GEOGRAPHIC DISTRIBUTION 
OF AUSTRALASIAN TEKTITES

Written By Aubrey Whymark 2018

Overview of Australasian tektite distribution

Tektites are restricted to distinct strewn fields, each strewn field relating to an individual impact event. The size and shape of the strewn field will be dependent on the magnitude and obliquity of the impact event. Optimal tektite production is achieved by impacts at 30 degrees to the horizontal (Artemieva & Pierazzo, 2003), so it is not surprising that all known tektite strewn fields are asymmetrical. The Australasian tektite strewn field is perhaps the most asymmetrical of all the strewn fields, resulting in all possible terrestrial tektite morphologies being formed.

Other tektite strewn fields are valuable for comparison as, unlike the Australasian strewn field, they have associated impact craters. The North American tektites are derived from the 85 km diameter (40 km diameter estimated post-impact size) Chesapeake impact crater. Proximal forms (georgiaites) are found in Georgia, whilst medial forms (bediasites) are found in Texas.  The Central European tektites are all proximal forms and are derived from the 24 km diameter Ries impact crater in Germany and are predominantly found in the Czech Republic, but also in Germany and Austria. The Ivory Coast tektites, again all proximal forms, are derived from the 10.5 km diameter Bosumtwi crater in Ghana and are found in the adjacent Ivory Coast, with microtektites extending into the Atlantic Ocean.

By this comparison one can observe that the indochinite tektites are proximal to the impact site, morphologically equivalent to georgiaites, moldavites and Ivory Coast tektites. Philippinites occur at medial distances, morphologically equivalent to the bediasites. No comparisons can be made for distal Australasian tektites, but it is easily inferred that these formed at higher velocities and, as such, greater distances from the impact site. This forms a continuous morphological sequence as one moves away from the crater.

If the distal ejecta pattern (distal ejecta encompassing all tektites, which in turn are sub-divided into proximal, medial and distal) is taken as a whole, it can be broken down into two related sections. Firstly one can investigate the overall pattern of the distal ejecta. Secondly one can investigate the pattern within the pattern i.e. the radial (and concentric) rays of concentrated ejected material.

All rays lead to the crater

Just as all roads lead to Rome, all rays lead to the crater. There are a few prominent rays - at least 3 rays in Australia, the most prominent of which is the Victorian-Tasmanian Ray. There is also a well developed ray in the Philippines known as the Manila-Bicol Ray. 
 

The Australasian ‘butterfly’ burst pattern

In discussing the overall distal ejecta pattern, Gault and Wedekind (1978) state that impacts ‘below 45 degrees exaggerate the preceding effects and lead to highly asymmetric ejecta deposits displaying “forbidden” azimuthal zones’. Decreasing the incidence below 45 degrees, a forbidden zone first appears down-range of the crater and then at shallower incidences a second zone appears up-range, both extending from the rims with bi-lateral symmetry about the path of the projectile trajectory. Generally, it appears that the two ‘butterfly’ rays to the sides move to become more perpendicular to the impactor trajectory with increasing obliquity of impact. At the same time, the forward projecting up-range lobe becomes more focused with obliquity.

Experiment

Get a flat board and place it under a fast running tap. Study how water is deflected depending on the difference in angle of the board and the water impacting the board.
 
With reference to the above image, in the Australasian impact event very prominent forbidden zones are apparent down-range and, to some degree, up-range also. The centre of the most prominent ‘butterfly’ rays are around 62 degrees to either side of the impact trajectory, with the forward projecting up-range lobe focused to around 30 degrees (15 degrees either side of the impact trajectory). Despite there being many variables present, such as target material composition and water depth at the Australasian impact site, sufficient is known to probably make reasonably accurate assumptions. Although beyond the realm of this book, it seems that the Australasian event should be mathematically modelled to accurately calculate the impact angle, velocity and crater morphology.

It should be theoretically possible to approximately calculate the impact angle from the angle of the down-range focal rays and probably also from the angle of the butterfly rays. The author is not aware of any formula, perhaps due to the number of variables. It is very clear, however, that with increasing obliquity the down-range rays and the lateral butterfly rays have a narrower angle, becoming more focused.  

From the available evidence, the author is willing to make the following comments. The impactor angle was almost certainly between 10 and 30 degrees, more probably between 15 and 25 degrees. Optimal tektite production favours the upper end of this range, whilst observations in distal tektites may favour the lower end of this range. The impactor came from the north-northwest along a trajectory of around 164 degrees.

It is assumed that the projectile impacted into rapidly deposited argillaceous sediment, most likely in a shallow shelfal sea of probably 25-100 metres water depth. The resultant crater is most likely close to circular, but probably slightly elliptical in detail with an atypical central peak and a higher rim on the down-range side. The crater wall is likely steeper on the up-range side compared to the down-range side (based on Gault & Wedekind, 1978). This off-perfect circular arrangement may be a reason why the crater has yet to be identified on seismic data. One must also consider soft sediment deformation. Deformation of surrounding, relatively uncompacted water bearing sediment, may differ somewhat from a typical crater. The unproven and somewhat controversial Silverpit crater in the North Sea may be an example of atypical cratering in soft sediment.