Apparent Cratering Rates

From the analyses of < 120 Ma craters, Grieve and Pesonen (1996) postulated that the size-frequency distribution of recognised terrestrial craters principally corresponds to the power law approximation Nc x D-2, where Nc is cumulative number and D diameter of impact structures. According to their analysis the estimate of the cratering rate is 5.5 ± 2.7 x 10-15 km-2 a-1 for D > 20 km, but there occurs a loss of small-size craters. In the SCD the occurrence of only two impact structures (Siljan and Lappajarvi) with D > 20 km is proved. Therefore, the Phanerozoic cratering rate for D > 20 km is as small as 1.2 x 10-15 km-2 a-1. Grieve and Pesonen (1992) have argued that the geological effects of impacts at simple structures (D < 2 -4 km) are visible to a depth of <1/3 the final rim diameter, whereas the depth / diameter ratio is shallower but the absolute depth are often greater. Applying these rules to the SCD show that simple craters may locally have not been penetrating the sedimentary cover because the thickness of sedimentary formations in the region has dominantly been between 0.5 and 3 km (Puura et al. 1996). The larger impacts with final diameter of >10 km have always truncated both the sedimentary cover (if was present at all) and the basement. Generally due (a) to erosion and final destruction of many impact structures and (b) probable existence of many unknown impact structures, hidden under surficial deposits in the platform areas and seabed, we are far from the determination of the real cratering rate in the 1.9 Ga history and frames of the SCD. Therefore, we discuss the apparent cratering rates (ACR) in the SCD and its regions for different epochs of its history.

Fig. 7. Apparent cratering rate of meteorite impact structures at the Svecofennian Crustal Domain by age. Solid line represents the total rates; dotted (dashed) line illustrates the rate at shield (platform) area. Abbreviations: CZ, Cenozoic; MZ, Mesozoic, PZ2, late Palaeozoic (Devonian - Permian); PZj, early Palaeozoic (Cambrian - Silurian); NP, Neoproterozoic; MP, Mesoproterozoic.

Fig. 7. Apparent cratering rate of meteorite impact structures at the Svecofennian Crustal Domain by age. Solid line represents the total rates; dotted (dashed) line illustrates the rate at shield (platform) area. Abbreviations: CZ, Cenozoic; MZ, Mesozoic, PZ2, late Palaeozoic (Devonian - Permian); PZj, early Palaeozoic (Cambrian - Silurian); NP, Neoproterozoic; MP, Mesoproterozoic.

Abels et al. (2002) demonstrated an uneven distribution of Scandinavian crater ages through Mesoproterozoic to Phanerozoic history. Figure 7 presents time- and location-depended rates of cratering in the SCD, whereas proven structures >1 km in diameter are taken into account. The average cratering rates for the 1.56 x 106 km2 SCD and those for shield and platform (0.88 x 106 and 0.68 x 106 km2, respectively) areas are calculated for six time intervals from Mesoproterozoic to Cenozoic.

According to the data given in Table 1 and Fig. 7, the observed ACR of structures with D > 1 km (Table 1, Fig. 7) within the SCD ranges within 10-15 -10-14 for Proterozoic, and 10-14 - 10-13 for Phanerozoic structures. The Mesoproterozoic ACR (1.07 x 10-15 km-2 a-1) is minimum based on one find (Iso-Naakkima) only. The Neoproterozoic ACR is higher (4.18 x 10-15 km-2 a-1), but still lower than the average Phanerozoic ACR (2.02 x 10-14 km-2 a-1). The maximum ACR (4.44 x 10-14 km-2 a-1) comes from Early Palaeozoic (from Cambrian to Silurian). There exists decreasing trend of observed cratering rates from Early Palaeozoic to Cenozoic (Fig. 7).

The average rates in the shield area are much higher than in platform area. To explain it, two factors have to be discussed: a) complete or partial destruction due to erosion and b) hiding under sedimentary cover (or seawater). As described above, erosion of soft sedimentary cover was essential and often recurrent process in many places of the SCD. Due to restricted vertical extent of craters they could not survive erosion of host sedimentary rocks. From another side, in cases of destruction of super- and infrastructure of craters down to the sub-crater levels, still survived impact-induced deformations are still overlooked. Burial is certainly another essential factor of observed cratering rates in sedimentary areas.

The observed Proterozoic cratering record is incomplete. The low Mesoproterozoic rates evidences destruction of impact structures in the shield area. In the platform area, the eroded sub-crater roots are deeply buried, and, thus, search is even more complicated.

Over a half of observed Phanerozoic craters are Cambrian to Ordovician in age indicating favourable conditions for survival of impact structures. Most of these impacts took place in shelf seas or close to the coast, and were rapidly buried under protecting sediments. Also, increased accretion rates of meteorites and cosmic dust for the Lower Ordovician is supposed (see Schmitz et al. 2001). Still, the observed cratering rate is probably less than the real rate during Phanerozoic as the rate of observed craters in the shield area is more than two-fold higher than that in platform area.

The observed Late Palaeozoic (since Silurian), Mesozoic and Cenozoic ACRs are three- to six-fold lower than in Early Palaeozoic. We suppose that beside of latest erosion of the basement in the shield area, the main reason of the low rate is erosion of largely distributed Devonian cover during the Tertiary. The hidden Devonian craters exist only in platform area with survived thick Devonian deposits.

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