Precise chitinozoan dating of Ordovician impact events in Baltoscandia

The chitinozoan biostratigraphy of four Ordovician impact craters has been investigated. Three of these (Tvären, Kärdla and Lockne) contain complete sequences of early Caradoc age, while the Granby crater contains rocks of late Arenig age yielding two bentonitic horizons at their top. Chitinozoans, together with graptolites and other planktic organisms, were the first to invade the craters after the impact event. It has therefore been possible to date the impact events with a precision of less than one million years. An immigration of graptolites from Australia during the late Arenig corresponds to an immigration of chitinozoans from Gondwana at this time. Two stratigraphically important taxa, not previously described or discussed, Lagenochitina sp. A aff. capax and Laufeldochitina sp. A aff. striata are commented upon. Three species. Cyathochitina hunderumensis, Spinachitina tvaerenensis and Tartuchitina granbyensis, are described as new.


INTRODUCTION
Five impact events of supposed Ordovician age have been reported from ELaltoscandia (see Henkel & Pesonen, 1992). Four of these were recently dated by Grahn & Nalvak (1993). A crater below the bottom of the Lumparen Bay in the h a n d archipelago, between Sweden and the mainland of Finland, was for a long time considered to be of tectonic origin (see Winkrhalter 1982), but Merrill (1980) regarded it to be an Ordovician or younger impact structure. More recent investigations show that the Lumparen Bay structure probably i:j an impact crater that was already deeply eroded when Ordovician sedimentation commenced in the area during the early Arenig. The Lumparen event might be of early Cambrian age since sandstones of this age, reminiscent of resurge deposits, are distributed in the area (Hagenfeldt. pers. comm. 1993). The oldest Ordovician crater, of late Arenig age, is situated at Granby in south Sweden (Fig. 1). The three other craters, of early Caradoc age, occur at Tvaren and Lockne in central Sweden, and at Kardla on the Island of IIiiumaa in northwest Estonia (Fig. 1).
The maximum thickness of the Ordovician sequences outside the craters is less than 200 m in Sweden and Estonia, and stratigraphical gaps occur frequently through the sequence. In contrast to the sequences surrounding the craters, those within the craters are virtually complete. The crater sequences are therefore excellent models for ecological studies of restricted environments and for high-resolution biostratigraphy. The present paper is a detailed study of the chitinozoan biostratigraphy in the four Ordovician craters investigated. The illustrated specimens are deposited in the collections of the Institute of Geology, Rennes (IGR) under the numbers 58601-58646. The coordinates are those o f the England Finder grid.

CHITINOZOAN BIOSTRATIGRAPHY
NBlvak &  recently published a chitinozoan biozonation comprising 23 zones and subzones for the Ordovician of Baltoscandia. All eight crater sections investigated from or close to the four craters yielded the index species and/or characteristic species for three of these zones, which allowed a very precise dating. The biostratigraphy from the sections related to the four craters will be described below.

Granby crater
The Granby crater is situated in the subsurface about 4 km southeast of Vadstena, a little town on the east shore of Lake Vattern. It has been discussed by Bruun & Dahlman (1982) and Grahn & Nalvak (1993). Two cores from the crater were investigated, viz. from the Fylla 9 borehole about 1 km east of the west crater rim, and the Fylla 3 borehole, about 800m further to the east (Fig. 2). The latter borehole was made near the centre of the crater above the central uplift (Bruun & Dahlman, 1982). The impact sediments in the crater are overlain by calcareous and glauconitic mudstone interbedded by grey limestone. The impact event is placed in the lower part of the Cyuthochitina regnelli chitinozoan Zone, since a characteristic chitinozoan assemblage from the Conochitina cucumis chitinozoan Zone is missing (NBlvak & Grahn 1993). The index fossil has not been found (Grahn & NZilvak, 1993) Lagenochitina esthonica (Pl. 3, fig. 9). Two bentonitic horizons found in the Fylla 9 core (Fig. 3)  1972), Svalbard (Bockelie 1980) and eastern Canada (Achab, 1986). It should be noted that in the graptolite Zone of Didymograptus hirundo (late Arenig) there is also evidence of an immigration of graptolites from Australia into Baltoscandia (Nilsson, 1983).

