Geology of the country around Flint : memoir for 1:50000 geological sheet 108 (England and Wales)

The district described in this memoir comprises both upland and lowland areas in the north-east of the former county of Clwyd (now included in parts of the unitary authorities of Denbighshire, Flintshire and Wrexham), and western parts of Cheshire. The Clwydian Range constitutes the highest part of the district and is cored by Silurian rocks. The range is fringed by Carboniferous strata including Dinantian limestones and the Silesian sequences of both the Flintshire and the northern part of the Denbighshire coalfields. Permo–Triassic strata underlie the Cheshire Plain to the east, and the Vale of Clwyd to the west. The lowland areas of the district are covered by extensive Quaternary deposits of variable thickness. The decline in traditional heavy industries and subsequent redevelopment of parts of the district has highlighted the need for up-to-date geological information, especially for planners and civil engineers. This memoir, in addition to earlier thematic reports, seeks to address this need. It details the results of a resurvey which, supported by numerous specialist investigations, has enabled a major re-evaluation of the area’s stratigraphy and geological structure. The cleaved and folded Silurian (Ludlow) rocks are composed of mudstone and sandstone turbidites, slumped and destratified strata, and of distinctive laminated deposits which record the background sedimentation. They were deposited in a deep water basin, the Denbigh Trough, part of the larger Lower Palaeozoic Welsh Basin. The memoir provides a detailed account of the local Dinantian rocks and of the palaeogeography of northeast Wales during this period. The earliest Carboniferous rocks comprise reddened fluvial and coastal plain conglomerates and mudstones. These are overlain by thick sequences of ramp and platform limestones, locally with intercalations of shallow-marine sandstone. Early Carboniferous deep-water deposits occur at depth in the north-east of the district. Southerly and northerly derived fluviodeltaic sandstones, together with sequences of cherts, and marine and nonmarine mudstones form the components of a complex Namurian succession which was influenced by movements in contemporary sea level. The overlying Westphalian sequence includes the productive Coal Measures and barren Red Measures of the Flintshire Coalfield and the northern part of the Denbighshire Coalfield. The memoir rationalises the nomenclature and correlation of both coal seams and overlying red-bed divisions. The Permo–Triassic rocks of the Cheshire Plain and Vale of Clwyd comprise thick sequences of aeolian and fluvial sandstones preserved in rift basins created by contemporary extensional tectonics. Tertiary ‘pocket’ deposits (clays and silts), preserved in solution hollows in the Carboniferous limestones, reflect an interval of fluviolacustrine sedimentation which followed a period of prolonged uplift, weathering and erosion. Glacial (Drift) deposits, the products of the Devensian glaciation, cover much of the district and erratics on the highest ground of the Clwydian Range indicate that ice-cover was total. Two opposing and contrasting ice masses, a Welsh ice sheet travelling eastwards and an Irish Sea ice sheet moving south to south-eastwards, met and interacted: this is reflected in the distribution, complexity and variety of resultant deposits. The geological structure of the district demonstrates the effects of the late Caledonian (Acadian), Variscan and Alpine orogenic episodes. This resurvey has also elucidated the role of syndepositional faulting in the Carboniferous, as a control on the distribution and thickness of sediments. Today the principal mineral resources are limestone and sands and gravels for aggregate. There has been limited investigation of the hydrocarbon potential of selected areas. Data on local aquifers, including the regionally important Permo–Triassic sandstones, are assessed.