Triassic sediments of the Inner Moray Firth, Scotland: early rift deposits

Abstract

Patterns of Triassic sedimentation in the Inner Moray Firth are examined in the light of a new model of continental rift sedimentation that has been developed in the Tertiary to Recent rifts of East Africa. The major tectonic control during the Trias was the Great Glen fault to the northwest, which can be shown to have been moving in dip-slip fashion at this time. The structure of the Firth appears to have resembled the simple half-graben that makes up the great East African rift system. Deposits both thicken and get finer towards the fault. Sediments found in off-shore wells are dominated by laminated fine sand and silty-clay with sporadic evaporites. Similar sequences are characteristic of, or associated with, ephemeral lakes in modern semi-arid continental rift settings. No alluvial fans have been detected close to the faulted margin. Again, this is consistent with general patterns of sedimentation against boundary faults in the Kenyan rift. The coarsest sedimentary sequences in the Moray Firth are on the unfaulted margin to the south around Burghead. They consist of fluviatile, lake shore and aeolian deposits. The fluvial sediments show evidence of tectonically induced cycles of sedimentation which are interpreted in terms of waves of detritus working down the drainage system. Beds dominated by pebbly horizontal laminae and reminiscent of steep, straight and shallow ephemeral streams alternate with troughed sands of deeper, more sinuous, but equally ephemeral rivers. These are the coarsest sediments of the unfaulted margin of the basin. However the lack of thick conglomerate sheets, which are a feature of East African sequences at equivalent positions within the rift basin, suggests only small-scale fault activity and a relatively subdued topography during Triassic times. This is supported by evidence of extensive calichification during the Rhaetic which produced a top-Trias marker horizon found throughout the Inner Moray Firth.