Since the early l990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago. Here, we report evidence from a previously uninvestigated core, Yaxcopoil-1, drilled within the Chicxulub crater, indicating that this impact predated the K-T boundary by Ϸ300,000 years and thus did not cause the end-Cretaceous mass extinction as commonly believed. The evidence supporting a pre-K-T age was obtained from Yaxcopoil-1 based on five independent proxies, each with characteristic signals across the K-T transition: sedimentology, biostratigraphy, magnetostratigraphy, stable isotopes, and iridium. These data are consistent with earlier evidence for a late Maastrichtian age of the microtektite deposits in northeastern Mexico.T he search for the K-T impact crater effectively ended in the early l990s with the discoveries of the Chicxulub crater and its estimated diameter between 180 and 280 km (1-4). The impact ejecta (microtektites) in Haiti and northeastern Mexico (5, 6) and melt rock in the Chicxulub cores have similar geochemistry, and the 39Ar͞40Ar ages are within Ϯ200,000 years (200 ky) of the K-T boundary (7,8). These observations made a convincing case for Chicxulub as the long-sought K-T boundary impact crater and cause for the end-Cretaceous mass extinction. But doubts persisted regarding the precise age and size of the impact crater (2-4, 9), the origin of the so-called megatsunami deposits (5,(10)(11)(12), and the nature of the mass extinction (13,14). To resolve these issues, the International Continental Scientific Drilling Program (ICDP) supported the drilling of a new core within the Chicxulub crater (Yaxcopoil-1 drilled between December 2001 and February 2002) with the stated objectives to determine the precise age of the crater and its link to the global K-T boundary layer, to unravel Chicxulub's role in the K-T mass extinction, and to study the cratering event and size of the impact crater (15).Chicxulub Core Yaxcopoil-1 (Yax-1) Yax-1 is located 40 km southwest of Merida, Mexico, and Ϸ60 km from the center of the Chicxulub structure (Fig. 1). A continuous core sequence was recovered from 400 to 1,511 m below the surface (15). † † ‡ ‡ Between 794.65 and 894 m, a 100-m-thick impact (suevite) breccia overlies a 617-m-thick sequence of horizontally layered shallow-water lagoonal to subtidal Cretaceous limestones, dolomites, and anhydrites. The first 85 m of the impact (suevite) breccia consist primarily of clasts from these underlying shallow-water lithologies, some crystalline rocks of continental basement origin, and devitrified glass (Cheto smectite) fragments and spherules. † † The upper 15 m of the breccia are stratified with alternating layers of upward fining clasts (3-5 cm at the base to 2-5 mm at the top), coarse cross-bedding structures, and gray friable sand layers in the top meter, all of which indicate...

Abstract-The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán-6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax-1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km-thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias.The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.

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Ancient secular variation in Central Mexico was determined from paleomagnetic measurements on 45 independent lava flows ranging in age (14C and K‐Ar dates) from 2,500 to 580,000 years B.P. All the analyzed flows are characterized by a normal polarity placing them within the Brunhes Chron. The paleosecular variation analysis yielded an angular standard deviation from the field of an axial dipole of 15.3° with 95% confidence limits of 13.4° and 17.9°, and that of the corresponding Virtual Geomagnetic Poles (VGPs) of 13.9° with confidence limits of 12.1° and 16.1°. These values are in agreement with those predicted by most statistical models of secular variation, but divergent from those obtained from low latitudes (19°N) such as Hawaii and Pagan Is.

This paper presents new paleomagnetic results from 24 independent cooling units in Tequila area (western Trans-Mexican Volcanic Belt). These units were recently dated by means of state-of-the-art 40 Ar-39 Ar method (Lewis-Kenedy et al., 2005) and span from 1130 to 150 ka. The characteristic paleodirections are successfully isolated for 20 cooling units. The mean paleodirection, discarding intermediate polarity sites, is I = 29.6• ,• , n = 17, which corresponds to the mean paleomagnetic pole position P lat = 85.8• , P long = 84.3• , K = 27.5, A 95 = 6.9• . These directions are practically undistinguishable from the expected Plestocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from western Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area since early-middle Plestocene to present. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving S F = 15.4 with S U = 19.9 and S L = 12.5 (upper and lower limits respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden et al. (1991) for the last 5 Myr. The interesting feature of the paleomagnetic record obtained here is the occurrence of an intermediate polarity at 671 ± 13 ka which may correspond the worldwide observed Delta excursion at about 680-690 ka. This gives the volcanic evidence of this event. Two independent lava flows dated as 362 ± 13 and 354 ± 5 ka respectively, yield transitional paleodirections as well, probably corresponding to the Levantine excursion.

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Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMS during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM-ARM1 and TRM-ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K-Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K-Ar ages determined are: I (13 f 2 Myr), 0.504 Oe; 11, 0.453 Oe; 111, 0.439 Oe; VI (52 f 10 Myr), 0.074 Oe; VII (14 ? 2 Myr), 0.187 Oe and VIII (12 f 2 Myr), 0.251 Oe.These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary.