THE AUSTRALASIAN CRATER RAY SYSTEM

Written By Aubrey Whymark 2018
One can study the rays that emanate from the crater, within the confines of the ‘butterfly’ tri-lobal pattern. The length and angle of these rays will help us to fully understand the impact event in terms of impact angle and velocity. The geochemistry of these rays also has the possibility to unravel some aspects of the stratigraphy at the impact site and possibly identify large-scale lateral and vertical homogeneities in the target sediment, which could aid discovery of the crater. The distal tektites in any given ray would have formed from the shallowest sediment, whereas the proximal tektites on the same ray would form from the deepest excavated sediments.

A number of problems present themselves when studying impact rays on the Earth. On geologically dead planetary bodies the light coloured rays are unmistakable from aerial views, whereas on Earth they are not evident. On Earth, the ejecta pattern is often inaccessible as it is hidden in the oceans, buried or eroded away – even within the brief ~788,000 years since the Australasian event. Water transportation can also move bodies considerable distances over relatively short time periods (in the order of tens or even hundreds of kilometres). With land discoveries being recorded and the deep sea record being uncovered by the ocean drilling projects a reasonably accurate, although inevitably partial and slightly ‘blurred’, impression of the Australasian strewn field can be gained.

Further problems are derived from the fact that tektites may be erased from the geological record, particularly small delicate specimens such as microtektites and small australites. This occurs in a wet tropical or sub-tropical environment where effectively the tektite is dissolved away (discussed in subsequent chapter). The deserts of Australia represent the best chances of preservation.

Humans must also be able to find the tektites, so the chances of finding a specimen are increased where vegetation is decreased. Chances are increased in populated areas (although excessive urbanisation leads to a reduction in finds as the ground becomes inaccessible in cities). Notable increases in the chance of tektite recovery comes where there are ground works, most notably road construction and mining operations and more-so mining of fluvial deposits. Another problem is human transportation of finds and poor reporting of where they came from, often giving only a general area, district or province. This may be due to poor record keeping, as importance of provenance is not realised, or deliberate in order to hide a monetarily valuable locality or conversely to enhance the monetary value by giving a false (rarer) locality. These factors lead to patchy occurrence maps.

One must also consider the motion of the Earth or Coriolis effect. At the impact site the tektite may be projected directly forward or to one side by the force of the impact (I find it valuable to break a single motion into a directly forward force and a perpendicular ‘side’ force). When the tektite parent material was on the Earth it was rotating at a given velocity dependent on latitude. In the case of the Australasian event, the 18°N latitude has a velocity of around 1,588 km/hr, whereas at the equator, over which many tektites passed, the velocity is 1,670 km/hr. The tektites are slightly skewed to the east. Then for some australites landing at say 38°S in Victoria, the Earth is rotating at 1,316 km/hr. This will very slightly skew the landing site back to the west. Given the high velocity and low flight time of tektites the net effect is, however, minimal (10’s of kilometers) and can be largely ignored.

The idea of tektite ‘rays’ is not new, even at times when a lunar origin of tektites was firmly favored. Dunn (1912) and numerous subsequent authors, including Fenner (1935), observed the linear patterns in Australia. Beyer (1936) recorded ‘strips’ of philippinites aligned ENE-WSW. This may have been partially genuine, although more localised, small-scale, distribution may have been controlled by river gravel deposits given the water-worn appearance of most Metro Manila specimens. Tektites in Indochina are not evenly distributed and are known to occur in rays. Izokh and An (1983) reported that tektites occurred in wide belts across Vietnam, like those of Australia. Some of these rays reportedly comprise solely Muong Nong-type layered impact glasses. The overall arrangement in Indochina is somewhat disguised by poor detailed locality data, often simply disclosing whole provinces or districts. Trnka (pers. comm., 2013), who has done field work in Indochina suggests the distribution of tektites may be more homogenous in the proximal setting. He comments that some of the apparent rays may not be genuine. This may well be the case in the proximal environment, but certainly, at distance, strong evidence of rays exists.

