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Most bolides that collide with the Earth hit the sea. Limited knowledge about marine-target impacts hampers predictions about their perilousness. This study presents geological features that are particular to craters formed at sea. The features are most likely a result of the influence of the target on the cratering process. Marine-target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed. When the crater diameter is large compared to the water depth, the crater resembles its counterparts that are formed on land. Craters formed in deeper water are concentric, and often lack melt sheets and rim walls, but have deposits and radial gullies formed by the resurge of the sea. Impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater.

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.

In Europe, a Lower Ordovician conodont zonation can be established for the calcareous, partially condensed succession of the Baltic Shield. Sporadic conodont occurrences in the graptolitic shale fades can be correlated with this zonation. The Tremadocian to Llanvirnian Series are dealt with, and eleven conodont zones are established in these series. From below, these are the zones of Cordylodus angulatus, Paltodus deltifer (both Tremadocian), Paroistodus proteus, Prioniodus elegans, P. evae (corresponding to the Arenigian extensus Zone), Baltoniodus triangularis, B. navis, Paroistodus originalis, Microzarkodina parva (corresponding to the main part of the Arenigian hirundo Zone), and Amorphognathus variabilis (top of hirundo Zone, Llanvirnian bifidus Zone). For handling the material, some new taxa have had to be described. The taxonomy is based on multielement species, where such can be established. The multielement species are defined on morphologic and statistical criteria, as well as on the pattern of evolution. The taxonomic approach allows a more "natural" classification on the suprageneric level than the form-taxonomy previously used for this material. An evolutionary pattern is discernible for the drepnodids (each species containing drepanodiform and oistodiform elements) and the prioniodids (species containing prioniodiform elements, branched compound elements with symmetry transition, and oistodiform elements). On a suggestion from Professor 0. H. Walliser, elements belonging to multielement species are distinguished by adding the ending "-form" to the name of the genus they would have been brought to in a purely formal taxonomy. The word formed in this 21 on July 10, 2015 memoirs.gsapubs.org Downloaded from 22 SYMPOSIUM ON CONODONT BIOSTRATIGRAPHY way is an adjective. The following new taxa are named: Drepanoistodus gen. nov., Paroistodus, gen. nov., Protopanderodus, gen. nov., Stolodus, gen. nov., Baltoniodus, gen. nov., Microzarkodina, gen. nov., Microzarkodina parva n. sp., M. ozarkodella n. sp. The following genera are redefined:

The well-preserved Tvären crater is noteworthy for being one of a small number of Early and Middle Ordovician impact structures formed in a marine environment. It is demonstrated to be an impact structure by the presence of a breccia lens, consisting of crystalline basement rocks, and shocked quartz. The breccia lens formed under dry-hot conditions after expulsion of sea-water by the impact. Resurging sea-water thereupon deposited a positively graded, 60 m thick turbidite-like unit. This graded resurge deposit is a previously unknown feature, to be expected in open-sea impacts. Breccia in the lower part of this graded deposit contains fragments of a remarkably complete orthoceratite limestone succession that existed at the site of impact, resting on non-lithified sand of probably Early to earliest Middle Cambrian age. A sedimentary succession was deposited inside the crater at depths decreasing from more than 200 m in the initial stages to some 100 m at the time of deposition of the youngest preserved beds. The environment within the crater thus favoured deposition of an essentially complete stratigraphic succession with depth-controlled palaeoecology for a significant time interval after the impact. Whereas planktonic members, like graptolites and chitinozoa, are present throughout the post-impact succession, and asaphids, almost as persistent, became established at an early date, burrowers were somewhat reluctant to enter and remopleuridids and small strophomenids came in at a late stage. We suggest as a result of this study that structures formed by impact may offer unique information about the palaeogeology and palaeoenvironment of the region hit by the impact.

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