PostGa Extraterrestrial Impacts

Continuous accretion of cosmic dust accretion from about 3.8 Ga, estimated from deep ocean pelagic sediment cores at the rate of 3 x 104 ton year-1 (Kyte, 2002), contributed a small fraction of about 0.2 x 10-7 of the Earth mass. The contribution of meteorites, asteroids, and comets can be roughly estimated from the impact frequency/size distribution (Fig. 8.1). Episodic heavy bombardment did not terminate with the LHB, but continued throughout Earth-Moon history at rates of 4.3 ± 0.4 x 10-15km-2 year-1 (for craters >20 km), i.e., two orders of magnitude less than during the LHB (Shoemaker

Fig. 8.1. Crater size vs. cumulative frequency plots for post-LHB times in Earth-Moon system. Moon post-LHB - post-LHB lunar maria craters and post-Martian plains craters (after Barlow, 1990). NEA - crater distribution extrapolated from observed Near-Earth Asteroids (Dc = 20DP). PHANEROZOIC - Phanerozoic impact rates (Grieve and Dence, 1979), showing loss of smaller craters. EARTH POST-LHB - average Earth cratering rate based on Table 8.1 and extrapolated to entire Earth surface for 20-km-diameter craters and cumulative crater vs. size-frequency relationships equivalent to lunar impact incidence rates; CONTINENTS - mean cratering rate on time-integrated continental crust (~20% of Earth's surface). OCEANS -mean cratering rate on time-integrated ocean crust (~80% of Earth's surface). E-LHB - Late heavy bombardment of the Earth, extrapolated from lunar data of Barlow (1990).

Fig. 8.1. Crater size vs. cumulative frequency plots for post-LHB times in Earth-Moon system. Moon post-LHB - post-LHB lunar maria craters and post-Martian plains craters (after Barlow, 1990). NEA - crater distribution extrapolated from observed Near-Earth Asteroids (Dc = 20DP). PHANEROZOIC - Phanerozoic impact rates (Grieve and Dence, 1979), showing loss of smaller craters. EARTH POST-LHB - average Earth cratering rate based on Table 8.1 and extrapolated to entire Earth surface for 20-km-diameter craters and cumulative crater vs. size-frequency relationships equivalent to lunar impact incidence rates; CONTINENTS - mean cratering rate on time-integrated continental crust (~20% of Earth's surface). OCEANS -mean cratering rate on time-integrated ocean crust (~80% of Earth's surface). E-LHB - Late heavy bombardment of the Earth, extrapolated from lunar data of Barlow (1990).

and Shoemaker, 1996) (Fig. 8.1). This agrees with the cratering rate inferred from the near-Earth asteroid (NEA) flux of 5.9 ± 3.5 x 10~15km-2 year-1 for asteroids >1 km. Based on these estimates, the impact incidence post-3.8-Ga involves some 390±36 craters > 100-km-large and 45±4 craters >250-km-large (Table 8.1). Due to Earth-Moon gravity cross section, it is likely that the mean impact incidence exceeded these estimates by x1.4. Based on the dominantly mafic/ultramafic composition and the lack of shocked quartz grains in Archaean and early Proterozoic impact ejecta (Simonson et al., 1998; Glikson and Allen, 2004), the majority of these impacts occurred in simatic/oceanic regions of the Earth. This conclusion is in agreement with estimates of the growth of continental crust with time based on rare earth elements (Taylor and McLennan, 1981) and Sm-Nd isotopes (McCulloch and Bennett, 1994).

That the projected impacts did not occur as a continuous or random flux is suggested by the clustering of impacts on the Earth and Moon (Fig. 8.2 A-E), corroborated in Precambrian terrains by impact fallout units (Table 8.2) and in Phanerozoic terrains by both impact craters and ejecta fallout units (Table 8.3). Established impact clusters occur at the K-T boundary (Chicxu-lub, Boltysh) and late Eocene (Popigai, Cheasapeake Bay). Possible, though

Table 8.1. Postterrestrial impact rates and estimated minimum numbers of craters on several crater diameter scales.

Crater diameters"

Dc > 20b

Dc > 100b

Dc > 250b

Dc > 500b

Dc > 1, 000b

[A]

8, 300 ± 770

390 ± 36

45 ± 4

17 ± 1.6

4 ± 0.3

[B]

10, 630± 5200 460±225

54 ± 26

19 ± 9.3

5 ± 2.4

(1)

2, 020 ± 990

90 ± 44

11 ± 5.4

4 ± 2

1 ± 0.5

(2)

8,100 ± 3970

360 ± 17

42 ± 21

15 ± 7.3

4 ± 2

(3)

40

6

2

NA

NA

(4)

30

10

NA

NA

(5)

0.38

1.3

3.8

NA

NA

(6)

1.9

6.6

18.0

NA

NA

Based on projected cratering incidence for craters Dc > 20 and empirically observed cumulative crater and asteroid size frequency plots N x D-1'8 (Shoemaker and Shoemaker, 1996). Estimated crater numbers are rounded; [A] Terrestrial cratering rate equivalent to the lunar cratering rate of R = 4.3±0.4x 10-15km-2year-1 at ~3.2 Ga; asteroid impact velocities assumed (Shoemaker and Shoemaker, 1996). [B] Terrestrial cratering rate of 5.5 ± 2.7 x 10-15km-2year-1 for craters of Dc > 20 km (Grieve and Pesonen, 1996); (1) predicted number of craters formed on continental crust based on a mean cratering rate; (2) predicted number of craters formed on oceanic crust based on a mean cratering rate; (3) number of actually observed continental craters; (4) absolute minimum number of craters deduced for the entire Earth as based on 3; (5) percent observed to predicted craters for entire Earth surface; (6) percent observed to predicted craters for a time-integrated continental crust area.

aEstimated cratering rate R x 10-15 km-2 year-1 craters with Dc > 20 km. ^Estimated number of craters of size > Dc (in km) formed < 3.8 Ga from the relation N x D-1'8. (N, number of craters; Dc, diameter).

