Sedimentology of the Mid-Visean limestones of the southern part of the Askrigg Block, North Yorkshire

Abstract

The earliest Carboniferous deposits, resting with profound unconformity on Lower Palaeozoic rocks, have been mapped across their entire outcrop area on the Askrigg Block. The sediments, comprising approximately half of the thickness of the Dinantian Great Scar Group, have been subdivided into four formations listed in ascending stratigraphlcal order:- the Thornton Force Formation and its lateral equivalent the Douk Gill Formation, the Raven Ray Formation and finally, the Horton Limestone.

Each of these formations has been described in great detail, noting the variations in thickness and the various rock-types contained therein. This data has been used to identify sixteen rock-types which occur in at least one and usually in all of the studied formations.

The depositional environment of each of these rock-types has been interpreted by means of palaeoenvironmental analysis of the fossil groups and sedimentary structures present and from the distribution of each of the rock-types. The diagenetic history of the carbonates has been studied by means of staining techniques.

The earliest deposits of the Thornton Force Formation were formed in a marginal marine environment. Although beach-nearshore sediments accumulated in an active environment, inundation of the Askrigg Block appears to have been a gradual and gentle process, allowing local preservation of soils and debris flow deposits in more protected pockets and hollows. The ridges of Lower Palaeozoic rock supplied detritus throughout deposition of nearshore shallow subtidal calcarenltes of the formation.

The Douk Gill Formation, restricted in outcrop to a local topographic hollow in Ribblesdale, is probably a lateral equivalent of the Thornton Force Formation. A ridge of Lower Palaeozoic rocks provided a protective barrier, allowing clastlc and later carbonate sediments to accumulate in the sheltered environment. Infilling of the lagoon resulted in the formation of tidal flats, and culminated in subaerial exposure and the development of a thin coal.

During deposition of the Raven Ray Formation a shelf-edge shoal must have formed, separating the Pennine Basin from the normal marine shelf lagoon of lime mud deposition. Small shoals occasionally developed in the extensive lagoon environment. Shoreline deposits formed around those Lower Palaeozoic ridges which persisted as islands.

The Horton Limestone represents an episode of deposition predominantly of cross-laminated calcarenites formed within surge depth. Eventually, shelf-edge shoals created a barrier which separated the Pennine Basin from a restricted marine, shallow lagoon of lime mud deposition. Gradual infilling of the lagoon led to the creation of a tidal flat environment. Tidal channels were common in this environment. Periodically the barrier shoals were breached and the lagoon-tidal flat environment overwhelmed by carbonate sand.

The principal mechanisms controlling sedimentation have been discussed. During the initial stages of inundation, the topography of the pre-Carboniferous rocks exerted a significant but dwindling influence on the rock-types deposited and on their distribution. Burial of the land surface eliminated this effect and the rates of sedimentation and subsidence became the most significant mechanisms controlling the type and distribution of Dinantian sediments.

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