Bryozoan-Algal Patch-Reefs in the Upper Permian Lower Magnesian Limestone of Yorkshire, Northeast England

ABSTRACT

Patch-reefs commonly 10 to 25 meters across and 3 to 8 meters thick are abundant in dolomitized skeletal oolite of the Upper Permian Lower Magnesian Limestone in Yorkshire, England, and are themselves dolomitic. They are roughly circular or oval in plan and irregular in section, with a common tendency for a shallow inverted cone to be surmounted by a gentle dome. All are stratigraphically younger than a widespread coquina which lies near the base of the formation and may have provided a stable foundation for reef forming organisms.

Most of the reefs comprise an untidy assemblage of sack-shaped bodies ('saccoliths'), each composed mainly of closely-packed sub-parallel remains of the ramose bryozoa Acanthocladia (generally predominant) and Thamnis-cus in a finely crystalline dolomite matrix, which commonly also contains a low diversity community of other invertebrates. It is tentatively suggested that each saccolith is founded on a singly colony of Acanthocladia anceps (Schlotheim). The reefs probably were formed entirely under water on a broad shallow tropical carbonate marine shelf, and the tops of most were less than 2 meters higher than surrounding contemporaneous sediment. Mud-trapping and binding by bryozoa appears to have been the main constructional process, with encrusting foraminifers and early submarine cements adding stiffening and bulk. Bryozoa die out in upper parts of the formation, where the reefs are composed largely or wholly of algal stromatolitic dolomite that was also probably formed subaqueously but in shallower water than the earlier bryozoan parts of the reefs.