Tom Flodén

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.

Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation

The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (∼140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin

In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of rapakivi magmatism. No major events destroyed the Svecofennian and rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.

Major earthquake at the Pleistocene-Holocene transition in Lake Vättern, southern Sweden

Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.

The Tvären Bay structure, an astrobleme in southeastern Sweden

Abstract Continuous seismic reflection profiling has revealed a 2000 m wide, almost circular depression in the crystalline bedrock of Tvaren Bay. The depression extends more than 200 m below the present sea level. Originally, it was probably 250–300 m deep. The physical parameters of the structure, which forms a foreign element in the tectonic regime of the Baltic coastal area, are consistent with those of known impact structures. Inclined sedimentary rocks of an assumed Ordovician age are unevenly distributed within the depression, partially smoothening the topographic relief of the structure. A late Lower or early Middle Ordovician time for the meteorite impact is suggested. Drillings into the ‘wall’ surrounding the impact structure and into the sedimentary bedrock inside the depression are proposed as a further step in this investigation.

A Silurian reefal succession in the Gotland area, Baltic Sea

Abstract Seismic reflection profiling east of Gotland has given information on a succession of four reefal units in the lower Ludlow stratal sequence ranging from the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), the Hammarudden Reef (Hemse Group ‘younger’ units a-c), the Östergarn Reef (Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef units form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland and in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds to a well-defined seismic unit with distinct facies zonality ranging from lagoon via reef barrier and biohermal slope to basin facies. The ‘fore-reef’ facies is in this paper termed biohermal slope to express its characteristic reefal features. Reconstruction of the post-depositional tilting of the sequence indicates lagoonal depths of up to 10 m. The reef barriers developed at depths of up to 20 m and the biohermal slopes at between 10 and 55 m. The reef barrier and the biohermal slope are generally separated by a debris fan of waste products from the reef, 1-2 km wide. The reef barriers are generally 1-3 km wide, which is about the same width as their lagoons. The biohermal slopes become successively wider towards the upper part of reef succession, ranging from 5-9 km at the Klinte Reef to 15-18 km at the Millklint Reef. Comparisons with the Gotland sequence show that the reef barriers are biostromal stromatoporoid reefs in a matrix of crinoid debris. The biostromes are of the Kuppen type. The bioherms are of the Axelro and Hoburgen types. The bioherms occur randomly on the seaward slope of the barriers.

Late Ordovician carbonate buildups and erosional features northeast of Gotland, northern Baltic Sea

Abstract The dimensions and form, structure and distribution of the Upper Ordovician carbonate buildups in the Baltic Sea, northeast of the Island of Gotland, have been determined by means of shallow-marine seismic reflection profiling. They are shown to decrease in size and number downslope, towards the deeper part of the Palaeobaltic Basin. The seismic signature variations, reflecting the structural and compositional changes of the carbonate buildups point to an increase in reef skeleton forming organisms and enhanced rigidity and stability of the buildups upslope. Supported by the tectonic setting, a local shallow-water break was formed on the slightly sloping carbonate ramp of the Late Ordovician sea, closely east of the Isle of Gotska Sandön. This depth development promoted the formation of a major linear reef of fringing or barrier character, separating a patch reef complex and reefs in the offshore belt adjacent to deep shelf muds from a beach or tidal flat. In the offshore area northeast of northern Gotland, the Late Ordovician sedimentary environment was entirely different from that farther east, offshore the Island of Hiiumaa, Estonia. The great diversity of the sediments, and the rapidly changing thicknesses of the layers, laid down on the fore-reef slope, were to a large extent governed by the numerous mud mounds and reef structures. Syn- and post-depositional compaction effects induced relative movements of the masses inside the deep-water carbonate buildups which led to their lateral spalling and collapse. The locally discordant reflectors from these rapidly changeable lithological units and strongly anomalous, chaotic reflector configuration around the collapsed carbonate buildups are intertwined with reflectors of erosional origin. Channel-like depressions, which probably form a unique set of ancient riverbeds, appear in the Late Ordovician sequence about midway between northeastern Gotland and Hiiumaa. They were formed before the formation of the Vormsi-Fjäcka layers, i.e., before the establishment of the Upper Ordovician carbonate buildup environment. The latest Ordovician erosional surface, most likely related to the Gondwana glaciation, is distinctly imprinted in the bedrock of the area.

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 ...

The structure and relief of the bedrock sequence in the Gotland-Hiiumaa area, northern Baltic Sea

Abstract Based on high resolution seismic reflection profiling, structural and relief maps of the sedimentary bedrock between Gotland and Hiiumaa in the Baltic Sea have been composed and analysed. The general structure and relief of the submarine Lower Palaeozoic succession reveal a westward extension of the homoclinal structure distinguished in the Estonian mainland. The main bedrock structures offshore are 1-4 km wide, and several tens of kilometres long, linear zones of disturbances. On the structural map, these disturbances appear as submeridional zones of contour changes, up to several tens of metres in offset. The seismic profiles usually reveal a faint flexure-like bending of the layers through the zone. Locally, this flexure can be intersected by small faults. These bedrock structures are ascribed to fault movements in the crystalline basement. Two different bedrock relief systems were superposed on the region during the Cenozoic uplift and the Pleistocene glaciations. The first event resulted in the formation of a subparallel cuesta-like system of alternating erosional scarps and plains. Glacial erosion created submeridional valleys and troughs. Today three large bedrock forms, namely the Baltic and the Silurian clints and the Ordovician plateau, characterize the area. The outlines of the cuesta relief, and the amount of eroded sediments, advocate a regional increase in erosional activity from the St. Petersburg district to the area of the Baltic-Bothnian mobile zone northeast of Gotland. This zone existed as a subsided meridional lower ground during the Cenozoic, accommodating a main river that collected water both from the craton margins and the inner platform areas.

Seismic correlation of the Cambrian sequence between Gotland and Hiiumaa in the Baltic Sea

Abstract Seismic reflection profiling in the Baltic Sea between Gotland and Hiiumaa reveals that the lower boundary of the Cambrian mainly coincides with the crystalline sub‐Cambrian peneplain here. In the Gotska Sandon area, the Cambrian sequence rests partially on Proterozoic sedimentary bedrock, whereas Vendian rocks may underlie the Cambrian at the Estonian coast. The sandy‐argillaceous Cambrian sequence is covered by Lower Ordovician limestones in the western part, and by an up to 10 m thick layer of Lower Ordovician siliciclastic rocks in the eastern part. At the end of Lontova time, the early Cambrian depocenter shifted westwards from the Moscow Basin to the Baltic Basin, prohibiting any detailed Trans‐Baltic seismic correlation. Two Cambrian reflectors extend westwards from Hiiumaa, and seven reflectors eastwards from Gotska Sandon. Only one reflector extends across the area. It is located near the base of the Cambrian sequence within the Nar Member shale unit in the Gotska Sandon drill core, and ...