Table 8.2. Precambrian impact fallout units and associated tsunami deposits.

Formation and impact unit Symbol symbol

Province Age (Ga) Number of units

Apex Basalt

Hoogenoeg Formation

Basal Fig Tree S2, S3, S4 Formation

Top Jeerinah Formation

ACM -1 Central Pilbara Craton, 3470.1±1.9

ACM-2 Western Australia Ma

HF Barberton greenstone 3470.4±2.3

belt, Transvaal, South Ma Africa

Barberton greenstone ~3.24-3.225 belt, Transvaal, South Africa

JIL Pilbara Craton, >2.629±5 Western Australia

Monteville MONT Griqualand West

Ghaap Basin, Kaapvaal

Formation Craton, Transvaal,

South Africa

Bee Gorge Member, Wittenoom Formation

Carawine Dolomite

SMB-1 SMB-2

SBMB

Hamersley Basin, 2561±8 northern Western Australia

East Hamersley Basin, ?~2.56 northern Western Australia

Dales Gorge S4 Macroband

DGS4 Hamersley Basin, northern Western Australia

Base Kuruman Iron Formation

KIF Kaapvaal Craton

Transvaal, South Africa

Graenseso KET

Formation,

Ketilidian

Orogen

Bunyeroo Ejecta BUN

Southern Greenland ~ 1.8-2.1

Flinders Ranges, South 0.58 Australia

Abbreviations: WA, Western Australia; WTR, Western Transvaal; BTR, Barberton Mountain Land, east Transvaal; SA, South Australia.

8 Extraterrestrial Impact Episodes and Archaean 259 Table 8.3. Listing of Phanerozoic impact clusters and possible impact clusters.

Impact structure and fallout

A. late Eocene cluster

B. late Cretaceous cluster

Early Danian —64.9

Chicxulub 180-280 64.98 ± 0.05

C. Cretaceous split projectiles

Tookoonooka,Queensland 55 128 ± 5

Tallunidilli, Queensland 30 128 ± 5

D. late Jurassic possible cluster

Mjolnir 40 143 ± 20

D. late Triassic possible cluster

Manicouagan 100 214 ± 1

Saint Martin 40 220 ± 32

Rochechouart 23 214 ± 8

E. Late Carboniferous split projectile

Cleawater East, Quebec 26 290 ± 20

Clearwater West, Quebec 36 290 ± 20

F. late Devonian possible cluster

Woodleigh, Western Australia 120 359 ± 4

Charlevoix, Quebec 54 42 ± 15

G. late Ordovician possible cluster

Calvin, Michigan 8.5 450 ± 10

Ames, Oklahoma 16 470 ± 30

Slate Island, Ontario —30 450

Based on crater and impact ejecta fallout data after [email protected] and Glikson (1996, 2001).

Fig. 8.2A,B. (A) Frequency distribution of U-Pb zircon ages of Archaean igneous rocks in the Pilbara Craton, Western Australia (based on data in Van Kranendonk et al., 2002); (B) Frequency distribution of U-Pb zircon ages of Archaean igneous rocks in the Kaapvaal Craton, South Africa (from Poujol et al., 2003). Asteroid impact ages are marked within frames.

Fig. 8.2A,B. (A) Frequency distribution of U-Pb zircon ages of Archaean igneous rocks in the Pilbara Craton, Western Australia (based on data in Van Kranendonk et al., 2002); (B) Frequency distribution of U-Pb zircon ages of Archaean igneous rocks in the Kaapvaal Craton, South Africa (from Poujol et al., 2003). Asteroid impact ages are marked within frames.

Fig. 8.2C,D. (C) Frequency distribution of Ar-Ar of lunar impact spherules after Culler et al. (2000); (D) Frequency distribution of Ar-Ar ages and Rb-Sr ages of mare baslts (after Basaltic Volcanism of the Terrestrial Planets, 1981).

yet subject to further isotopic age definitions, occur in the late Ordovician, late Devonian, late Triassic, and late Jurassic. Double craters, probably representing split projectiles, are recorded at Tookoonooka-Tallunidilli (Queensland), Kara-Kara-Ust (Ukraine) and Clearwater East-Clearwater West (Quebec) (Table 8.3). Precambrian impact fallout units include multiple impacts at 3.47 Ga (Pilbara and Kaapvaal cratons; Lowe et al., 2003; Byerly et al., 2002; Glikson, 1993, 1996, 1999, 2001; Glikson et al., 2004) and 2.56 Ga in the Hamersley Basin (Glikson, 2004) (Table 8.3). In evaluating the environmental and biological consequences of multiple impacts, the cumulative synergy effect of these events needs to be taken into account - the total effect being greater than the sum of individual events.

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