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Figure 1. Plate tectonic
reconstruction of Carib-
bean region near K/T
time modified from Pin-
dell and Barrett (1990).
Bold lines—fault zones
between plates; arrows
and triangles indicate
relative motions. Solid
triangles—subduction
zones; open triangles—
thrusting. V pattern—sub-
duction-related island-
arc volcanism. Dashed
line—paleoshoreline on
North American conti-
nent. Diagonal-rule pat-
tern—areas of possible
impact-wave deposits.
Dots—Deep Sea Drilling
Project sites: impact-
wave deposits are found
at sites 151, 153, and
603B. Stars—positions of -50-cm-thick K/T ejecta layers found at sites in Haiti and Mexico.
Circle—Chicxulub crater on Yucatan platform.
Chicxulub Crater: A possible Cretaceous/Tertiary
boundary impact crater on the
Yucatan Peninsula, Mexico
SPECIAL
REPORT
Alan R Hildebrand*
Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721
Glen T. Penfield
Aerogravity Division, Carson Services Inc., 32A Blooming Glen Road, Perkasie, Pennsylvania 18944
David A. Kring
Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721
Mark Pilkington
Geophysics Division, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada
Antonio Camargo Z.
Gerencia Exploracion, Petroleos Mexicanos, Avenida Marina Nacional 329, Mexico D.F. 11311, Mexico
Stein B. Jacobsen
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge,
Massachusetts 02138
William V. Boynton
Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721
ABSTRACT
We suggest that a buried 180-km-diameter circular structure on the Yucatan Peninsula,
Mexico, is an impact crater. Its size and shape are revealed by magnetic and gravity-field
anomalies, as well as by oil wells drilled inside and near the structure. The stratigraphy of the
crater includes a sequence of andesitic igneous rocks and glass interbedded with, and overlain
by, breccias that contain evidence of shock metamorphism. The andesitic rocks have chemical
and isotopic compositions similar to those of tektites found in Cretaceous/Tertiary (K/T)
ejecta. A 90-m-thick K/T boundary breccia, also containing evidence of shock metamorphism,
is present 50 km outside the crater's edge. This breccia probably represents the crater's ejecta
blanket. The age of the crater is not precisely known, but a K/T boundary age is indicated.
Because the crater is in a thick carbonate sequence, shock-produced CO
2
from the impact may
have caused a severe greenhouse warming.
INTRODUCTION
The debate concerning the postulated impact
at the Cretaceous/Tertiary (K/T) boundary
(Alvarez et al., 1980) may be resolved by the
discovery of the impact site and/or the thick
proximal deposits of the impact (Hildebrand
and Boynton, 1990a, 1990b). An -50-cm-thick
K/T boundary ejecta layer in Haiti and prox-
imal impact-wave deposits in the Caribbean re-
gion suggest that the K/T boundary impact
occurred between North and South America.
Circular anomalies, -200 km in diameter, in
both magnetic and gravity fields (with asso-
ciated andesitic rocks) on the northwestern mar-
gin of the Yucatan peninsula of Mexico (Fig. 1)
have been interpreted as representing a volcanic
center (Lopez Ramos, 1975) or an impact crater
with associated extrusive material (Penfield and
Camargo, 1981). We describe geophysical, strat-
igraphic, and petrologic evidence indicating that
this structure is a large impact crater of possible
K/T boundary age.
GRAVITY AND MAGNETIC DATA
The circular structure is buried in the middle
of the Yucatan carbonate platform in a region of
*Present address: Geological Survey of Canada,
Geophysics Division, Observatory Crescent, Ottawa,
Ontario KIA 0Y3, Canada.
anomaly might be best fit by a center slightly
farther northeast. The gravity-field anomaly is
truncated to the north by an east-northeast-
trending lineament that crosses the Yucatan plat-
form north of the present coastline. A negative
anomaly trails -100 km to the south from the
circular anomaly. Its internal circular structure is
disrupted near both the truncating lineament
and the southward-extending trough.
