POSSIBLE AUSTRALASIAN TEKTITE SOURCE CRATER?

This is a March 2013 update on the possible Australasian tektite source crater. I have written an abstract for the 44th Lunar and Planetary Science Conference. You can download it for free here:

Review of the Australasian Tektite Source Crater Location and Candidate Structure in the Song Hong-Yinggehai Basin, Gulf of Tonkin.

I will go into more detail on the subject in my forthcoming book.

A summary of the principal lines of evidence is found below:

Microtektite regression: The abundance of microtektites, as with macrotektites, increases as one moves towards the crater. The advantage of microtektites is that they are small and therefore only a small volume of rock is required to obtain a representative sample. The number of mi-crotektites (over 125 µm) per cubic centimeter can then be calculated. Microtektite layers have been studied at many deep sea and oceanic drill sites. An iso-concentration map can then be drawn up and the values will, on average, increase towards the impact site. Microtektite regression suggests a crater in the eastern part of Indochina or Gulf of Tonkin (Glass & Koeberl, 2006; Prasad et al. 2007).

10Be regression: Ma et al. (2001) and Ma et al. (2004) highlight the use of 10Be in order to identify the likely source crater area. The first formed and most distally ejected tektites contain the highest 10Be con-tent. The last formed are the Muong Nong-type layered impact glasses. These were excavated from the deepest sediments and hence contain the lowest 10Be content. The lower the 10Be con-tent, the closer to the crater one is. Ma et al. (2004) found the lowest 10Be concentrations for tektites fall on or within a contour centred offshore Vietnam, in the south of the Gulf of Tonkin (17° N, 107°E), but also encompassing two other locations in the area of north-eastern Thailand previously proposed for the site of a single tektite producing impact.

Chemistry of source rock: Glass et al. (2004) state that the composition of normal Australasian microtektites fall on the border between greywackes and lithic arenites. Lee et al. (2009) concluded that the best fit for tektites from the Wenchang and Kon-Kai areas was a mixture of 47% shale, 23% sandstone, 25% greywacke and 5% quartzite.

Boron: Chaussidon and Koeberl (1995) concluded that the high boron and lithium abundances and δ11B values of (-4.9 to +1.4 ‰) are consistent with pelagic and neritic sediments as well as river and delta sediments. They note that the 10Be and Rb-Sr data clearly point to a continental crustal source rock, excluding pelagic and probably neritic sediments as a source. They note that river and deltaic sediments, which are continental crustal material with a higher clay content that have acquired boron from the seawater would satisfy the 10Be, Rb-Sr and B data.

Rb-Sr dating: It is evident that the sediment that was melted to produce tektites is probably far from ideal for the purpose of whole-rock Rb/Sr dating. In selecting a rock for Rb/Sr dating one is looking for a slowly deposited, fully marine, fine grained shale comprising mainly illite (Dicken, 2005). So, whilst the ~170 Ma age is correct for the last major Rb enrichment event, it may not be representative of the last erosion, transportation and depo-sition event. Whole rock Rb/Sr dating should be carried out on Plio-Pleistocene sediments in the Gulf of Tonkin and sur-rounding deltaic settings for the purpose of elimination. It is suggested that the Plio-Pleistocene sediments of the Gulf of Tonkin may have an averaged middle Jurassic Rb/Sr 'remnant' age. If so, the Australasian tektite source sediment can be much younger than the middle Jurassic estimate.

High 10Be content: Tektites derived from depth should contain little or no 10Be unless they were derived from sediments deposited within the last few million years (Pal et al., 1982). In order to see the ob-served concentrations of 10Be in tektites derived primarily from a middle Jurassic source rock, thorough mixing with soils/sediments within the zone of meteoric water percolation and/or recent surficial fluvial deposits must be implied (Blum et al., 1992). Thorough mixing is less probable in the last formed, deepest excavated, Muong Nong-type layered impact glasses. A more satisfactory explanation is that the target rock is a thick column of rapidly deposited Plio-Pleistocene sediment with high sedimentation rates of at least 0.02 cm/yr-1 (Pal et al., 1982).

Muong Nong-type layered impact glass: Muong Nong-type layered impact glasses clearly formed in close proximity to the impact site as they represent the least melted and least homogenised of the ‘tektites’. The lower 10Be content compared with more distally ejected tektites (Aggrey et al., 1998) indicates that they came from the greatest depths. Schnetzler (1992) plotted a distribution map of Muong Nong-type impact glass. It was noted that there was a higher concentration of Muong Nong-type impact glass on the eastern side of the Indochinese Peninsula (see Figure 13.1). Schnetzler (1992) then took the study further by also plotting the geographical dis-tribution of SiO2 concentration, Na2O concentrations and CaO concentrations in Muong Nong-type impact glass. Schnetzler (1992) concluded that the likely source crater was on the eastern side of Indochina or offshore to the east of Vietnam. The favoured area was within 125 km of 16° N, 105°E, eastern Thailand or neighbouring southern Laos.

