Postimpact Alteration of Impact Structures

The present boundary of the Fennoscandian Shield with the neighbouring northwestern part of the Russian Platform represents an important frontier between meteorite impact structures with different post-impact histories.

Only a few young or buried (Table 2) impact structures have preserved their primary impact-produced features. Occurring in different succession and intensity, the main factors changing and hiding such features are: (a) erosion, single or recurrent, (b) burial under sediments, single or recurrent,

(c) submergence under water, and (d) deformation (in tectonically mobile areas).

In Fig. 4, individual histories of craters of different age groups from Mesoproterozoic through Holocene are sketched with respect to (a) geological setting of the impact site (sedimentary, crystalline, or at sea), (b) presence of high resistant impact melt rocks in crater interior, (c) burial of the structure, immediate after impact or posterior to primary erosion, (d) erosion histories of cover complex, sedimentary target rocks, and top of crystalline basement, and (f) planation of crater sites or hollowing of craters through the Pleistocene glaciations.

Preservation level according to Dence (1972) of any particular structure is given in Table 2, demonstrating the morphological signatures of crater sites and present setting of craters in an order of increasing PL. Impact structures within the platform area appear to have their PL smaller, i.e., are more preserved compared to structures in the present shield area. Recent (Holocene) craters are best preserved having their ejecta still present (PL = 2). Most of the pre-Pleistocene impact structures of the SCD are preserved at the level 4 - 7, with exceptions of two (Kärdla, Neugrund, and Lockne) at the level 2 and two (Dobele and Vepriai) at the level 3. All these five structures were buried soon after the impact, whereas two, Neugrund and Lockne, were only lately re-exposed. Caledonian orogeny deformed the shape of Lockne structure, but tectonic nappes overriding the impact site protected the crater against erosion (Lindström et al. 1996, Ormö et al. 2002). Craters at the PL 4 (Logoisk and Mizarai) are preserved in platform area, but also the youngest (Cretaceous) structures Mien and Dellen in the shield area. All the rest of craters at PL = 5 - 7, except Mishina Gora, are located in the present shield area, but were, except Iso-Naakkima and Suvasvesi N & S, formed in past platform areas.

The craters formed in past land areas were subjected to erosion immediately after their formation. Depending on the resistance against erosion, their superstructure as well as the surrounding sedimentary blanket were eventually destroyed and removed. In the result, shallowed and smoothed crater sites with partially survived crater interiors are figured out (Dobele, Vepriai, Logoisk, and Mishina Gora).

The smoothing of crater sites before the Pleistocene glaciations was considerable, but not perfect. In many cases in the shield area, after glaciations, the topography of great majority of exposed crater sites became much more complicated than before. Lakes and bays in crater sites are the most typical features. In Fig. 5 and 6 and Tables 1 and 2, it is shown that all craters located on shield at PL = 3 - 7 are hollowed (Fig. 6) or, at least, include glacial incisions (Lumparn, Fig. 5). In platform area, the submarine, re-exposed Neugrund crater is subjected to erosion, which

Fig. 5. Some types of past and present morphologies of craters formed at sea (left) and subsequently buried under sedimentary deposits, then exhumed and differently eroded during late Phanerozoic, including Pleistocene glaciations (right). (a) presently buried crater that formed in shallow sea, whereas its rims were slightly eroded immediately after the impact. (b) presently partially submerged incised structures in which the impact and postimpact deposits are survived in the interior only. PL, preservation level after Dence (1972).

Fig. 5. Some types of past and present morphologies of craters formed at sea (left) and subsequently buried under sedimentary deposits, then exhumed and differently eroded during late Phanerozoic, including Pleistocene glaciations (right). (a) presently buried crater that formed in shallow sea, whereas its rims were slightly eroded immediately after the impact. (b) presently partially submerged incised structures in which the impact and postimpact deposits are survived in the interior only. PL, preservation level after Dence (1972).

created deep annular incision channels (Suuroja and Suuroja 2000). Subsurface craters are not subjected to surface lowering, except Kardla structure, whose post-impact cover of walls, locally composed of hard limestones, is sculptured by glaciers to form drumlins (Fig. 5).

Already the exposed impact structures were eroded during the periods of continental exposition, the selective re-sculpturing by Pleistocene glaciers appears to be mainly responsible for the present shapes of many presently exposed structures. In the region, the glaciers have been formed several valleys along the relatively soft pre-Cambrian fault zones (e.g., Landsort trench in the Baltic Sea) as well as selectively removed the softer impactites and post-impact sediments. The removed clastic impact material, scoured from crater structures by the latest glaciation, is found to form extensive fans in Pleistocene tills south of, e.g., Neugrund (Suuroja and Suuroja 2000), Saaksjarvi (Papunen 1992), and Lappajarvi (Koivisto and Korhonen 1997).

