TEKTITE STREWN FIELDS

Written By Aubrey Whymark 2018
There are a number of major geographically restricted tektite strewnfields, alongside a number of minor strewnfields containing impact glasses, grading towards tektites. One question often asked is why there are not more strewnfields. The answer is that the target material must be a silica-rich rock, which implies a sandstone, siltstone or shale - silica-rich sedimentary rocks and their metamorphic equivalents. This type of rock is concentrated in restricted regions of the Earth's surface, primarily continental shelfal regions. So, not every impact produces tektites. Secondly tektites are made of glass, which is unstable over extended periods of geological time. Tektites will only be found in relatively recent 'Tertiary' impacts. 

Strewnfields

There are more craters than tektite strewnfields. This is because tektites only form from silica-rich rocks and are geologically unstable. So tektites are only found from impacts into silica-rich (primarily sedimentary and metasedimentary rocks) of a young geological age.
 
ABOVE: The distribution of tektite strewn fields (grey shading) and associated or probable source craters (black dot). Modified from Montanari and Koeberl (2000).

Ivory Coast Tektites

Ivory Coast tektites were ejected from the 1.07 million year old, 10.5 km diameter, Bosumtwi Crater in Ghana (Koeberl et al., 1997) (see Figure 2.1). All Ivory Coast tektites are termed Ivorites. Locally, in the Baoulé language, they are known as ‘Agna’ (Lacroix, 1935) or ‘Anja’ (Gentner, 1966).

Central European Tektites

Central European tektites were ejected from the 24 km diameter Ries impact crater in Germany 14.34 (±0.08) million years ago (Laurenzi et al., 2003) (see Figure 2.1). Central European tektites are usually referred to as moldavites regardless of whether they originate in the Czech Republic, Germany or Austria. Moldavites were first found in deposits near Týn nad Vltavou in the Czech Republic. The name ‘moldavite’ is derived from the Vltava River, which, in German, the chosen language of many earlier scientific publications, is the Moldau River. Within the Czech Republic, moldavites are termed vltavín. A detailed explanation is given in Bouška (1994). Moldavites, which typically show various shades of green are the only true 'gem' quality tektites. The relationship of tektite localities to the known (and accessible) source crater is another valuable observation. All moldavites are proximal tektites, but clear differences can be seen between the most proximal Bohemian moldavites and the slightly further ejected, but still proximal, Moravian moldvaites.

Largest Strewnfield

The Australasian strewnfield is the largest macrotektite strewnfield. It covers over 10% of the Earth's surface. It is also the youngest strewnfield at 786 thousand years.
 

North American Tektites

Georgiaites are proximal tektites from the 40 km diameter (estimated post-impact size) Chesapeake Crater, which formed 35.5 million years ago. The name georgiaite is derived from Georgia County. These proximal forms are comparable in morphology to recent proximal Australasian tektites found in Indochina.

Bediasites are medial tektites from the same impact event and the name bediasite is derived from the Bedel Indians and the town of Bedias in Texas (Povenmire, 2003). County. These medial forms are comparable in morphology to recent medial Australasian tektites found in the Philippines, Borneo and Belitung Island.

Single, isolated, North American tektites have also been found at Martha’s Vineyard (Kaye et al., 1961), Cuba (Garlick et al., 1971) and South Carolina (Povenmire, 2010). Microtektites belonging to the North American strewn field have been found in Barbados (Glass et al., 1984) as well as a number of oceanic sites.

Owing to the age of the North American tektites, the majority of the strewn field is buried by more recent strata. The tektite distribution pattern we see today is largely controlled by the outcrop of strata of the correct age (Upper Eocene or younger) in which tektites have been deposited or reworked into. These strata are then eroded and the tektites are found in surfcial deposits (Albin, 1999).

The terms amerikanite and colombianite refer to terrestrial obsidian that resemble tektites. They are unrelated to, and should not be confused with, true North American tektites.

Central American Tektites

The first clue to the existence of Central American tektites came from the presence of 11 tektites found in the Maya ruins at Tikal in Guatemala (Essene et al., 1987; Hildebrand et al., 1994). Tektites were also reportedly found at El Pilar ruins in Belize (Cornec, 2010, after Ford, 1994). These tektites were found in an archaeological context and therefore their origin was unknown. Cornec (2010) announced the discovery of in situ tektites in western Belize. Izett and Meeker (1995) dated two tektites from western Belize at 0.82 million years old. A suggested, but unconfirmed, source crater is the 14 km Pantasma Crater in Nicaragua (see Figure 2.1), around 540 km from the suspected centre of the Belize tektites (Povenmire et al., 2011). In the future one might expect this strewn field to extend into Guatemala, Honduras and even possibly southern Mexico.