Tvaren crater
The Tvaren crater is situated below the bottom of the Tvaren Bay, about 72 km south-southwest of Stockholm. It has been discussed by FlodCn et al. (1986) and Lindstrom et al. (1994). Two boreholes, Tvaren 1 and 2, were drilled in the structure (Fig. 5). The former borehole was placed in the crater rim, while the latter (Fig. 6)   . Luufeldochitina sp. A aff. striata was also found in a coarse breccia formed by the resurge turbidite immediately after the impact. This places the base of the Tvaren post-impact sequence, and the impact event, within beds corresponding to the lower Peetri Member of the Viivikonna Formation

Kardla crater
The Kardla crater is situated in the subsurface, just east of the Khrdla city, and on the north coast of the Island of Hiiumaa. It has been discussed by Puura & Suuroja (1992) and Grahn & NGIvak (1993). The chitinozoan biostratigraphy in two boreholes drilled in the structure has been investigated. Palukula 383 ( Fig. 8) is situated within the crater near the northeastern rim, and Mannamaa ( Fig. 9) about 20 km southwest of the crater (Fig. 7). The Palukiila 383 borehole terminated just above the first post-impact sediments, which consist of calcareous mudstone with rare limestone intercalations. Howcver, ejecta from the impact are spread over large areas in northwest Estonia. They consist of a quartz-rich limestone, known as the Kisuvere Member of the lower Tatruse Formation (PKlma ef al. 1988). This layer is present at 164.82-16493 m and has been dated in the Mannamaa -Tvaren event 0 E 20 rn borehole. It gives a precise dating of the first post-impact sedimentation in the crater, as well as the impact event (Grahn & NKlvak, 1993

Lockne crater
The Lockne crater is situated at Lake Lockne, about 20 km southeast of the town of Ostersund in the province of Jamtland. The crater has been described by Simon (1987), Lindstrorn & Sturkell (1992) and Grahn & Niilvak (1993). Three sections outside the west rim of the crater were investigated southwest of the village of Tandsbyn (Fig. 10). The first section, described by Thorslund (1940, fig. 21), is a railway-cut at Lappgrubban about 1.1 km southwest of the Tandsbyn church (Fig. 11). The second section is along the stream Ynntjarnsbacken, about 400 m east of Lake Ynntjarn (Fig. 12), and the third section is a railway-cut, described by Simon (1987;Fig. 23), situated about 400m west of Lake Ynntjarn (Fig. 13). All three localities exhibit the sandy resurge deposit locally known as 'Loftarstone', which is followed upwards by the first post-impact sediments, a limestone with interbedded calcareous shales. Locally the limestone contains patch reefs that grew on the crater rim. Characteristic of the chitinozoan fauna is the presence of the index species Lagenochitina dulbyensis (PI. 3, fig. 3) together with L.agenochitina sp. A aff. capax in two of the localities (Lappgrubban and Ynntjarnsbacken). This means that the Lockne event (Grahn & Niilvak, 1993), and the first post-impact sedimentation in the crater, took place in the early Caradoc, and correspond to the lower part of the L. dalbyensis chitinozoan Zone. The Lockne event may be coeval with the Kardla event, but most probably is slightly younger. The occurrence of Belonechitina hirsuta in the section at Ynntjarnsbacken (Fig. 12) suggests that beds younger than those corresponding to the L. dalbyensis Zone are also present at this locality.  fig. 1).

CONCLUSION
The immediate appearance of chitinozoans in the craters after the impact events, and their rapid evolution, have made it possible to achieve a high-resolution biostratigraphy of virtually complete sequences related to the impact craters. The planktic nature of the chitinozoophorans (Grahn, 1981) may also make it possible to date other types of natural hazards in marine environments (e.g. volcanic eruptions, earthquakes, etc.), from Ordovician through Devonian, sometimes with a precision of less than one million years. Chitinozoans occur in most lithologies, except for coarse sandstones, reef limestones, carbonate mounds and also marine redbeds and dolomites. The environments within the craters were restricted. The first Ordovician reefs known from Baltoscandia started to grow on the rim of the Lockne crater during early Caradoc (lower Idavere Stage) times (Fig. 14). The climate was obviously warm enough for the formation of reefs, and the absence of other contemporary reefs in Baltoscandia indicates that the seas were probably deeper (Lindstrom 1971) than generally believed earlier. (For a summary see Jaanusson, 1982.)

