Depositional Environments of Norphlet Formation (Jurassic) of Southwestern Alabama: ABSTRACT

Abstract

The Norphlet Formation in southwestern Alabama has become a primary target for oil and gas exploration. Isopach data show that Norphlet deposition was affected by a subsiding Mississippi Interior Salt basin, early movement of the Louann Salt, and stable Appalachian ridges and paleohighs, such as the Conecuh ridge and the Wiggins arch. The formation is over 1,000 ft (350 m) thick in parts of Washington, Clarke, Baldwin, and Mobile Counties. The Norphlet thins or is absent over areas of penecontemporaneous salt movement and is absent over the stable Wiggins arch. The Denkman Member of the Norphlet is underlain by a red-bed sequence that grades updip into a conglomerate. The Denkman consists of clean, fine-grained, well-sorted sandstone, including a lower cross-stratified unit and an upper massive unit. The red-bed sequence is mostly sandstone with shale, shaly sandstone, and siltstone at the base. The shales, present in Escambia and Choctaw Counties, were probably deposited as distal portions of alluvial fans. Wadi gravels were deposited adjacent to the Appalachian highlands and grade downdip into the red-bed sequence. Both the gravels and red beds were deposited by sediment-choked braided streams and subjected to reworking and deflation, producing the Denkman Member. The Denkman consists of eolian sands, reworked at the top by the Smackover t ansgression into a massive sand up to 70 ft (21 m) thick. Thin, massive and horizontally laminated units in the cross-stratified sand indicate that narrow interdunes separated the broad dune sands of the Norphlet. A generalized sequence of diagenetic events affecting porosity may have been compaction, quartz overgrowth, and carbonate cementation, and possible selective dissolution of cements followed by deep cementation. End_of_Article - Last_Page 1435------------