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.