Research School of Earth Science,

Australian National University

Canberra, ACT 0200

Evidence is accumulating that terrestrial crustal evolution has been

severely perturbed by post-Late Heavy Bombardment (LHB - post-3.8 Ga)

impact by large asteroids and comets, as predicted from the cratering flux

in the solar system and increasingly evidenced by the geological record.

The research program summarised below includes (1) a search for distal

impact fallout deposits in Archaean terrains, identified by microkrystite

spherule condensates from impact-released silicate vapour; (2)

mineralogical and geochemical relics of the LHB in early Archaean

sediments; (2) elucidation of the origin of the Permian-Triassic boundary

extinction, with reference to impact event/s; (3) study of Australian

multi-ring impact structures, notably the newly proven 120 km-diameter

Woodleigh impact structure, of possible Permian-Triassic or late Triassic

age, and (4) study of strewn crater fields - the late Eocene Carolyn crater

field, Timor Sea, possibly signifying a cometary breakdown event.

1. Early Archaean extra-terrestrial impact signatures

A search is in progress for impact fallout components in early Archaean

detrital sediments from the Western Gneiss Terrain of the Yilgarn Craton

(with D.R. Nelson), southwest Greenland (with A. Nutman), and the Slave

Province, Northwestern Territories, Canada (with W. Bleeker). Principal

search criteria include (1) occurrence of shocked quartz and zircon; (2)

Cr-rich chondritic spinels; (3) Ni-rich chromites (with high Co, V, Zn),

where Ni levels are one to two orders of magnitude higher than terrestrial

Ni-chromites associated with Ni-sulphides; (3) PGE anomalies showing

enrichment in refractory Ir and Ru and depletion in the volatile Pd and Au

- as contrasted with the volatile PGE-enriched terrestrial profiles,

excepting depleted mantle harzburgite.

Three 3.26-3.24 Ga-old units of spherulitic microkrystite condensates

identified at the base of the Fig Tree Group, Barberton Mountain Land,

eastern Transvaal, are characterised by marked PGE anomalies,

quench-textured and octahedral resorbed Ni-rich chromites, iridium

nano-nuggets, and negative 53Cr/52Cr ratios diagnostic of C1 chondrites.

These units define the oldest well-defined mega-impact cluster known in the

Archaean record to date. Scanning electron microscopy coupled with energy

dispersive spectrometry of Ni-chromites-bearing spherules (with G.R. Byerly

and D.R. Lowe) indicate internal Ni zonation of the chromites, to be

followed by ICPMS study of the PGE. Assuming global distribution of

fallout layers, mass balance calculations of Ir and Cr can be used to

estimate projectile composition and dimensions. For spherule unit S4 the

model suggests projectile diameter >30 km, consistent with estimates based

on recent vapour-liquid equilibria by J. Melosh and by O'Keefe and Aherns

(1982). Crater size scaling suggest a >600 km-diameter-large structure,

likely to have formed in a simatic crustal environment in view of the lack

of shocked quartz fragments, thus constituting an Archaean terrestrial mare

equivalent.

3.26-3.24 Ga events are well recorded in the Pilbara Craton of Western

Australia, including the Sulphur Springs Group which consists of komatiite,

andesite, dacite and chert interbeds. This sequence is overlain by a

yet-undated granitoid clast-bearing siltstone-banded ironstone sequence of

the Gorge Creek Group. This break and the contemporaneous break between

the Onverwacht Group (komatiite-tholeiite-felsic volcanic assemblage) and

Fig Tree Group (turbidite-felsic volcanic assemblage) in the Barberton

Mountain Land, representi fundamental changes from simatic to sialic

volcanic-sedimentary facies. The onset of rifting and exposure of

granitoids following the impacts suggest strong vertical tectonic

movements, with implications for the potential consequences of Archaean

mega-impacts. Modelling of the effects of very large impacts on thin

thermally active oceanic crust overlying shallow asthenosphere predict

development of propagting lithospheric faults and regional to global

magmatic and tectonic effects. The post-LHB impact flux indicated by lunar

cratering data and the modern asteroid and comet flux underpin the

significance of these events for crustal evolution.

2. Woodleigh structure, Carnarvon Basin, Western Australia: a 120

km-diameter multi-ring impact basin of Late Triassic or Permian-Triassic

boundary age

The relations between the extinction of ssome 85 percent of species

(trilobites, rugose and tabulate corals) at 251 Ma, Siberian Norilsk flood

volcanism (248.4+/-2.4 Ma), contemporaneous oceanic anoxia, and observed

extraterrestrial impacts (c. 250 Ma-old 40 km-diameter Araguinha crater,

Brazil; shocked quartz fragments in sediments from Antarctic and New South

Wales) poses an enigma. Candidate mega-impacts based on geophysical

anomalies in the Falkland Plateau (M.Rampino) and Bedout rise, Northwest

Australian shelf (J. Gorter), remain unconfirmed. The discovery of the 120

km-diameter large Woodleigh multi-ring impact structure, south Carnarvon

Basin, Western Australia, provides a potential resolution of this problem.

