Stratigraphy, facies development, and depositional dynamics of the Late Wenlock Fröjel Formation, Gotland, Sweden.

Abstract

A lithostratigraphic subdivision of the topmost Slite Beds on western Gotland is proposed, viz. the Fröjel Formation (9-11 m) including the Svarvare Mudstone Member (2-3 m) and the Gannarve Siltstone Member (7-8 m). The Fröjel Formation resulted from increased siliciclastic deposition within an intracratonic carbonate platform setting during the mid-Homerian (Wenlock, Silurian). The temporal and spatial development of sedimentary facies and the characters of formation boundaries can be explained with a sequence stratigraphic approach to depositional dynamics. The accelerating influx of siliciclastic material was connected to a sea-level fall which therefore initiated the formation of a gradationally based parasequence (the Gannarve Member). The depositional trend is supported by both the facies (upward decrease of accommodation space) and faunal development. The Svarvare Mudstone Member reflects a late highstand period, when hemipelagic deposition and weak, small-scale density currents contributed to sedimentation. The subsequent short period of shallowing resulted in deposition of the overlying Gannarve Siltstone Member, consisting of siltstone tempestites and associated fair-weather mudstones deposited in successively decreasing palaeodepth. The shallowing culminated with the formation of an unconformity, which marks the top of the Cyrtograptus lundgreni Biozone, and which is overlain by oolites. This unconformity correlates to the transgressive surface in offshore settings. The siliciclastic deposition is regarded as regional for this part of the Baltic Basin, influencing an area from offshore east Öland to Saaremaa west of Estonia. The depositional history of the Fröjel Formation is discussed in terms of relative and eustatic sea-level changes and with respect to regional correlation of the mid-Homerian eustatic regression