PRELIMINARY MEASURED STRATIGRAPHIC SECTION AT MARTHA'S CHAPEL ROAD, FARRINGTON 7.5-MINUTE QUADRANGLE, DURHAM SUB-BASIN, DEEP RIVER BASIN, CHATHAM COUNTY, NORTH CAROLINA

Non-marine, fluvial, siliciclastic deposits of the Upper Triassic Chatham Group in the Durham sub-basin of the Deep River basin are well exposed along the shore of B. Everett Jordan Lake adjacent to Martha's Chapel Road. Recent detailed field mapping partially funded through the STATEMAP Program, of the Triassic sedimentary rocks in the Farrington 7.5-minute Quadrangle provided an opportunity to measure this exposed section in detail. A stratigraphic section 604 feet thick is almost continuously exposed along 2291 feet of shoreline.

Previous geological mapping in adjacent quadrangles has attempted to subdivide the Durham Sub-basin siliciclastic deposits into Lithiofacies Associations based on overall lithology and accessory minerals. The siliciclastic deposits studied consists of medium- to coarse-grained, light gray to white, and light grayish-pink, micaeous, arkose and lithic arkose sandstones that fine upward through medium- to fine-grained sandstone into reddish brown to maroon, bioturbated siltstone and mudstone. The entire 604 feet thick section is very cyclical in nature. Overall, 23 sandstone units are overlain by 24 siltstone and mudstone units. The lower portions of the sandstones are typically trough crossbedded. Planar crossbedding typically overlies the trough crossbedded portions of the sandstones. Normally graded beds and inversely graded beds are also present. Muscovite and biotite are common in almost all of the sandstone units. The lower contact of the sandstone units is usually scoured into the underlying siltstone. The upper contact of the sandstone units is sharp and grades quickly over a few inches. Bioturbation appears to have destroyed all bedding in the siltstone and mudstone units. There is possible faulting in the upper portion of the measured section.

The well developed, cyclical, fining-upward sandstone into siltstone and mudstone are characteristic of a fluvial system of laterally-aggrading point bar deposits and crevasse splay deposits surrounded by a vegetated floodplain. The abundance of muscovite and biotite throughout the entire section suggests that the presence or absence of these accessory minerals is not characteristic of any individual Lithofacies Associations.