In view of its size, it is likely that Woodleigh, by analogy with other

mega-impacts (Manicouagan - late Triassic; Morokweng - J-K boundary;

Chicxulub - K-T boundary; Popigai and Chesapeake Bay - late Eocene) was

associated with extinction. The pre-lower Jurassic post mid-Permian age of

Woodleigh therefore suggest it may be of either late Triassic or of

Permian-Triassic boundary age. A hint supporting the latter possibility

may be provided by a thermal high at 280-250 Ma indicated by apatite

fission tracks.

Following a suggested impact origin (Mory and Iasky, 1998; Glikson, RSES

Annual Report for 1998), Woodleigh has been proven by drilling into (1) the

~25 km-diameter central granitoid uplift, and (2) breccia-bearing >600

m-deep rim syncline. Petrological, SEM, EDS, and Laser Raman (T.P.

Mernagh) studies of samples from the shocked granitoid core demonstrate

planar deformation features (PDF) in quartz and feldspar, microbrecciation,

pseudotachylite veining, and pervasive vitrification of feldspars.

Pseudotachylite veins contain highly refractory micron-scale comminuted

breccia and (?) glass enriched in the refractory elements (Mg, Al, Ca) and

depleted in relatively volatile elements (K, Si) (Fig. X). Experimental

laser fusion/volatilisation studies (Yu.P. Dikov) and relations between

chemical fractionation and boiling points militate for a role of

shock-induced volatilisation. High Mg levels (<3.0% MgO) in glass

inclusions within MgO-free feldspars remain unexplained. (with A.J. Mory,

R.P. Iasky, F. Pirajno, T.P. Mernagh).

ICPMS analyses of shocked pseudotachylite-injected granitoid samples

indicate that, while the lithophile and chalcophile trace metals (Pb, Sn,

Wo, Mo, Bi, Ag, Sb, Zn, Cu, Au) vary by factors in the range of X0.1 to

X10.0 relative to average granite, the siderophile trace metals are

strongly enriched, including V (< 390 ppm; X6-20AG), Cr (< 125 ppm; X30AG),

Co (< 48 ppm; X20-50AG), and Ni (< 66 ppm; X70-120AG). These enrichments

are consistent with EDS analysed MgO levels of < 3.0 % in the

pseudotachylites. Ni/Cr ratios fall in the range 0.12 - 0.53 and are more

similar to typical mantle pyrolite values (Ni/Cr ~ 0.75) than chondritic

values (Ni/Cr ~ 4.0). Ni sulphide collection ICPMS analysis of the PGE

suggests Ni/Ir values in the range of 17 000 - 33 000, more similar to

chondrites (Ni/Ir ~ 23 000) than to mantle pyrolite (Ni/Ir ~ 600 000). The

evidence tentatively suggests the introduction of a chondrite-contaminated

component through volatile condensation and melt transport, represented by

the pseudotachylites. Selective volatilisation of the projectile and mixing

with granitoid components may not allow direct identification of original

meteoritic chemical parameters.

3. The Carolyn crater field: Asteroid breakdown or SL9-type cometary

fragmentation?

Seismic reflection array studies of the North Bonaparte Basin, Timor Sea,

disclose an ENE-striking 120x25 km-large swathe of more than 40 circular

features excavated in the pre-Miocene - post-Eocene erosional surface and

buried by Langhian sediments. The larger craters include central uplifts,

circular troughs and raised rims, whereas smaller circular features (Dc <

2.0 km) consist of crater-form and bulge-form structures. An impact origin

of many of these craters is suggested by their similarity to Fohn impact

structure, located in the north-eastern part of the crater field, where 350

meter-thick PGE-rich melt breccia lens was drilled within the ring

syncline. The smaller circular features are interpreted in terms of deeper

erosion of analogous structures. Limits on the degree of erosion are

allowed at Fohn impact structure from stratigraphic and morphometric

estimates. The Timor Sea crater field may alternatively represent (1)

high preservation rate of the 37.5-24 Ma impact flux, 2) an atmospheric

breakup of a low-angle asteroid, or (3) a cometary fragmentation event

similar to the Shoemaker-Levy-9 event on Jupiter.

Publications

Glikson, A.Y., 1999. Oceanic mega-impacts and crustal evolution, Geology,

27:387-390.

Gorter, J.D. and Glikson, A.Y., 1999. Origin of a late-Eocene to

pre-Miocene buried crater and breccia lens at Fohn-1, North Bonaparte

Basin, Timor Sea: a probable extraterresrial connection. Meteoritics and

Planetary Science (in press).

Glikson, A.Y., 1999. Can shatter cones and penetrative deformation lamella

form due to explosive volcanic processes? A discussion of "Argument

supporting explosive igneous activity for the origin of "cryptoexplosion"

structures in the mid-continent, United States. Geology, March, 1999.

Glikson, A.Y., 2000. Spherulitic microkrystites and early terrestrial

maria. The Australian Geologist (in press).

Gorter, J.D. and Glikson, A.Y., A terrestrial analogue of the

Shoemaker-Levy-9 comet fragmentation event: the late Eocene - pre-Miocene

strewn crater field, Timor Sea, northern Australia. Science (in press).

Mory, A.J, Iaski, R., and Glikson, A.Y. The Woodleigh structure, Carnarvon

Basin, Western Australia: a multi-ring impact structure of 120 km diameter.

Earth Planetary Science Letters (in press).