Precise chitinozoan dating of Ordovician impact events in Baltoscandia

Abstract

-The chitinozoan biostratigraphy of four Ordovician impact craters has been investigated. Three of these (Tviiren, Kardla and Lockne) contain complete sequences of early Caradoc agc, 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 ycars. 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.

Chitinozoan biostratigraphy of the early Caradocian Lockne impact structure, Jämtland, Sweden

“…3) in the Tandsbyn Gully (Lindstrom et a/., 1996) ( i e . , West of Ynntjarn 1, Ynntjbsbacken 1 and Lappgrubban 1, now believed to be situated in an erosional channel through the outer crater (Tandsbyn Gully)) have previously been discussed by Grahn and NBlvak (1993) and Grahn et al (1996). In this study, four boreholes, the Lockne drillings DBH 1 to 4 in the central part of the Lockne impact structure (Fig.…”

“…12b) in samples of MAlingen I, and the absence of chitinozoan species from the underlying biozones, strengthen the arguments that the impact event occurred in the lower part of the L. dalbyensis zone. The finds of Laufeldochitina stenfor in boulders within the Lockne Breccia (basal resurge deposits) at Ynntjmsbacken 1 (Grahn et al, 1996) indicate that the youngest preimpact sediments hitherto found belong to the lower part of the Dalby Limestone. The sedimentary infill in the Lockne impact structure seems to have ceased at a time corresponding to the upper Belonechitina hirsuta Zone.…”

“…A section from the uppermost nodular and argillaceous Dalby Limestone, and the lowermost Or& Shale, was sampled at this locality (Fig. 5 ) (Grahn et al, 1996). The locality has been described or discussed by Thorslund (1940), Simon (1987), Grahn and N6lvak (1993) (Fig.…”

“…At this locality, the upper nodular and argillaceous Dalby Limestone overlies the Loftarstone. The sequence corresponds to the lower part of the Lagenochitina dalbyensis zone (Grahn et al, 1996). The locality has been LOCKNE DRILLING DBH 4 FIG.…”

“…At HBllnaset I , in the southern part of the impact structure, and within Bergbole-Loke Gully, the index species B. hirsuta (Fig. 12a) Thorslund (1940), Simon (1987, Grahn andNBlvak (l993), Sturkell et al (1994), andGrahn et al (1996).…”