GEOLOGIC FEATURES OF THE MAUMEE QUADRANGLE, MARION AND SEARCY COUNTIES, ARKANSAS

The Maumee 7.5-minute quadrangle is located on the southern flank of the Ozark Dome in a region of overlap of the Salem and Springfield plateaus. A 370-m-thick section of mostly carbonate sedimentary rocks is exposed and comprised of 14 mappable units representing the Ordovician, Silurian and Mississippian. Ordovician and Silurian rocks are best exposed in the SE where dissection of the terrain reaches a maximum near the Buffalo River. Silurian limestones are discontinuous and often found in troughs of low-amplitude folds and overlain by undeformed Missississippian rocks. Silurian rocks, Upper Ordovician Fernvale Limestone and Middle Ordovician Plattin Limestone and St. Peter Sandstone successively thin and pinch out northward, an indication of relative uplift N of the area before deposition of Mississippian limestones.

Abundant faulting is present in the southern half of the quadrangle. The Tomahawk and South Tomahawk faults are WNW-trending, N- and S-bounding faults, respectively, for a small graben that down-drops, and has preserved, the only Upper Mississippian Fayetteville Shale, including the Weddington Sandstone Member, in the quadrangle. This graben system transitions to a more NW trend farther E with the down-to-the-SW North Rocky Creek fault forming the NE boundary and two discontinuous down-to-the-NE faults forming the SW boundary of the graben. A NE-trending zone of left-stepping en echelon faults and intervening monoclines crosses the NW part of the quadrangle and accommodates as much as 45-m of down-to-the-SE throw and undetermined right-lateral slip. Erosion of the NW footwall of this zone exposes the oldest rocks of the quadrangle, a limestone-rich, lower part of the Middle Ordovician Everton Formation that locally hosts an artesian spring.

Old gravel terraces representing former channel positions of the Buffalo River have been identified at two elevation ranges above the current river. The lower terrace is about 25-35 m above base-flow level of the river and correlates with similar deposits identified farther west along the Buffalo River. Two remnants of older gravel deposits composed of pebble- to cobble- sized clasts of well-rounded fine-grained sandstone and chert have been identified about 72-80 m above base-flow on narrow point bars of tight meanders.