Per Ahlberg

Hummeln is a simple impact structure located in south-eastern Sweden. It is approximately 1.2 km in diameter and almost completely covered by a lake. Here, we present the first detailed investigation of impactites and mapping of the 164.25 m deep drill core Hummeln-1 with a focus on impact metamorphism and the impact process. We find that the drilling has penetrated a complex sedimentary succession representing syn- to postimpact crater fill. It consists of (from base to top) lithic impact breccia (Unit 1), overlain by diamictite and graywacke with an overall fining upward trend grading from sandy into silty to clayey turbidites (Units 2, 3), and, lastly, suspension dominated marine clays and limestone (Units 4, 5). The crater fill was deposited mostly as gravity slides and sediment gravity flows (debris flows, (hyper)concentrated density flows and turbidity flows), which transported sediment into the crater as a series of fan lobes prograding toward the crater center. We have identified shocked quartz in 12 samples covering the interval of 160.69–56.60 m in the drill core and in samples of polymict and suevitic breccia obtained during fieldwork. Shocked quartz grains dominantly record planar fractures (PFs), with an average of 1.5–3.5 sets per grain. We measured a total of 122 PF sets in 54 grains, with orientations parallel to the (Formula presented.), (0001), and (Formula presented.) orientations being most common (30%, 26%, 21%, respectively). In the same samples, we also measured and indexed 14 sets of planar deformation features (PDFs) in eight grains, oriented parallel to the basal plane (50%), as well as rhombohedral planes (Formula presented.), (Formula presented.), and (Formula presented.) (21%, 21%, and 7%, respectively). Feather features occur associated with PFs in seven of the samples. The quartz grains with shock microstructures in the drill core occur exclusively in beige graywacke–diamictite interbedded with the basal lithic breccia unit and in distinct graywacke–diamictite beds in the late syn- to early postimpact crater fill. We suggest that Hummeln was formed just prior to the deposition of parallel bedded marine mudrock with trilobites of the species Ellipsocephalus polytomus, indicating an early “middle” Cambrian (Wuliuan) age for the impact.