Total magnetic-field data (Penfield and Ca-
margo, 1981; Lopez Ramos, 1975) show -210-
km-diameter, circular, dipolar anomalies with
large horizontal gradients and some concentric
structure nearly coincident with the gravity
anomaly. Large-amplitude, short-wavelength
anomalies (up to -1000 nT) occur over the cen-
tral gravity high, but extend farther, to a radius
of -35 km. An outer zone of weaker (5 to 20 nT)
short-wavelength anomalies extends to a radius
of -105 km, but has an irregular margin. The
subsequent tectonic quiescence. Bouguer gravity
data show an -180-km-diameter, circular, 30
mgal, negative anomaly (Fig. 2) similar in shape
to those found over large impact craters (Fig. 3).
A center 10 km east of Progreso, near the town
of Chicxulub Puerto, best fits a 20 mgal central
high (20 km radius) and two internal concentric
lows (35 and 60 km radii); the margin of the
GEOLOGY, v. 19, p. 867-871, September 1991
867
100
50
0
km
0
25
Figure 2. Contour plot (contour interval = 2 mgal) of Bouguer gravity data (Gravity Anomaly Map
Committee, 1988) covering northwest corner of Yucatan Peninsula, Mexico. Outermost heavily
dashed circle—margin of circular negative gravity anomaly; two other circles—concentric lows
within negative anomaly; cross—center of anomaly. Dotted line—east-northeast-trending re-
gional lineament that truncates anomaly. Two light dashed lines indicate positions of profiles
shown in Figure 3. Dark solid line (top) shows approximate position of seismic reflection profile.
Exploration wells drilled by PetrOleos Mexicanos: C-1 = Chicxulub-1; S-1 = Sacapuc-1; Y-1, Y-2,
Y-6 = Yucatan-1, 2, and 6; T-1 = Ticul-1.
magnetic-field anomalies extend to the north
without significant disruption across the linea-
ment that truncates the gravity anomaly. The
central anomalous zone is slightly elongated in a
northwest-southeast direction. Modeling of the
magnetic-field anomalies places the top of the
magnetic source bodies at a depth of -1100 m
(Penfield and Camargo, 1981).
STRATIGRAPHY
The subsurface stratigraphy of the northern
Yucatan peninsula is known primarily from pe-
troleum exploration drillholes (Murray and
Weidie, 1967; Lopez Ramos, 1975, 1983;
Marshall, 1974; A. E. Weidie, Jr., et al., unpub-
lished cross section). A structurally uncompli-
cated platform sequence of nearly horizontal
Lower Cretaceous to Quaternary carbonates
and evaporites overlies a poorly known crystal-
line basement of probable Paleozoic age. In the
northern part of the peninsula the platform se-
quence is at least 3500 m thick. Three deep
exploration wells (1527 to 1631 m total depth)
have been drilled within the margins of the geo-
physical anomalies (Fig. 2). The upper sequence
ranges from Pleistocene to Paleocene in age and
is a flat-lying, conformable sequence with no
known significant stratigraphic breaks (Fig.
4).
The sedimentary facies and fauna reported from
these wells indicate a deeper-water environment
than that found elsewhere on the platform. The
rocks are fossiliferous limestones and marls with
minor shale, bentonite, and chert. The unit
underlying the Tertiary sequence is composed of
fossiliferous limestone, marl, shale, and minor
bentonite, and has been previously dated as Late
Cretaceous.
The wells penetrated coarse breccias and clas-
tic and andesitic igneous rocks at depth (Fig. 4).
In wells C-1 and S-1, limestone and bentonite
breccias (containing Cretaceous fossils), are in-
terbedded with marlstone, shale, and sometimes-
dolomitic limestones. Thin intercalations of an-
desitic glass are present in the lower part of this
unit. Well Y-6 recorded sandstone interbedded
with shale, marl, and bentonite at this level. Thick
units of andesitic glass underlie the breccias in
wells C-1 (with minor interbedded tuffs) and
S-1. Well Y-6 intersected microcrystalline ande-
sitic rock before bottoming in laminated anhy-
drite. Well C-1 also intersected microcrystalline
andesitic rocks below the andesitic glass unit.
