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

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