MIAC
Chicxulub Crater, Mexico, and
the Cretaceous - Tertiary boundary


Gravity measurements over the Chicxulub crater

Regional Bouguer gravity anomaly map showing most of the Yucatán peninsula (North is up.). The outline of the peninsula is indicated by a thin dark line. Colour indicates the value of the gravity anomaly - warm colours (purple and red) represent gravity highs, cool colours (green and blue) represent gravity lows. The horizontal gradients of the Bouguer gravity anomaly (the positions of changes in the gravity anomaly values) are shown as grey shading with darker shading corresponding to larger changes. This gravity data set was compiled by the Decade of North American Geology project and consists of survey results gridded at ~6 km spacings. Note that approximately half of the Yucatán block of continental crust (which is outlined by a thick, nearly black gradient feature) is covered by a shallow sea.

The Chicxulub crater lies buried, straddling the northwest coastline of the Yucatán peninsula. In this plot three concentric gradient features outline a circular gravity low ~180 km in diameter. The crater's negative anomaly is complicated by regional anomalies, including a low extending to the south and fan-shaped anomalies extending to the north. The crater was named Chicxulub because its center lies near the coastal town of Puerto Chicxulub (Image courtesy Geological Survey of Canada).



Three-dimensional Bouguer gravity anomaly map over the Chicxulub crater (North is up.). Compare with the regional Bouguer map also showing the shaded horizontal gradient. The crater is represented by the near-circular low central to the figure (with contained concentric structure). Notice that regional gravity anomalies interfere with the circular pattern (fan shape to the north and double low to the south) and that a gravity low expressed as a trough occurs to the east side. The origin of most of the regional anomalies is not yet known, but correlations to regional magnetic anomalies suggests that many are related to variations in the basement rocks of the peninsula.

Note that this image is not showing the shape of the crater; the negative gravity anomaly of the crater corresponds to the relatively low densities of the rocks within the crater (breccias and the melt sheet) and the Tertiary sediments filling the crater. The double humped central gravity high corresponds to the central uplift buried deep within the crater. The Chicxulub crater has no central topographic feature - the central peak characteristic of smaller complex craters. Chicxulub is a peak-ring crater with a poorly known topographic ring occurring at 40 to 45 km radius from the crater's center. A few minutes after the impact the crater probably looked like a slightly larger version of the 165 km-diameter peak-ring crater Strindberg on Mercury . Subsequent erosion probably substantially removed the rim of Chicxulub before it was buried. (Image courtesy Geological Survey of Canada)


Horizontal gradient map of the Bouguer gravity anomaly over the Chicxulub crater (North is up.). The coastline is shown as a white line. A striking series of concentric features reveals the location of the crater. This image was constructed from gravity measurements taken by Petróleos Méxicanos beginning in 1948 in the course of petroleum exploration augmented by recent work of researchers from the Geological Survey of Canada, Athabasca University, the Universidad Nacional Autónoma de México, and the Universidad Autónoma de Yucatán. These recently acquired data were taken to map out detailed crater structure. All data were gridded at 750 m intervals before the horizontal gradient was computed. Most of the concentric gradient features can be related to inferred structural elements of the buried crater, including the central uplift (note the radial features revealed in the uplift), the collapsed transient cavity edge, faults in the zone of slumping, and the edge of the topographic basin - the now buried crater.

White dots represent the locations of water-filled sinkholes (solution collapse features common in the limestone rocks of the region) called cenotes after the Maya word dzonot. A dramatic ring of cenotes is associated with the largest peripheral gravity gradient feature. The cenotes of the ring are typically larger than those found elsewhere on the peninsula. The sinkholes developed when sea level was lower during the Pleistocene glaciation, becoming water-filled when sea level returned to its present level. The ring represents a zone of high permeability where groundwater can flow to the sea creating coastal freshwater springs at the east and west sides of the crater. The origin of the cenote ring remains uncertain, although the link to the underlying buried crater seems clear. The cenotes of the ring are developed in near-surface Tertiary limestones overlying the crater, and are not directly related to the rocks of the crater. Somehow the crater is able to reach up through several hundred metres of sediment, and tens of millions of years of time, to influence groundwater flow. Some form of subsidence controlled by peripheral structure of the crater may have induced fracturing in the much younger rocks that cover the crater. The fracturing could then initiate the groundwater flow that caused the cenotes to form. This subsidence may be continuing today. Note that the crater is able to influence modern erosion of the sediments that bury it. The edges of the crater correspond to a notch in the coastline in the east, and to a sharp bend southwards in the west. Also, the cenote ring corresponds to a topographic low of up to 5 metres along much of its length. (Image courtesy Geological Survey of Canada)



A perspective plot of the ~180 km-diameter Chicxulub crater with a cutaway view showing a cross section of the crater as revealed by seismic reflection data. This view is looking to the south; the Yucatán coastline is shown by a thin dark line. Note the different horizontal and vertical scales such that the cross section has a vertical exaggeration of approximately ten times. The vertical scale is approximate as seismic velocities vary with depth and rock type.

