Allen Bay and overlying Read Bay shelf carbonate rocks and their lateral northward transition to basinal clastic rocks of the Cape Phillips are well exposed on Cornwallis and Griffith Islands. The exposures of Silurian shelf to basin strata on Cornwallis may be unexcelled anywhere in the world for this part of the stratigraphic record.

Allen Bay carbonate rocks, approximately 6,000 ft (1,830 m) thick, consist largely of dolomite. Brachiopods suggest a Silurian (Wenlockian to Ludlovian) age for the middle and upper Allen Bay.

Detailed outcrop studies led to the recognition of five main depositional facies and 15 lesser facies, despite pervasive dolomitization. Vertical and lateral changes of facies are described in terms of four transitional depositional stages. Deposition took place in environmental settings ranging from restricted interior algal flats and shelf-edge sabkha islands to unrestricted basins. Normal marine deposition on the carbonate shelf and shelf edge occurred in a variety of high- and low-energy environments.

Between shelf and basin, slopes were very low, particularly during lower Allen Bay deposition, but may have been more pronounced through deposition of the middle Allen Bay when a dolomitized shelf-edge reef complex provided a relatively effective barrier. In general the sedimentation rate in the basin and slope apparently kept pace with that of the carbonate shelf throughout Allen Bay and lower Read Bay deposition.

The amount, type, and degree of thermal alteration of organic matter in the Allen Bay and transitional Allen Bay-Cape Phillips facies appear to have been optimum for generation of both oil and gas.

Greater values of porosity in the Allen Bay are particularly common at the shelf edge and in dolomitized carbonate sands and patch reefs of the interior shelf. Reduction in porosity in a shelfward direction, particularly for the middle Allen Bay, is interpreted as related primarily to change from dolomitized carbonate sand to finer textured protected-shelf and algal-flat facies.

Dolomitization of Allen Bay carbonate rocks is related primarily to intrastratal mixing of marine and hypersaline water with fresh water.