Sandstones and phosphatized calcareous sediments of the Lower Cambrian Rispebjerg Sandstone, Bornholm, Denmark

Abstract

The Rispebjerg Sandstone on the island of Bornholm, Denmark, consists of sandstone layers intercalated with phosphorite layers. The phosphorite lithology is composed of micritic phosphorite containing varying amounts of terrigenous grains and minor amounts of calcareous and non-calcareous allochems. Mineralogically, the micritic phosphorite is chiefly composed of carbonate-apatite, according to X-ray diffractograms. More or less complete calcite replacement by phosphate occurs both in the micrite and in the calcareous allochems. Bioturbation must have been intense as most depositional sedimentary structures have been destroyed. The most plausible explanation of the phosphorite lithology is that calcareous mud was deposited in a barred lagoon in which storms blew back sand from the barrier. Sand and mud were then mixed by bioturbation. Penecontemporaneously with the deposition of this sediment, phosphatization of calcite by phosphorous from sea water is judged to have occurred.

The sandstone lithologies are calcitic siliceous submature glauconitic quartzarenite, calcitic submature quartzarenite and siliceous submature quartzarenite. Gravel-sized grains are present in the two latter sandstone types. All the sandstones are characterized by good roundness and moderate sorting. These textural parameters indicate textural inversion. The roundness is interpreted as inherited from older sediments. The sandstones may have been deposited in the tidal complex on the landward side of the sandy barrier.

Scarce calcareous and phosphatic fossil fragments were locally found in rocks of all lithologies. Deposition of the Rispebjerg Sandstone was interrupted by periods of erosion, one of which originated the formation of an intraformational unconformity.

Several diagenetical replacements - phosphatization, chertification, calcification, glauconitization, chloritization, pyritization - occurred during and after deposition of the Rispebjerg Sandstone.