PETROLOGY AND GEOCHEMISTRY OF UPPER TRIASSIC DAN RIVER GROUP SANDSTONES, DAN RIVER-DANVILLE BASIN, N.C. AND VA

Dan River Group sandstones are nonmarine rift valley deposits of continental block provenance. Sandstones are poorly sorted, immature plagioclase arkose, lithic arkose, and feldspathic litharenite. Mean QFL=58:36:6, Q/F=1.98, and P/F=0.62. Framework composition and textural and mineralogical immaturity suggest derivation from a nearby terrane composed of gneiss and schist with subordinate granite, amphibolite, and metavolcanic rock. Average major element composition/1s for 73 sandstone samples is (percent): SiO₂, 69.1/6.5; TiO₂, 0.69/0.30; Al₂O₃, 14.7/1.9; Fe₂O₃, 2.70/1.63; FeO, 1.27/1.37; MnO, 0.11/0.08; MgO, 0.93/0.77; CaO, 1.47/1.86; Na₂O, 5.01/1.16; K₂O, 2.41/1.12; P₂O₅, 0.17/0.10; H₂O+, 0.34/0.16; CO₂, 0.32/0.98. Minor elements detected/1s in 262 samples include (ppm): U, 2.7/1.4; Th, 12/5; Hf, 8/4; Ce, 94/50; Sc, 12.4/6.1; V, 98/52; Dy 6.1/3.4; Eu, 3.2/1.6; La, 52/23; Sm, 10/4; Yb, 5/3; Lu, 0.5/0.3. The major oxides are relatively high for TiO₂, Al₂O₃, Fe₂O₃, FeO, and Na₂O, but low for SiO₂ and CaO compared to the average arkose (Pettijohn, 1963). The chemical index of alteration (CIA) for sandstone samples averages 55.1 and is nearly identical to that of interbedded mudrocks (54.3). These values are characteristic of unweathered igneous and metamorphic rocks of the source terrane and suggest minor chemical modification of the sediment prior to transport and burial. Based on tectonic discrimination plots (SiO₂ vs K₂O/Na₂O, log (K₂O/Na₂O) vs log (SiO₂/Al₂O₃), and La-Th-Sc) the setting of the source terrane appears to reflect an active continental margin and continental island arc. Small La/Th and TiO₂/Al₂O₃ ratios and relatively large Hf contents indicate derivation from crust of predominantly felsic composition.