Subject: Recent paper suggesting KT impact wasn't at boundary
Date sent: Mon, 12 May 1997 22:53:12 -0500 (CDT)

The April 1997 issue of the Bulletin of the Geological Society of America
contains a paper entitled "Age, stratigraphy, and deposition of near-K/T
siliciclastic deposits in Mexico: Relation to bolide impact?" by
G. Keller, Department of Geosciences, Princeton University;
J. G. Lopez-Oliva, Facultad de Ciencias de la Tierra, Universidad Autsnoma
de Nuevo Leon, Mexico;
W. Stinnesbeck, Geologisches Institut, Universitdt Karlsruhe, Germany; and
T. Adatte, Institut de Giologie, Switzerland. I've appended the abstract

The authors suggest that the Chicxulub-forming event occurred up to 150,000
years prior to the KT boundary event. Also, deposits previously interpreted
to have been laid down by impact-generated tsunamis exhibit internal
bioturbation. This indicates they were not laid down in a few hours or days
as one would expect if the cause was a single impact-generated tsunami.
If this evidence holds up, it would indicate that the Chicxulub
structure is not the crater of the KT boundary impactor (assuming there
was one :-}).

I will be interested to see what followup this paper generates.

-- Phil "Pib" Burns
Northwestern University, Evanston, IL. USA


The abstract reads as follows:

Examination of 10 K/T boundary sections in northeastern and east-central
Mexico, and new data presented from 7 sections, permit the following

(1) The globally recognized K/T boundary
and mass extinction in planktic foraminifera is
stratigraphically above, and separated by a thin marl layer of
Maastrichtian age, from the siliciclastic deposit that is commonly
interpreted as a short-term (hours to days) K/T-impactgenerated
tsunami deposit. A similar relationship between the K/T
boundary and siliciclastic or breccia deposits is observed at
Brazos River in Texas, Beloc in Haiti, and Poty Quarry in Brazil.

(2) Stratigraphic control indicates that deposition of the siliciclastic
member occurred sometime during the last 150 k.y. of the
Maastrichtian, and ended at least several thousand years prior to
the K/T boundary.

(3) At least four discrete horizons of bioturbation have been observed
within the siliciclastic deposit that indicate episodic colonization
by invertebrates over an extended time period.

(4) The glass- and spherule-rich unit, which has been linked to
the Haiti spherule layer and the Chicxulub structure, is
at the base of the siliciclastic deposit and thus significantly
predates the K/T boundary event. The stratigraphic separation of
the K/T boundary and siliciclastic deposits and the evidence of
long-term deposition between them, suggests the presence of two

(1) a globally recognized K/T boundary (impact) event marked by
Ir anomaly and the mass extinction, and

(2) a Caribbean event (impact or volcanic and probably linked to
the Chicxulub structure) that predates the K/T boundary and
is marked by glass and siliciclastic or breccia deposits.

CCCMENU CCC for 1997