Stratigraphic development of an U pper J urassic deep marine syn‐rift succession, I nner M oray F irth B asin, S cotland

The stratigraphic development of an U pper J urassic syn‐rift succession exposed at outcrop in the I nner M oray F irth B asin has been investigated using high‐resolution biostratigraphy and sedimentology. A continuous 970 m thick section, exposed in the hangingwall of the H elmsdale F ault was logged in detail. The succession spans 8 Ma and contains eight lithofacies types, which indicate deposition in a deep marine setting. Boulder beds contain large, angular clasts, with bed thicknesses typically >2 m and poor sorting suggesting deposition by debris flows. An inverse clast stratigraphy is observed; the oldest boulder beds contain sandstone clasts of U pper O ld R ed S andstone ( ORS ) with younger debris flows containing clasts of M iddle ORS calcareous siltstone. A marked change from siliciclastic to carbonate dominated sedimentation occurred during the E arly T ithonian, interpreted primarily as a result of change in lithologies in the footwall catchment from sandstone to calcareous siltstone, which reduced supply of siliciclastic sediment. Secondary factors are identified as increased aridity in the E arly T ithonian, which reduced sand supply from the hinterland and a third‐order E arly T ithonian eustatic sea‐level rise, which trapped coarser clastic sediment within the hinterland. Biostratigraphy allows calculation of variations in sedimentation rates with recognition of: (1) an early rift phase characterised by sandy turbidite deposition, when sedimentation rates averaged 0.08 m/ky, (2) a rift climax phase from the E arly K immeridgian where sedimentation rates increased steadily to a maximum of 0.64 m/ky in the Early Tithonian, with strata dominated by boulder scale clast‐supported debris flows and (3) a late stage of rifting from the mid T ithonian, where sedimentation rates decreased to 0.07 m/ky. Overall sedimentation rates are comparable to those of other deep marine rift basins. Unroofing a resistant lithology on the footwall of a rift has important implications for siliciclastic sediment supply in rift basins.