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PAUL A. THAYER AND MARY K. HARRIS
moldic formed by the dissolution of aragonitic pelecypods.
Permeability is very low owing to high percentage of mud in
this lithofacies.
Muddy, Calcareous Quartz Sand
This lithofacies is abundant in both wells and consists of
subangular, fine quartz sand containing 10 to 40% skeletal
allochems set in a terrigenous mud and micrite matrix that
ranges from 10-30% of whole rock volume. The unit grades
into muddy, quartz-rich packstone and grainstone.
Allochems are gravel and sand sized fragments of oysters,
other pelecypods, echinoderms, foraminifers, bryozoans,
barnacles, sponge spicules, diatoms, ostracodes, and gastropods.
Most of the mollusc and echinoderm fragments are
abraded. Sparry calcite cement forms 2-5% and occurs
mainly as syntaxial overgrowths on echinoderm fragments; it
also partly fills interparticle and moldic pores. Porosity
ranges from 3-10% and is mostly interparticle and moldic.
Permeability is low because terrigenous mud and micrite
occlude pores and pore throats.
Muddy, Quartz Rich Packston
This lithofacies, which occurs in both wells, consists of
skeletal packstone containing 10-30% fine quartz sand and
5-15% terrigenous mud matrix. Gravel and sand sized oysters,
echinoderms, other pelecypods, and bryozoans are the
main allochems; most are abraded. Also present are foraminifers,
gastropods, serpulid worm tubes, sponge spicules, barnacles,
ostracodes, and finely comminuted shell debris.
Micrite forms 10-30% of whole rock volume and is locally
pelleted. Other constituents include glauconite, collophane,
muscovite, carbonate intraclasts, heavy minerals, and woody
organic debris. Sparry calcite forms overgrowths on echinoderm
fragments and partly lines moldic and intraforam
pores. Porosity ranges form 5-22% and is moldic and intergranular
with minor vug and intraparticle in foraminifers and
bryozoans. Permeability is low because pores are isolated
from each other bymicrite or terrigenous mud.
Muddy, Quartz Rich Grainstone
This minor lithofacies occurs as thin beds intercalated
with quartz rich packstone and calcareous quartz sand. It
consists of abraded gravel and sand size skeletal fragments
mixed with subordinate fine quartz sand and minor terrigenous
mud. Skeletal fragments include oysters, other pelecypods,
echinoderms, bryozoans, barnacles, and foraminifers.
Minor sparry calcite occurs as overgrowths on echinoderm
fragments and partly fills intraparticle pores in foraminifers
and bryozoans. Porosity of this lithofacies averages 21% and
is mainly intergranular. Permeability is high because of the
large number of well connected interparticle pores.
Diagenesis
Diagenetic events in the Griffins Landing Member
include: 1) marine phreatic – micritization of skeletal
allochems and minor glauconitization; and 2) freshwater
meteoric dissolution of opaline silica and aragonitic skeletal
grains, along with sparry calcite cementation in primary and
secondary pores and as overgrowths on echinoderm fragments.
Opal-CT lepispheres and authigenic clinoptilolite
crystals that line the margins of secondary moldic pores indicate
that they formed after dissolution of aragonitic
allochems. The dissolution of sponge spicules and diatoms is
the likely source of silica for formation of opal-CT lepispheres
and chalcedony replacements of molluscan shells.
Depositional Environment
Huddlestun and Hetrick (1986) proposed a marginal
marine, coastal environment for the Griffins Landing Member.
The abundance of terrigenous quartz and mud suggests a
nearby source terrane, as does the presence of woody organic
debris. Faunal elements in this unit indicate relatively clear
marine waters of normal salinity. The presence of abraded
gravel and sand sized skeletal allochems suggest that high
energy conditions alternated with quiet water deposition in
which muds accumulated.
REFERENCES
Fallaw, W.C., Price, V., and Thayer, P.A., 1992, Stratigraphy if the
Savannah River Site, South Carolina, in Zullo, V.A., Harris,
W.B., and Price, V., eds., Savannah River region: transition
between the Gulf and Atlantic Coastal Plains-Proceedings of
the Second Bald Head Island Conference on Coastal Plains
Geology: Wilmington, N.C., The University of North Carolina
at Wilmington, p. 29-32.
Gohn, G.S., 1988, Late Mesozoic and early Cenozoic geology of
the Atlantic Coastal Plain: North Carolina to Florida: in Sheridan,
R.E., and Grow, J.A., eds., The geology of North America,
v. 1-2, The Atlantic Continental Margin, U.S., Geological Society
of America, p. 107-130.
Harris, W.B., and Zullo, V.A., 1992, Sequence stratigraphy of Paleocene
and Eocene Deposits in the Savannah River region, in
Zullo, V.A., Harris, W.B., and Price, V., eds., Savannah River
region: transition between the Gulf and Atlantic Coastal Plains
– Proceedings of the Second Bald Head Island Conference on
Coastal Plains Geology: Wilmington, N.C., The University of
North Carolina at Wilmington, p. 134-142.
Huddlestun, P.F., and Hetrick, J.H., 1985, Upper Eocene stratigraphy
of central and eastern Georgia: Georgia Geologic Survey
Bulletin 95, 78p.
Nystrom, P.G., Willoughby, R.H., and Price, L.K., 1991, Cretaceous
and Tertiary stratigraphy of the upper Coastal Plain,
South Carolina, in Horton, J.W., Jr., and Zullo, V.A., eds., The
geology of the Carolinas: Knoxville, Tenn., The University of
Tennessee Press, p. 221-240.
Powell, R.J., 1984, Lithostratigraphy, depositional environment,
and sequence framework of the middle Eocene Santee Lime-
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