These andesitic igneous rocks occur only in-
side the circular zone of the geophysical anoma-
lies; several other nearby drillholes (Fig. 2) have
not encountered these rocks. Possible analogues
to the limestone and bentonitic breccias com-
pose the uppermost Cretaceous stratigraphy in
other holes. For example, the Yucatan-2 (Y-2)
well, located 135 km southeast of the center of
Figure 3. Two east-northeast Bouguer gravity
profiles (parallel to coastline) across Chic-
xulub structure showing symmetrical negative
anomaly with central high (see Fig. 2 for posi-
tions of profiles). Profiles are drawn through
data points spaced at -6 km intervals. Re-
gional field decreases from east to west, as
shown by dashed line. Bottom profile is lo-
cated -10 km south of upper (central) profile.
Upper profile has been displaced 25 mgal
above its true position to separate the two.
Center of crater, two concentric gravity lows,
and suggested crater rim are indicated along
central profile. Profile from Manicouagan cra-
ter shown for comparison is from Sweeney
(1978); regional background was removed.
Vertical and horizontal scales on Manicoua-
gan profile have been expanded for compari-
son purposes.
the anomalies (Fig. 2), intersected a unit of ben-
tonitic limestone breccia from 240 to 330 m.
PETROLOGY
To investigate the origin of the andesitic igne-
ous rocks and carbonate breccias within the area
of the circular anomalies, we studied some of the
few remaining samples from petroleum explora-
tion using optical microscopy, X-ray diffraction
(XRD), and electron-microprobe techniques
(Kring et al., 1991).
Sample Y6 N14 (Fig. 5A) came from a depth
of 1208 to 1211 m in the Y-6 well (Fig.
4),
from
a predominantly sandstone unit. However, this
sample is a partly chloritized, polymict breccia
with angular to subrounded igneous and sedi-
mentary clasts in a fine-grained calcite matrix
(-25%). The sedimentary clasts (-5%) are an-
hydrite and carbonate rocks; there are also trace
amounts of small polycrystalline quartz clasts.
The igneous clasts (-65%) are dominated by a
microcrystalline groundmass consisting of alkali
feldspars (Ab
93
0r
2
An
5
to Ab
14
0r
85
An
1
), pla-
gioclase feldspars (An
1
3Ab
84
0r
3
), and augite
(Wo
5
0En
39
Fs
1 1
). About half of the igneous
clasts contain lithic inclusions and/or ropy-
868
GEOLOGY,
September 1991
C-
1
S-1
Distance
to
center
of
Structure
(km)
Y-6
20
30
4
50
um;
Pliocene-Pleistocene
1_u
ava.
waria
.....
u
mom
......
•-•
N9
N12
..Wi
■
...e.
mom.
swim Limestone
Breccia
......
emeem
.8.....
Miocene
rte
-
-
MEM=
---
----
Marl
F.71Ande5ific
--,,"-
glass
&lee
rm..
MEMO
____
-
=7
.-
=---
Ofigocene
---
-.
-
--
-
"
"
V
.
3;:d
Sandstone
:r,4+,Andesite
-
Immo
_...„_,_,„
Rtil:
memo
-
'--
Eocene
-
Paleocene
,....
umme,
.....1
_.
-
____________________
?-------_-
IINNINIMI
A
■
il
Wm=
2-
--
"Ig:2-4
--
_--
ow
MN
MIN=
IMIMIN
=ME
r..
---
-------
--
T.z
7
....... ........._ ..........
-
-,
? -------
r
inne. la
MEM/
a Nom...
MIA
MIMI
N14
..
.
-7
8-8mofAnhyddte
Figure 4. Stratigraphic columns from three deep interior wells in Chicxulub crater (see Fig. 2).
Vertical exaggeration x10. Drillholes are spaced according to their distance from estimated
center of structure. Arrows show locations of samples from well Y-6, together with sample
numbers. Ages have not
been
given to units at base of drilled section; these units have pre-
viously been regarded as Upper Cretaceous.
