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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 4. (April)

First Page: 688

Last Page: 701

Title: Origin of Conglomerates in Silurian Red Mountain Formation of Central Alabama; Their Paleogeographic and Tectonic Significance

Author(s): Denny N. Bearce (2)


The Lower Silurian Red Mountain Formation in central Alabama consists of interbedded sandstone and siltstone and four prominent intervals of sedimentary iron formation. Conglomerates consisting of discoid limestone cobbles in sedimentary hematite ore matrix commonly are present in one or more beds within the formation. Elongate cobbles have preferred northeast and southeast orientations. Crossbedding within the conglomerate matrix suggests southwestward-directed paleocurrents. Cobbles are deformed by soft-sediment draping. Bioclastic limestone lenses within and directly below the conglomerate intervals were apparently the source material.

Although exposures of the Red Mountain Formation are insufficient to prove a beltlike distribution of the conglomerates, or the trend of such a belt, the conglomerates are known to be present along the strike of the southeastern limb of the Birmingham anticlinorium from Bessemer to Gadsden, Alabama, a distance of more than 70 mi. Width of the possible belt may be as much as 2 mi.

The Red Mountain Formation is absent in the Helena thrust block southeast of the Birmingham thrust block. Northwest of the Birmingham anticlinorium, the formation grades into calcareous shale and limestone.

Clay cobbles similar in size and shape to those in the Red Mountain Formation are forming at a site in the Mississippi Sound on the north coast of the Gulf of Mexico. The clay cobbles originate through the combined effects of desiccation and cracking of clay lenses in the intertidal zone and gentle wave abrasion.

It is suggested that during Early Silurian time a slowly rising arch at the site of the Birmingham anticlinorium separated land on the southeast from the open sea on the northwest. The cobbles originated in an intertidal zone on the arch. Southwest-oriented crossbedding foresets evidently resulted from back-bar longshore currents during tidal withdrawal. The arch never was sufficiently emergent to cause erosion during Early Silurian time, and during deposition of the greater part of the Red Mountain Formation the arch remained submerged. The conglomerate zones mark the times of maximum emergence. Basement involvement in the development of the Birmingham anticlinorium is suggested by this evidence of uplift early in the history of Paleozoic sedimentation.

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