Detecting multiscale karst features including hidden caves using microgravimetry in a Caledonian nappe setting: Mefjell massif, Norway.

Abstract

Cave passages and karst features form negative density contrasts expressed in gravity field anomalies. We present an interpretation approach for microgravimetry, applicable for challenging geological settings with heterogeneous lithologies. The workflow covers not only detection of large cave rooms, but also deals with minor karst features (epikarst) in carbonate rocks. Challenges due to heterogeneous infill of large cavities and variations in carbonate facies are addressed. We used 3D forward modelling of surface gravity measurements to investigate a large karst cave complex, known as the Svarthammarhola cave, in the Caledonian nappe setting of Nordland. The most important result relates to distinct gravity lows detected in the survey, which are interpreted as hitherto unknown and inaccessible cave rooms, some of them of a very large size. These correspond with known collapse and sediment infill features both at the surface and inside the Svarthammarhola cave system. This expands the known cave in an eastward and northward direction. Combining the cave’s position at the top of the hinge of a large antiform where we also modelled uncommonly high densities, together with the cave’s outstanding size, opens for new interpretations of the speleogenesis of the Svarthammarhola cave. The study also has wider implications on how gravity field data can be used for the understanding of complex subsurface karst features.