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Mississippian (Chesterian) Sequence Stratigraphy in the Black Warrior Basin: Pride Mountain Formation (Lowstand Wedge) and Hartselle Sandstone (Transgressive Systems Tract)
Frank W. Stapor (1), A. W. Cleaves (2)
The Pride Mountain Formation (Gasperian Stage) is primarily a shelfal shale containing two deltaic sandstone members, the Lewis and the overlying Evans. Punctuating this siliciclastic section are thin limestones that mark interruptions in siliciclastic supply. Sand-rich deltaic facies occur to the north and east whereas distal, shale-dominated facies occur to the south and west. Isolith and isopach maps describe A) dip elongate, river-dominated lobes in the Lewis; and B) cuspate, wave-dominated lobes with SW/NE trending shorelines in the Evans. Both sets of lobes thin downdip to the south and west. Petrographic analysis of these quartz arenites indicates a cratonic rather than orogenic source. Southward flowing rivers, draining the craton, supplied sediment to the prograding Pride Mountain deltas.
The overlying Hartselle Sandstone (Hombergian Stage) consists of relatively thin, less that 10 m thick, wave-dominated deltas with E/W to NW/SE trending shorelines; progradation was to the south and southwest. In northwest Alabama these deltas are arranged in an aggradational pattern; beginning in northeast Alabama they are in a backstepping or transgressive pattern that produces a nearly continuous sheet sandstone that extends across Tennessee to south-central Kentucky. Beginning in north-central Alabama the Hartselle disconformably overlies subaerially weathered Monteagle Limestone. Paleosol remnants have been identified in the uppermost Monteagle from Huntsville, Alabama, north to Cookeville, Tennessee.
The subaerial weathering of the shallow-water, ramp carbonates of the Monteagle Limestone indicates a pronounced fall in relative sea level. This drop exposed the carbonate ramp as far south as northern Mississippi and Alabama. The Pride Mountain Formation and the Hartselle Sandstone of northwest Alabama represents a lowstand "wedge" deposited after sea level stabilized. One incised-valley fill associated with the Lewis Sandstone Member has been identified in the north Alabama subsurface; the Bethel channel-fill of north Kentucky is equivalent in age and may be related to this hypothesized Alabama, lower Chesterian valley-fill complex. The Hartselle Sandstone of northeast Alabama and central Tennessee is a transgressive systems tract that was deposited during the subsequent relative sea-level rise as the siliciclastic shoreline moved back to the north.
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