There are a couple of ways one can pick rays. One is simply to note where tektites occur, where high abundances of tektites occur and where the largest tektites occur. Then simply draw a line along the highest abundance and largest specimens. Project the line towards the proximal tektites and it should cross the crater. We can do this with the Australian rays, most notably the Victorian-Tasmanian Ray. We can also do this with the Manila-Bicol Ray in the Philippines. This gives an approximate location of the crater. One can then attempt to infill the pattern - there may be a reasonably consistent angle of seperation of rays in a particular area of the field. This can be used to predict locations where you might expect higher abundances of tektites. This manual approach is, however, somewhat subjective.

A more scientific method to establish the crater location is to plot each and every tektite locality that is deemed reasonably accurate and then project this locality back to a proposed source. With regards the distal and medial ejecta one would expect to see a bilateral (or butterfly) symmetry. One can establish the crater location in the position where the highest degree of symmetry is achieved and where the greatest overlap of lines into rays is achieved. The more asymmetrical and the less overlap, the further you are from the source crater. With regards the proximal ejecta one expects to see a more radial symmetry (as the crater opened up). So one expects the proximal ejecta to be reasonably equi-distant from the crater. One must take into account, in doing this, that some areas are largely devoid of samples as they are oceanic. Therefore overlap of lines is probably the best guide.

Butterfly Symmetry

Distal ejecta from an oblique impact forms a distinctive pattern. In order to locate the crater position, find the point that develops the greatest bilateral symmetry. The last formed ejecta would be expected to form a radial pattern as the crater opens up.
 
Figure 0: Australasian tektite localities. Crater locality unknown.
Figure 1: Australasian tektite localities projected to proposed source crater in Stauffer, 1978. Plotted Latitude: 10.57° N, Longitude: 106.50° E.

Overlap

Ejecta forms rays - you can see them on any 'dead' planetary body like the Moon. The Earth is no different. Plot the tektite occurrences and then project a line to various possible crater localities. The point where you have the greatest degree of line overlap is the crater location. 
 
