PETROLOGY AND DIAGENESIS OF UPPER TRIASSIC
FLUVIAL SANDSTONES, SUBSURFACE DUNBARTON BASIN,
S.C.
THAYER, Paul A., Department of Earth Sciences, University
of North Carolina at Wilmington, 601 S. College Road,
Wilmington, NC 28403-3297
SUMMER, Neil S., Department of Geology, University of
Florida, 1112 Turlington Hall, Box 117340,
Gainesville, FL 32611-7340
Study of core from five wells in the buried Dunbarton Basin indicates
that the sandstones are braided stream and alluvial fan rift-valley
deposits of continental block provenance. The sandstones are
moderately to poorly sorted, fine to very coarse-grained plagioclase
arkose, lithic arkose, and feldspathic litharenite. Modal analysis of 45
thin sections from two wells (DRB-10 and DRB-11) shows mean QFL
= 51:38:11 and Q/F = 1.34. Whole-rock chemical composition of 19
sandstone samples average: SiO2, 69.5%; Al2O3, 12.5%; CaO,
4.47%; MgO, 0.94%; Na2O, 3.73%; K2O, 0.89%; FeO, 0.30%;
Fe2O3, 2.36%; MnO, 0.07%; TiO2, 0.59%; P2O5, 0.11%; and LOI,
4.02%. Textural and mineralogical immaturity, chemical composition
and framework mineralogy suggest derivation from a nearby
metamorphic terrane composed of gneiss and schist with minor
amphibolite, metavolcanic rock, quartzite, and granite.
Total porosity of 36 sandstones averages 7.2 ± 5.2%, whereas
mean effective porosity >1 mm is 4.1 ± 2.4%. Geometric mean
permeability (horizontal) averages 0.1 mD. Study of mercury
injection curves, SEM micrographs, and blue dyed, epoxy-
impregnated thin sections demonstrates that most pores are micro-
sized with pore-aperture radii less than 0.5 mm. Isolated grain-
dissolution pores comprise less than 30% of total pore space and
formed by removal of feldspar, unstable heavy minerals, labile rock
fragments, and calcite replacements of feldspar. Low matrix
permeability results from high percentages of allogenic and authigenic
clays, tight grain packing, squeezing of ductile rock fragments into
primary intergranular pores, and cementation.
Triassic sandstones have undergone several miles of burial and
are in the advanced stage of diagenesis. Diagenetic events included
physical and chemical compaction, cementation, and dissolution of
grains and cements. Albite and calcite are the major cements and
replacements; others include quartz, chlorite, kaolinite, hematite,
sphene, pyrite, and laumontite. Electron microprobe analyses show
pervasive albitization of detrital feldspar within the cored intervals
(1,000-5,000 ft. bmsl). A representative analysis of the authigenic
albite is 98.32 mole % Ab, 0.26 % Or, and 1.42 mole % An. By
weight percentage, the representative albite contains 68.84% SiO2,
20.21% Al2O3, 0.03% Fe2O3, 0.28% CaO, 10.76% Na2O, and 0.04%
K2O. The source of Na+ for authigenic albite is unknown; it may
have came from weathering or fluid-dominated alteration of sodic
plagioclase from the adjacent source terrane or deep basinal brines
derived from compaction of interbedded clays.
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