Radiomagnetotelluric mapping of marlstone and limestone in the Silurian bedrock of Gotland

Abstract

The Silurian of Gotland is composed of a series of stacked carbonate platforms. Airborne electromagnetic measurements show areas with high electrical resistivity, dominated by dense limestone, and areas with lower resistivity, dominated by marlstone. On Gotland, three main regional, up to 80 m thick, limestone-dominated bedrock packages overlie marlstone-dominated strata. The thickness of the limestone and the subsurface relationship between limestone and marlstone was investigated in the area of Ale–Ardre on central Gotland by use of the radiomagnetotelluric (RMT) method. A significant difference in resistivity between the limestone (1000–10 000 Ωm) and the marlstone ( < 100 Ωm) renders favorable RMT conditions. Four profiles, up to 4.3 km long, were measured across the upper part of the Klinteberg Formation and lower part of the Hemse Group (late Wenlock–early Ludlow). The results show a distinct change in resistivity at 50–80 m depth, correlating with the transition between biostromal–biohermal limestone and shelf marlstone–wackestone lithofacies, as observed in boreholes. The measurements were used to create resistivity models that make it possible to map the subsurface lateral relationship between limestone- and marlstone-dominated sequences. The results also provide important information on distribution and bedding, which give supporting information regarding the sequence stratigraphic model for the Klinteberg Fm–Hemse Group in the Ala–Ardre–Sigsarve area. The RMT method utilized in this study proved to be a very useful method for mapping subsurface bedrock in carbonate-dominated geological settings such as those on Gotland.

Keywords::

• geophysical methods
• radiomagnetotelluric
• mapping
• carbonates
• Klinteberg Formation
• Hemse Group

Acknowledgements

Thanks are expressed to Anna-Karin Rasmussen who assisted in the field work, and the municipality of Gotland, especially Jonas Aaw, for letting us use their well information. Olof Sandström and Niklas Juhojuntti are thanked for fruitful discussions and comments on the manuscript. The work has been funded by the R&D program at the Geological Survey of Sweden.