An area exists to the south and east of Savannakhet, Laos where only Muong Nong-type impact glasses are found and splashform tektites are absent (Schnetzler and McHone, 1996). Logically, this has often been taken to indicate that the source crater is within this area. One should note, however, that this is not a circular arrangement, but appears to extend and converge towards the Gulf of Tonkin. One must also consider the arrangment of interpreted crater rays. The figure below demonstrates the most prominent distal rays (two butterfly and one down-range). The occurrence of only Muong Nong-type impact glasses in the absence of splashform tektites, closely aligns with this pattern. The pattern observed may simply be that of a prominent butterfly ray heading into the Indian Ocean, as oppose to indicating this precise region contains the source crater.

ABOVE: The distribution of Muong Nong-type layered impact glass (black dots). The area encompased in a dashed area is believed to contain only Muong Nong-type layered impact glass and no splashform tektites (after Schnetzler and McHone, 1996). The grey areas are prominent distal rays.

Size of Crater: General consenus appears to be that a 32 km diameter crater is a realistic, but likely minimum, value. A crater in the order of 40 km diameter (Glass, 1993, 2003) appears most probable. The higher values of around 100 km crater diameter are unrealistic given that the Australasian microtektite strewn field is not global like the late Eocene spherule layer believed to be derived from the ~100 km diameter Popigai structure (Ivanov et al., 2004).

Interpretation of Crater Ray System: Crater rays can be interpreted manually, taking into account similar patterns, such as large tektites, high concentrations, layered tektites and simply the pattern observed, A mirror image can be assumed and so gaps can be filled in. This gives the following pattern:

ABOVE: An attempt to interpret the Australasian tektite crater ray system, based on an assumed crater location at 17°45'20"N, 107°50'30"E. Black dots represent tektite occurrences.

This method of interpretation is, however, somewhat subjective. To make it less subjective one can plot a line from every single known tektite occurrence back to various source craters. In doing so one must make allowances for missing data in seas, oceans and poorly published areas. One can then make the following assumptions 1) The down-range ejecta in an oblique impact should have a reflection symmetry along the down-range axis. This should enable one to left-right position (east-west in this case) of the crater. 2) Ejecta is distributed in rays and therefore the more the lines fall on top of one another and clearer the rays are defined then the closer to the crater one must be. 3) The up-range ejecta is produced by an explosion (which creates a circular crater). This creates a circular area of ejecta up-range and to teh sides of the impact. The crater should be at an equidistant point. This fixes the up-down (in this case north-south) position of the crater. Ultimately the crater positioning can be narrowed down. The largest draw-back is missing data in seas and oceans that creates a margin of error.

ABOVE: All tektite data (see key references listed at end of Chapter 3), excluding generalised locations of Fenner (1935). Focal point is Indochina, with Philippines to the east. Note that the prominent southerly rays lack con-siderable data to the west (Indian Ocean), refer to the proximal occurrences for a fuller, although still incomplete, picture. Each tektite location is pro-jected back to a) TOP: Tonle Sap, 13°1'60"N and 103°56'0"E as per Hartung and Koeberl (1994). One can note the extreme lack of symmetry along the down-range-axis. Furthermore, the easterly butterfly ray does not cross the highest concentrations of tektites in the Philippines. b) MIDDLE: A position in the Savannakhet area of Laos, 16.35°N and 106.15°E as per Schnetzler and McHone (1996). Although no evidence of a crater was found at this point, the general Savannakhet area remains a popular crater hunting ground. One can observe a much better symmetry along the down-range-axis, but the slight asymmetry of the data would still suggest the true axis lies to the east. This area is probably just within the area of faesibility if one concludes considerable data is missing due to seas/oceans. c) BOTTOM: A position based on the GETECH gravity anomaly at 17°45'20"N and 107°50'30"E. Symmetry appears to be very good, indicating this is probably very close to the true source of the Australasian tektites. The true axis may lie very slightly to the west of this point. Again, some degree of uncertainty exists due to missing data in the seas and oceans and possibly in mainland China.

Another point to add about the crater ray system is that a deep sea will reduce the tektite distribution whereas a shallow marine sea will very much aid the production of microtektites. Shuvalovi & Dypvik (2004), after O’Keefe J. D. et al. (2002), states that ‘volatile-rich sediments (typical for sea-covered targets) provide a more extensive expansion of shock compressed material than in drier, subariel materials’. It is noted that this effect is limited to shallow waters where sea depth is much less than projectile size. Also refer to Artemieva (2013).

