Linking laboratory seismic velocity measurements with the minerlogical content and (micro)structures of the COSC-2 drill core, central Scandinavian Caledonides

The deep erosion of the Scandinavian Caledonides provides a unique opportunity to study the interior of an orogen. The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to better understand orogenic processes and to verify interpretations of the Scandinavian Caledonides based on subsurface geophysical investigations. The second drill hole of the project (COSC-2) is located near Järpen in central Jämtland, Sweden (central Scandinavian Caledonides). Based on seismic images, the ∼2.3 km deep drill hole was assumed to transect the Lower Allochthon, the main décollement located in the Alum shale formation, the footwall sedimentary succession, and the underlying basement. Although a deformation zone in the Alum shale formation is found between ∼775 and ∼820 m depth, its related structures dip moderately towards ESE to E, which does not fit a décollement that is expected to dip gently to the west. The recent detailed description of the COSC-2 core also revealed a mostly continuous sedimentary succession deposited on top of a porphyry sequence, with no abrupt transition from autochthonous to allochthonous units.

The discrepancy between the interpretation of the seismic image and the drilled lithologies highlights the need to determine seismic properties of the drill core. The P-wave and S-wave sonic downhole logging performed after drilling may provide a first indication in high spatial resolution. However, laboratory seismic velocity measurements are required to link seismic velocities with mineralogical composition, (micro)structures, and associated anisotropy. We determine the P- and S-wave velocities of six samples covering main lithologies of the drill core: (1) a sand-to claystone (turbidite) from ∼380 m depth, (2) a sandstone from ∼690 m depth, (3) a phyllitic shale (Alum shale) from ∼815 m depth, (4) a fine grained conglomerate from ∼1175 m depth, (5) a porphyry from ∼1255 m depth, and (6) a dolerite from ∼1655 m depth. The seismic velocities are measured in three mutually perpendicular orientations, at different confining pressures up to 250 MPa. Measurements at pressurized conditions are used to simulate in-situ conditions and to estimate the intrinsic (crack-free) velocities. For all samples, we determine the density and describe the mineralogical composition as well as textures that may lead to seismic anisotropy. With the resulting data, we will be able to constrain the origin of the seismic velocity changes and associated reflections found in the seismic image. Furthermore, we can derive basic petrophysical properties such as seismic anisotropy and dynamic elastic moduli, which may serve as a basis for future studies related to similar tectonic settings.