Geological 3D Modelling ofthe File Hajdar Quarry, Slite

Abstract: Located on the island of Gotland, Sweden, the File Hajdar limestone quarry is an important locality for domestic cement production. With depositional settings generally related to shallow sea carbonate ramps, the lithologies encountered on File Hajdar range from reef-associated limestone units to more basinal deposition of marls. As the depositional setting is related to the CaCO3-content (carbonate), which is an important property to consider during cement production, geological investigation of the area through conceptual 3D modelling could be a very powerful tool to enhance the general knowledge of the area or for planning future prospecting campaigns. Since the quality of the 3D model is related to the amount and quality of data utilized, data acquisition and management are very important steps in the construction of a 3D model. Two main campaigns with drill core extractions were done in 1993 and 1999, respectively. Along with resistivity data collected from skyTEM-measurements and ground surveys, subsurface geological information is quite extensive in the area. Using overall data and a reclassification of the lithologies, three main areas associated with reef complex deposition have been identified on File Hajdar. Corresponding to elevated resistivity measurements as well as classification in drill cores, two reef complexes centered around the quarry, have been described as the central- and southeastern reef complexes. Based on high resistivity anomalies in skyTEM-data along with field interpretations, the emplacement of a third reef complex is also interpreted in the northwestern parts of the study area (northwestern reef complex). However, as overall data is lacking at the location of the northwestern reef complex, further investigation is necessary before the relationship between the central- and northwestern reef complex can be fully investigated. Due to the generally indistinct transition between clay-rich limestone and marl, the subsurface boundary to the marl is rather insinuated. However, corresponding to low resistivity anomalies, outcroppings of the surfacing marl unit are outlined at the base of the File Hajdar rise. Future utilization of the produced 3D model could include the implementation of additional data or as a foundation during the planning of further prospecting campaigns in the area. Corresponding to the locations of the central- and northwestern reef complexes, these two areas are suggested to be most suitable for further investigation.