The sedimentology and diagenesis of the Asbian limestones of north Derbyshire
The Asbian (Dinantian) succession of the Buxton area, North Derbyshire, consists of 230 m of cyclically developed shallow water shelf facies limestones with interbedded clays and contemporaneous igneous horizons. Twenty one Asbian sequences have been successfully correlated using the distribution of interbedded clay wayboards, lavas and Davidsonina septosa faunal horizons as principal criteria. Clay wayboards represent periods of subaerial emergence of the platform sediments and they are commonly underlain by palaeokarstic surfaces and caliche profiles. The caliche profiles display a variety of subaerial diagenetic textures including laminated crusts, brecciation, root tubules and alveolar texture and these have been directly compared with Pleistocene caliche profiles in the Florida Keys. Such textures are considered to have formed beneath a soil cover with high evaporation rates and plant activity having been of particular importance in their development. Stylolites are abundant, in the Asbian limestones and six distinct styles of pressure-solution have been recognized. Some major stylolites were generated along former subaerial emergent surfaces and now contain clay wayboard material along their seams. Pressure-solution took place when the rocks contained only about 5% pore-space and represented an estimated maximum vertical loss to the succession of 10%. The stylolites formed prior to late Carboniferous-early Permian deformation of the platform under an Upper Carboniferous and Brigantian overburden of several hundred metres. Microfacies analysis of the limestones has revealed that sedimentary cyclicity is complex but that the cycles seem to be generally asymmetrical with an "ideal" fully developed cycle reflecting a shallowing upwards sequence of environments as follows : - SUBAERIAL EMERGENCE (clay wayboards etc.) TIDAL FLAT/SHOREFACE (Mudstone, crinoid-foram wackestone, peloidal grainstone). SHALLOW SHELF LAGOON (crinoid-algal/crinoid-peloid packstone). "DEEP" SHELF LAGOON (crinoidal packstone/wackestone, crinoid-foram packstone). SUBAERIAL EMERGENCE (clay wayboards etc.) Very shallow water deposits, indicative of tidal flat and shoreface environments, are extremely rare at the base of cycles and are also commonly absent from the tops of cycles. This indicates rapid transgression of the sea immediately after subaerial emergence and also, in many cases, rapid regression leading to subaerial emergence thereby inhibiting the development of extensive intertidal facies. There are a total of 29 correlatable major cycles with many laterally impersistent sub-cycles in the Asbian of North Derbyshire. The cycles were generated by an interplay between: local tectonics; regional or plate tectonics; eustasy; and sedimentation. Later growth stages display apparently non-crystallographic "planar" growth surfaces (pseudofaces; Walkden Berry, 1984a). Syntaxial overgrowths on crinoids from different geological settings are quite distinct and show textures which reflect their differing diagenetic histories. The cements of the Asbian platform limestones consist of four distinct Zones (1, oldest - 4, youngest) which are identified by cathodoluminescence. These may be recognized in Asbian shelf limestones throughout Britain and perhaps further afield.