| The
Iturralde structure is possibly the Earth's most recent ″big″ impact
event recording a collision with a meteor or comet that might have
occurred between 11,000 and 30,000 years ago. The most convincing
evidence for the existence of a crater comes from the Shuttle Radar
Topography Mission (SRTM) image of the structure. The feature appears
as a quasi-circular closed depression about 20 meters in depth
resembling a cookie cutter cutting heavily vegetated soft sediments and
pampas at the edge of the Bolivian Amazon. A barometric traverse from
inside the crater to outside the rim supports the 20 meter depth
contrast suggested by the SRTM data.A magnetic survey across the crater
was able to define a symmetry along an east-west axis with the center
being lower than the rim. Base station records were able to identify
the Equatorial Electrojet (EEJ) geomagnetic diurnal signal attributed
to a narrow electric current sheet flowing eastward along the magnetic
dip equator. The soil magnetic susceptibility remains low and
featureless until about 1 meter depth where lateritic nodules
identifiable with the downward migration of iron are found. As yet
there are no identifiable evidences of shocked quartz in the quartz
sand collected in soil pits inside the crater, along river cuts, and at
the ″rim″. The expedition did not yield the ″smoking gun″ required for
verification.There does exist oil exploration geophysical survey data (
gravity, magnetics) which include the Iturralde structure in the survey
areas and there are seismic lines near the structure. We hope to be
able to obtain this information perhaps by presentation time. The
vegetation inside the crater appears different from that outside the
crater but this may simply reflect the inundated nature of the
depressed structure which may be underwater for a good portion of the
rainy season. A return to the structure to drill for evidence of
shocked material may be the only way to prove that the structure is a
crater. |