Geoscience Reference
In-Depth Information
TABLE
A
Evidence for Impact at the Big Five, Eocene-Oligocene,
and Jurassic-Cretaceous Extinction Boundaries
Extinction
boundary
Age
(m.y.)
Evidence
of impact
Age
(m.y.)
Size
(km)
Crater?
Eocene-
Oligocene
33.7
±0.5
Tektites,
microtektites,
shocked
quartz, coesite
Popigai,
Siberia;
Chesapeake
Bay, U.S.
35.7±0.2
35.5±0.6
100
85
Cretaceous-
Tertiary
65.0
High iridium,
shocked minerals,
microtektites,
spherules, spinel,
diamond
Chicxulub,
Mexico
65.0
170-
300
-145
High iridium,
shocked
quartz
Morokweng,
South Africa
145
70-
340
Jurassic-
Cretaceous
Shocked
quartz, weak
iridium peak,
fern spike
Manicouagan,
Canada;
Puchezh-
Katunki,
Russia
214±1
Triassic-
Jurassic
-202
100
80
220±10
40
Permian-
Triassic
-250
Weak iridium
peak, shocked
quartz,
microspherules
reported
Araguinha,
Brazil
247±5.5
Late
Devonian
367
Microtektites,
weak iridium
peak
Siljan,
Sweden
368±1
Ordovician-
Silurian
-438
Weak iridium
peak
None known
of this age
When we study the geologic sections that mark the late Devon-
ian interval in detail, we find not a single extinction boundary but
several smaller ones spaced over a few million years. Several iridium-
rich layers are found at this horizon. One expert, George McGhee
