40Ar/39Ar Dating of Detrital Muscovite and Sediment Compositional Analysis of the Pottsville Formation in the Black Warrior basin in Alabama: Implications for Tectonics and Sedimentation
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Geology and Geography
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The Greater Black Warrior basin, including the Cahaba and Coosa synclinoria and the Black Warrior basin, contains the thickest preserved accumulation (ca. 2 km) of sediment in the Appalachian-Ouachita foreland. 40Ar/39Ar detrital muscovite ages, sandstone modal analysis, and conglomerate composition provide a powerful basis for delineating ancient depositional patterns, tectonic activity, and the provenance of ancient sediment. The Pennsylvanian Pottsville Formation was deposited in the Black Warrior basin between ca. 320 and 308.5 Ma, the latter age evidenced by dating of an ash layer in the upper Pottsville in Mississippi. Sandstones from the Cahaba synclinorium are compositionally immature and dominated by litharenite, with a mean composition of Qt58F8L34 and Qm51F8Lt39. Detrital muscovite modal ages of ca. 450, 375, and 325 Ma are recorded from the Cahaba, recording a dominant Appalachian source for metamorphic muscovite. The compositionally immature sediment and varied detrital geochronology indicate that sediment in the Cahaba synclinorium was deposited during rapidly changing tectonic regimes during Lower Pennsylvanian. Additionally, sediment from the Black Warrior basin varies in composition and detrital muscovite ages over various stratigraphic levels. To investigate the character of sediment in the Black Warrior basin, samples were collected from surface exposures and from two cores in Alabama: (1) the Hendrix core, located in a local depocenter of the basin containing more than 1 km of sediment from the upper Pottsville; and (2) the Brooks core, located in the central part of the basin and sampling 500 m of the lower Pottsville. Sandstones from the lower Pottsville are compositionally and texturally more mature than those of the upper Pottsville Formation. Conglomerates of the lower Pottsville Formation are dominated by quartz pebbles. In contrast, conglomerates in upper sections of the basin depocenter contain clasts similar to lithologies of the nearby Valley and Ridge, Piedmont and Blue Ridge provinces, consistent with a dominant local source for the upper Pottsville. Laser 40Ar/39Ar analyses for detrital muscovite from samples of the lower Pottsville (Brooks core) yield mainly Ordovician, Taconic ages of ca. 450 Ma, and a broad distribution of Devonian ages (ca. 390-360 Ma). Younger sandstones in the basin are dominated by Upper Mississippian to Lower Pennsylvanian muscovite (ca. 335-310 Ma). The compositional and geochronologic data are interpreted to indicate that large amounts of sediment were provided to the Black Warrior basin from Taconic terranes in the central and northern Appalachians during Lower Pennsylvanian. This interpretation requires that quartz arenites near the base of the Pottsville Formation were deposited by a Ganges-scale river system draining the central and northern Appalachians Mountains, while immature sediments (abundant in the upper Pottsville) were deposited by deltaic systems that drained the nearby southern Appalachians. Alleghanian structural development may have placed terranes containing abundant Acadian and Alleghanian muscovite in positions favorable to provide sediment to the sequences of the upper Pottsville Formation, while precluding accumulation of significant contribution of sediment from terranes with abundant Taconic muscovite.