TABLE 1. COMPOSITION OF CHICXULUB MELT ROCKS AND HAITIAN TEKTITES
0
500
E
1000
1500
90G RANGE
60.3-67.6
0.7- 0.8
13.7-15.3
4,7-10.9
2.4- 3.8
4.7- 5.7
0.1- 0.2
2.4- 3.6
1.0- 1.8
BE*
63.1 ±2.1
0.7 ±0.1
15.2 ±0.3
7.3 ±1.7
2.7 ±0.3
5.4 ±0.4
N17
63.2 /60.5
0.4 / 0.4
12.6 /13.6
10.2 /10.5
3.1 / 3.2
4.5 / 5.0
4.0 / 4.7
1.9 / 1.9
0.08/ 0.1
Oxide
G6*
Si0
2
63.2
TiO
2
0.72
Al
2
0
3
15.5
Ca0
7.9
Mg0
2.6
Fe0
5.4
Cr
2
0
3
0.01
Mn0
0.16
Ni0
0.01
Na
2
0
2.7
K
2
0
1.7
P
2
0
5
0.27
11M*
63.2
0.8
14.7
7.1
2.8
5.3
3.3
3.6 ±0.3
1.5
1.6 ±0.1
0.2
0.14±0,06
0.1 / 0.1
Total
100.17
98.8
99.74
100.08/100.09
Note: All analyses in weight percent. Sample G6: glass from Haitian ejecta
layer residue (this work); 90G RANGE, 11M: Haitian glasses described by Izett
et al. (1990); BE: average Haitian glass (lo variation; Sigurdsson et al.,
1991); N17: slightly-altered, andesitic, melt-rock sample from the Y-6 well,
Chicxulub crater. All analyses by electron microprobe, except N17 which
was
analysed by X-ray fluorescence (reported on a sulfur-free basis). No numbers
- not determined.
* Values given are averages of multiple analyses.
textured phyllosilicates. Quartz grains in two of
these xenoliths exhibit multiple sets of planar
elements that are indicative of shock metamor-
phism. Quartz crystals (up to 1.25 mm long)
with up to eight sets of planar elements are also
present in grain residues of three breccia splits
(Fig. 5B). Debye-Scherrer XRD studies of single
grains confirm the visual identification of shock
deformation (Florz and Quaide, 1973) and indi-
cate that the quartz grains have been subjected
to shock pressures ranging from --10 to 20 GPa.
Sample Y6 N17 came from between 1295.5
and 1299 m in the Y-6 well (Fig. 4) and is
compositionally similar to andesites (Table
1). It
consists predominantly of a microcrystalline
groundmass of plagioclase feldspars (An30Ab64
Or
6
to An
4
9Ab
48
0r
3
), alkali feldspars (Ab
99
Or
o
An
i
to Ab
17
0r
82
An
1
), and augite
(Wo
4
5En
44
Fs
11
to Wo
49
En
t7
Fs
34
)• Minor
amounts of magnetite, ulvospinel, and apatite
occur interstitially. Large quartz grains or aggre-
gates of
quartz
grains are scattered throughout
the rock, each surrounded by coronas of
medium-grained augite and feldspar that appar-
ently nucleated on the preexisting quartz. Sim-
ilar coronas are common in impact-melt rocks
(e.g., Grieve, 1975), although they have also
been observed around xenoliths and quartz xe-
nocrysts in volcanic andesites and basalts (Sato,
1975). This rock lacks plagioclase phenocrysts,
but does contain pods of microcrystalline feld-
spar. Small pockets of secondary anhydrite and
calcite and quartz veins pervade the sample, in-
dicating that the unit has been altered.
Sample Y2 N6 is a series of core fragments
taken from 301 to 303 m in well Y-2 (Fig. 2).
The sample is from a 90-m-thick bentonitic
breccia, which is the uppermost Cretaceous unit
in the well (A. E. Weidie, Jr. et al., unpublished
cross section). The rock is a poorly sorted poly-
mict breccia with angular to rounded fragments
of calcareous and dolomitic limestones (some-
C
Figure 5. A: Polished slab showing polymict
breccia sample Y6 N14. Specimen is 4 cm
across.
B:
Shocked quartz grain (0.32 mm
across) from polymict breccia Y6 N14. Grain
shows eight sets of lamellae when rotated: two
sets are visible
here.
