Lithostratigraphy and sedimentary evolution of the Triassic, Jurassic and Lower Cretaceous of Bornholm, Denmark

Abstract

A new lithostratigraphic subdivision of the Triassic to Lower Cretaceous sequence of Bornholm is presented including 2 new groups, 3 new formations and 10 new members. The Late Ladinian-Carnian variegated clay, sand and conglomerates of arid flood-plain origin with thin marine intercalations are tentatively referred to the Kagerod Formation. The Jurassic sediments are placed in the Bornholm Group (new) comprising a lower Hettangian-Sinemurian Rønne Formation (new) which contains a succession of lacustrine clay, fluviatile sand and coal, and tidal flat and marsh heteroliths and sand. The overlying Lower Pliensbachian Hasle Formation (new) includes shallow marine fossiliferous sandstone, while the Bajocian-Bathonian Baga Formation (new) is characterized by fluviatile fining-upwards sand-clay-coal sequences and resedimented conglomerates. This is followed by the Lower Cretaceous Nyker Group (new), comprising a basal Rabekke Formation (Lower Berriasian, possibly extending down into the Tithonian and up into the Upper Berriasian) which contains fluviatile coarse, sometimes ferruginous sand and sandy clay, and swamp and lagoonal dark grey clay. This is overlain by the Robbedale Formation (Upper? Berriasian) which comprises tidal flat, beach and lagoonal fine-grained sand and clay and rather coarse-grained barrier beach and shelf sand. Then follows the Jydegard Formation (uppermost Berriasian - Valanginian) comprising back-barrier and washover fan sand, and lagoonal clay, clay-ironstone and silt with thin sands. The Triassic-Lower Cretaceous sequence thus reflects deposition in a variety of coastal environments, and while eustatic sea-level changes seem to have controlled the main facies evolution, the local depositional environments and breaks in sedimentation seem to reflect the interplay between eustacy and synsedimentary tectonic phases.