Abstract: Petrology, Stratigraphy, and Depositional History of the Upper Triassic-Lower Jurassic Eagle Mills Formation, Choctaw County, Alabama

Donald E. Burch, Jr., A. E. Weidie

In southwestern Choctaw County, Alabama, the Eagle Mills Formation consists of alluvial fan red beds deposited in a Mesozoic graben. The Exxon 1 Smith Lumber Co. well penetrated the upper 2210 ft of the Eagle Mills and is divided into eight lithostratigraphic units designated A through H from bottom to top.

Unit A consists of numerous fining- and coarsening-upward sequences of sandstones, conglomeratic sandstones, and conglomerates of variable thickness separated by red mudstones. Sediments were deposited on the distal part of the fan by ephemeral streams and sheetfloods and are composed of low-grade metamorphic rock fragments probably derived from a source area to the east or south. Unit B consists of red mudstone with interbedded fining- and coarsening-upward sandstones and conglomerates with abundant calcite concretions and paleocaliche. Unit B was deposited on the distal fan by sheetfloods and ephemeral streams. Faulting and igneous activity related to rifting occurred during the latter stage of deposition of Unit B. Units C through G are dominantly conglomerates; Units C, D, and F c ntain abundant igneous rock fragments. Unit C contains plutonic (syenite) and volcanic (rhyolite and dacite) rock fragments, whereas the igneous clasts of Units D and F are volcanic. Units C and D exhibit a general coarsening upward of sediments indicating progradation of the fan. Unit E coarsens upward to a sedimentary (chert and limestone) and metamorphic (dolomite marble and lesser amounts of phyllite and schist) rock fragment-bearing cobbly pebble conglomerate. Unit F is a mixed volcanic and sedimentary-rock-fragment-bearing conglomerate. Unit G is a massive sedimentary and metamorphic-rock-fragment-bearing cobbly pebble conglomerate. Units D through G were deposited in the midfan subenvironment by braided streams. In Unit H, high log porosities and well rounded sand grains in drill uttings indicate the presence of eolian deposits. Log responses and the presence of anhydrite in drill cuttings show that Eagle Mills is overlain by the Werner Formation.

The provenance for Eagle Mills sediments was the southern Appalachian system, with sedimentary rock fragments being derived from folded Paleozoic rocks and metamorphic rock fragments coming primarily from metasedimentary rocks of the Talladega Slate belt. Igneous rock fragments were derived from rocks emplaced in the upper-fan area as a result of igneous activity associated with rifting.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994