CALDON LOW EARL STERNDALE CASTLETON

km T Ey I

0 10 km

apparently overlie a series of major tilt blocks dipping to the southwest and separated from each other by faults. It is immediately apparent from this structure why the sequences encountered by the Woo Dale Borehole, which penetrated only a thin limestone cover to reach volcanic basement, differ from those in the Eyam Borehole, which penetrated more than 1800 m of carbonates to reach basement. The two boreholes had been drilled into two different tilt blocks. The third borehole, the Caldon Low Bore- hole, did not reach basement at all but bottomed within a sequence of pebbly sandstones, siltstones and mudstones at the base of the fault wall of a third tilt block further to the southwest. The material here is similar to that at the base of the wall produced by the fault (now called the Bakewell Fault) between the Woo Dale and Eyam Tilt Blocks.

The subsurface structure shown in Fig. 2, having been derived from borehole, seismic and gravity data, is naturally consistent with them; but it also provides explanations of other phenomena not used in the derivation. For example, there are numerous pro- ducts of early Carboniferous volcanism in Derbyshire, generated by known and inferred volcanic centres.

These centres are now seen to correspond broadly with the position and trend of the Bakewell Fault in accordance with the general proposition that volcan- ism is most likely to occur along zones of weakness in the crust. Mineralization also coincides generally with the Bakewell Fault. The fault may be presumed to have exerted some control over the circulation of the mineralizing fluids, producing the sort of correlation with structure that has already been observed in connection with mineralization in Ireland.

Subsurface geology has an unnerving tendency to reveal greater complexity as it becomes more accessi- ble to study. The real structure beneath Derbyshire is therefore probably less simple than is represented in Fig. 2. But even the simple version has considerable explanatory power and, as Smith and his colleagues note, ‘may contribute towards the development of new ideas about the formation of British Upper Palaeozoic basins’. (A detailed account of the chrono- stratigraphy of Derbyshire limestones may be found in ‘A standard nomenclature for the Dinantian forma- tions of the Peak District of Derbyshire and Stafford- shire’, by N. Aitkenhead and J.I. Chisholm, Report of the Institute of Geological Sciences, No. 8218, 1982.)

Conference report

Reefs ancient and modern Produced chiefly by the dead skeletons of countless coral colonies, reefs form impressive and complex structures in today’s oceans. Internally, however, in-place coral and other skeletons are relatively unim- portant volumetrically in comparison with the amount of sediment formed by the constant attack of the reef framework by boring and grazing animals, as well as by the vagaries of storms and currents. Indeed, the intensity of these destructive processes on modern reefs is likely to form a sedimentary rock which may be difficult to recognise as the product of a once-rigid, wave-resistant reef (Fig. 1).

From their Phanerozoic history, two main reef framework types can be recognised, each formed by different types of organisms and each requiring differ- ent ecological and environmental conditions to allow them to thrive. First, there are the soft substrate reefs. These are formed in quieter and often deeper water conditions by the baffling of sediment, general- ly by unlinked branching organisms. This results in mounds of predominantly fine-grained sediment and, if it is preserved, an open organic framework.. Second, there are the hard substrate, or rough-water, reefs. In these, wave action removes most of the fne-grained

Fig. 2. The inferred geological structure along the line AB in Fig. 1, showing three tilt blocks beneath the limestone cover. The three boreholes are plotted in their appropriate positions. The Tournaisian, Chadian, Arundian, Holkerian and Asbian are successively younger stages within the Dinantian. Kv = Kevin Limestones; Hp = Hopedale Limestones; Mi = Milldale Limestones; EcL = Ecton Limestones; WDL = Woo Dale Limestones; BLL = Bee Low Limestones.

Ancient and. Recent Reefs, University College, Cardiff, 13 November 11’385

GEOLOGY TODAY Mar-Apt 1986141

Fig. 1. Coral reef abbk wa&c?d on to shore &ter a major storm (from the Yucatan Peninsular, Mexico). The many destructive processes affecting modern reefs may produce a sedimentary rock which may be diffimlt to recognise as the prodm of a once-rigid, wave-resistanf reef.

sediment, leaving a rigid, and sometimes solid, framework. In both, high growth rates for the animals or plants constructing the framework are important, in the !%st case to prevent their burial by sediment and in the second to repair biological and physical damage.

The physical abrasion of modern coral-reef frameworks, along with the many types of organisms that encrust it, contribute to different grain-size classes in the surrounding sediment, depending on their skeletal architecture. As well as causing massive damage, the action of boring and grazing animals and endolithic algae on the reef framework is also a major sediment producer. Chips cut off the coral skeletons by the boring sponge Cliona, for example, form up to 30 per cent of the cavity-filling sediment under some reefs. Other, non-framework reef dwellers may also contribute significantly to the reef sediment. A stand of small bushes of the calcified alga Halimda, which is common in many of today's reefs, can produce between 5 to 15 kilogrammes of carbonate sediment per square metre per year. The texture and grain-size of the resulting sediment may also be affected by animals living and burrowing within it, such as the shrimp Calianassa.

Much of the rigidity of modern reefs is due more to the growth of inorganic cements within the sediment than to the intrinsic strength of the framework. Cement, which is generally in the form of aragonite, dso occurs as crusts within reef cavities. Inorganic cementation is also important in some fossil reefs: the Permian Capitan reef of West Texas and New Mexico is an example.

COA-~S are nor the only significant modern reef &riIders. Coralline red algae, which also secrete dense calcaceaus skeletons, form reefs in high and low energy conditions and in both temperate and tropical climates. Generally, coralline algal reefs flourish under environmen,tal conditions in which corals do not fare sb well. In tropical reefs, the areas of maximum wave energy - the reef crests - are formed

by massive algal ridges, while in the quieter water back-reef lagoons, more delicate finger-like growths form soft-substrate mounds. Similar, soft-substrate mounds are also known from more temperate cli- mates, where coral reefs are unknown. The coralline algal mounds from Mannin Bay, off the west coast of Ireland, are an example. As in their coral equivalents, bioerosion greatly modifies and often largely destroys their original reefal framework.

On a broader scale, many factors control the mor- phology and type of modern reefs: antecedent topography, climatic, oceanographic and biogeo- <graphic circumstances, eustatic changes of sea level and tectonic setting. The extent to which each of these has moulded any one reef is frequently difficult to distinguish. Different combinations of factors may produce reefs with similar morphologies. Comparison of Recent and fossil reefs is, therefore, difficult and may be an unreliable basis for the facies interpretation of che ancient forms. The effects of the Holocene transgression, for example, may make modern reefs anomalous. The setting of most Recent reefs in high-energy environments may reduce their preserva- tion potential. If similar reef types are preserved, the combinations of biological and physical attack on the framework may make them difficult to recognise. As yet, modern analogues do not exist for many of the well-preserved reefs and coral beds common in the fossil record. More non-uniformitarian synthetic models for ancient reefs are needed: the present is not always a reliable key to the past.

The seminar upon which this report is based was organised by Dr R. Riding, with contributions by Dr D.W. J. Bosence (University of London, Goldsmiths' College), Dr R. Riding (University College, Cardifo, Dr B.R. Rosen (British Museum, (Natural History)) and Dr T.P. Scoffin (University of Edinburgh).

PETER ELLIS Research Fellow, Department of Earth Sciences

The Open University

42lGEOLOGY T0DA.Y Mar-Apr 1986