Chalk deformation and large‐scale migration of calcium carbonate

Abstract

Sixty Upper Cretaceous chalk exposures were examined and sampled in Dorset, the Isle of Wight and Guildford, South England, in order to investigate the effects of tectonic deformation on the chalk fabric. Light and scanning‐electron microscopes were used extensively, and chemical, mineralogical and isotopic analyses ware carried out. Two types of fabric modification were distinguished. The first type involves more than 90% volume loss by mechanical compaction at the early stages of deformation followed by dissolution and removal of calcium carbonate in the advanced stages of deformation. Chalks which have undergone this type of fabric modification are dense and consist of well rounded 0.5–3 μ calcite crystals with well developed pressure‐solution contacts. These chalks contain a high proportion of calcispheres (many of which are plastically deformed) but very few well preserved planktonic foraminifera and coccoliths. They are relatively enriched in insoluble constituents and depleted in strontium. The second type of fabric modification involves introduction of calcium carbonate into the pore spaces. Chalks which have undergone this type of modification are dense and contain a high proportion of 3–5 μ polygonal interlocking calcite crystals. Fossils are virtually never deformed, and delicate foraminifera and coccoliths are well preserved. Insoluble constituents similar to those found in the unmodified chalks occur in low concentrations. Removal of calcium carbonate at crystal contacts has taken place in chalks which have been subjected to high effective tectonic stresses and at crystal peripheries in chalks which have been subjected to high pore fluid pressure. Dissolution at crystal peripheries is responsible for the extensive calcium carbonate losses, and it is termed herein ‘confining pressure solution’. Introduction of calcium carbonate occurs in rocks which were under low tectonic stresses. During deformation calcium carbonate migrates from chalks under high stresses towards those under tow stresses, while some goes into solution in sea water. Well developed joints and high pore fluid pressure might increase the rate of calcium carbonate removal by three orders of magnitude. Petrographic and isotopic data suggest‐that all fabric modifications studied took place in meteoric water.

Geochemical and stable isotope patterns of calcite cementation in the Upper Cretaceous Chalk, Uk: Direct evidence from calcite-filled vugs in brachiopods

“…The role of the increasing deformation on the hardness and composition of the Middle (Turonian) and Upper ('Senonian') Chalk associated with post-Cretaceous steep, asymmetrical east-west folding in southern England (Surrey; Isle of Wight; Dorset) and Yorkshire has been investigated by Mimran (1975Mimran ( , 1977Mimran ( , 1978, making comparison to his own findings (ideas) on the unfolded chalk of Northern Ireland (Mimran 1978) which differ significantly from those of Wolfe (1968) and Scholle (1974). Mimran (1977Mimran ( , 1978 modelled the effects of increasing deformation using measurements of acid insoluble residue, bulk specific gravity (intact dry density), concentration and distortion of calcispheres (assumed to be originally spherical in form), values of various elements (Sr, Mg, Ti, Ba), and stable isotopes (δ 18 O, δ 13 C) in bulk chalk calcite, and microfabric analysis.…”

“…The exceptionally hard chalks of Northern Ireland overlain by a thick basalt pile were a topic of much interest -ideas ranged from baking by the volcanic pile to the possibility that the Irish Chalk had a higher proportion of aragonite in the original sediment (Hancock 1963). In Yorkshire and southwest England, the association of faulting and high dips was thought to be related to the general hardening (Mimran 1977(Mimran , 1978.…”

“…Mimran (1977Mimran ( , 1978 modelled the effects of increasing deformation using measurements of acid insoluble residue, bulk specific gravity (intact dry density), concentration and distortion of calcispheres (assumed to be originally spherical in form), values of various elements (Sr, Mg, Ti, Ba), and stable isotopes (δ 18 O, δ 13 C) in bulk chalk calcite, and microfabric analysis. Isotope evidence suggested that the hardening process in southern England, Yorkshire and Northern Ireland all took place with porewaters of differing Sr contents but of meteoric origin.…”

Diagenesis of Miocene Sandstone in the District Sudunhoti and Poonch, Azad Jammu and Kashmir, Pakistan

“…The dissolution of grain under lithotomic stress is an important criterion for the recognition of rock deformation (Mimran, 1975;Mimran, 1977, Alvarez et al, 1976. The pressure solution in the rocks was categorized into the intergranular and stylolitization Fig.…”

Evolution of pore space in the Poza Rica trend (Mid‐Cretaceous), Mexico

“…Weller, 1959;Pray, 1960;Bathurst, 1975Bathurst, , cf. 1980) but has been challenged by recent experimen- tal work (Shinn et al, 1977;Bhattacharyya & Friedman, 1979, Shinn & Robbin, 1983) and field observations (Matter, 1974;Mimran, 1977;Buxton & Sibley, 198 1). Evidence for mechanical Compaction, such as grains broken in situ, is virtually absent in Tamabra rocks, although a few pelagic limestones show flattening of burrows (Enos, 1977a, p. 295).…”