Sedimentology of the Mid-Visean limestones of the southern part of the Askrigg Block, North Yorkshire.
University of Southampton, Faculty of Science Geology,
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 equlvalent 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
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 rldges 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
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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