THE CONDITIONS OF GROWTH OF THE WENLOCK REEFS OF SHROPSHIRE (ENGLAND)

Authors


ABSTRACT

Distribution and size of the reefs in the Wenlock Limestone (Mid-Silurian, Wenlock area) indicate that the most suitable site for growth was at the seaward fringe of the reef belt. The reef development increased through time as clay-mineral concentration reduced. Many reefs colonized small mounds of coarse crinoidal debris but growth also started on soft muddy substrates. The Wenlock structures have similar characteristics to modern patch reefs: they are small—the average width is 12 m and the thickness 4.5 m—and roughly oval in section. During growth reef surfaces were gently convex and reached heights of between 50 cm and 3 m above the sea bed.

The organic framework of all reefs examined consisted of massive and branching tabulate corals, stromatoporoids, branching rugose corals and bryozoans; with stromatolites, laminar corals and stromatoporoids and bryozoans playing the role of reef binders. Most reef builders grew contiguously and are preserved in growth position. Biotic zoning within reefs was not detected. Biomicrites accumulated in interstices near the surface of the reef at the time of, and just shortly after, organic growth. The sea was probably less than 30 m deep and extreme shallowing at the close of the Wenlockian terminated reef development. However, reefs were not broken up by wave or current action. The inter-reef sediments, which were deposited at the same rate as reef growth, are predominantly argillaceous crinoidal biomicrites alternating with thin shale bands. Very few beds of reef-derived talus occur (Fig. 13–15). High influxes of clay minerals—especially bentonites—brought about local reef death.

Diagenetic processes of dissolution, recrystallisation and compaction greatly modified the original fabric of the reefs.

Ancillary