Near-surface brecciation of chalk, isle of thanet, south-east England: a comparison with ice-rich brecciated bedrocks in Canada and Spitsbergen

Abstract

Chalk on the Isle of Thanet, Kent, is brecciated to depths of a few metres beneath the ground surface. The brecciation commonly comprises (i) an undeformed layer of angular, platy blocks more or less parallel to the surface overlain by (ii) a deformed layer containing small open folds, typically with vertical axial planes. Above the brecciated chalk is an involuted layer (∼0.5 to 2.0 m thick) of chalk diamicton and brickearth. By analogy with brecciated ice-rich limestones, arkoses and shales in areas of continuous permafrost in Arctic Canada and Spitsbergen, it is suggested that brecciation of the Chalk resulted primarily from ice segregation in perennially frozen bedrock, and repeated segregation formed an ice-rich layer just beneath the former permafrost table. Subsequent thaw consolidation of this layer is thought to have formed an involuted layer through soft-sediment deformation. Three implications arise from this study: (i) near-surface brecciation of the Chalk probably took place during conditions of continuous permafrost; (ii) the growth and thaw of the ice-rich layer in chalk was probably an important element in the geomorphological evolution of the English Chalklands, heaving and brecciating the Chalk during permafrost conditions, and deforming or redepositing the overburden during periods of active-layer deepening; and (iii) repeated ice segregation near the top of permafrost may have brecciated other bedrocks in the British Isles.