Maurits Lindström

When a cosmic impact strikes the sea bed

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.

The marine impact crater at Lockne, central Sweden

Abstract The Lockne crater (63°00′20″N, 14°49′30″E), dated as Middle Ordovician, Caradoc, chitinozoan Zone of Lagenochitinal dalbyen‐sis, conodont Subzone of Baltoniodus gerdae, consists of a 7.5 km wide inner crater that is surrounded by a 3 km wide outer crater. The inner crater has been identified by drilling on both sides of Lake Lockne. Its floor is strongly crushed crystalline basement with a variable thickness of monomictic breccia. It is filled with more than 200 m of resurge deposits and nearly 100 m of secular, post‐impact Dalby Limestone, parts of which are covered by a thin Caledonian nappe outlier. The outer crater formed at the top surface of the crystalline basement that it exposes in a state of strong crushing and with a patchy cover of monomictic breccia. The presence and structure of the outer crater suggest that Lockne may be a nested crater with the basement acting as a strong, lower layer, and the sediment cover and the water acting together as a weak upper layer, the thickness of whi...

Post-impact deposits in Tvaren, a marine Middle Ordovician crater south of Stockholm, Sweden

The well-preserved Tvaren 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.

Geology of the Early Palaeozoic Lockne impact structure, Central Sweden

Abstract The diameter of the crater at Lockne as seen on topographic maps is 7–8 km. The Tandsbyn Breccia, the principal evidence of impact, consists of strongly crushed local basement. Its distribution follows the crater margin. On the north margin, basement granite rests on an inverted sub-Cambrian erosion surface on the lowermost Middle Cambrian. Tangential faults occur at the periphery of the structure. After the impact occurred under the sea during the middle Ordovician, there was a resurgence of ejecta-loaded sea water which deposited a chaotic breccia (Lockne Breccia), which has the appearance of a debris flow. The Lockne Breccia, together with a less coarsely grained turbidite (“Loftarsten”), which lies immediately above it, predominantly consist of clasts of lower to middle Ordovician limestone and, mainly subordinate, inclusions of Tandsbyn Breccia and isolated clasts of crystalline basement. Fragments of impact melt are important components of the turbidite. A protective cover of sediments formed after the impact, is, in turn, overlain by an outlier of a Caledonian overthrust nappe, which occupies the central and topographically deepest part of the crater. Shocked quartz has been identified in the Loftarsten.

Hydrocode Simulation of the Lockne Marine Target Impact Event

In this study 2D and 3D numerical simulations are used to model the formation of the Lockne crater (centered at 63°00′20″ N, 14°49′30″E) during Middle Ordovician times, about 455 Ma ago. We study a possible mechanism of shallow excavation to explain the concentric structure of the crater, as well as the interaction between basement the ejecta curtain and the water ejecta curtain, to explain the final ejecta distribution on the Earth’s surface. We also consider different angles of trajectory inclination to understand how obliquity can influence the cratering flow in a marine target impact. Comparison between the results of numerical simulations and field studies allows us to estimate a water depth at the time of the impact of about 700–900 m.

The Lockne Crater: Revision and Reassessment of Structure and Impact Stratigraphy

The Lockne impact crater in central Sweden has features characterizing a relatively deep marine environment. Recently, improved outcrop (road construction, lumbering, etc.) has favoured examination of important features, resulting in greater precision and understanding. Under a water depth of at least 500 m, the target seabed consisted of 80 m Cambrian and Ordovician sediments. About one-half of this succession was lithified limestone; the other half was soft claystone and semi-lithified limestone. This succession rested on a peneplain cut into crystalline basement. The crater in the basement is just over 7 km wide. The sub-Cambrian peneplain can be followed all the way up to this crater, which thereby is shown to lack any significantly raised rim. The peneplain next to the crater was stripped of most sediment by the expanding water crater. Instead, there is a roughly 50 m thick brim of ejected crystalline rock, resting on the peneplain and on some remaining sediment. The brim is 2.5 km wide in the west, but less than 1 km in the east. Crystalline ejecta bodies, each with a volume of thousands of cubic meters, are scattered towards westerly directions as far as 9 km from the crater centre. An oblique impact from an easterly direction is indicated (Shuvalov et al., this volume). Radial furrows through the brim (“resurge gullies”) might have originated during ejection but were utilized by resurging seawater. Target carbonates in the wider surroundings were brecciated during the ejection and excavation stage. During the resurge stage, another breccia was deposited including both limestone and redeposited clasts of crystalline ejecta.