C:
Altered
ejecta clast in
polymict breccia sample Y2 N6. Note con-
torted internal fabric and crosscutting gypsum
veins. Scale shows centimetre divisions.
times fossiliferous), anhydrite, and marl; there is
also one 5-cm-diameter, crudely laminated, disc-
shaped clast of very poorly ordered smectite
(Fig. 5C). Grain residues from two different
samples of the breccia contain quartz grains with
rare grains (up to 1.1 mm long) containing sin-
gle and multiple planar lamellae typical of shock
deformation. The large smectitic clast contains
-2% quartz and feldspar grains that also have
single and multiple planar lamellae typical of
shock deformation. Debye-Scherrer XRD stud-
ies on individual quartz and feldspar grains
confirm the visual identification of shock
deformation.
MORPHOLOGY
Because the rocks that cause the geophysical
anomalies are deeply buried, the shape of the
GEOLOGY, September 1991
869
presumed circular structure is not well known.
The K/T boundary within the structure is de-
pressed —600 to 1100 m relative to its —500 m
depth in the Ticul-1 (T-1) well (Lopez Ramos,
1975), located 5 km outside the suggested
margin of the structure. A multichannel seismic
reflection line —45 km north of Progreso (Fig. 2)
shows that a rough acoustic reflector, which
probably corresponds to the K/T boundary, oc-
curs at a depth of —1500 m (PetrOleos Mexica-
nos, unpublished data). These indicators of a
depressed K/T boundary in the area of the cir-
cular anomalies, together with the presence of
relatively deep water facies, suggest that a basin
existed there in early Tertiary time. No obvious
topographic or bathymetric expression of the
circular structure exists now, although the struc-
ture may influence the near-surface ground-
water regime (Pope et al., 1991).
ORIGIN
We believe that the geophysical, stratigraphic,
and petrologic evidence described here strongly
indicates the presence of a buried impact crater.
The circular negative Bouguer-gravity anomaly,
with a central high and two concentric lows, is
consistent with an —180-km-diameter peak-ring
crater. The andesitic glasses and microcrystalline
rocks interbedded with polymict breccias are
probably a sequence of interbedded impact-melt
rocks and impact breccias. Assuming a specific
gravity contrast of 0.3 g/cm
3
(based on values
determined for breccia sample Y6 N14 vs. the
overlying sediments), an —2 km breccia thick-
ness would be required to produce the observed
—25 mgal negative gravity-field anomaly.
Therefore, the 200-450-m-thick breccias overly-
ing the andesitic rocks may produce the rever-
sals in the gravity profiles by forming two peak
rings, but most of the anomaly must be the result
of some other mass deficiency, such as fractured
basement beneath the crater. The breccias and
the andesitic rocks may be the cause of the
magnetic-field anomalies. Samples Y6 N14 and
N17 have magnetic susceptibilities of 2.5 x 10
-4
and 1 x 10
-3
(cgs units), respectively, which is in
contrast to the 10
-5
to <10
-6
(cgs units) suscep-
tibilities of the overlying sediments. Therefore,
the andesitic rocks may produce, at least in part,
the central high-amplitude anomalies, whereas
the intracrater breccias and proximal ejecta may
produce the small-amplitude anomalies that ex-
tend —15 km past the margin of the gravity
anomaly (and suggested crater rim). The depres-
sion of the K/T boundary by —600 m indicates
the presence of a basin with a depth consistent
with the strength-limited depth of —500 m ex-
pected for terrestrial impact craters (Melosh,
1989).
The Y6 N14 and Y2 N6 polymict breccias
contain shocked quartz, indicating production
by impact. The presence of both shocked and
unshocked material in the Y6 N14 breccia and
the absence of a glassy matrix (although a glass
matrix could have been altered) suggests that the
unit is a sedimentary breccia, which is consistent
with a marine impact. The underlying Y6 N17
rock does not contain unambiguous evidence of
shock-induced melting, but the presence of
quartz and quartz-aggregate xenoliths, and the
absence of plagioclase phenocrysts and ortho-
pyroxene, are unusual for volcanic andesites.
