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CAROLINA GEOLOGICAL SOCIETY
Guidebook for 1992 Annual Meeting
Pages 77-81
PETROLOGY AND POROSITY PERMEABILITY CHARACTERISTICS OF TERTIARY AQUIFER SANDS,
SAVANNAH RIVER SITE REGION, SOUTH CAROLINA
Paul A. Thaye 1 , Andrew D. Smits 2 , and Rolf K. Aadland 3
1 Department of Earth Sciences, UNCW, 601 South College Road, Wilmington, NC 28403-3297
2 Science Applications International Corporation, 360 Bay Street, Augusta, GA 30901
3 Environmental Sciences Section, SRTC, Westinghouse Savannah River Co., P.O. Box 616, Aiken, SC 29802
INTRODUCTION
The purpose of this paper is to describe the textural, petrographic,
and porosity permeability characteristics of Tertiary
sands that comprise the terrigenous portions of the
Floridan and Floridan Midville aquifer systems in the Savannah
River Site (SRS) region (Fig. 1). The Gordon and Upper
Three Runs aquifer units represent the updip clastic phase of
the Floridan aquifer system and are separated by the Gordon
confining unit (Aadland and others, 1992; this volume). In
the northwestern part of the SRS region, where the Gordon
confining unit becomes too thin to be an effective barrier to
flow, the Gordon and Upper Three Runs aquifer units coalesce
to form the Steed Pond aquifer of the Floridan Midville
aquifer system (Aadland and others, 1992) (Fig. 1).
Figure 1. Tertiary hydrostratigraphic units in the Savannah
River Site region (from Aadland and others, 1992).
GORDON AQUIFER
The Gordon aquifer consists of the sandy portions of the
Paleocene Williamsburg Formation and sands of the overlying
lower and middle Eocene Fishburne and Congaree Formations.
(Aadland and others, 1992). The Gordon is an
unconsolidated quartz sand that contains thin, discontinuous
layers of mud and minor zones of opal and chalcedony
cemented quartzarenite (Robertson, 1990). Figure 2 shows
the percent gravel, sand, and mud for 443 Gordon samples.
Figure 3 shows the percent sand, silt, and clay for 10 representative
Gordon sands. The average grain size of 410 Gordon
sands is 1.66 + 0.04φ (lower medium). Sands range from
upper very coarse (-0.74φ) to very fine (3.97φ); about 80% of
the sampled population is coarse and medium grained. The
mean sorting of 410 Gordon sands is 1.32 + 0.04φ, which is
poorly sorted on the Folk and Ward (1957) scale; sorting
ranges from 0.44φ (well sorted) to 4.77φ (extremely poorly
sorted). About 70% of the sampled population is moderately
and poorly sorted. Robertson (1990) showed that sorting of
Gordon sands is controlled chiefly by the percentage of mud
matrix. That is, the greater the amount of mud the poorer the
sorting. Mean skewness of 410 Gordon sand is 0.19 + 0.01
(fine-skewed). Skewness ranges from –0.53 (strongly coarseskewed)
to 0.84 (strongly fine-skewed), and is controlled by
the amount of mud and gravel in the sands. Sands with high
mud contents are usually fine and very fine-skewed, whereas
those with high percentages of gravel are coarse and strongly
coarse-skewed. Mean kurtosis of 410 Gordon sands is 1.52 +
0.04, which is very leptokurtic according to Folk and Ward’s
(1957) verbal scale. Kurtosis varies from 0.69 (platykurtic)
to 5.61 (extremely leptokurtic); most sands are leptokurtic
and very leptokurtic, indicating that the sorting in the central
two-thirds of the distribution is better than that in the “tails”.
Porosity and permeability of 382 Gordon sands were
determined from sieve analyses using the Beard and Weyl
(1973) empirical method, based on Trask (1930) median size
and sorting. Mean porosity of Gordon sands is 35.1 + 2.5%
and ranges from 27.3 to 40.2%. The distribution of porosity
is skewed toward lower values, although it appears to follow
a near normal distribution (Fig. 4). Calculated permeabilities
range from 2 to 220 Darcies (D) (5 to 537 ft/day); 75% of the
permeability values fall between 16 and 128 D (39 to 312 ft/
day) (Fig. 5). The geometric mean permeability of the 382
sand samples is 39.6 D (97 ft/day). Calculated permeabilities
of Gordon aquifer sands are skewed toward high values and
appear to follow a log normal distribution (Fig. 5).
Robertson (1990) and Robertson and Thayer (1992)
constructed facies maps showing the distribution of textural
parameters and permeability for the Gordon aquifer. The
facies maps showed the following downdip (i.e., toward the
southeast) changes: mean grain size and coarsest percentile
(1 st ) decreased; sorting became poorer; kurtosis became
more leptokurtic; and percent mud increased. Geometric
mean permeability also decreased in a downdip direction,
and varied from 90 D (220 ft/day) in the north to 20 D (49 ft/
day) in the south. The decrease in permeability toward the
south results from a decrease in mean grain size coupled
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