ABSTRACT A major linear reef ultimately more than 100 meters high protected the seaward edge of the carbonate shelf of the Upper Permian Middle Magnesian Limestone in northeastern England, and is overlain by an extensive stromatolite biostrome. Rocks of both structures are almost completely dolomitized. The reef is founded on a patchy lenticular coquina, and much of its lower parts is formed of typically unbedded bryozoan biolithite which appears to have formed subaqueously and grew mainly upwards. Some contemporaneous lithification and rigidity is indi? cated by the presence of biolithite debris in associated talus. Middle stages of reef growth are characterized by a progressive increase in the proportion of algal rocks and laminar organic or inorganic encrustations at the expense of the bryozoa that dominated in early stages, and a tendency towards bedding may indicate shallowing towards the end of this phase. Stromatolitic and other laminar rocks became dominant in latest phases of reef growth, where the evidence of active contemporaneous erosion, roughly horizontal bedding and lateral rather than upwards growth is thought to indicate proximity of the top of the reef to sea level. The growth and increasing asymmetry of the reef led to progressively more complete separation of environments to landward and seaward of the reef, culminating when the reef approached sea level in the formation of a lagoon and a starved or semi-starved basin. Scattered small patch-reefs occur locally in lagoonal beds in the north of the area and considerably larger masses of reef rock in the same area are probably also patch-reefs but could be outliers of a much widened main reef. The stromatolite biostrome is a relatively uniform tabular body up to 30 meters thick formed of finely-laminated subtidal algal stromatolites on the flat top of the Middle Magnesian Limestone reef. Algal growth forms are diverse only at the lagoonal and basinal margins of the biostrome, the basinal margin also being varied by the presence, in its lower part, of conglomerates composed of rolled cobbles and boulders of biolithite possibly derived from the underlying reef.

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.