Figure 2: Australasian tektite localities projected to proposed source crater in Hartung & Rivolo, 1978, 1979; Ford, 1988. Plotted Latitude: 13.47° N, Longitude: 106.34° E.
Figure 3: Australasian tektite localities projected to proposed source crater in Walter, Schnetzler & Marsh, 1986. Plotted Latitude: 9.30° N, Longitude: 107.30° E.
Figure 4: Australasian tektite localities projected to proposed source crater in Walter, Schnetzler & Marsh, 1986. Plotted Latitude: 7.30° N, Longitude: 107.30° E.
Figure 5: Australasian tektite localities projected to proposed source crater in Walter, Schnetzler & Marsh, 1986. Plotted Latitude: 13° N, Longitude: 111° E.
Figure 6: Australasian tektite localities projected to proposed source crater in Schnetzler, Walter & Marsh, 1988. Plotted Latitude: 13.55° N, Longitude: 110.37° E.
Figure 7: Australasian tektite localities projected to proposed source crater in Burns & Glass, 1989. 4-5° N, 100-102° E. Plotted Latitude: 4.30° N, Longitude: 101° E.
Figure 8: Australasian tektite localities projected to proposed source crater in Hartung, 1990; Hartung & Koeberl, 1994. Plotted Latitude: 13° N, Longitude: 104.5° E.
Figure 9: Australasian tektite localities projected to proposed source crater in Schnetzler & Garvin, 1992. Plotted Latitude: 16.35° N,, Longitude: 106.15° E.
Figure 10: Australasian tektite localities projected to proposed source crater in Schnetzler & Garvin, 1992. Plotted Latitude: 16.55° N,, Longitude: 104.90° E.
Figure 11: Australasian tektite localities projected to proposed source crater in Schnetzler & Garvin, 1992. Plotted Latitude: 16.60° N,, Longitude: 105.50° E.
Figure 12: Australasian tektite localities projected to proposed source crater in Schnetzler, 1992. Plotted Latitude: 16° N,, Longitude: 105° E.
Figure 13: Australasian tektite localities projected to proposed source crater in Glass, 1993. Plotted Latitude: 11.5° N,, Longitude: 106° E.
Figure 14: Australasian tektite localities projected to proposed source crater in Schmidt & Wasson 1993. Plotted Latitude: 10°25'54.12"N,, Longitude: 105° 2'18.70"E.
Figure 15: Australasian tektite localities projected to proposed source crater in Glass & Pizzuto, 1994. Plotted Latitude: 12° N,, Longitude: 106° E.
Figure 16: Australasian tektite localities projected to proposed source crater in Hildebrand, Rencz & Graham, 1994. Plotted Latitude: 13.57° N,, Longitude: 106.32° E.
Figure 17: Australasian tektite localities projected to proposed source crater in Chaussidon & Koeberl, 1995. Plotted Latitude: 9.30° N,, Longitude: 107.30° E.
Figure 18: Australasian tektite localities projected to proposed source crater in Schnetzler & McHone 1996. Plotted Latitude: 16.35° N,, Longitude: 106.15° E.
Figure 19: Australasian tektite localities projected to proposed source crater in Schnetzler & McHone 1996. Plotted Latitude: 16.6° N,, Longitude: 105.5° E.
Figure 20: Australasian tektite localities projected to proposed source crater in Schnetzler & McHone 1996. Plotted Latitude: 16.55° N,, Longitude: 104.9° E.
Figure 21: Australasian tektite localities projected to proposed source crater in Schnetzler & McHone 1996. Plotted Latitude: 16.38° N,, Longitude: 105.05° E.
Figure 22: Australasian tektite localities projected to proposed source crater in Dass & Glass, 1999; Glass, 2000; Southern Laos or adjoining Thailand or Vietnam. Plotted Latitude:  16°32'29.94"N, Longitude: 105°48'49.23"E.
Figure 23: Australasian tektite localities projected to proposed source crater in Schnetzler, Fiske, Garvin & Frawley, 1999. Plotted Latitude:  13.6°N, Longitude: 110.5° E.
Figure 24: Australasian tektite localities projected to proposed source crater in Lee & Wei, 2000. Plotted Latitude:  12°N, Longitude: 106° E.
Figure 25: Australasian tektite localities projected to proposed source crater in Glass, 2003. Plotted Latitude:  15°N, Longitude: 105° E.
Figure 26: Australasian tektite localities projected to proposed source crater in Ma et al., 2001; Ma et al., 2004. Plotted Latitude:  17°N, Longitude: 107° E.
Figure 27: Australasian tektite localities projected to proposed source crater in Glass & Koeberl, 2006. Plotted Latitude:  22°N, Longitude: 104° E.
Figure 28: Australasian tektite localities projected to proposed source crater in Prasad, Mahale, Kodagali, 2007. Plotted Latitude:  18°N, Longitude: 104° E.
Figure 29: Australasian tektite localities projected to proposed source crater in Trnka et al., 2009. Plotted Latitude:  16°20' N, Longitude: 106°8' E.
Figure 30: Australasian tektite localities projected to proposed source crater in Whymark, 2013. Plotted Latitude:   17°45'20.00"N, Longitude: 107°50'30.00"E.
Figure 31: Australasian tektite localities projected to proposed source crater in Kenkmann et al., 2014. Plotted Latitude:   23°56'16.00"N, Longitude: 111°36'30.00"E.
So, have a look through the images above. You decide where you think the crater is, taking into account absence of data in many oceanic areas. In the medial (e.g. Philippinites, Billitonites) and distal settings (Australites) you are looking for bilateral, or butterfly symmetry. In the proximal (Indochinite) setting you are looking likely looking for a radial symmetry.

For me it is clear that the crater must lie in the Gulf of Tonkin or eastern-most part of Indochina (which can be excluded for a host of other reasons).