Macro-tektite morphology: Very similar proximal splashform tektite mor-phologies and Muong Nong-type layered impact glass-es are found in north-eastern Thailand, central-southern Laos and Vietnam, to the west, and on Hainan Island and the Leizhou Peninsula, to the east. The east-west equidistant centre of the bilaterally symmetrical distri-bution pattern is in the Gulf of Tonkin; not the Indo-chinese Peninsula, which lacks tektites to the west.

Absence of a crater: Much smaller tropical craters of comparable age, such as Bosumtwi Crater (10.5 km Ø, 1.07 Ma (Koeberl et al., 1997)) and Lonar Crater (1.8 km Ø, 0.656 ±0.081 Ma (Jourdan et al., 2010)) form very evident lakes. The implication is that the Australasian tektite source crater must have either been very significantly eroded or buried. The former is highly improbable given the ±0.788 Ma age (Channell et al., 2010; Lee and Wei, 2000), whereas the latter is highly plausible if the crater were located in the Song Hong-Yinggehai (SHY) Basin. The depo-centre of the SHY Basin has accumulated 17 km of sediment since the middle Eocene (Luo et al., 2003), or on average 0.035 cm/yr-1. This is 275 m in the last 0.788 Ma, probably an extremely conservative estimate given that peak sedimentation rates are recorded in the Plio-Pleistocene (Yan et al., 2011). Lei et al. (2011) give a sedimentation rate of 780 metres per million years for the Pliocene to Quaternary interval, or 615 metres in the last 788,000 years.

SUMMARY OF CRATER EVIDENCE

So to recap, we are looking for a crater on the eastern side of the Indochinese Peninsula, to more likely central Gulf of Tonkin. The source rock is likely to be rapidly deposited silica rich Plio-Pliestocene sediment. The impact site was possibly in a shallow marine environment. These data largely exclude the eastern part of the Indochinese Peninsula and effort must be focused in the Gulf of Tonkin.

The Song Hong-Yinggehai basin, between central Vietnam and Hainan is a pull-apart basin. It has incredibly high sedimentation rates. Source rock-wise and geochemically this basin could not be more perfect for production of the Australasian tektites. Other suitable basins can largely be ruled out due to the various regression patterns strongly suggesting a crater in this area.

SEARCH FOR THE CRATER

Assuming the crater was in the Song Hong Yinggehai Basin, Gulf of Tonkin a search was made in published literature. One thing that immediately jumped out was the circular arrangement of shale diapir-like features in the Song Hong-Yinggehai Basin. These can best be seen in the latest paper Lei et al. (2011) - click here. The pattern can also be observed in older papers (Zhang, 1994 (not seen); Hao et al., 1998; Xie et al., 1999; Hao et al., 2002; Luo et al., 2003; Yin et al., 2004; Clift & Sun, 2006; Wang & Huang, 2008). This made me think whether these features could be related to a soft-sediment impact.

A company called GETECH were kind enough to share some gravity anomaly data with me for the area of interest. In the middle of this pattern was a prominent circular gravity anomaly that was 43 km (+/- 3 km) diameter. You'll have to look at my abstract here to see this gravity anomaly as I do not have permission to reproduce. The x limits of the gravity anomaly map are 107.505 to 108.49 and the y limits are 17.45 to 18.58. The circular feature is centred on 17°45'20"N, 107°50'30"E.

Following this up, I need seismic data to create an even stronger case for an impact crater at this point. Unfortunately I cannot obtain any publically available data for this area. It is interesting to note that in Yan et al. (2011), after Gong and Li (1997, 2004), a question mark exists on Late Miocene and Pliocene sediment isopachs maps atop this circular feature, but not deeper (Early Miocene). This may lead to the question of whether chaotic sediment exists at this point in the younger sediment. Other interesting features observed are layer-bound faulting increasing in frequency towards the depo-center (and the gravity anomaly) after Lei et al. (2011). Also thrust faulting was noted in Yin et al. (2004) which may warrant further investigation.

To date, no hydrocarbon wells have been drilled over this structure. A well drilled directly over the structure would provide definitive proof (with the central uplift possibly being a good hydrocarbon trap). The nearest wella are around 12-15 km east of the rim of the gravity anomaly (Zhu et al., 2009). At this distance an ejecta layer perhaps 10's of metres in thicknes may be expected, but may not be recognised if not actively looking for such a feature. 30 km from the similar sized Mjølnir Crater rim the ejecta was a mere 80 cm thick (Tsikalas and Faleide, 2003) and this would probably not be detected in routine drilling operations.

The search goes on.... but I am confident that I'm seeing something interesting here. If you could help with geophysical data then this project can move forward.

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