Origin of solution seams and flaser structure in upper cretaceous chalks of southern England

Abstract

Flaser chalks consist of small ellipsoidal bodies or lenses of relatively pure chalk surrounded by clay-rich solution seams. The latter may be simple, individual clay partings or composite aggregations of clay partings. These flaser structures formed during late burial diagenesis in response to mechanical compaction and pressure dissolution of calcium carbonate. Dissolution preferentially affected the coccolith-rich, fine-grained matrix of the flaser chalks, leaving coarser skeletal particles largely unscathed; in addition, solution was most intense in those parts of the chalks which were originally most argillaceous. Variations in flaser chalks resulted when the above mentioned processes acted on and modified the products of early diagenetic lithification such as nodular chalks, intraformational conglomerates and incipient hardgrounds. True stylolites produced by pressure solution are present only in these early lithified chalks. The most probable range of burial depths at which flaser structure formed in chalks was approximately 300–2000 m.