Figure 5 represents the present configuration of a group of craters with similar starting structures, but different post-impact histories. In the sketch (a), an intact structure reminding the Ordovician Kardla structure (Puura and Suuroja 1992), penetrating into the basement through Middle Ordovician and Cambrian sediments, is given. An impact at sea formed a crater filled with allochthonous breccia and seawater (left side of the figure). Subsequently, laminated carbonate mud (layer I) and limestones (layers II and III) filled and covered the crater. Local specific hard limestones deposited on top and slopes of the elevated crystalline rim wall behaving as shoals at sea, whereas in the crater proper and its surroundings much softer clayey limestones accumulated (Ainsaar et al. 2002). During the Tertiary erosion the structure was almost uncovered. Pleistocene glaciers drumlinized the bedrock relief forming remnant highs composed of hard limestones covering the crystalline rim walls, and lows in-between and in surroundings (see Suuroja et al. 2002). The PL 2 is due to early post-impact partial destruction of the proximal ejecta layer from the elevated parts of the crater rim that formed an atoll-like feature in a shallow sea.

Sketch (b) represents a structure in which allochthonous impact breccias and post-impact deposits are survived in the crater depression only. These structures, pre-Cambrian in age, may be formed into (i) pure crystalline (Iso-Naakkima) or composite (pre-Cambrian crystalline rocks covered by Jotnian sediments; Paasselka, Janisjarvi, Lumparn, and Bjorko) target. These structures (PL = 5) were eroded either before the burial under Cambrian and Ordovician deposits, and/or afterwards, during the Tertiary unroofing, Quaternary glacial (incisions), and post-glacial erosion.

Analysing the structures formed in the basement before, in the course of, or after the formation of the Phanerozoic sedimentary cover, different, but not historically predicted relationships between the impact and postimpact lithological units may be presented. The most characteristic cases of craters are presented in Fig. 6. For the present platform areas, craters survived (almost) intact (a; PL = 1 - 2) or influenced by erosion (b, PL = 3 - 7) are sketched. The PL of these buried craters should have been reached before the covering platform sedimentation started. In the present shield area (Fig. 6 c-e, g), the crater structures may have been formed either directly in the crystalline basement (e.g. Suvasvesi N & S) or in the

Fig. 6. Most typical types of craters penetrating into basement, subsequently buried under Palaeozoic or other sedimentary rocks in platform areas (a and b), and under Quaternary sediments in shield areas (c - g). a and b indicate craters buried under sedimentary rocks, whereas the structure (a) was not and (b) was previously eroded, (c - f) illustrate craters hollowed and incised due to Quaternary glacial erosion and recently buried under Quaternary sediments and waterbodies (lakes or bays): (c) includes resistant melt-rock bearing interior, (d) includes Phanerozoic post-impact rocks, (e) indicates deeply eroded impact structure with brecciated bottom. Cases (f) and (g) represent deeply eroded impact structures, where allochthonous breccias are totally removed whereas the crater-like structure has survived (f) or not (g).

Fig. 6. Most typical types of craters penetrating into basement, subsequently buried under Palaeozoic or other sedimentary rocks in platform areas (a and b), and under Quaternary sediments in shield areas (c - g). a and b indicate craters buried under sedimentary rocks, whereas the structure (a) was not and (b) was previously eroded, (c - f) illustrate craters hollowed and incised due to Quaternary glacial erosion and recently buried under Quaternary sediments and waterbodies (lakes or bays): (c) includes resistant melt-rock bearing interior, (d) includes Phanerozoic post-impact rocks, (e) indicates deeply eroded impact structure with brecciated bottom. Cases (f) and (g) represent deeply eroded impact structures, where allochthonous breccias are totally removed whereas the crater-like structure has survived (f) or not (g).

past composite target. The most common are craters partly hollowed, incised, and filled by impactites with (Fig. 6 c, e; e.g., Jänisjärvi, Sääksjärvi, Mien, Dellen, Lappajärvi) or without melt rocks (Iso-Naakkima, Paasselkä, Ävike-bukten, Granby, Hummeln). All those may be filled with post-impact Phanerozoic sedimentary deposits, but are partially or completely covered with Quaternary deposits and water. Impact structures that are eroded, subsequently filled with Palaeozoic sediments, partially incised during glaciations, and flooded by sea are considerably rare (Fig. 6 d; Lumparn). Siljan is the only proved structure, where the ejecta and within-crater allochthonous breccias are completely destroyed. In Siljan, the down-lifted outer moat is partially incised and filled with water, and the structure is partially covered with Quaternary deposits (Fig. 6 f). Impact structures without distinct morphological signatures, with all the superstructure and crater fill completely destroyed (Fig. 6 g, PL > 7), but with brecciated and fractured sub-crater basement are not yet recognized in the region.

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