The 0.82 Ma Central American tektites were unofficially named zapotectites by Brian Burrer. The name is derived from the Zapotec civilisation. This reflects their initial recovery in Mayan ruins of two countries. Mayanite was a term already in use as an unofficial name for an iridescent quartz crystal: Rainbow Mayanite Quartz (Burrer, pers. comm., 2012). Brian then switched to a more specific moniker for the local Mayans; the Zapotec, although strictly speaking the Zapotec civilisation is from the southern Mexico and not from the area in which tektites are currently found (Ford, pers. comm., 2012). Note that this term deviates slightly from traditional nomenclature in that it has an additional ‘t’: Other groups end in ‘ite’ rather than ‘tite’. Care should be taken to use the originally proposed term ‘zapotectites’ and not the incorrect variants ‘zapotecite’ and ‘zapotektite’.

The 65.5 million year old Cretaceous-Tertiary (K-T) impact spherules/altered tektites are also found in Belize and are not to be confused with this much younger and separate tektite-forming event.

Australasian Tektites

The Australasian tektite strewnfield is the largest and most recent strewnfield. It covers in excess of 10% of the Earth's surface. The Australasian tektite strewnfield is dated by Mark et al., 2017 as 786 ± 2 ka. The stratigraphic position pre-dates the glacial Termination IX, which is positioned at 785.6 ± 0.7/0.8 ka. So this means that the impact took place prior to termination of a glacial period, i.e. at a time of lowered sea levels. The Australasian impact was highly oblique, probably under 20 degrees inclination. All known types of tektites are present in this impact - proximal, medial and the only known crater to form distal tektites. The only thing we are missing is a crater, but numerous regression methods for geochimical, morphological, distribution and numerical attributes, combined with geologically suitable source areas, very strongly indicate the crater is buried in the Yinggehai / Song Hong Basin in the Gulf of Tonkin. Wildly speculative suggestions have been made that the crater is further afield, but these tend to ignore the evidence we have.

Proximal tektites are found in Vietnam, NE Thailand, Laos, Cambodia and southern China (Guangxi, western Guangdong, Hainan). These are typical splashforms exhibiting teardrops and flattened discoidal forms. Testimony to a viscous melt that has heavily interacted with the atmosphere.

Medial tektites are found in the Philippines, Malaysia and northern Indonesia. These are more spherical bodies, although clearly exhibiting some flattening through interaction with the atmosphere, they never form discs. Due to higher re-entry velocities of cooled tektites the anterior surfaces are usually spalled. U-grooving is a common texture.

Distal tektites are found in southern-most Indonesia and Australia. They are typically smaller bodies and exhibit ablation from re-entry, usually followed by distinct spallation to form equatorial cores.

South Ural Tektite-like Glass

Only a single South Ural tektite-like glass specimen has been found near Magnitogorsk (53°37' N, 60°10' E). The occurrence with quartz pebbles implies reworking and redeposition. The glass is light green in colour, rounded, and weighs ~90 grams. It has an extremely low water content (< 0.09 wt%). The specimen is dated at ~6.2 Ma. (Deutsch et al., 1997; after Koroteev et al., 1994).

Urengoites, West Siberia

Urengoites were first reported by Masaitis et al. (1988) from the northern part of West Siberia (78° E, 66° N). Three specimens from two sample localities were found, some 40 km apart. The deposits in which they were found were Late Pleistocene sandy fluvioglacial deposits (Masaitis et al., 1988). The glass is bottle-green and pale green (Ostermann et al., 1996). Urengoites are virtually H2O free at 0.008 to 0.024 wt% (Beran and Koeberl, 1997). Urengoites have been fission-track dated to 22.1 ±2.6 Ma to 26.0  ±2.8 Ma and one K/Ar determination of 24.0 ±1.2 Ma (Komarov et al., 1991).

Impactites: 

Darwin Glass, Libyan Desert Glass, Irghizites and Zhamanshinite, South Australian High Na/K 'Australite' Glass, Lonar Crater Glass, Waber Glass and Aouelloul Glass, plus a range of other glasses are considered to be impact glasses as oppose to true tektites. They have very proximal strewnfields, usually related to known craters.
Distribution of Darwin Glass, Tasmania, Australia.