Abstract: Contrasts in Passive-Margin Evolution on Rifted and Transform Margins

William A. Thomas, Brian M. Whiting

During rifting, promontories and embayments of continental crust are framed orthogonally by rift segments and transform faults. Rifted margins are characterized by extensional faulting and crustal attenuation. Opening of an ocean adjacent to a transform margin of continental crust involves migration of the end of a spreading ridge and an associated thermal uplift along the transform. During late Precambrian-Cambrian rifting, the Alabama promontory on the southeastern margin of North American continental crust was framed by a rift on the southeast and a transform fault on the southwest. The Cambrian-Mississippian stratigraphic succession records late syn-rift and complete passive-margin history of the continental promontory, thereby providing an opportunity to test predictable contrast in stratigraphy of rifted margins and transform margins.

Along the rifted (southeastern) margin of the Alabama promontory, a laterally variable succession of Upper to lower Upper Cambrian clastic and carbonate rocks records late syn-rift graben filling. A thick carbonate unit of Upper Cambrian and Lower Ordovician age indicates relatively rapid post-rift

subsidence along an evolving passive margin. Thin, shallow-marine deposits document decreasing subsidence rates through time and very slow subsidence from Upper Ordovician through Lower Mississippian.

Along the transform (southwestern) margin of the Alabama promontory, thick Cambrian and Lower Ordovician carbonate rocks reflect rapid post-rift subsidence, but presently available chronostratigraphic data are not adequate to resolve possible migration of a thermal uplift along the margin. In contrast to the rifted margin, no syn-rift clastic deposits are preserved. As along the rifted margin, subsidence rates decreased through time, and slow subsidence is indicated by a thin Upper Ordovician through Lower Mississippian shallow-marine succession.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994