Download Guidebook as .pdf (1.8 Mb) - Carolina Geological Society

HYDROSTRATIGRAPHY OF THE SAVANNAH RIVER SITE REGION
Figure 7. Isopach map of the Gordon aquifer unit.
Gordon Confining Unit
The Gordon confining unit consists of fine grained,
glauconitic, muddy sand and clay of the Warley Hill Formation
(Fig. 2). In the study area, Warley Hill strata are generally
less than 50 feet thick. The Gordon confining unit has
been informally termed the “green clay” in updip areas of the
SRS. The Gordon confining unit is not defined in the northwestern
part of the SRS (Figs. 7 and 8); instead, discontinuous
lenses of Warley Hill Formation from the “green clay”
confining zone within the Steed Pond aquifer.
Upper Three Runs Aquifer Unit
The Upper Three Runs aquifer consists of all strata
above the Warley Hill Formation, and includes the sandy and
sometimes calcareous sediments of the Santee Limestone
and all of the overlying Barnwell Group from the water table
down to the top of the Gordon confining unit (Fig. 2). The
Upper Three Runs aquifer is the updip terrigenous equivalent
of the upper Floridan aquifer (Fig. 1). The Upper Three Runs
aquifer varies from less than 50 to more than 200 feet thick
in the SRS region (Fig. 8).
Floridan Midville Aquifer System
In the northwestern part of the SRS region, the Floridan
and Dublin Midville aquifer system coalesce to form the
Floridan Midville aquifer system (Figs. 1, 2, and 3). The
Floridan Midville is divided into three aquifers separated by
two confining units (Fig. 2).
Figure 8. Isopach map of the Upper Three Runs and Steed
Pond aquifer units.
Steed Pond Aquifer Unit
Where the Gordon confining unit becomes too thin to be
an effective confining unit, the Gordon and Upper Three
Runs aquifers merge to form the Steel Pond aquifer (Figs. 1
and 2). The Steed Pond includes strata from the water table
down to the clays and muddy sands of the Ellenton Formation
and upper Peedee Formation (Fig. 2). The Steed Pond
varies from less than 30 to more than 150 feet thick in the
SRS region (Fig. 8).
CONCLUSION
The hydrostratigraphic classification described herein
utilizes a four-level hierarchy to delineate the hydrostratigraphy
of the SRS region. The Piedmont hydrogeologic province
is overlain by Cretaceous and Tertiary strata of
Southeastern Coastal Plain hydrogeologic province. Downdip,
the Coastal Plain sequence is divided into three aquifer
systems and three confining systems. Updip thinning of the
sedimentary sequence results in the coalescence of the threeaquifer
systems into one at the Fall Line.
The classification system allows subdivision of the
hydrostratigraphic units into as much detail as the local geology
demands while keeping within a regional framework.
This feature makes the system especially attractive for use in
site specific hydrostratigraphic studies where a complete
description of the hydrologic regime is required.
75