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JRASC October 2004 - The Royal Astronomical Society of Canada

Figure 6 d — Two dumbbell shaped splash-form type tektites, length 8-10 cm, showing sculptures such as pits and grooves (O’Keefe 1976).6. Formation of the North American, CentralEuropean, and Ivory Coast fieldsThe common characteristics that exist among the four tektite-strewnfields indicate that all originated from a single source. All are dated towithin the last 35 million years of Earth’s history, and all strewn fieldshave splash-forms and microtektites as a common morphology. Inpetrology they differ from terrestrial rocks; they are non-crystalline,homogeneous, and deficient in water and volatiles; and all have a lowferric/ferrous ratio, similar isotopic properties, and identical corrosionpatterns. Analysis of the chemical composition of the tektites in thestrewn fields shows that a first-order trend exists among the oxides ofsilica, magnesium, and calcium (silica is negatively correlated) and thatthe range from one field to another is less than the range within a field(O’Keefe 1976). Also a linear trend can be observed in neodymium/strontiumvalues in the tektites of all four strewn fields (Glass et al. 1998). Thesecommon characteristics, which encompass both chemical and physicaldomains, indicate that the tektites in all four strewn fields belong to asingle family and thus originated from a common source.The North American, Central European, and Ivory Coast tektitesare chronologically (though not necessarily in chemical composition)linked to three impact craters, namely Chesapeake Bay for the NorthAmerican, Ries Kessl for the Central European, and Bosumtwi for theIvory Coast. The distribution of the tektites is asymmetrical in relationto these craters, and aerodynamically shaped tektites, similar to australites,have not been found in these three strewn fields. These fields consistmostly of splash-forms; layered Muong-Nong type tektites are scarce.Only one georgiaite and some layered-type tektites in deep sea coresare found in the North American field, and a few layered moldavitesare known in the Central European field (O’Keefe 1976; Heinen 1998).In the North American field, the tektites are distributed south ofthe crater and fan out towards the equator (Figure 7). This strewn fieldcovers a wide area stretching from New Jersey to Barbados. All tektiteshere are found north of the equator converging on the crater, and noneare found further north of the crater (Heinen 1998). Based on theseobservations, it is possible to model the formation of the North Americanfield in relation to the orbiting tektite ring cluster at the time of theChesapeake Bay impact event. The impactor, moving from southwestto northeast, either collided or gravitationally interacted with theorbiting ring cluster. The bolide then entrained the tektites andmicrotektites from their orbit and dispersed them in a southerly directionbefore impacting on the Earth’s surface. Since the ring arc was in anFigure 7 — The distribution of tektites (checked areas) in relation to theChesapeake Bay crater {CB} in the North American strewn field (Heinen1998).equatorial orbit, the strewn field would extend north of the equatorand converge towards the impact crater.The distribution of the tektites in the Ivory Coast field in relationto Bosumtwi crater also gives credence to this proposition. In this case,the impactor moved from west to east, passing the ring arc along theequator, with the resulting tektite field stretching west of the crater inan equatorial direction; tektites in the existing field have not been foundeast of the crater. Microtektite distribution in the Atlantic Ocean alsoshows a large strewn field west of the Bosumtwi crater along the equator(Figure 8).In the Central European field the distribution again is asymmetricalin relation to the relevant crater; the tektite strewn field is found eastof the crater, and none are found towards the west, indicating that thepassage of the impactor within the gravitational domain of the ring arcwas from southeast to northwest. Unlike the tektites in the other twofields mentioned here, these tektites are found in close vicinity to thecrater, about 200 km east, and constitute a small strewn field in massand distribution (O’Keefe 1976).Figure 8 — The distribution of tektites in relation to the Bosumtwicrater in the Ivory Coast strewn field (O’Keefe 1976).These three fields mostly contain splash-form tektites andmicrotektites, which are the main components of the proposed ringcluster. In this impactor scenario, the entrained particles in the ringarc did not undergo natural decay through the Earth’s atmosphere as196JRASC October / octobre 2004

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