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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 9. (September)

First Page: 1690

Last Page: 1690

Title: Depositional Environments and Regional Stratigraphy of Jurassic Norphlet Formation in South Alabama: ABSTRACT

Author(s): Richard P. Wilkerson

Article Type: Meeting abstract


In the western Gulf coastal plain area the Norphlet Formation is typically characterized by nonmarine and red bed lithofacies. In south Alabama, the Norphlet consists of an updip conglomerate, a basal shale, red beds overlying the shale, and an upper quartzose sandstone, the Denkman Member. The Norphlet unconformably overlies either salt, anhydrite, red beds, or Paleozoic rocks. The Smackover Formation overlies the Norphlet with a sharp contact over most of south Alabama, except in parts of Mobile County where the contact is gradational.

The conglomeratic lithofacies is discontinuous in areal extent, and is present in cores from Escambia, Monroe, and Wilcox Counties. It consists of red and gray sandstone, conglomerate, and conglomeratic sandstone. The shale lithofacies also appears to be discontinuous in areal extent, and is present in cores from Escambia County. It consists of mostly black shale, with some brown and red shale. The red bed lithofacies was penetrated in wells in Escambia and Clarke Counties. It consists of red, brown, and gray, very fine to coarse-grained sublitharenite and subarkose, with an average composition of 64% quartz, 13% feldspar, 8% rock fragments, and 10% matrix. It is characterized by low-angle planar cross-beds and discontinuous laminae, along with interbedded silt and coarse sand. The qu rtzose lithofacies (or Denkman Member) is present in cores from Mobile, Baldwin, Escambia, Clarke, Choctaw, and Washington Counties. It attains a thickness of from 400 to over 700 ft (122 to over 213 m) in parts of Choctaw, Washington, and Mobile Counties, and thins to the northeast and east (Clarke, Monroe, Conecuh, and Escambia Counties) where Norphlet red beds and conglomerates predominate. The quartzose lithofacies consists of gray and brown, very fine to medium-grained subarkose with an average composition of 76% quartz, 12% feldspar, 3% rock fragments, and 2% matrix. It is characterized mainly by low to high-angle planar cross-beds, and also contains slump structures, wavy discontinuous laminae, and massive intervals.

Norphlet deposition in south Alabama occurred in an arid climate. The lower shale probably was deposited in lagoons or mud flats left from a retreating hypersaline sea which had deposited the Louann salt. Accompanying the retreat of this sea were climatic and/or tectonic changes which resulted in clastics being shed from exposed paleo-highs. Initial clastic deposition occurred in alluvial-braided stream environments which are represented by sediments of the conglomeratic and red bed lithofacies. These sediments were reworked into downdip areas and deposited in desert dune and inter-dune environments. A transgression near the end of Norphlet time resulted in reworking of underlying sediments and deposition in intertidal environments. These deposits may be partial landward equivalents o seaward Smackover carbonates. Dune, interdune, and intertidal environments are represented by the quartzose lithofacies or Denkman Member. Basement paleo-highs not only were a source of sediments but also controlled Norphlet deposition in that the formation thins or is absent over them.

The Norphlet Formation is an important reservoir in south Alabama. Stratigraphic relations indicate that lower Smackover Formation carbonate mudstone provide the petroleum source rocks. Reservoirs are facies selective, occurring mainly in Norphlet intertidal, eolian, and braided-stream deposits. Traps are due to a combination of favorable stratigraphic, structural, and diagenetic development.

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