Abstract

Tertiary synrift deposits along the northern margin of the Gulf of Aden, Republic of Yemen, provide new information on the controls on deposition and stratigraphic relationships during the transition from continental to marine rift sedimentation. Intracontinental rifting in the region commenced in the Oligocene, developing into a marine extensional margin in the Miocene. The outcropping part of a small subbasin at Wadi Hiru, west of Mukalla, contains carbonate platform, alluvial-fan, fan-delta, and raised-beach deposits that record the development of the rift margin. The spatial relationships between carbonate and clastic units, their location adjacent to rift-border faults, and timing of sea-level changes indicate that tectonics was the overriding control on relative sea-level changes. Accommodation space created by fault movement and the morphology of older sediment bodies dictated the three-dimensional architecture of carbonate platforms and clastic fans. Fault movements are documented by changes in clast provenance in fanglomerates. Rift-parallel and rift-perpendicular faults formed a trap-door configuration that controlled long-lived pathways of terrigenous clastic supply throughout rifting. Five local depositional sequences, bounded by erosion surfaces, are recognized. Sequences have a distinct tectonic signature in the form of sequence boundaries overlain by thick fanglomerates followed by transgressive and highstand carbonates. Two of these sequences were dated using 87 Sr/ 86 Sr isotope dating. The relative sea-level curve derived from this study bears little relationship to published "eustatic" curves.

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