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Geologists and archaeologists searching in 1987 for limestone sinkholes (cenotes) used as water sources at ancient Mayan sites in northern Yucatán discovered the perimeter of the 200 km-wide Chicxulub impact crater that, some 65 million years ago, may have played a major role in the extinction of the dinosaurs. These giant reptiles (fig.1) flourished during the Mesozoic era (Triassic, Jurassic, and Cretacous geological periods) when much of the Earth's landmass was joined in supercontinents called Laurasia and Gondwanaland, and large portions of today's North American Atlantic coast were underwater.
[Fig.1: Triceratops, a horned dinosaur from the Late Cretaceous (US National Museum; photo Athena Review).]
During the 160 million years of the Mesozoic era several episodes of impact cratering are known. The idea of linking the dinosaurs' demise at the end of the Cretaceous era (135-65 myr) to a giant, catastrophic impact event which caused sudden cooling of the atmosphere and an increase in sulfur content gained widespread acceptance with the 1980 Nobel Prize-winning theory by Louis and Walter Alvarez. This focused on the marked increase of iridium and other rare earth elements found in stratigraphic layers at the geological division of the Cretaceous and Tertiary eras, called the K-T boundary. The fact that iridium is more abundant in comets and asteroids than on the Earth strongly suggested an extraterrestrial, intrusive source for these elements in the time frame of the K-T stratigraphic boundary.
[Fig.2: Positions of Earth's continents at the end of the Cretaceous era, 65 million years ago.]
The giant meteor causing the Chicxulub Crater in Yucatán, becoming better known through study of subsurface gravitational anomalies, is presently the best candidate for that source. This crater, variously estimated at 180-300 km in diameter, is one of the largest impact structures known on Earth, or other planets in the solar system such as Venus, where many craters have been studied. The center of the Yucatán crater has been located at latitude 21º30' N, longitude 89º50' W, near the village of Chicxulub on the Caribbean coast near Progresso. While there is general agreement on the chronology of the impact event some 65 myr ago, based on results of dating the isotopic decay of argon and potassium in the rocks, determining the size and outer perimeters of the crater have been more difficult, since visible outcrops of the impact on land are limited to secondary, erosional features including cenotes. These sinkholes or natural wells (Maya dz'onot) are 30-500 m in diameter (fig.4).
[Fig.3: Chicxulub crater in northern Yucatán, and major archaeological sites.]
Cenotes are among the most common features in the limestone topography of the Yucatán peninsula. Hundreds have been found across NW Yucatán with clusters around the perimeter of the ancient crater (figs.4,6). The gradual dissolving of upper limestone layers overlying broken-up rocks, or breccia, around the crater's original impact rim has resulted in the creation of a large number of cenotes. These cenotes later provided water sources in northern Yucatán between the Puuc hills and Chichén Itzá, used by ancient Maya towns and ceremonial centers. Other sites in the Puuc region such as Kabah and Sayil which lacked cenotes used man-made cisterns (chultuns) or reservoirs (aguadas). Chultuns are common at lowland Maya household sites in both Yucatán and the Peten of northern Guatemala.
[Fig.4: The cenote at Chichén Itzá, a Late Classic and Postclassic Maya site in Yucatán (photo: Athena Review).]
The Yucatán peninsula is a low-lying limestone platform which arose out of shallow seas during the Pleistocene, 2 million years ago and later. The crater site is thus buried under Quaternary (Pleistocene and Holocene) carbonate sediments from 0.6 to 1.0 km thick, lying over Tertiary sandstones and volcanic rocks which, in turn, immediately overlie the impact site. The northern half of the now-buried crater today lies in shallow Caribbean waters of the Sea of Campeche (figs.3,5).
[Fig.5: Map of the shorelines of 65 million years ago at the end of the Cretaceous (green), vs. today's coastlines (grey).]
Survey work related to oil exploration by Glen Penfield and Antonio Camargo in 1981 first identified concentric rings of gravitational anomalies around the northwest coast of Yucatán. Independently, fragments of melted glass and rock from the ancient impact have been found throughout the Caribbean and Gulf of Mexico, and as far as Wyoming. These glass fragments, or tectites, at the K-T boundary were used to support the original Alvarez theory and suggested a major impact around Yucatán even before the Chicxulub Crater was discovered. Besides the value of the Chicxulub impact to explain catastrophic events causing the end of the dinosaur era on earth, of equal importance is comparative data useful for planetary geology. According to recent views, when the huge meteor or asteroid fragment hit the carbonate platform of the Yucatán shelf, large amounts of sulfur and carbon were released. Grieve (1990) proposed that droplets of sulfuric acid blocked out the sun and poisoned the oceans, while carbon dioxide enhanced the greenhouse effect warming the earth. A partly similar effect is thought to have caused the thick, sulfur-laden atmosphere of Venus, whose surface shows many large impact craters.
Since initial discovery of the crater, a number of projects have converged to identify its size and nature. Examining the available gravity data, Hildebrand (1991) located two concentric rings which suggested an outer rim of 180 km, matching the ring of cenotes first mapped by Charles Duller in 1987 while searching satellite imagery for ancient Mayan water sources. In a more detailed survey, Sharpton (1993) found 4 concentric rings of Bouguer anomalies, indicating a 200 km-wide basin with inner rings 104 and 154 km in diameter, and a 300 km periphery. Hildebrand (1995), while mapping shallow structures in the crater, rather than the deeper underlying strata, showed six rings within his proposed 180 km diameter. He also noted that the cenotes along the rim are generally larger than those found elsewhere in the Yucatán peninsula.
[Fig.6: Gravity map of Chicxulub crater, showing outline of Yucatán coast. White dots are cenotes, often concentrated along the crater rim (after Hildebrand et al 1995).]
More recent research by Morgan et al. (1997) employed seismic reflection data from the offshore portion of the crater to obtain a clearer picture of the crater's shape and size. The transient crater, or hole from the initial impact (fig.3), appears to have been 85 km in diameter, caused by a 10-14 km meteor. The overall crater would have included three rings: a peak ring 80 km in diameter, a 130 km inner ring, and a 195 km outer ring. When newly formed, this structure would have resembled other multi-ringed craters, as on Venus, Mercury, Europa, or the Moon. Beads of altered green glass called tektites probably related to the formation of Chicxulub Crater have also been found in Belize 480 km from the crater (Ocampo and Pope 1998). Similar tektites, formed from the heat of the Chicxulub impact, are scattered as far afield as Haiti and north Mexico.
From such evidence we learn that Earth, like its neighboring planets Mars and Venus, is covered with ancient craters, usually much obscurred by weathering, vegetation, or water, yet detectable through gravitational, radar, seismic, and other remote sensing techniques. In Yucatán, these methods have been combined productively with on-the-ground surveys to identify remains of a buried colossus, responsible for the major change in lifeforms at the start of the Tertiary, and which much later provided a central focus of both ancient and modern Maya settlements.
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