contents

Basin, and are tilted about 10˚ or so to the northeast. The wholesequence is interrupted by a structure that can easily be seen fromspace. It is 30 km in diameter and the rocks are dropped down inpart of a ray-shaped depression; because they are buried under stillhigher strata, they do not appear dark in the illustration. The wholesequence is then turned on end in an inner collar of the structurewhich surrounds an uplift of Archaean rocks. The structure that wesee is therefore a ringed structural depression with a central uplift.That is the typical structural signature of a deeply eroded complexcrater; the crater itself has long since disappeared. The feature isabout 1.6 billion years old, whereas the whole region was tilted andflanked off perhaps 1.2 billion years ago. The structure has been ator near the surface of the crust in Australia since then. It is the oldestimpact feature yet recognized in Australia. The very complicatedstructure of the feature must be deciphered by finding the outcropsFig.3 — Oblique aerial view of the low-impact Spider structure showingbeds near the centre skidded successively and stacked in ridges along thrustfaults (Shoemaker & Shoemaker 1996).of older rocks. They are turned up more or less on end and are coveredwith folds, much like the folds created when one pulls a napkinthrough a napkin ring. They are also bifurcated by many faults. Allof these are diagnostic structural features.Figure 3 illustrates another kind of structure that Carolyn andI mapped in detail — the Spider structure in the Kimberley districtof Western Australia. In the centre there are ridges of quartzite rocks,three units of which have been duplicated along faults and acrossthe centre. Visible in the image is the central uplift, and it is alsopossible to see why it is called Spider since it looks much like a twentyleggedspider. On a geological map of the whole structure the spideris in the middle and is surrounded by a complex structural depression.It appears in this case that there was a weak impact from the north,so that the beds near the centre were skidded successively and stackedup as plates as they repeated along a whole series of thrust faults.The central uplift punched up through those plates. This was a lowimpactstructure.Shatter cones can be detected in the field for each of the structuresshown here. Shatter cones are beautiful structures visible only inimpacted rock. Figure 4 illustrates one found on the Spider structure,where they are located on the central uplift. Shatter cones have conicalsurfaces, or portions of conical surfaces, that are decorated withfantail or horsetail kinds of striations formed by shock. They can becreated by shock on a small scale in laboratory experiments; theyFig. 4 — Large shatter cones on the central uplift of the Spider structureare conical features with horsetail striations formed in nature only as a resultof the shock pressures generated by extraterrestial impact (Shoemaker &Shoemaker 1996).can also be found in craters formed by explosives, so we have a goodidea of the range of shock pressures needed to form them. Shattercones are diagnostic features that, when found in the field, providedirect evidence that the rock has been shocked. They are abundantin the central 2 km of the Spider structure, which measures about11 km 13 km across.Another crater that Carolyn and I have studied is the Lawn Hillstructure in northwestern Queensland. It has a central uplift surroundedby a structural moat that is concealed by limestone breccias. It wasa target that was already structurally complex and folded with middleProterozoic rocks when the crater was formed, after which materialeroded away. The structure was next covered by seawater for sometime, during which limestone rocks were laid down. The limestonesare Cambrian rocks about 500 million years in age; even today LawnHill is recognized as a crater-formed feature. There is a centraldepressed region with ranges surrounding it. At present, Lawn Hillis basically an exhumed crater about 20 km across.One of the larger structures in Australia is Lake Acraman — asalina. It is located on the Eyre Peninsula in South Australia. GeorgeWilliams, who discovered it (Williams 1994a,b), has described it asa very large crater; he believes that the outlying ring structures foundthere might define the original crater. The beds in the surroundingranges are basically flat lying and undeformed, with no shatter cones;Fig. 5 — Density of noble metals (iridium, gold, platinum, palladium) andchromium, another iron-loving metal, reveal elevated abundances in theLake Acraman fallout layer (Wallace et al.1990).300JRASC December/décembre 1998