Planetary Society Expedition to Belize

In Search of the K /T Boundary and other Adventures

by Paul Dudley
In January 1998 the Planetary Society, head quartered in Pasadena, California, conducted its third expedition to Belize to investigate the material ejected from a large impact crater caused by a huge meteorite or comet that struck the earth 65 million years ago. The site of the impact was at the northern tip of the Yucatan peninsula, centered approximately at the village of Chicxulub.

Academy members Paul and Jutta Dudley participated in this expedition and present this report to the Academy as the first original on-line publication. It is hoped that such contemporary on-line publications will continue to supplement some of the Academy's classic publications that have been re-published in this forum. All RAS members are encouraged to submit articles and photos for for inclusion.

A Little Background Information

65 millions years ago, the Cretaceous period and the Mesozoic era of geologic time came to an abrupt end. Geologic time intervals are defined by major changes in the fossil record -- when large numbers of living organisms suddenly become extinct and are replaced by new forms. The end of the Mesozoic era is most famous for marking the extinction of the last of the dinosaurs, but it also saw the demise of the ammonites and other marine creatures, and approximately 75 percent of all the species then living on the earth. The new era, the Cenozoic, began with the Tertiary period in which mammals replaced the dinosaurs as the dominate land animals (in terms of size anyway as some would argue that the insects are more numerous and diverse).

The reason for this sudden "revolution" has long been a subject of speculation amongst geologists and paleontologists. Various theories have been proposed relating to climate, volcanism, drifting crustal plates, etc. However recent evidence seems to have settled the issue -- the Chicxulub impactor and its widespread effect on the earth's ecosystems.

The key to solving the riddle was the discovery of a layer of clay at the boundary between the Cretaceous and Tertiary periods worldwide. (Known as the K/T boundary -- geologists abbreviate the Cretaceous with the letter K to differentiate it from the Cambrian and Carboniferous periods of the Paleozoic era.) This K/T boundary clay has higher concentrations of the element iridium than found in rocks and sediments normally found in the earth's crust. However, meteorites also have higher concentrations of iridium. This discovery led to the idea that a large impactor from space must have resulted in the rapid die-off that ended the Cretaceous.

The Chicxulub crater is now completely buried by more recent sedimentary rocks and is not visible. But in northern Belize, 364 kilometers (225 miles) from the center of the crater, faulting of the rock has resulted in the K/T boundary being exposed where it can be studied. These rocks preserve dramatic evidence for the enormous blast that resulted from the impact.
[Picture of Albion Island Quarry] This is the Albion Island quarry in northern Belize. It has the best exposure of the "ejecta blanket" from the Chicxulub blast. Overlying the Cretaceous dolostone is an orange-colored layer of fine material that contains rounded carbonate particles called spherules. The spherules when cut show evidence of being formed by accretion very much like hailstones. It has been hypothesized that the y formed in the atmosphere from the condensation of carbonate rocks vaporized by the impact. Above the spherule bed is a thick layer of jumbled rocks of all sizes in a fine matrix. This unsorted material is called diamictite. The diamictite contains large boulders, as well as numerous cobbles. Many of the rocks are polished, faceted, and striated and closely resemble rocks with similar features that can be found in the glacial tills of western New York state. Except they are found in tropical Belize! Instead of being transported by moving glacial ice, this diamictite material was pushed outward by the blast of the impact .
[Click on any of the pictures on this page to see enlargements. ]
One feature that can be seen in Mexico near the buried Chicxulub crater a partial ring of sinkhole lakes called cenotes. The cenotes formed when limestone caves collapse and fill with water. The Chicxulub cenote ring was first noticed on satellite images and helped in location of the crater. The reason they are aligned concentric to the crater is because the impact up warped the cave-sinkhole producing limestone beds, bringing them near the surface. Here is a similar cenote found just outside the Albion Island quarry, although it has no relation to the impact structure itself. [Picture of  Cenote at the Albion Island Quarry]
[Picture of  Giant Accretionary Mudball] Within the overlying diamictite ejecta blanket there are a large "mudballs." These mudballs appear to have been formed from some accretionary process as the ejecta blanket was forced along the earth's surface by the tremendous force of the impact blast. They are very much like giant snow ball that grew larger as they rolled along. This mudball (outlined in green) has a diameter of approximately 5 meters, and has a large mass of dense clay as its core.
This is a close-up of the spherule bed. This exposure is in a cut through the ejecta blanket at one location in the quarry "highwall." The spherule bed is about 1 meter thick and is semi-consolidated. The spherules are rounded grains from millimeter to several centimeters in diameter. Their interior structure shows concentric layering, similar to that seen in hail stones. They have been interpreted as having condensed in the blast cloud from the vaporized rock that was expelled high into the atmosphere and rained down over a wide area. Because the Albion Island site in Belize is only 364 kilometers from the center of the Chicxulub crater, these particles are much larger than the fine clay-sized particles that were deposited further away (including numerous sites in Europe). [Picture of Spherule Bed]
[Picture of Paul Digging  Spherules] Paul worked up a sweat in the tropical heat while excavating small trenches around a block of the spherule bed material for collection. Several similar oriented samples were collected for laboratory analysis. The goal of this analysis will be to see if there is any preferred orientation of the individual spherules that may indicate direction of movement as they were deposited. This sample was taken at the base of the spherule bed -- immediately on top of the late Createceous rock. Note person standing and quarry truck tire tracks at left for reference of the distance to quarry floor.
Jan Smit of the Netherlands, a noted authority on the K/T boundary, was one of the scientists on the expedition. He will conduct the study on these spherule bed samples. Here Dr. Smit has just marked the top of the sample to show the direction to magnetic north. This will be used as the reference for determining any preferred orientation of the spherules. Dr. Smit has proposed an alternate theory that the spherules may have formed by a rolling action at the base of the diamictite as it was moving horizontally out from the blast at the crater. [Marking Spherule Sample Orientation]
[Drilling Geo-Mag Sample Core] Jaime Urrutia, Chairman of the Geophysics Institute of the Universdad Nacinnal Altonomas de Mexico, participated in the expedition by collecting oriented rock cores for paleomagnetic studies. Such studies can sometimes be useful in determining whether rocks formed when the earth's magnetic was Normal (like the present) or with Reversed polarity. Here Dr. Urrutia is operating the gasoline-powered portable coring apparatus while boring into crystalline rocks in the Maya Mountain of central Belize. Unfortunately most of the limestones and dolostones at the K/T boundary sites lacked sufficient magnetic materials to allow geomagnetic studies.
After the core bit bores around a plug of rock, this brass compass device is used to mark the cores orientation with respect to the earth's current magnetic field. [Orientating Geo-Mag Sample Core]
[Hunting Pooks's Pebbles] In the Cayo District of central Belize, some interesting polished stones are found. These limestone pebbles and cobbles are highly polished and have interesting striations, gouges, and signs of plastic deformation. These "Pook's Pebbles" (named after the Pook's Hill Jungle Lodge near this site) are believed to be rocks blasted into space at high velocity by the force of the Chicxulub impact. As they reentered the earth's atmosphere, they were subjected to intense heating by friction against the air.
This is a view of some of the team members exploring the Tertiary strata above the Pook's pebble containing layer. [Exploring Tertiary Deposits and Jungle View]

On Sunday January 25, we had a day off on. Not wishing to sit around idle, we set off on an adventure to an ancient Mayan Ceremonial cave.