The approximately circular
geophysical anomaly, now known to define the Chicxulub structure, was originally
identified on the northern edge of the Yucatan Peninsula during oil surveys
in the 1950's. Several exploratory wells, all dry, were subsequently
drilled in and around the structure, which is buried beneath ~1km of Tertiary
carbonate platform sediments. Two of the boreholes penetrated a crystalline
melt rock,
which was initially interpreted
to be an extrusive andesite.
Chicxulub
Drilling Sites
This map shows the locations
of wells drilled into the Chicxulub crater and surrounding impact ejecta.
The red circles show boreholes drilled by Petroleos Mexicanos (PEMEX) as
part of their oil exploration program and the locations of boreholes from
the scientific drilling program conducted by the Universidad Nacional Autonoma
de Mexico (UNAM).
Additonal geophysical data, collected many years later, led researchers Penfield and Camargo Z., working for Petroleos Mexicanos (PEMEX), to suggest the structure might be the result of a mid-plate igneous plume or extrusives associated with an impact event. Later petrologic and geochemical studies by Hildebrand, Kring, and Boynton at the University of Arizona, working with Penfield, Camargo Z., Pilkington from the Canadian Geological Survey, and Stein Jacobsen from Harvard University, proved this hypothesis correct. Seismic, gravity and magnetic data define a structure ~180 km in diameter.
The seismic reflection map
shows the locations of offshore reflection lines in solid blue. The white
dots show the locations of wide-angle seismic receivers. Adapted from
Figure 1 Morgan et al. in Nature (1997).
The seismic refraction map shows the location of the Leicester Univ. seismic array. Broadband seismometers are indicated by green squares and short period seismometers are shown in blue. Adapted from Figure 1, P.K.H. Maguire et al. in Meteorites: Flux with Time and Impact Effects (1998).
The magnetic anomaly map shows
the location of magnetotelluric profiles as green lines, with each sounding
station marked with a green dot. Adapted from Figure 4, Campos-Enríquez
et al. inThe Leading Edge (1997).
The Bouguer gravity anomaly at Chicxulub is largely negative and is similar to those at other impact craters which are produced by relatively low-density allochthonous breccia deposits filling the craters and by underlying autochthonous units which have relatively low densities because they were fractured and brecciated during the impact event. The Chicxulub structure also consists of a centrally-located positive anomaly defined by two concentric negative anomaly troughs with diameters of ~70 and 120 km, reflecting the peak-ring structure typical of complex impact craters. The central positive gravity anomaly and the surrounding ring(s) are probably produced by relatively high density crystalline rock that was uplifted during the rebound of the crater cavity.
Gravity
Anomaly Map
Bouguer gravity data from
the northwest corner of the Yucatán Peninsula, Mexico. Adapted from
Figure 2, Hildebrand et al. in Geology 1991. The semi-circular blue and
yellow feature is produced by the impact crater. The linear blue structure
south of the crater is a deeper structural feature in the Earth's crust
that existed before the impact event.
One of the wells, Yucatan-6, penetrated the flank of the ~100 km ring and samples of the polymict breccia and melt rock were recovered. The polymict breccia consists of igneous-textured microcrystalline clasts with shocked quartz xenoliths, and vesicular and ropy textured phyllosilicate clasts which look like altered fragments of glass, both of which were produced by shock melting. The breccia also contains clasts of sedimentary carbonates and evaporites, recrystallized sandstone, granitic gneisses and mica schists in a carbonate-rich matrix.
Examples of an unshocked quartz
grain (left) and a shock-metamorphosed quartz grain (right). The planar
features cross-cutting the shock-metamorphosed grain are essentially microcrystalline
faults. The shock metamorphosed grain was ejected from the Chicxulub crater
and found in Haiti.
Photo micrograph of unshocked quartz grain (left) and shock metamorphosed
quartz grain (right).
