The lower Wenlock reef sequence of Gotland: Facies and lithostratigraphy

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Abstract
The sequence which was previously described by J. E. Hede as Lower Visby Marl, Upper Visby Marl, Högklint Group, and Tofta Limestone is a shallowing marine sequence dominated by the development of numerous large stromatoporoid-tabulate-algal-cyanobacterial patch reefs and disconformably overlain by restricted, locally high energy, oncolitic and stromatoporoidal limestones associated with erosion surfaces. The total sequence at any one point is generally approximately 45 m thick, and is well-exposed in sea-cliffs along much of the north-west coat of Gotland. It represents the lowest part of the exposed Gotland successon and is of latest Llandovery to earliest Wenlock age. The sum of the maximum thicknesses of the individual formations is approximately 56 m. The conformable underlying sequence is known from boreholes. The following revised lithostratigraphic scheme is proposed: (1) Visby Formation: 1 Ygne Member (11 m), 2 Rövar Lilja Member (6.6 m). These members are approximately equivalent to the Lower and Upper Visby Beds, respectively, of Hede. (2) Högklint Formation: 1 Korpklint Member, 2 Millingsklint Member, 3 Irevik Member, 4 Brissund Member, 5 Hallshuk Member, 6 Domkyrka Member, 7 Kappelshamnsvik Member, 8 Högklint Patch Reefs. The Högklint Formation as here defined is equivalent to the Högklint Group of Hede. Its maximum thickness at any one place is generally about 35 m. (3) Kopparsvik Formation: 1 Stajnkrogen Member, probably equivalent to Hede's Högklint d “Pterygotus” beds, 2 Gutevägen Member, 3 Plägan Member, 4 Halsjärnet Member, 5 Visborg Member, 6 Käringen Member. The Kopparsvik Formation is approximately equivalent to the Tofta Limestone of Hede, and is generally approximately 7.5 m thick. It is overlain disconformably by the Slite Group. The members of the Högklint Formation exhibit lateral equivalence due to their origin in a patch reef complex. In exposures in the central and southwestern area (between Ygne and Häftingsklint) the Högklint passes laterally into the upper part of the Visby Formation. In the north-east (near Hallshuk) the Korpklint and Irevik members are absent and the Högklint sequence commences with the Brissund Member. The Stajnkrogen, Gutevägen-Plägan, Halsjärnet, and Visborg-Käringen members of the Kopparsvik Formation are all underlain by disconformities.