Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 236

small as 3 km are capable of producing atmospheric blow-outs (Whitehead et al. ,
2000)
Spherules are small fragments of melted rock blasted into the atmosphere
after bolides impact with the ground surface. They are deposited into thin layers
at various locations across the Earth's surface. They occur across a variety of geo-
logical time periods but appear to be most common within Precambrian strata
of South Africa and Western Australia. The Precambrian spherule layers were
deposited between 3.46--3.23 and 2.64--2.49 Ga (billion years ago). Strata from
thePhanerozoic, the geological time period following the Precambrian, contains
fewer spherulelayers. The most famous of these, however, are the thin layers
associated with the Cretaceous--Tertiary (K--T) boundary impact event. Simonson
and Harnik (2000)recognised that the K--T boundary layer is usually only a few
millimetres thick but some deposits, thousands of kilometres away from the
Chicxulub crater, can be 60--100 mm thick.
Tektites are glass bodies, lacking internal crystals, formed by the melting of
terrestrial surface deposits during extraterrestrial impacts. They are found in
four major strewn fields, these being the Australasian, Ivory Coast, Czechoslo-
vakian and North American fields. Tektite layers are associated with unmelted
impact ejecta in the form of shocked quartz, feldspar, coesite and stishovite in
both the North American and Australasian strewn fields. The source crater for
thetektites and microtektites in the Australasian strewn field is in the Indochina
region (Glass and Wu, 1993)(Fig.9.5). Coesite and shocked quartz are absent
from the Ivory Coast strewn field. This may be due to a smaller-size event and
the distanceofthesamplesites from the impact area.
Microkrystites consist of a combination of crystals (primarily clinopyrox-
ene) and glass (usually replaced) (Simonson and Harnik, 2000). Deep-sea micro-
spherules consisting of microtektites and microkrystites are also found globally
in upper Eocene rocks, around 40 million years ago, and are generally believed
to have been generated by an extraterrestrial impact melting and ejecting ter-
restrial material (Wei, 1995). The number and exact age of these layers varies
between locations.
The Hammersley Basin of Western Australia has four spherule layers, which
formed from extraterrestrial impacts between 2.49 and 2.63 Ga (Hassler and
Simonson, 2001). It would appear that these spherule layers were deposited
below the wave base in a basinal or a deep-shelf marine environment because
they are:

(1) interbedded with sedimentary rocks that were originally clastic or

chemical mud, including carbonate, shale, chert, and/or banded iron
formations;