Regional Jurassic Lithostratigraphy of Alabama: ABSTRACT

Abstract

Jurassic units in the subsurface of Alabama include the Werner Formation, the Louann Salt, the Norphlet Formation, the Smackover Formation, the Haynesville Formation (including the Buckner Anhydrite Member), and the Cotton Valley Group. These units range in age from early Callovian to late Tithonian, (Middle to Late Jurassic) with the Jurassic-Cretaceous boundary possibly occurring in the upper part of the Cotton Valley Group. Deposition was controlled by pre-Jurassic paleohighs, diapiric salt structures, and the peripheral fault system that rims the Gulf Coast basin. Climates during the Jurassic ranged from hot and dry during Early Jurassic to hot and humid during Late Jurassic. The Werner Formation consists of a lower sandstone and shale sequence, overlain by an evapori ic unit, and lies disconformably either on the Paleozoic basement or the Eagle Mills Formation of Late Triassic age. Deposition of the Werner represents the initial transgression of marine waters into the Gulf Coast basin. The Louann Salt is a massive halite unit that formed a southward tilting ramp on which younger Jurassic strata were deposited. The Norphlet Formation is a sandstone sequence with thin interbedded shale that lies disconformably on the Louann. The Smackover Formation consists of two units, a lower laminated mudstone and brown dense limestone, and an upper dolomitic grain-supported limestone. The Smackover lies conformably on the Norphlet, and in places is gradational with it. The Haynesville is a sequence of calcareous, anhydritic sandstone and interbedded, anhydritic, m caceous shale. The Haynesville may consist of either a sequence of interbedded, calcareous mudstone, anhydritic shale and sandstone, limestone, dolostone, and salt stringers, or a massive anhydrite (Buckner) at the base. In places the Buckner is gradational with the upper Smackover. The Cotton Valley Group is a sequence of fine to very coarse-grained to conglomeratic sandstone, interlayered with silty micaceous shale, very thin limestone beds, and in the Mississippi Interior Salt basin, thin coal and lignite beds. The Cotton Valley was deposited in a terrestrial to littoral environment and the absence of fossils, to date, makes it difficult to distinguish from the sandstone units of the overlying Hosston of Early Cretaceous age. End_of_Article - Last_Page 1438------------