Microcrystalline melt rocks, apparently resulting
from impact-melt bodies of sufficient size to re-
crystallize, are known at much smaller impact
craters, such as the 28-km-diameter Mistastin
Lake crater (Grieve, 1975). The required vol-
umes of melt could easily be produced at a
crater of this size; using a melt-volume scaling
relation (Melosh, 1989) and a transient crater
diameter of 110 km, we calculate a lower limit
to the ratio of melt volume to the displaced-mass
volume of 0.3. The 6 to 8 m of anhydrite inter-
sected at the base of 380 m of andesitic rocks in
well Y-6, located —50 km from the crater cen-
ter, may indicate that this is the outer and
thinner part of the melt sheet.
We suggest that the breccias recorded at the
K/T boundary exterior to the crater, such as the
90-m-thick breccia in well Y-2, represent the
proximal ejecta blanket, because the Y2 N6
breccia contains evidence of shock metamor-
phism. The large smectitic clast is probably al-
tered impact-melt ejecta, which may be analo-
gous to Muong Nong tektites or to the Ries
crater suevite bombs. The ejecta was probably
mixed with locally derived sediment when it
was deposited and/or was disturbed by impact
waves. The back surge filling the crater may also
have extensively modified parts of the ejecta
blanket.
AGE
Although the available stratigraphic informa-
tion provides some bounds, the age of the crater
is not precisely known. Former work suggested
a Late Cretaceous age for the 60 to 170 m of
limestone and marl overlying the presumed im-
pact breccias and melt rocks (Lopez Ramos,
1975). However, G. Keller and W. Sliter inde-
pendently determined a late Paleocene (P3) age
(based on foraminifera) for sample Y6 N12
(Fig. 4) from the limestone-marl unit (from
1000 to 1003 m depth), although the preserva-
tion is not good. Therefore, the earlier age as-
signment is probably invalid, and the top of the
breccias could be of K/T boundary age. In addi-
tion, the 90-m-thick boundary breccia in well
Y-2 probably corresponds to the crater's prox-
imal ejecta blanket, thus indicating that the
crater formed at the KIT boundary.
RELEVANCE TO THE
K/T BOUNDARY
The size and location of the Chicxulub struc-
ture come close to satisfying the characteristics
necessary for the KIT crater (Hildebrand and
Boynton, 1990b). Because its calculated maxi-
mum excavation depth is —15 km (Melosh,
1989) and the crust under the Yucatan platform
is at least 30 km thick (Ewing et al., 1960), the
crater could have excavated only continental
crust. It is an attractive candidate for supplying
the shocked quartz and alkali-feldspar grains
found in the boundary layers (Izett, 1990), and it
is well located to produce the observed size dis-
tribution and fluence of shocked grains, which
are largest at Caribbean K/T sites (Hildebrand
and Boynton, 1990a). The proportion of the
shocked grains in the mineral residue from the
Y6 N14 breccia and the Y2 N6 ejecta clast are
27% and 31%, respectively, within the range of
12% to 47% reported for 12 K/T sites (Izett,
1990).
The Chicxulub crater is also an attractive
source for the K/T boundary tektites (Izett et al.,
1990; Sigurdsson et al., 1991), particularly be-
cause the Haitian K/T tektites are deficient in
silica relative to most tektites (Koeberl, 1986),
and the Chicxulub crater has melt rocks of the
necessary intermediate composition (Table 1).
The composition of the Chicxulub andesitic
rock is within the range observed for the K/T
tektites, except for its lower Al
2
03 and TiO
2
content. In addition, the carbonate rocks of the
Yucatan platform are a good candidate source
for the high-Ca tektites reported from the Hai-
tian K/T sites (Sigurdsson et al., 1991).
However, the Chicxulub crater may not be
the sole source of the boundary layers, because
isotopic data (Shaw and Wasserburg, 1982)
suggest that a mantle component occurs in the
boundary layers, and the presence of unaltered
pyroxene spherules (Smit, 1990) establishes that
there is at least a small ultramafic component.