16-17.
Holotype. P1. 1, fig.1. IGR 58614 (L. 39/4). Type locality. Fylla 3 borehole, core sample at 254.30m, Granby crater, Ostergotland, south Sweden. Description. A small species of Cyathochitina. Vesicle smooth with a characteristic bell-like to conical shape. The maximum width is at the margin. The margin is provided with a short thickened carina. A basal scar is present. The neck is cylindrical, and shorter than half the total length. Aperture straight. Dimensions. The dimensions given by Grahn (1980, p. 27) are characteristic also for the specimens from the Granby crater. H e noted that also the main parts of the populations fall within the 1ength:width ratio 1.25-2:l. From the type level 30 flattened specimens were measured. A coefficient of 0.7 was used to restore the diameter of chamber and neck. The total length is 1.56-280 p m (holotype 233 p m , mean value 199 pm),   maximum wid.th 73-124 p m (holotype 123 p m , mean value 94 pm), width of neck 42-53 p m (holotype 48 p m , mean value 47,um) and the length of the neck 50-94pm, (holotype 9 0 p m , mean value 7")pm). The ratio of vesicle length/chamber diameter for specimens from the type level is shown in Fig. 15, and the ratio of chamber diameter/neck 'diameter in Fig. 16.
Discussion. The small C. hunderumensis sp. nov. are easily distinguished from typical specimens of Cyathochitinu campanulaeformis as defined by Eisenack (1931Eisenack ( , 1962    to the top of Kunda Stage (upper Arenig-lower Llanvirn). The species is known from Oland (Eisenack, 1976;Grahn, 1980) and Narke (Grahn, unpublished data), Sweden and from the Granby crater (this paper). It occurs also in North Estonia (reported but not illustrated from the Suhkrumagi section in Tallinn by Grahn 1984   close to the margin. The neck is cylindrical and indistinct with a straight aperture. Flexure and shoulder absent. The greatest width is at the margin, which is provided with about 20 robust short and simple conical processes, elongated parallel to the vesicle axis with their proximal end (insertion zone). A mucron is present. The vesicle wall is smooth aperturewards the margin.
achabae from the late Arenig of western France is probably a synonym to T. granbyensis sp. nov. They are of a similar length and the vesicle is frequently curved along its longitudinal axis. T. granbyensis sp. nov. may be confused with Rhabdochitina gracilis Eisenack when the carina is strongly eroded. The latter is also commonly curved along the long axis of the vesicle. Occurrence. Tanuchitina granbyensis sp. nov. has so far only been found in late Arenig strata in the Granby cratcr, Ostergotland, south Sweden. It is probably present   fig. 3) and with Lagenochitina deunffi Paris 1974. The stratigraphic range is identical ( L . capax is slightly younger than L. deunfi but they may overlap; Paris 1981), and it cannot be excluded that L. sp. A aff. capax is conspecific with one of these species. The size falls completely within the range of Lagenochitina deunffi, but the neck of L. deunfi is more narrow than that of L. sp. A aff. cupax. For a population of 40 flattened specimens (flattening corrected by a coefficient of 0.7) from 144.40 and 144.99-145.00m the total length is 88-122pm (mean value 103pm), maximum width 38-51 p m (mean value 44 p m ) , width of aperture 23-33 p m (mean value 3 0 p m ) , and length of the neck 13-22pm (mean value 16 pm). The ratio vesicle length/chamber diameter for specimens from level 144.40 and 144.99-145.00m is shown in Fig. 18, and the ratio vesicle length/neck length in Fig. 19 Mannil, 1986). Its occurrence in the Tvaren 2 borehole is the first safely established in Sweden. Remarks.. Cha.racteristic Luufcldochitinu sp. A aff. striuru differ from Laufeldochitirza striutu (Eisenack,1Y37) in possessing a predominantly smooth wall. A striate ornamentation restricted to the basalmost part may occur on some specimens (Pl. 2, fig. 9). The dimensions fall within tht: range of Laufeldochitinu striata.