Early Carbonate Fabrics in Silurian Reefs of Gotland, Sweden: ABSTRACT

Abstract

The Hogklint reefs of Gotland occur within a shallowing-upward sequence and are developed in the outer zones of a carbonate wedge adjacent to a shale belt. Shoal and peritidal sequences are the major carbonates which occur in the inner parts of the wedge and are preferentially associated with erosion surfaces. The Hogklint reefs show carbonate fabrics that are inferred to indicate synsedimentary marine cementation of the reefs. These fabrics are: (1) pseudofibrous calcites that are believed to have replaced acicular/fibrous cements with original aragonite and/or Mg-calcite mineralogy, (2) pelmicsparite-pelmicrite areas, (3) stromatolite-like crusts, and (4) early micritic cements. The reefs also show evidence of subaerial exposure by the occurrence of erosion surfaces at the top of the reefs, dissolution cavities, and vadose crystal silt infills in cavities in the algal biofacies which cap the reefs. End_Page 640------------------------------ The distribution of the various cement types within the reefs can be linked with the shallowing-upward nature of the limestone sequence as there is a predominance of early marine cement fabrics in the middle and upper parts of the reefs, particularly in the algal biofacies. The spatial and sequential nature of the various carbonate diagenetic fabrics show that the Hogklint reefs went through four main diagenetic events. Microstalactitic cements, vadose crystal silts, grain-contact cements, and pseudofibrous isopachous cements are developed within cross-bedded peloidal grainstones at the top of the shallowing-upward sequence and indicate cementation within the freshwater vadose zone and intertidal and/or marine phreatic zone. End_of_Article - Last_Page 641------------