2. The Impact Controversary

Figure 4. Yucatan impact crater location, and magnetic gravity anomaly map of the circular subsurface structure at Chicxulub.

The Introduction to the Field Guide of 1994 to Northeastern Mexico’s Cretaceous-Tertiary (KT) sequences with impact ejecta deposits is as current Today as it was then. In that introduction, Keller et al. (1994a) wrote: The controversy over the nature of the KT transition and the causes of the associated global faunal and floral changes was altered fundamentally in 1980 with the discovery of the now-famous iridium anomaly at the KT boundary at Gubio, Italy (Alvarez et al., 1980). The discovery of similar anomalies elsewhere and the proposition that these anomalies and the KT extinctions resulted from the impact of a large extraterrestrial bolide have spurred over a decade of unparalleled research on the physical and biological events at and near the KT boundary. Within a short time, the controversy resolved itself into two contrasting schools of thought:  (1) the KT events reflect the catastrophic effects of a large (10-km) bolide colliding with the earth, and (2) the KT extinctions were the culmination of long-term changes in the earth’s biota reflecting major changes in the global climatic system and resulted from extreme, but still normal terrestrial processes, mainly volcanism, which may have been accelerated by a bolide impact at KT boundary time.”

We expressed the hope that “some issues of basic geology might be resolved by discussions on the outcrops and that an interdisciplinary approach might be taken towards some of the contentious issues of their interpretations.”

Unfortunately, that did not happen either during the 1994 field trip attended by about seventy scientists, or in the years since. Instead, interpretations of the KTB age of the Chicxulub impact, Chicxulub as the single cause for the KTB mass extinction, and the tsunami scenario to explain any discrepancies became the lore in the scientific community and popular wisdom.

Despite a virtual taboo on questioning the KTB impact hypothesis, pesky little facts that could not be reconciled with this hypothesis surfaced in the literature. The evidence was multidisciplinary and ranged from the extinction records of dinosaur to microfossils, from sedimentology to geochemistry, including stable isotopes, trace elements, and platinum group elements (PGEs). Though at first largely ignored by the scientific community, they eventually added up to a sizeable body of evidence that was incompatible with the KTB impact hypothesis. Today, this body of evidence is the source of contentious arguments regarding the age of the Chicxulub impact on Yucatan and whether this impact did or did not cause the KTB mass extinction. It is this body of evidence that calls for a long-overdue re-evaluation of the KTB impact-kill hypothesis and a new look at the other catastrophe: Deccan volcanism as potential cause for the KTB mass extinction.

Eureka - The Smoking Gun

After a ten-year search for a likely impact crater, the search centered on a circular magnetic and gravity anomaly subsurface structure on the northwestern margin of the Yucatan peninsula, Mexico (Hildebrand et al. (1991). A decade earlier Penfield and Camargo (1981) identified this structure as a possible impact crater but it failed to garner much attention. The crater diameter was about 170km and the crater was named Chicxulub (devils tail) (Fig. 4).

The Chicxulub crater was linked to the KT boundary based on shocked quartz and an Ir anomaly in the impact breccia, which was never confirmed. Impact glass spherules from Haiti and NE Mexico and melt rock from the crater breccia yielded 40Ar/39Ar ages that spanned the KT boundary though with an error margin 1% or 650 ky (Izett et al., l99l; Swisher et al., l992; Smit et al., 1992; Dalrymple et al., l993). Back in the early 1990s the case seemed sealed; Chicxulub was the long-sought KTB impact crater and the cause for the end-Cretaceous mass extinction. Many scientists believed that the smoking gun had been found. Substantiating this belief proved difficult and has fueled the controversy ever since.

The Case of the Vanishing Cores

Proving that Chicxulub was the KT killer still lacked critical evidence – namely, the age of the sediments overlying the impact breccia in the Chixculub crater. Without this age control, all that could be said with confidence was that the breccia was deposited sometime within the rather large error margin (± 650ky) of 40Ar/39Ar ages of impact glass in the breccia of the crater. This was insufficient to claim a cause-and-effect scenario with the KTB mass extinction. But in the irrational impact exuberance of the time, this critical detail was considered inconsequential.

Age data was available from a series of cores drilled by Mexico’s oil company PEMEX across the crater area in the past. Analyses from oil company scientists and consultants (Lopez Ramos (1973, 1975, including Meyerhof) had determined a late Maastrichtian age for the 60-170 m of limestone above the impact breccia in two Chicxulub wells (C1 and Y6, Fig. 5). This clearly presented a problem for declaring this impact the smoking gun. In the irrational impact exuberance the only acknowledgment of this problem was in the first announcement of the Chicxulub crater rediscovery stating that determining the precise age of the crater was impossible from available stratigraphic data (Hildebrand et al., 1991).

At this time Alan Hildebrand sent a single sample from well Y6 (N12 at 1000-1003 m depth) about 70 m above the impact breccia to G. Keller, who shared it with W. Sliter for age determination. Both reported a late Paleocene zone P3 age, millions of years above the KTB. Based on this age, Hildebrand et al. (1991, p. 870) erroneously reported that a KTB age is indicated for the impact breccia and that the earlier age assignment of Lopez Ramos (1975) was probably invalid.

Figure 5

Figure 5. Correlation of Yucatan wells modified from Ward et al. (1995). See Ward et al. for e-log and planktic foraminiferal biostratigraphy for upper Maastrichtian marls or limestone overlying the impact breccia at wells T1, Y6, C1 and Keller et al., 2004 for data of Yaxcopoil-1, the new core drilled in 2001-2002.

The Lopez Ramos (1975) biostratigraphic age report could not be verified because that core interval had vanished. Likewise, Hildebrand had no samples for the 70 m between the impact breccia and sample Y6 N12. It was rumored that the PEMEX warehouse where the cores were stored had burned down, destroying all cores, except for the few samples analyzed by the small group that announced the smoking gun studies.

When Chicxulub cores reappeared a few years later at the University of New Orleans, a biostratigraphic and sedimentologic study of all existing PEMEX wells of the Chicxulub crater area was done and published by Ward, Keller and Stinnesbeck (1995). This study revealed two critical points: 1) the sediments underlying the impact breccia are essentially undisturbed fossil-rich limestone, dolomite and evaporites – rather than the expected melt trock and completely disturbed, fractured rocks of an impact crater. 2) Above the impact breccia is a minimum 18 m of undisturbed late Maastrichtian limestone in wells Y6 and C1, which was subsequently confirmed in the newly drilled well Yaxcopoil-1 with a 50 cm thick limestone layer between the breccia and KT boundary (Fig. 3). Wared et al. (1995) cautioned that it was impossible to substantiate Chicxulub as the KTB impact crater based on biostratigraphy of existing PEMEX well samples. The evidence for a KTB age for the impact crater was flimsy and unsubstantiated.