Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

Abstract

Sedimentological analysis of resurge deposits at the Lockne and Tvären craters:Clues to flow dynamics Abstract-The Lockne and Tvären craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement. The target water depth for Tvären (apparent basement crater diameter D = 2 km) was probably not over 150 m, and for Lockne (D = 7.5 km) recent best-fit numerical simulations suggest the target water depth of 500-700 m. Lockne has crystalline ejecta that partly cover an outer crater (14 km diameter) apparent in the target sediments. Tvären is eroded with only the crater infill preserved. We have line-logged cores through the resurge deposits within the craters in order to analyze the resurge flow. The focus was clast lithology, frequencies, and size sorting. We divide the resurge into "resurge proper," with water and debris shooting into the crater and ultimately rising into a central water plume, "anti-resurge," with flow outward from the collapsing plume, and "oscillating resurge" (not covered by the line-logging due to methodological reasons), with decreasing flow in diverse directions. At Lockne, the deposit of the resurge proper is coarse and moderately sorted, whereas the anti-resurge deposit is fining upwards and better sorted. The Tvären crater has a smoothly fining-up section deposited by the resurge proper and may lack anti-resurge deposits. At Lockne, the content of crystalline relative to limestone clasts generally decreases upwards, which is the opposite of Tvären. This may be a consequence of factors such as crater size (i.e., complex versus simple) and the relative target water depth. The mean grain size (i.e., the mean -phi value per meter, ϕ) and standard deviation, i.e., size sorting (σ) for both craters, can be expressed by the equation σ = 0.60ϕ -1.25.

Facies distribution of post‐impact sediments in the Ordovician Lockne and Tvären impact craters: Indications for unique impact‐generated environments

“…1) took place at a water depth of about 500-700 m (see summary of the water depth discussion in Ormö et al 2007) at the time of carbonate sedimentation. A crystalline granitic basement overlain by about 30 m of Cambrian shale (commonly referred to as "alum shale") and 50 m of Ordovician limestones constitute the target rocks, the youngest sediments being the Dalby Limestone, during which deposition the impact occurred.…”

“…The target sequence at Tvären consisted of Ordovician carbonates resting on non-lithified sands of Early to earliest Middle Cambrian age covering a crystalline basement (see Fig. 5 in Ormö et al 2007). Drillings in the structure showed a similar succession as at Lockne with a crystalline breccia overlain by 60 m of fining-upward resurge deposits, and 80 m of secular sediments of carbonate mudstone representing the Dalby Limestone (Lindström et al 1994;Ormö et al 2007).…”

“…Although Lockne and Tvären are different in crater size and actual water depth, at the target site they demonstrate similarities in age (Lindström et al 1994;Grahn et al 1996;Grahn 1997), resurge sediments (Ormö et al 2007), and post-impact secular depositional patterns. The understanding of the craters' influence on the post-impact sedimentation and biofacies development is important for understanding other marine-target craters such as Chicxulub (Mexico), Chesapeake Bay (USA), and Mjølnir (Norwegian Barents Sea).…”

“…To the west, the area was transitional to a slope environment. Sedimentological and paleoecological evidence favors a relatively deep shelf position, rather than shallow water at the Lockne impact site (Lindström et al 1996(Lindström et al , 2005a(Lindström et al , 2005bOrmö et al 2007). Deposition at this time was characterized by very slow and, on average, constant sedimentation rates.…”

“…4 in Ormö et al (2007). The target seabed at Lockne was located on the western margin of the sea that extended over southern Scandinavia and the Baltic (Männil 1966;Jaanusson 1973).…”