The Ritland impact structure: : Characteristics and distribution of the ejecta layer and associated Lower Paleozoic sedimentary succession

The newly discovered Ritland impact structure is located in Hjelmeland Municipality in Rogaland County, Western Norway. The structure is 2,7 km in diameter, about 350 meters deep and probably of Early/Middle Cambrian age. In this study, parts of the Lower Paleozoic succession, which include a layer of material ejected during impact, has been mapped and logged east of the impact structure. The depositional environment has been interpreted based on the combined sedimentary field data, including sedimentological logs and general mapping, as well as petrographical and mineralogical studies including thin sections, XRD, SEM and CL analysis. In addition to discuss the depositional environment of the Early Cambrian succession overlying the sub-Cambrian peneplain, special emphasis has been on the ejecta layer which is observed some meters above the basement. During the Cambrian transgression, the Ritland area became a part of an extensive sea which at the time covered most of Scandinavia, as well as the Baltic. The stratigraphy reveals that the environment was quite calm and in periods stagnant, although bioturbation frequently occurred. Although the overall development seems to be transgressive, a shorter period of regression seems to have occurred as well. A layer found up to a few meters below the ejecta layer is characterized by calcite precipitation and stromatolites, suggesting minor water depth. The term “ejecta” describes the debris expelled from the crater during impact and crater formation. The ejecta layer is matrix-supported, consisting of clasts of basement rocks of various sizes; from blocks and boulders down to sand and silt-sized fragments in shale. The average clast size decreases both away from the crater centre as well as upwards within the ejecta bed. For most impacts, ejecta are emplaced ballistically. The fact that the ejecta layer is found within marine deposits some meters above the sub-Cambrian peneplain strongly suggest that the Ritland area was covered by a shallow sea at the time of impact. The presence of water at the time of impact probably increased the extent of the ejecta distribution. In addition, the presence of water may have caused early post-impact reworking of the ejecta layer by wave action, tsunamis, currents or resurge flows. The ejecta layer has been observed as far as 4,9 km (3,6 crater radii) from the crater centre. Originally it most likely extended much farther out when comparing the ejecta distribution at Ritland with other craters of similar configuration and size. Both microscopic and macroscopic evidences of shock metamorphism have been found within the ejecta layer. Several samples collected in the area revealed planar deformation features (PDFs) in quartz grains, while a cluster of shatter cones were observed at another locality. Such features only form by shock pressures of several GPa, and are considered evidence of meteorite impact. The Silurian-Devonian Caledonian Orogen thrust nappes altered the stratigraphic column overlying the peneplain. Thus, there is an uncertainty regarding the presently observed height of the ejecta layer above the basement, as well as its original thickness. The Lower Paleozoic successions overlying the sub-Cambrian peneplain were in addition further exposed to later uplift, erosion and glaciation compared to the sediments found within the crater. Consequently the ejecta layer is only partly preserved and the field observations have been restricted to locations east of the crater.