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Brazos River
M oscow Landin g
Sediment Type
Marl
Silt-sand
100 cm
90 cm
Rippled and
Beloc, Haiti
layer sands
Turbidites
Gravels
60 cm
Chalk
Deep ocean
mud
60 cm
50 cm
Backwash
Turbidite
Cobbles and
boulders
ï¿¿
30 cm
Spherule-rich
basal unit
30 cm
Marl
Iridium-rich
fallout
0 cm
Backwash
Chalk
Marl
0 cm
0 cm
Mimbral
Marl
Rippled and
layer sands
Parris Basin
300 cm
Chicxulub
crater
Turbidites
200 cm
Ocean basin
Spherule-
rich
basal unit
100 cm
Shelf
K/T sandstones
Major fault
Marl
0 cm
Fig. 9.8 Location of the Chicxulub impact crater and stratigraphic sections of tsunami deposits surrounding the proto-Gulf of Mexico. Based on
Bourgeois et al. ( 1988 ), Bohor ( 1996 ), and Smit et al. ( 1996 )
initial blast wave of compressed air from the impact, water
flowing into the cavity and large scale landslides. The most
significant factor was generation of the crater. This pro-
duced a tsunami 200 m high that rolled across the southern
United States for a distance of 300 km to a maximum ele-
vation of 300 m (Fig. 9.1 ) (Alvarez 1997 ; Matsui et al.
2002 ). Within an hour of the impact, the burning forests that
had been flattened and then ignited by the initial blast were
picked up, mixed with uncompacted sediment and ripped-
up bedrock, and then driven inland hundreds of kilometers
over the flat coastal plains adjacent to the proto-Gulf.
Backwash re-entered the Gulf as a wave tens of meters high,
accompanied by channelised backflow that eroded channels
across the wide shelf. This slurry spread along the seabed as
a turbidity current, depositing a chaotic mixture of ejecta,
bedrock clasts, sand, and terrestrial vegetation across the
abyssal plain. Reflection of this wave back and forth across
the Gulf ensured that landmasses were repetitively swept by
tsunami diminutively over the next few days. Slowly, sed-
iment suspended in the water column and dust put into the
atmosphere settled to the seabed over the next few weeks as
quiescence returned to a lifeless ocean. This latter sediment
contained the iridium-rich signature of the cosmogenic
source. Millions of years later as the modern Gulf formed,
deposits laid down by the passage of the tsunami were
uplifted and then exposed around the Chicxulub impact
center—along the Mexican Gulf coast, in exposures on the
Brazos River in Texas, at several sites in Alabama, and on
the Island of Haiti (Fig. 9.8 ) (Bourgeois et al. 1988 ; Bohor
1996 ; Smit et al. 1996 ).
The nature of the turbidite signature varies depending
upon its proximity to the initial impact and its relative
location to the shoreline of the proto-Gulf (Fig. 9.8 ). The
deposits have been attributed to turbidity currents generated
by submarine slides or to bottom currents generated by the
passage of tsunami waves (Bohor 1996 ). The enormous
height of the wave would have ensured that even the
deepest part of the Gulf was in shallow water and that
cobble-sized material was moved. All deposits contain sand
that must have been brought there from the shelf by back-
wash generated by the tsunami or by turbidity currents.
Because tsunami have wavelengths tens if not hundreds of
kilometers long, current velocities at the seabed generated
by the oscillatory nature of the wave must have been
 
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