Distal alluvial fan sediments from the Upper Jurassic of Portugal: controls on their cyclicity and channel formation

Abstract

The Lusitanian Basin of west-central Portugai is an Iberian Atlantic margin basin. During a late Jurassic rifting event, a series of alluvial fans distributed coarse, basement-derived clastics, southeastward from the western margin of the basin. The distal part of this sequence, which is about 140 m thick, consists of heterolithic, massive sandstone and massive mudrock facies, arranged in crude fining-upwards cycles 1–15 m thick. These form discrete packets of sediment in a strongly aggrada-tional sequence and are interpreted as the products of sheetflooding as the active area of sedimenta­tion on the fan surface switched. Channelized flow was not necessarily an early feature of cycle development. Considerable fluctuations in discharge occurred and were probably related to climatic factors. Large quantities of tractional and suspended sediment were transported at high stages, and were rapidly deposited as flows waned. Caliche soil profiles indicate a seasonal semi–and climate. However, other features such as widespread soft-sediment deformation, the lack of desiccation structures and the absence of evaporites suggests that areas of active sedimentation often remained wet. Three fining-upward megacycles are distinguished. These indicate rapid advance of fan toes triggered by tectonic activity followed by gradual retreat.