The Chicxulub Impact Structure, Yucatán Peninsula, México. Studies in the Eastern Merida -Valladolid Sector

Jaime Urrutia Fucugauchi Director, Instituto de Geofísica, UNAM. Email: juf@tonatiuh.igeofcu.unam.mx José María Chávez Aguirre Supervisor de Geología, Depto. de Geología, GEIC-CFE . Email: jose.chavez@cfe.gob.mx José Luis de la Rosa Residente, Residencia de Geohidrología, CFE, Mérida, Yucatán. Email: respenin@mda.com.mx

Abstract

The Chicxulub crater is located in the northwestern sector of the Yucatan peninsula, buried by a

thick sequence of Tertiary carbonate rocks. The crater was first identified from geophysical studies

carried out during oil exploration surveys in the peninsula. For several years and particularly since 1980

the Federal Commission of Electricity (CFE) has conducted geohidrological and geotechnical

investigations in Yucatan. Studies have concentrated on the groundwater aquifers and karstic landforms.

The semicircular distribution of sinkholes has been related to the crater structural features (the cenote

ring correlates well with circular gravity anomaly pattern). Chemical analyses of water samples from the

saline intrusion/groundwater interface show high concentrations of elements like arsenic, cobalt, nickel,

and strontium. Studies are being conducted to document their sources. As part of the CFE program,

several boreholes were drilled in the eastern sector between Merida and Valladolid. Distribution of

sinkholes presents a more complex pattern in this area (as compared with the well-defined ring to the

south and west) and there is little information on the shallow stratigraphy and occurrence of impact

breccias. Here we report results from petrographic studies on samples from eleven boreholes. Boreholes

reach depths down to >300 m, and cut a thick carbonate sequence that has been divided into the Carrillo

Puerto and Chichen Itza Tertiary Formations. Beneath about 250 m depths, the carbonate rocks show

effects of tectonic deformation and fragmentation. An interval of about 34 m of carbonate breccias has

been recovered, which may correlate with the carbonate breccia sequence observed in the Santa Elena

and Tekax UNAM boreholes. Apparently undisturbed limestone strata are observed beneath the breccia

interval.

Introduction

The Chicxulub structure is currently interpreted in terms of a large bolide impact, occurring some

65 Ma ago (Hildebrand et al., 1991; Sharpton et al., 1992). The impact produced a large >200 km

diameter crater, as well as significant structural deformation on the target area and adjacent extensive

carbonate platform. The Chicxulub impact has been linked to the worldwide distributed Ir-rich clay layer

that has been the stratigraphic

marker of the Cretaceous/Tertiary

(K/T) boundary. The K/T layer

formed as a result of the

deposition of fine-grained

particle material sent out high

into the atmosphere by the

impact, which blocked incoming

solar radiation causing severe

global environmental and

climatic effects on the life

support systems (Alvarez et al.,

1980).

The Chicxulub crater is

located in the northwestern sector

of the Yucatan peninsula, buried

by a thick sequence of Tertiary

carbonate rocks (Figure 1).

Figure 1.- Chicxulub

crater and cenotes location in the

Yucatan peninsula.

The crater was first identified from geophysical studies carried out during oil exploration surveys

in the peninsula (Penfield and Camargo, 1981). Samples from the impact breccias and melt were

recovered from the exploratory PEMEX drilling program (e.g., Lopez Ramos, 1983). Samples from this

program are available in limited amounts, however. More recently, UNAM conducted a drilling program

in the southern sector of the crater, and in three boreholes the thick breccia sequence was sampled.

Analyses indicate that the breccia is formed by two distinct sequences: an upper breccia rich in basement

and melt fragments and a lower breccia rich in carbonate fragments. The upper breccia presents high

magnetic susceptibility, low seismic velocities, low density and low porosity and permeability (e.g.,

Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyra et al., 2000). The upper breccia appears similar to the

suevitic breccias documented in the Ries crater.

For several years and particularly since 1980 the Federal Commission of Electricity (CFE) has

conducted geohydrologic and geotechnical investigations in Yucatan. Studies have focussed on the

groundwater aquifers and karstic landforms, and also include an exploration shallow drilling program

(Figure 2). In this paper we report on the initial results of the CFE studies in the eastern sector of the

crater in the area between Merida and Valladolid.