Another potential difficulty is that the Chic-
xulub andesitic rock Y6 N17 yields an
ENd
(65
Ma) of —5.0, which is lower than values of —2.9
(65 Ma) found in altered boundary clays
(Hildebrand and Boynton, 1988) or —2.8
(66 Ma) found in tektite glass (Promo and lzett,
1991). If the Chicxulub crater were the source of
the boundary ejecta, some Nd isotopic hetero-
geneity would be required in the andesitic melt
rocks. However, the
Sr
E
(65 Ma) of the andesitic
rock (+55) is similar to that of the Haitian tek-
tites (+62).
The Chicxulub crater is well located to pro-
duce the known K/T boundary ejecta. This
crater is approximately equidistant between the
two thickest known deposits, the 0.5-1.2-m-
thick K/T ejecta found on Haiti (Hildebrand
and Boynton, 1990b), which was to the south-
east at K/T time, and the 0-1.0-m-thick ejecta
layer found in northeastern Mexico by
J.
Smit,
A. Montanari, N. Swinburne, and W. Alvarez
(W. Alvarez, personal commun.). The Chic-
xulub crater might not have produced the
observed suite of impact-wave deposits (Fig. 1),
870
GEOLOGY, September 1991
because it formed on a shallow-water platform a
minimum of -200 km from the deep sea, and
the size of impact-induced waves is limited by
the water depth at the impact site. Nevertheless,
because the Chicxulub impact would be seismi-
cally equivalent to a magnitude 10 to 11 earth-
quake (McKinnon, 1982), it may have triggered
the formation of the deep-water slump and
turbidity-current deposits (which may be over-
lain by crater ejecta) observed at the K/T
boundary at Deep Sea Drilling Project (DSDP)
Sites 94, 95, 536, and 540, adjacent to the
Campeche bank (Fig. 1; Buffler et al., 1984).
The coarse boundary deposits in the trough
present at KIT time between Cuba and the Ba-
hamas may be seismically triggered slumps, as
suggested by Pszczolkowski (1986). However, it
might have been difficult for this impact to pro-
duce the distant impact-wave deposits, such as at
Brazos River, Texas, or DSDP Sites 603B, 151,
and 153, if the ocean on the Yucatan platform
was too shallow at K/T time; therefore, a
nearby, deep-water impact may have occurred
at the same time.
The Chicxulub impact, having presumably
produced the largest impact crater on Earth,
must have caused a mass extinction, assuming
that any impact can do so. In addition to prompt
lethal effects, it should have produced a dra-
matic greenhouse warming by the shock produc-
tion of CO
2
from the >3-km-thick carbonate
target rocks, as suggested by O'Keefe and
Ahrens (1989). Crater scaling relations (Melosh,
1989) and the size of the Chicxulub crater sug-
gest that an order of magnitude increase in CO
2
above the current atmospheric inventory would
have occurred. A temperature increase of -10
°C for 10
4
to 10
5
years could have resulted
(O'Keefe and Ahrens, 1989), causing an ex-
tended period of extinctions. The period of
warmer climate could also allow for increased
continental erosion to produce the K/T bound-
ary
8
7
Sr/
86
Sr spike (Hess et al., 1986).
CONCLUSIONS
The Chicxulub crater is the largest probable
impact crater on Earth. Its position and target-
rock composition satisfy many of the character-
istics required for the K/T crater, and it may
have a K/T boundary age. This impact may
have caused the K/T extinctions.
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ACKNOWLEDGMENTS
Supported in part by NASA Grant NAG9-37. We
thank G. Keller and W. Sliter for paleontological stud-
ies. A. Weidie provided access to samples and unpub-
lished stratigraphic information. Petroleos Mexicans
allowed discussion of proprietary seismic reflection
data. X-ray fluorescence data are courtesy of A. Hein-
rich, R. Rousseau, and the Geological Survey of Can-
ada. Magnetic susceptibility data are courtesy of
G. Calderone and the U.S. Geological Survey.
Manuscript received February 26,1991
Revised manuscript received May 22,1991
Manuscript accepted May 30,1991
Reviewer's comment
Proposes the long-sought K/T crater-the "smoking gun."
GEOLOGY, September 1991
Printed in U.S.A.
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