SEARCH FOR THE AUSTRALASIAN TEKTITE SOURCE CRATER: MAP OVERLAY PROJECT

Written By Aubrey Whymark 2018
One of the biggest tektite questions these days is 'Where is the Australsian tektite source crater?' It's the largest known strewnfield and youngest of the large strewnfields. This presents a bit of a problem. Fortunately there have been many studies of geochemical isoconcentrations, microtektite and macrotektite regressions, morphological distribution, inclusion distribution, etc. Each study will point to a general area. 

So now we have a big pile of data. How do we make sense of this? One method may be map overlay. McHarg (1971) was one of the pioneers of the Map Overlay Concept. In order to establish the best route to build a new road he overlain maps of various features such as surface drainage, slope map, bedrock foundation, soil foundation, soil drainage, susceptibility to erosion, historic value, land values, recreational values, residential values, scenic values, water values, wildlife values, etc. By overlaying these maps into a composite map the sensible route for the road becomes apparent.

In the same way we can overlay maps of where the crater may be based on suitable geology, sediment thickness, isochemical concentrations, microtektite regression, morphological distribution, etc. Now some of the data ends up being conflicting, and maybe you can reject some pieces of data based on other information (I have not done this). But regardless, what you end up with is a 'heat map' where the darkest colours represent the most likely position of the impact crater and the lighter colours represent increasingly less probable positions for the crater. If you like, you can then apply some logic such as 'if there was a 40 km diameter crater on land in this area then we'd know about / it would be a big lake, thus removing these areas. This is a start - it says 'concentrate your efforts in these areas'.

Overlay Mapping

McHarg (1971) was one of the pioneers of overlay mapping. He used it to work out were to position a road. Here we use it to determine the most probable crater location.
 

The Study Area

Whilst some data falls outside of the mapped area, the vast majority of data is focused on the eastern Indochinese area in SE Asia. 
ABOVE: SE Asia, focused on the Indochinese area. Base map from Geo-MapApp (http://www.geomapapp.org).
ABOVE: The Indochinese area chosen for the base maps. Base map from Geo-MapApp (http://www.geomapapp.org).

The Final Composite Map

Composite Maps

By combining all the data in one place, ven though some is contradictory (and therefore some must be wrong), we create a 'heat' map of the most probable crater location.
 
ABOVE: By combining all the data, the most likely crater position is in the deepest red area. This is generally in the Laos - mid Vietnam - western Gulf of Tonkin area.
ABOVE: If we combine all the data, but then remove the land area on the basis that if a c. 40 km crater were present it would form a very obvious landform and yet none are present, then we end up with the map above. This indicates that the crater is likely in the western-mid part of the Gulf of Tonkin.
Now I would like to add a final comment here that some of the above data is contradictory and therefore some conclusions are clearly wrong, on average though the data should indicate the highest probability area for the crater. Some of the data is also slightly scewed, pushing results to the west. This is because of the readily available data to the west on the Indochinese Peninsula, but lack of data to the east due to the presence of the South China Sea. We get a pretty good picture and now we need to examine the details. The details, by applying logical conclusions or assumptions can remove large swathes of land / sea, as in the case of removing all land areas as a crater would be a very obvious landform and is simply not present. Similarly one could go further and make assumptions about sediment type, age and thickness to remove large areas, increasingly though one is working on sensible assumptions, but also assumptions that might have a small chance of being incorrect.

Individual maps used to make up the composite map.

ABOVE: Mesozoic sediment from Cao L., Jiang T., Wang Z., Zhang Y., Sun H., 2015. Suggested by Blum et al., 1992. In reality, the 10Be content, combined with evidence that mixing was very limited, actually suggests a rock of much younger age with a remnant average Jurassic age (i.e. the precursor sediment).
ABOVE: Jurassic sediment from Lee Y. T., Chen J. C., Ho K. S., Juang W. S., 2004. Suggested by Blum et al., 1992. In reality, the 10Be content, combined with evidence that mixing was very limited, actually suggests a rock of much younger age with a remnant average Jurassic age (i.e. the precursor sediment).
ABOVE: 10Be isoconcentrations from Ma P., Aggrey K., Tonzola C., et al. 2004. This is one of the most important papers one should read in order to understand the positioning of the source crater locality. One can infer a great deal of information about the range of ages of sediment incorporated into tektites, the likely sediment thickness and likely sediment deposition rate. In order to calculate these it is pushing the data and based on logical assumptions, possibly inaccurate, but worth exploring.  
ABOVE: Locations of layered and intermediate type tektites, which are assumed to be the most proximal types of tektites.  From Schnetzler C. C., 1992.
ABOVE: Absence of a crater indicating it is below water on the continental shelf.
ABOVE: Based on 10Be data the crater is likely to be where there is a thick accumulation of rapidly deposited sediment such as the Yinggehai and Hanoi depression. After Lei C., Ren J., Clift P. D., et al., 2011.
ABOVE: Based on 10Be data the crater is likely to be where there is a  thick Pliocene Sequence. After Yan Y., Carter A., Palk C., Brichau S., Hu X., 2011.
ABOVE: An area of chaotic seismic. Possibly compatible with impact origin, but many other reasons such ass diapirism exist. After Yan Y., Carter A., Palk C., Brichau S., Hu X., 2011.
ABOVE: Microtektite regression after Glass B. P., Koeberl C., 2006.
ABOVE: Microtektite regression after Prasad M. S., Mahale V. P., Kodagali V. N., 2007.
ABOVE: Microtektite regression after Glass B. P., Pizzuto J. E., 1994.
ABOVE: Crater ray regression after Whymark, 2016.
ABOVE: Proximal tektite distribution (and central area of distribution).
ABOVE: Muong Nong-type tektite lower melt temperatures. After Glass B. P., 1999.
ABOVE: Mineral inclusions in Muong Nong-type tektites. After Dass J. D., Glass B. P., 1999.
ABOVE: Highest CaO concentrations in Muong Nong-type tektites. After Schnetzler C. C. 1992.
ABOVE: Highest Na2O concentrations in Muong Nong-type tektites. After Schnetzler C. C. 1992.
ABOVE: Lowest SiO2 concentrations in Muong Nong-type tektites. After Schnetzler C. C. 1992.​​
ABOVE: Occurrence of Muong Nong-type tektites only (no splashforms). Note that I have heard splashforms do occur in this region, but evidently they aree sparse. After Fiske P. S., Schnetzler C. C., McHone J., Chanthavaichith K. K., Homsombath I., Phouthakayalat T., Khenthavong B., Xuan P. T. 1999.​​
ABOVE: Occurrence of Muong Nong-type tektites only (no splashforms). Note that I have heard splashforms do occur in this region, but evidently they aree sparse. After Schnetzler C. C., McHone J., 1996.