This study is a first attempt to compile sedimentological features of synimpact to postimpact marine sedimentary successions from marine-target impact craters utilizing six well-studied examples (Chesapeake Bay, Gardnos, Kärdla, Lockne, Mjølnir, and Wetumpka). The sedimentary formations succeed autochthonous breccias and, in some cases, allochthonous suevites. These late synimpact and early postimpact depositional successions (on top of the suevites) appear to be in comparable stratigraphic developments and facies in marine-impact craters. Their composition reflects common mechanisms of sedimentation; they were developed from avalanches/scree, slides, and slumps through sequences of mass-flow–dominated deposition before ending with density currents and fine-grained sedimentation from fluidal flow and suspension. With detailed study, it may be possible to separate the late synimpact and early postimpact successions based on their clast composition relative to target stratigraphy. The process-related comparisons presented here are highly simplified, including characteristics of moat, central peak, and marginal basin sedimentation of both simple and complex craters.

The Eyreville A and B cores, recovered from the “moat” of the Chesapeake Bay impact structure, provide a thick section of sediment-clast breccias and minor stratified sediments from 1095.74 to 443.90 m. This paper discusses the components of these breccias, presents a geologic column and descriptive lithologic framework for them, and formalizes the Exmore Formation. From 1095.74 to ~867 m, the cores consist of nonmarine sediment boulders and sand (rare blocks up to 15.3 m intersected diameter). A sharp contact in both cores at ~867 m marks the lowest clayey, silty, glauconitic quartz sand that constitutes the base of the Exmore Formation and its lower diamicton member. Here, material derived from the upper sediment target layers, as well as some impact ejecta, occurs. The block-dominated member of the Exmore Formation, from ~855–618.23 m, consists of nonmarine sediment blocks and boulders (up to 45.5 m) that are juxtaposed complexly. Blocks of oxidized clay are an important component. Above 618.23 m, which is the base of the informal upper diamicton member of the Exmore Formation, the glauconitic matrix is a consistent component in diamicton layers between nonmarine sediment clasts that decrease in size upward in the section. Crystalline-rock clasts are not randomly distributed but rather form local concentrations. The upper part of the Exmore Formation consists of crudely fining-upward sandy packages capped by laminated silt and clay. The overlap interval of Eyreville A and B (940–~760 m) allows recognition of local similarities and differences in the breccias.

The Gardnos structure in Hallingdal, Norway, is a 5 km in diameter, eroded impact crater of probably late Precambrian age. Within the structure, both impactites and postimpact crater sediments are preserved. The sediments comprise a wide range of siliciclastics: (1) postimpact breccias, (2) coarse conglomerates, (3) conglomeratic sandstones, (4) sandstones, and (5) interbedded fine sandstones, silt-stones, and shales. The sedimentary succession reveals a shifting depositional environment. The post-impact breccias covering the crater floor were deposited by rock avalanches. Directly after impact, loose rock debris slid down the crater wall and the central uplift, settling on top of the newly formed impactites (suevite and lithic breccia known as Gardnos Breccia). The overlying conglomeratic and sandy sequence shows significant local thickness variations, consistent with coalescing fan-shaped deposits along the lower crater wall and on the crater floor. Probably the resurging water breached the crater rim at its weakest parts, initiating series of screes and debris flows, which built out into an eventually water-filled crater. Sand-enriched density flows then dominated in the water-filled crater basin. Above fine-grained sandstones, siltstones and shales were deposited, representing the reestablishment of quiet conditions, possibly similar to the preimpact depositional conditions. Carbon-enrichments in the impactites, and partly deformed clasts of sedimentary origin in the strata just above, suggest that the crystalline basement of the target area was covered by a thin layer of organic-rich sediments. This supports a scenario with a target area in shallow, stagnant water.