Section at Daping reveals Sino-Baltoscandian parallelism of facies in the Ordovician

Abstract A bed-by-bed record of a 92m thick section through the Arenig to lower Ashgill at Daping near Yichang, Yangtze Platform, is the first Ordovician section from China to be presented internationally in this detail. The predominant facies is identical to what is known in northern Europe as Orthoceratite limestone. Among features shared between the Chinese and Baltoscandian carbonate facies are frequently occurring, bored and/or burrowed, mineralized discontinuity surfaces, buckled beds, perched cephalopod conchs, thin seams of small ferrugineous stromatoids, and biocalcarenitic components dominated by arthropods and echinoderms. The Chinese as well as the Baltoscandian sections show evidence of a major regression about the transition from the Arenigian to the Llanvirnian. Contrasting with Baltoscandian sections, the Daping section begins with tropical to subtropical carbonate platform facies.

Two Ordovician Conodont Faunas found with Zonal Graptolites

Abstract The conodont faunas treated in this paper are from the zone of Didymograptus balticus in South-Central Sweden and the zone of Climacograptus wilsoni in SW Scotland. On conodont evidence, the Didymograptus balticus zone is correlated with limestone strata intermediate between the Lower and the Upper Planilimbata Limestone. The conodont fauna of the Climacograptus wilsoni zone is described and figured, but no new species is named.

The Lower Palaeozoic of the probable impact crater of Hummeln, Sweden

Abstract A 1.2 km wide, sub-circular hole in Lake Hummeln (57°22′N, 16°15′E) has been suggested to be an impact crater. The lake has furthermore been suspected to contain an outlier of Cambrian and Ordovician sedimentary rocks. A shallow seismic survey, followed by core drilling to 164.25 m below the lake surface, demonstrated the presence of a strongly slumped succession consisting, from below, of shattered crystalline basement rock, Lower and Middle Cambrian sandstone and claystone, Lower Ordovician limestone, and Middle Cambrian clay-stone. The lithogenesis and original stratigraphy are closely similar to coeval beds on Oland 40 km to the east. Dating was done through acritarchs in the Cambrian, and through conodonts and chitinozoans in the Ordovician. An impact is postulated although no quartz with PDF has been identified in the available lithologies. The drilling had to stop, apparently without reaching the crater floor. If dated by the oldest sediments present, the impact would be Early Cambrian. A ...

Concentric impact structures in the Palaeozoic of Sweden – the Lockne and Siljan craters

The Ordovician age Lockne and Devonian Siljan craters are two of the largest impact structures in northern Europe. Both formed in targets with a thick, low-strength upper layer. This target configuration is known to generate concentric crater structures with an outer, shallow crater in the low-strength layer, surrounding a central, deeper crater in the more resistant substrate. The concentric craters of Lockne and Siljan are excellent models for studies of similar concentric craters on Earth and elsewhere in the Solar system. Several structural issues remain, and drilling through the craters within the Swedish Deep Drilling Program intends to address the following: the extent of the craters with respect to the time of impact; the effects of cratering on the basement; and the role of basement structure for the crater formation. A problem for the Lockne crater is the relation to the Caledonian orogeny and the lateral extension of the ejecta blanket – the rim is interrupted by a radial depression that has been interpreted both as primary and secondary, tectonically induced. A second feature to study is the deeper and older (1.82–1.80 Ga) NNW–SSE shear zones that cut the basement. In the Siljan area the development of mega block associations comprising the infilling of the graben is disputed. The blocks may either be formed by sagging of peripheral parts of the fault blocks or alternatively by major radial movement involving kilometre long transport.