Ejecta Deposits

Adriana Ocampo and Kevin Pope of the Jet Propulsion Lab, and Alfred Fischer (who moved from Princeton to the University of Southern California) have discovered the closest ejecta deposit to ground zero.30 In a quarry on Albion Island in the Hondo River in Belize, 360 km from Chicxulub, at the top of the Cretaceous, they have come upon a double layer reminiscent of those found elsewhere around North and Central America, except that here some of the rock fragments are truly on a giant scale. The lower layer is about 1 m thick and contains abundant rounded spherules, 1 mm to 20 mm in size, composed of dolomite, a magnesiated limestone. Above it lies a 15-m layer containing broken fragments of a variety of rocks of Cretaceous age; some of the chunks are as big as a car.

Albion Island is important not only because it is the nearest Chic-xulub deposit to ground zero, but because unlike all the others so far discovered, it appears to have been deposited above sea level, without the stirring and mixing effects produced by deposition in water.

The possibility that the impactor might have struck in the ocean and generated a tsunami, which in turn would have left behind characteristic deposits, was first suggested in '985 by Smit and Romein.3' They studied the K-T boundary in the Brazos River country of Texas, famed in American song and story, and found a rock known as tur-bidite (from the Latin turbidus, or disordered; with the same root as bioturbation). Turbidites are thought to be produced when sediment, jarred loose at the head of a submarine canyon by an undersea earthquake, cascades down as a submarine "landslide" and flows outward for hundreds and even thousands of miles. Turbidity currents produce layers of silt and sand several meters thick, with the coarsest material, which settles out first, at the bottom, grading upward to the finest sediment at the top. However, a giant wave, such as would be produced by a large meteorite striking near shore, would generate an earthquake which would also stir up sediments and produce a turbidite-like effect. It struck Smit and Romein as important that the only occurrence of such a rock type in the Brazos country was exactly at the critical K-T boundary, especially when that boundary also contained an iridium anomaly. Sedimentologist Joanne Bourgeois of the . University of Washington concluded that the Brazos River turbidite was created by a tsunami 50 m to '00 m high.32 Subsequent field-work identified many other examples of possible impact-generated sedimentary deposits in the Gulf-Caribbean region. The one that by now is the most thoroughly studied is at Arroyo el Mimbral, one of several outcrops in northeast Mexico that expose the boundary layer.

"The significance of the Mimbral section lies in the combination of altered and unaltered glassy tektites, shocked minerals, anomalous iridium abundance, continental plant debris, and evidence for deep-sea disturbances and coarse-sediment transport precisely at the K-T boundary," according to Jan Smit and co-authors.33 Charles Officer, Wolfgang Stinnesbeck of the University of Nuevo Leon, and Gerta Keller of Princeton were quick to disagree.34 They too had studied Mimbral and "found no evidence of a nearby impact." Each piece of evidence presented by Smit and his co-authors, Stinnesbeck and company explained away. A key point in the disagreement was the position of the K-T boundary, which they placed above the level of Smit's uppermost unit, inevitably causing them to conclude that the rocks below it were older than, and therefore could not have been caused by, the K-T event. According to Smit, however, Stinnesbeck and colleagues placed the K-T boundary at the appearance of new Tertiary foraminifera. This is the way geologic boundaries have traditionally been set: at the level of first appearance of one or more abundant new species. However, a mass extinction caused by impact might delay the evolution of new species for tens of thousands or hundreds of thousands of years, until conditions were again hospitable. Therefore, Smit argued, to use the first appearance of new species to mark the boundary in the case of impact will always place the boundary too high, making it appear younger than it really is and by definition making the rocks immediately below it appear older than they really are. In contrast, ejecta and turbidites produced by impact would settle within a few hours or days of the event and would have exactly the same age as the impact.

On the contrary, Stinnesbeck and company argued, the deposits at Mimbral and elsewhere formed through normal geologic processes, possibly as coastal sediments slumped into deeper water. In that view, the Gulf of Mexico K-T sections, including Mimbral, were not impact-generated, but are of pre-K-T boundary age and were probably deposited by turbidite or gravity flows.35 Thus the debate turns on whether the Gulf K-T deposits, as Alfred Fischer put it in an address at the Snowbird III conference, "formed in 100,000 seconds [1.15 days] or 100,000 years."36

The controversy resembles the one over the sharpness of the iridium peak at Gubbio. In both cases, experts viewing the same evidence come to opposite conclusions and debate the matter at length in the literature. But having seen the example of the blind test at Gubbio, the solution to the Mimbral controversy became obvious, though it needed a modification: Since the outcrop could not be brought to the geologists, the geologists had to be brought to the outcrop. Accordingly, a field trip to Mimbral was organized at the time of the 1994 Snowbird III conference, held not in Utah but in Houston. In the party were four former presidents of the Society for Sedimentary Geologists, including Robert Dott of the University of Wisconsin, regarded by many as the dean of American sedimentolo-gists. According to Richard Kerr, the Science magazine reporter who has made the impact controversy a specialty, Dott spoke for these experts on sedimentary rocks: "We were impressed with the evidence that this sequence was very rapidly deposited. [It must have taken] closer to 100,000 seconds than 100,000 years."37

According to Kerr, the assembled sedimentologists concluded that the most likely cause of what they had seen was an impact-generated tsunami. Keller, ignoring Mark Twain's advice never to get into a contest with a man who buys ink by the barrel, took vigorous exception to Kerr's account. In her rebuttal, she claimed that "The impact tsunami scenario did not win the day. . . . Sedimentologists generally disagreed with Smit's model of tsunami wave deposi-tion."38 Kerr replied, "Of the five sedimentologists on the trip other than Lowe that I interviewed for the story, four of them agreed that the deposit is consistent with waves from an impact and that no proposed alternative can reasonably explain the deposit."39

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  • anita
    Why are there turbidites in Mimbral?
    1 year ago

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