, and
CO2 analyses from sidewall samples. The best
results were obtained by examining both electric
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Grothaus, B. & Howard, N. Correlation of alluvial deposits at the Nevada Test Site, report, September 30, 1977; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1055748/m1/4/: accessed May 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.