The top surface shows the horizontal gradient of the Bouguer gravity anomaly similar to that shown in figure 3. The circular gradient features that correspond to crater structures lose definition near the coast because no gravity field surveying has been done in the coastal shallow water, leading to an approximately 20 km-wide gap in data coverage. The interpretation of seismic reflection data shown on the face of the cutaway is from Camargo and Suarez (1994), and is shown along the line where it was collected by a shipbourne survey. The seismic data reveal much of the crater's structure including the ~1 km-thick Tertiary sediments filling the crater, the crater edges, the peak ring, and the down-dropped blocks in the crater's zone of slumping. Note the contortion of the sediments in the down-dropped blocks. The central zone where no deep structures may be seen corresponds to the crater's collapsed transient cavity. (Image courtesy Geological Survey of Canada)


Tektites produced by the impact

Altered tektites produced by the Chicxulub impact as preserved at the Dogie Creek, Wyoming, Cretaceous-Tertiary boundary locality. The scale bar shows divisions of millimetres. At this locality the ejecta layer from the Chicxulub crater is ~2 cm thick and at some places preserves the shapes of the individual tektites that compose the layer. Originally composed of glass, the tektites have been pseudomorphed (altered and replaced by a secondary mineral while their shape is preserved) by phosphate-bearing minerals (most commonly goyazite) at this site. The tektites are predominantly spheres at this locality; this photograph includes only one egg-shaped individual. (Image courtesy Geological Survey of Canada)


Unaltered tektite glass separated from the Cretaceous-Tertiary ejecta layer as preserved at the Mimbral locality, Tamaulipas state, Mexico. Although the tektite glass has been altered at most localities, a handful of sites preserved in marine sediments have been found where a remnant of the glass is preserved, particularly in carbonate-rich parts of the ejecta layer. The southernmost sites found near Beloc, Haiti contain the greatest abundances of unaltered tektite glass. Mimbral represents the second most prolific locality for tektite glass recovery.

This photograph shows the dark brownish green glass which is the most commonly preserved material. The yellow and amber grains represent partially hydrated glass, the hydration is apparently the first step towards alteration/replacement. (Taken by Alan Hildebrand)


Shocked quartz grains produced by the impact

A 0.32 mm shocked quartz grain from intracrater breccia sample Y6 N14 of the Chicxulub crater. The drill hole Yucatán-6 was located ~50 km from the crater's center and penetrated ~500 metres of impact melt and breccias at its base. The melt and breccia units contain clear evidence of production by impact, including mineral grains showing evidence of shock metamorphism. In the mineral quartz the passage of a strong shock wave can cause dislocation of the grain's crystal structure along preferred crystallographic orientations. This quartz grain shows at least 8 sets of planar deformation features when rotated; two strong sets (and part of a third set) of shock lamellae are visible in this orientation. The lamellae are decorated with inclusions. Impact is the only natural process known to produce shock waves of sufficient strength to cause deformation of this type. (Taken in cross-polarized light by Alan Hildebrand)


Outcrop of the Cretaceous-Tertiary (K/T) boundary

Outcrop of strata crossing the Cretaceous-Tertiary (K/T) boundary at Brazos River, Falls County, Texas; note pick and shovel for scale. The pick rests on a rippled sandstone bed, one of several coarse units found immediately below the paleontological K-T boundary at this locality. The base of the coarse beds is an erosional sequence that is overlain by a sedimentary breccia containing clasts up to one metre in size. The sequence fines upwards to a 0-7 mm dark, calcareous mudstone with associated geochemical anomalies (iridium, gold, rhenium, arsenic, antimony and selenium) that are characteristic of the K/T fireball layer.

The sequence of coarse beds is unique in seven million years of Upper Cretaceous and Lower Tertiary stratigraphy exposed along the Brazos River near here and represents a highly unusual geological event. The enclosing strata form a nearly monotonous sequence of mudstones deposited on the middle to outer continental shelf in 100 to 150 metres of water. The coarse boundary sequence apparently represents the deposits produced by a series of giant waves produced by the Chicxulub impact as they rolled across the continental shelf of North America before washing up on the shore. The thin bed with trace-element anomalies apparently represents the fine fraction of the fireball layer that settled out on top of sediments transported and deposited by the giant waves. (Image courtesy Geological Survey of Canada)

The coarse boundary sequence would all comprise part of the Chicxulub formation as defined by Hildebrand (1992); "...the Chicxulub Formation is defined as the units of rock deposited by the Chicxulub impact including material ejected from the crater, fragments of the projectile if they exist, and secondary deposits produced by the effects of the impact such as seismicity and impact waves." Historically, the significance of this Brazos River deposit is that it is the first locality recognised as having an impact wave deposit. This represented the first step in the process which located the K/T impact to the region between the Americas. (Taken by Alan Hildebrand)



Text and images by Alan Hildebrand, Geological Survey of Canada.. Edited by Michael Higgins