The critical factors controlling sequence development and infill architecture of extensional, tilt-block sub-basins are: (1) the accommodation space created by block rotation and background subsidence, and, to a lesser extent, by eustatic sea level change, and (2) sediment supply, which is critically important during all phases of rotation, and is commonly meagre in sub-basins farthest from the hinterlands, unless there are intra-basinal drainage areas.

Cycles of initially increasing then decreasing rates of rotation tend to produce initial sand abundance, then shaleprone conditions and finally increased sand deposition as sediment supply again outpaces subsidence and rotation rates. A general effect of rotation, local subsidence and the ‘steep' slopes associated with this type of basin is for the transgressive tracts to be thickness enhanced.

Middle Jurassic extension and slight block tilting in Scotland's Hebridean Province is indicated by (1) locally thick (up to 600m) clastic successions, (2) marked updip-downdip patterns of sandstone-shale partitioning on some of the broader (40km) tilt blocks, and (3) evidence of strong tidal current enhancement on sedimentation within some of the narrow (less than 5km across), strait-like sub-basins.

Ten transgressive-to-regressive sequences, and four sequence sets have been recognised in the Middle Jurassic succession of the North Skye sub-basin, along the margin of the Sea of the Hebrides Basin. The sequence sets are believed to represent deposition during episodes of slight tilting of the sub-basin floor.

Downdip thickness expansion of each sequence (Fig. 1) is accommpanied by a decreasing sandstone/shale ratio. This arises partly because of the rapidly expanding shale packages in the regressive intervals, and partly due to a tendency for sand to have been preferentially stored updip in the transgressive intervals. The thickest and coarsest sandstone units accumulated at the bases of sequence sets, probably as tectonically generated lowstand wedges. Some of the thickest sandstone successions, occurring across the updip reaches of the block, accumulated in transgressive, estuarine channel complexes.

The North Skye sub-basin accumulated a tectonic tilt of less than 10 degrees during four phases of slight rotation. Two of the narrowest tilt blocks, associated with the North Raasay and Camasunary Faults respectively, created tidal straits which accommodated the thickest most sand-prone successions.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah