Dunes Galore: Tidal Signatures in Sandprone, Tectonically-Generated Straits

Stuart Blackwood and Ron Steel
University of Texas, Austin, TX

The superbly well exposed Mid-Jurassic Bearreraig Formation on the Isles of Skye and Raasay in NW Scotland displays a spectacular, overall upward coarsening hierarchy of sand rich, tide-generated bedforms exhibiting four-fold organization:

1. 2D and 3D dunes (up to 2m high),

2. Compound dunes (up to 10m high),

3. Tidal bars/ridges (up to 30m high),

4. Tectonically-generated sequences (up to 100m thick).

Dunes and compound dunes form tidal bars / ridges, which in turn constitute unconformity bounded tectonically generated sequences.

Previous depositional interpretation is one of alternating tide-dominated deltaic and macro-tidal estuarine cycles with high-frequency regressions and transgressions. However, the anomalously large size of the bedforms (implying relatively deep water), the intensely sand-rich character of the succession, and the high bioclastic content of broken shell debris suggests a narrow, high-energy depositional setting.

Sedimentation occurred in NNE-SSW-aligned, North Skye/Raasay and South Skye half-grabens, defined by Aalenian-Bajocian faulting associated with the initial opening of the North Atlantic. Bimodal (S. Skye) and unimodal (N.Skye/Raasay) fault-parallel paleocurrent directions suggest that the faults were active and had enough topographic expression to enhance the tidal regime in these very narrow, block-tilted zones. The character and scale of the bedforms and bars, the absence of pro-delta, lagoonal or delta-plain deposits together with the mutually-evasive flood- and ebb-dominated channels suggests a series of seaways or tidal straits generated by active tilting of narrow fault blocks. This was a precursory phase to Late-Jurassic rifting in the region.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005