The most recent cataclysmic asteroid impact event happened about 770,000 years ago. For many, this is no longer a matter of speculation.
Much evidence, such as the presence of Iridium in geological soil layers associated with the event, support the conclusion that it was this impact that created the Australasian tektite strewn field in Indochina, Southeast Asia (including the Philippines), and Australia.
Chemically, these tektites were found to be similar in composition to the Irghizites related to the Zhamanshin asteroid impact site in Kazakhstan. This particular impact site also had the same age as the Australasian tektites (Philippinite shown at left), which made it a good candidate as the source. Scientists speculated that the ejecta from the impact were hurled high enough beyond the atmosphere to fall as far away as Asia in a process involving remelting.
The problem with connecting the Zhamanshin crater (left) to the Australasian tektites is that the crater may be too far away and it too small (some 15 kilometers) to have created enough energy to have created the strewn field. Research in the past twenty years have given scientists new insights into the origins of the Australasians and the distribution of the tektites, including those microtektites found in sea floor core drillings pointed to a likely source in Indochina, possibly in Southern Laos, Eastern Thailand, or Northern Cambodia (top picture).
Scientists even have the coordinates established: 12 degrees North latitude and 106 degrees East Longitude. The crater size is estimated to be around 100 kilometers. The estimates are based on established data concerning tektite distribution and characteristics in relation to distance traveled. Do you see any possible craters in the top picture? Actually, there is quite a few possible candidates, but they have all been studied. So what are the scientists missing? The crater must be staring them right in the face if the coordinates are correct. The crater must be there.
On site research was done on the Tonle Sap lake vicinity in Cambodia (left) to determine if evidence would be found for it to be identified as an impact crater, such as shock metamorphism, but there was none discovered. Possible craters in Northern Cambodia and Southern Laos were also surveyed but the results were negative. It's suspected that the crater is hidden by accumulated debris or is severely eroded to be recognized.
Thus, despite all the evidence pointing to a specific area to search which includes the size of the crater, the source of the Australasian tektites is still unidentified. It is one of the biggest geological and astronomical mysteries that need to be solved as the impact event (it may have been a multiple impact) is said to have caused a major global extinction that could have likely led to the extinction of animals like the stegodon and the nomadic paleolithic people who hunted them in places like the Cagayan Valley in the Philippines.
Coincidentally, the stone tools (following picture, below) found in the same stratigraphic layer of soil as the remains of stegodon pygmy elephants (left) in Cagayan Valley was also dated to be the same age as the tektites. Indeed, the tektite found in the same layer was pivotal in the dating of the artifacts, which could have been made by people contemporaneous to Homo Erectus.
You can see the many research papers and abstracts done on Australasian tektites, which include the Philippine tektites, at the SAO/NASA Astrophysics Data System. You can find the references to the information presented here in this post. Just type in the search bar keywords such as Philippine, Australasian, tektites, microtektites, and strewn field. A little bit of digging won't hurt. You may also check out this UK tektite site which has compiled much of the latest research on the subject.
FACT: as of this writing, the Australasian tektite strewn field (or much of it), is the only strewn field in the world with no associated source crater.
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