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OUTLINE OF STRATIGRAPHY AT THE SAVANNAH RIVER SITE
lachians. They include slate, phyllite, schist, gneiss, volcanics
and metavolcanics, granite, and some mafic bodies. The
rocks are generally rich in feldspar, providing a source for
the kaolinite which is abundant in much of the updip Coastal
Plain section.
Rocks similar to those exposed in the Piedmont lie
beneath the Coastal Plain sediments within most of SRS.
The southeastern part of the Site is underlain by mudstones,
sands, and conglomerates of the Triassic Newark Supergroup
in the Dunbarton basin (Siple, 1967, p. 22 – 23; Marine and
Siple, 1974). The basement surface, the sub-Cretaceous
unconformity, dips at about 50 ft/mi to the southeast at SRS.
UPPER CRETACEOUS
Outcropping Upper Cretaceous
Lithology and stratigraphic terminology.
Outcropping Cretaceous sediments consist mostly of
medium to very coarse, poorly sorted grayish sands with
common to abundant kaolinite and muscovite. Pebbly sands
and gravel layers are common, as are clay clasts. Bedding is
irregular with facies changes occurring over short distances.
Cross-bedding is well developed in places. Clay laminae are
common within the sands, and large lenses of “soft” kaolin
are mined in the area (Buie and Schrader, 1982, p. 5 – 12).
Detailed descriptions of the outcropping Cretaceous are in
Nystrom and Willoughby (1982) and Nystrom and others
(1986).
The exposed Cretaceous strata have been mapped as
Hamburg and Middendorf by Sloan (1908), Middendorf by
Cooke (1926) and Snipes (1965); Middendorf and Black
Creek by Colquhoun and others (1983); Tuscaloosa by
Cooke (1936), Lang (1940), Siple (1967), and Prowell and
O’Conner (1978). Christopher (1982) observed that the Cretaceous
sediments in the area were younger than the type
Tuscaloosa, and the use of the term in the area as a formal
formation name has declined. Nystrom and Willoughby
(1982) and Nystrom and others (1986) mapped the deposits
as simply “Cretaceous”.
Much of what was included before 1970 as Cretaceous
strata in Georgia and South Carolina updip from SRS is now
assigned to the Tertiary based on paleontologic data (Scrudato
and Bond, 1972; Abbott and Zupan, 1975; Tschudy and
Patterson, 1975; Buie, 1978) and detailed mapping (see
Nystrom and Willoughby, 1982; Nystrom and others, 1986,
1991.
Paleontology, age and correlation
Fossils are rare in the outcropping Cretaceous. Leaves
have been found (Berry, 1914), and pollen from an outcrop
of dark clay on Interstate Highway 20 near Aiken indicated a
late Campanian age (see Nystrom and Willoughby, 1982, p.
86; Nystrom and others, 1986, p. 7), correlative with the
Black Creek Formation which crops out in northeastern
South Carolina, and the Blufftown and Cussetta formations
of Alabama. There are probably older and younger sediments
within the exposed Upper Cretaceous Series in Aiken
County.
Environment
Most the sands were probably deposited in fluvial and
deltaic environments. The large clay bodies suggest deposition
in oxbow lakes or in interdistributary bays.
SUBSURFACE UPPER CRETACEOUS
Introduction
The Cretaceous section in the subsurface at SRS is
divided from older to younger into the Cape Fear, Middendorf,
Black Creek, and Steel Creek formations. The thickness
of the Cretaceous section is about 400 ft at the
northwestern boundary of SRS and 800 ft at the southeastern
boundary. Siple (1967) assigned all the Cretaceous strata in
the vicinity of SRS to the Tuscaloosa Formation, the type
locality of which is in Alabama. They type Tuscaloosa is
now thought to be Cenomanian or Turonian (Christopher,
1982; Valentine, 1984; Sohl and Smith, 1985). Fossil age
determinations from SRS are younger. Colquhoun and others
(1985a) subdivided the subsurface Cretaceous in the SRS
area.
Cape Fear Formation
Lithology and stratigraphic terminology.
The basal unit of the Coastal Plain stratigraphic section
at SRS is composed of poorly sorted, silty to clayey quartz
sands and interbedded clays. The sands are commonly
medium and coarse. They are arkosic in places and pebbly
zones are present in many parts of the section. Gray, yellow,
orange, red, brown, tan, and blue colors are common. The
Cape Fear Formation is more indurated than the other Cretaceous
formations because of high clay content and abundance
of cristobalite in the matrix (Prowell and others,
1985a, p.8). In general, bedding thickness varies from about
5 to 20 ft, with sands being thicker than clays. Lithologic
similarity and fossils indicate a correlation with the type
Cape Fear Formation in southeastern North Carolina. Prowell
and others (1985a) assigned these sediments to their UK1
unit.
The Cape Fear is about 30 ft thick at the northwestern
SRS boundary and thickens to about 200 ft near the southeastern
boundary. Regional dip of the upper surface is about
35 ft/mi to the southeast. In the northwestern and central
parts of SRS, the Cape Fear lies nonconformably on metamorphic
rocks of the Appalachian orogen. In the southeastern
part of the Site, it lies on red mudstones, conglomerates,
and sands of the Triassic Newark Supergroup.
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