An integrated study of core and borehole image data: palaeokarst development in late Permian siliceous dolomites and the structural evolution of the Loppa High from the Gohta discovery well, SW Barents Shelf

Abstract

Abstract The Gohta discovery well 7120/1-3, drilled on the Loppa High in the SW Barents Shelf in northern Norway encountered a meteoric karst system hosted by the late Permian lower Røye Formation that formed during a period of sub-Triassic exposure. The karst system is preserved as a series of interbedded collapse breccias representing formerly open palaeocaves, matrix-rich breccias that represent palaeocave fills with in situ and reworked stalactites and stalagmites. Layers of ‘Swiss cheese’ texture are interpreted as zones of dissolution in marine-meteoric mixing zones that may have controlled the levels of palaeocave development within the Gohta structure. These other cavities and the matrix of karst breccias were infilled by layered fine doloarenite matrix interpreted as an internal fill generated by collapse. This was followed by a later matrix of dark grey argillaceous mudstone that is interpreted as an external fill that pre-dates the overlying Klappmyss Formation. The karst system has undergone complex early diagenesis that includes extensive dissolution, early leaching of silica, the precipitation of pyrite on internal surfaces and sediment and the precipitation of carbonate and silica as speleothems and vadose cement. A borehole imaging log (BIL) was run across the lower Røye and Triassic Klappmyss formations, including the cored interval that provides a record of inclination and azimuth of the karst fills and in situ Røye Formation. The core provides information about the origin and timing of the different karst fills, and the BIL provides information about their azimuth and inclination. This integrated use of core and BIL data allows the incremental tilting history of the Loppa High structure during exposure to be reconstructed. At present, the Gohta structure has a finite tilt of 5–10° to the SE. The plunge of in situ stalagmites and stalactites suggests that the karst system was initiated while the Røye Formation was slightly tilted to SE. The dip of karst components and the host stratigraphy indicate several phases of incremental tilting during deposition of the doloarenite and argillaceous mudstone karst matrix. There appears to have been a reversal of tilt of the Gohta structure prior to its onlap by the Klappmyss Formation.