Abstract

A new core from the Black Warrior Basin of Alabama contains a newly discovered mound lithofacies in the Tuscumbia Limestone (Meramecian). The Schlumberger-Alabama Power 1 Plant Gorgas well contains 37.2 m (122 ft) of carbonate rock assigned to the Tuscumbia. This formation overlies the Fort Payne Chert (Osagean), which was penetrated but not cored in this well. The Tuscumbia underlies calcareous shale and limestone of the Chesterian Pride Mountain Formation.

Three lithologic units have been defined in the cored part of the Tuscumbia. The basal 2.8 m (9.25 ft) of the core consists of sponge-microbial boundstone (unit 1). This is overlain by 20.6 m (67.6 ft) of mixed carbonate strata dominated by mixed-particle grainstone, which increases in abundance upward (unit 2). Some of the grainstone is brecciated, suggesting exposure and paleokarst development following mound formation. Grainstone is interbedded with thin units of argillaceous cherty peloidal carbonate, sponge-microbial boundstone, and mixed-particle rudstone. Glauconite is common in the basal part of unit 2. Unit 2 is abruptly overlain by 13.8 m (45.2 ft) of bryozoan crinoid grainstone (unit 3) containing breccia beds, low-angle cross-strata, and immature carbonate paleosols. This unit is sharply overlain by the Lewis limestone of the Pride Mountain Formation, which here consists of 0.5 m (1.7 ft) of interbedded fossiliferous shale and mixed-particle packstone. The Lewis limestone is overlain disconformably by fenestrate-bryozoan-rich shale.

The basal part of the core records growth of a sponge-microbial mound in relatively shallow water below normal wave base. The mound is not Waulsortian: stromatactis is absent, and matrix material is grainy. The upper contact is sharp, and borings are locally abundant. Abundant authigenic glauconite indicates reducing pore waters. The mound was buried by foreshoal grainstone, much of which is brecciated. Upper Tuscumbia bryozoan-crinoid grainstone formed in a mobile shoal that quickly aggraded to sea level. Although the upper Tuscumbia here is typical of the formation, the shoal buried a mound facies not previously reported from the Tuscumbia.

Early diagenesis was dominated by marine cementation, syndepositional alteration, and fracturing. Burial diagenesis was dominated by calcite cementation, dissolution of siliceous spicules, stylolitization, chert formation, and later, emplacement of hydrocarbons and their subsequent transformation to pyrobitumen. Today, original interparticle voids are filled with a mixture of calcite cement, replacive chert, and solid hydrocarbons. Irregular nodules of chert replaced parts of the mound and the overlying heterogeneous unit. Dolomite partially replaced heterolithic strata of unit 2. Patches of once porous rock in units 1 and 2 contain abundant solid hydrocarbons, but porosity (0.7%–5.6%) and permeability (1–78 μd) are low. In-place heating led to gas generation and to concomitant in-place solidification of liquid hydrocarbons.

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