Lake-mire deposition, earthquakes and wildfires along a basin margin fault; Rønne Graben, Middle Jurassic, Denmark

Abstract

Regional uplift of structural highs took place in the greater North Sea region in early Middle Jurassic times, resulting in a change from marine shelf deposition in the Early Jurassic to widespread emergence, erosion and localized deposition in grabens. The Middle Jurassic Bagå Formation exposed on the island of Bornholm in the Baltic Sea was deposited in the Rønne Graben, which forms a dog-leg pull-apart basin linking the northern and southern branches of the Tornquist Zone. The studied succession is situated on the hanging wall immediately adjacent to the main eastern graben margin fault and the footwall consists of deeply weathered granite. It is the only locality along the northern branch of the Tornquist Zone where active tectonic influence on Middle Jurassic deposition can be demonstrated in outcrop. Small alluvial fans composed of poorly sorted washout sands and muddy debris flows with weathered granite boulders fringed the footwall scarp. Syn-depositional soft-sediment deformation and chaotic bedding structures interpreted as seismites caused by earthquake shocks are characteristic at several levels. On the hanging wall further away from the active fault the depositional environment was dominated by shallow freshwater lakes with deposition of clay. Lake deposition outpaced subsidence and open, oxygen-deficient freshwater mires were formed during periods of no or only incremental fault-movement. The mires were only occasionally isolated from clastic input due to doming and dense vegetation, resulting in accumulation of pure peat beds. The content of spores and pollen shows that the mires were covered by a vegetation of arborescent and herbaceous Filicopsida (ferns), Lycopsida (club mosses), Cycadales/Bennettitales, Ginkgoales, Caytoniales, Taxodiaceae or Cupressaceae, and Araucariaceae. Wildfires were common in the mires and also on the footwall probably followed by episodes of increased run-off and erosion of the upland. The presence of common washout sands, debrites, seismites and abundant lacustrine flooding surfaces indicating sudden drowning of the mires was caused by abrupt movements on the fault with associated earthquakes. The succession thus provides an example of temporal changes in deposition in response to changes in tectonic conditions. The study illustrates how regional uplift of structural highs caused a restriction of deposition to narrow grabens like the Rønne Graben. Facies patterns changed dramatically from laterally extensive uniform packages controlled by sea-level changes in the Early Jurassic to strongly localized deposition with marked facies changes controlled by faulting and earthquakes in the Middle Jurassic. The succession thus serves as an excellent example of tectonically controlled deposition under humid conditions in a continental graben.