The sedimentology and palaeo-environmental analysis of the Dinantian limestones of the Wirksworth-Grangemill area, Derbyshire
The Dinantian carbonates of the Wirksworth-Grangemill area were deposited in a broad platform to platform margin setting with superimposed cyclic sedimentation mechanisms. Four lithostratigraphic formations have been studied in detail. The oldest, the poorly exposed Griffe Grange Formation (Holkerian/Asbian) consists of a suite of variable low and high energy intertidal sediments with less common shallow subtidal and supratidal facies. The Asbian-age Hoptonwood Formation, up to 102m thick, consists of a series of fifteen laterally continuous major (thick) cycles and up to twelve locally developed minor (thin) ones. Although the cycles are somewhat cryptic, each one appears to be symmetrical and consists of variable marginal marine high energy and shallow subtidal sediments, the regressive acme represented by well developed subaerial palaeosols and palaeokarsts. Most of the formation was deposited in a platform setting with a well developed 'reef' belt-type platform margin. The minor thin cycles formed in an inner platform area, probably in response to local tectonism/subsidence superimposed on the regional eustatic-subsidence- tectonic mechanisms which formed the major cycles. The 'reef' belt is absent in late Asbian times and the platform margin attains the character of a poorly developed ramp (alternating middle subtidal and shoal sediments). The top of the formation equates with a major regression (the Asbian/Brigantian boundary palaeokarst). The palaeobathymetric profile developed in late Asbian times was further exaggerated during deposition of the lower Brigantian Matlock Formation (up to 66m thick) within cyclic platform to deeper water ramp-type platform margin settings. The platform cycles are strongly asymmetric, each composed of a condensed trangressive sequence followed by a well developed regressive phase from deep subtidal to subaerial exposure via middle subtidal, shallow subtidal shoaling and intertidal lagoon environments. Traced into the deeper areas of the ramp, the sequence thins and the shallower facies interdigitate with deep subtidal sediments containing locally developed submarine channels and slumped horizons. At the break in slope on the platform/ramp margin, extensive shoals were developed. The little studied upper Brigantian Cawdor Formation (up to 55m thick) consists of a reef-belt platform margin, apparently developed on top of the southern edge of the Matlock Formation cyclic platform sediments, with platform lagoon sediments to the north. South of the reef-belt an upper Brigantian channel or gorge was formed in the vicinity of Wirksworth on the slopes into the basin. The overall stratigraphic pattern is a sequence of carbonates which appears to be net transgressive. Three 'minor' transgressions represent each of the Asbian and lower and upper Brigantian stages. These are in turn subdivided by minor pulsed transgressions and regressions resulting in the cyclic nature of the platform sequences.