Figure 2.- Chicxulub impact and UNAM and CFE boreholes location.

Eastern Sector of Chicxulub Crater

The semicircular distribution of sinkholes has been related to the crater structural features (the

cenote ring correlates well with circular gravity anomaly pattern). Chemical analyses of water samples

from the saline intrusion/groundwater interface show high concentrations of elements like arsenic, cobalt,

nickel, and strontium. Studies are being conducted to document their sources. As part of the CFE

program, several boreholes were drilled in the eastern sector between Merida and Valladolid. Here we

report results from petrographic studies on samples from eleven boreholes. Boreholes reach depths down

to >300 m, and cut a thick carbonate sequence that has been divided into the Carrillo Puerto and Chichen

Itza Tertiary Formations (Figure 3).

m

Recrystalized limestone

Dolomitic limestone

Green mudstone

Fossiliferous limestone

Figure 3. Carbonate sequence after CFE boreholes.

Beneath 250 m depths, the carbonate rocks show effects of tectonic deformation and

fragmentation. An interval of about 34 m of carbonate breccias has been recovered, which may correlate

with the carbonate breccia sequence observed in the Santa Elena and Tekax UNAM boreholes (Figure 4).

Apparently undisturbed limestone strata are observed beneath the breccia interval.

Figure 4.- CFE exploratory boreholes between Valladolid and Mérida.

Discussion

The Chicxulub crater is located in the northwestern sector of the Yucatan peninsula, buried by a

thick sequence of Tertiary carbonate rocks. The Chicxulub impact has been linked to the worldwide

distributed Ir-rich clay layer that has been the stratigraphic marker of the K/T boundary. The K/T layer

formed as a result of the deposition of fine-grained particle material sent out high into the atmosphere by

the impact, which blocked incoming solar radiation causing severe global environmental and climatic

effects on the life support systems. The crater was first identified from geophysical studies carried out

during oil exploration surveys in the peninsula. Samples from the impact breccias and melt were

recovered from the exploratory PEMEX drilling program. Samples from this program are available in

limited amounts, however. More recently, UNAM conducted a drilling program in the southern sector of

the crater, and in three boreholes the thick breccia sequence was sampled. Analyses indicate that the

breccia is formed by two distinct sequences: an upper breccia rich in basement and melt fragments and a

lower breccia rich in carbonate fragments. The upper breccia presents high magnetic susceptibility, low

seismic velocities, low density and low porosity and permeability. The upper breccia appears similar to

the suevitic breccias documented in the Ries

crater. 256.00m

Federal Commission of Electricity has

conducted geohidrological and geotechnical

investigations in Yucatan. Studies have

focussed on the groundwater aquifers and

karstic landforms. The semicircular

distribution of sinkholes has been related to the

crater structural features (the cenote ring

correlates well with circular gravity anomaly

pattern). Chemical analyses of water samples

from the saline intrusion/groundwater interface

show high concentrations of elements like

arsenic, cobalt, nickel, and strontium. Studies

are being conducted to document their sources.

As part of the CFE program, several boreholes

were drilled in the eastern sector between

Merida and Valladolid. We report results from

petrographic studies on samples from eleven

boreholes (Figure 5).

Figure 5.- Samples of the exploratory borehole BE-4, C.T. Valladolid, Yuc. 278.40m

Boreholes reach depths down to >300 m, and cut a thick carbonate sequence that has been divided into

the Carrillo Puerto and Chichen Itza Tertiary Formations. Beneath 250-m depths, the carbonate rocks

show effects of tectonic deformation and fragmentation (Figure 6). An interval of about 34-m of

carbonate breccias has been recovered, which may correlate with the carbonate breccia sequence

observed in the Santa Elena and Tekax UNAM boreholes.

Brecciated limestone Recrystalized brecciated limestone (274.00m) (274.40m)

Intraclastic brecciated limestone Dolomitic micrite (279.70m) (280.40m)

Figure 6.- Microscopic views of thin sections of samples of the BE-4 CFE exploratory borehole.

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