Chicxulub and the Exploration of Large Peak- Ring Impact Craters through Scientific Drilling David A. Kring, Lunar and Planetary Institute, Houston, Texas 77058, USA; Philippe Claeys, Analytical, Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels 1050, Belgium; Sean P.S. Gulick, Institute for Geophysics and Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758, USA; Joanna V. Morgan and Gareth S. Collins, Dept. of Earth Science and Engineering, Imperial College London SW7 2AZ, UK; and the IODP-ICDP Expedition 364 Science Party. ABSTRACT proving the structure had an impact origin. to assess the depth of origin of the peak- The Chicxulub crater is the only well- The buried structure was confirmed by ring rock types and determine how they preserved peak-ring crater on Earth and seismic surveys conducted in 1996 and were deformed during the crater-forming linked, famously, to the K-T or K-Pg mass 2005 to be a large ~180–200-km–diameter event. That information is needed to effec- impact crater with an intact peak ring tively test how peak-ring craters form on extinction event. For the first time, geolo- (Morgan et al., 1997; Gulick et al., 2008). planetary bodies. gists have drilled into the peak ring of that The discovery of the Chicxulub impact The expedition was also designed to crater in the International Ocean structure initially prompted two scientific measure any hydrothermal alteration in Discovery Program and International drilling campaigns. In the mid-1990s, a the peak ring and physical properties of the Continental Scientific Drilling Program series of shallow onshore wells up to 700 m rocks, such as porosity and permeability, (IODP-ICDP) Expedition 364. The deep were drilled by the Universidad to calibrate geophysical data, test models Chicxulub impact event, the environmen- Nacional Autónoma de México (UNAM; of impact-generated hydrothermal sys- tal calamity it produced, and the paleobio- Urrutia-Fucugauchi et al., 1996) to sample tems, evaluate the habitability of the peak logical consequences are among the most near-surface impact breccias in the ejecta ring, and investigate the recovery of life in captivating topics being discussed in the blanket surrounding the crater. In 2002, a sterilized portion of Earth’s surface. The geologic community. Here we focus atten- the International Continental Scientific recovered rocks also make it possible to tion on the geological processes that Drilling Program (ICDP) also sponsored a evaluate shock deformation of Earth’s shaped the ~200-km-wide impact crater deep drilling project, producing a 1511 m crust, including the vaporization of rocks responsible for that discussion and the borehole between the peak ring and the that may have contributed to climate-alter- expedition’s first year results. crater rim. Continuous core beneath 404 m ing effects of the impact. A large number of geological, environmental, and biologi- INTRODUCTION included Tertiary marine sediments, poly- mict impact breccias, an impact melt unit, cal results will emerge from the expedi- The Chicxulub crater (Hildebrand et al., and one or more blocks of Cretaceous sedi- tion. Here, we focus on the planetary geo- 1991) on the Yucatán Peninsula of Mexico mentary target rocks. We refer readers to science findings: how the peak-ring crater was produced by a terminal Cretaceous two special issues of Meteoritics & formed and what peak-ring and multi-ring impact that has been linked to regional and Planetary Science (Jull, 2004a, 2004b) for craters can reveal about deep planetary global K-T or K-Pg boundary deposits (see the major results of that ICDP project, but crusts. As the borehole pierced only a sin- reviews by Smit, 1999; Kring, 2000, 2007; note that the project left unresolved, gle location within the crater, we begin by Schulte et al., 2010). The subsurface struc- among other things, the geologic processes looking at a fully exposed peak-ring crater ture was initially detected with geophysi- that produced the peak-ring morphology of on the Moon, which provides a picture of a cal techniques (Cornejo Toledo and the crater. similar structure to that targeted by Hernandez Osuna, 1950). While exploring The Chicxulub crater is the best-pre- Expedition 364. the source of those anomalies, Petróleos served peak-ring impact basin on Earth, so Mexicanos (PEMEX) drilled three explo- it is an essential target for additional study. EXPOSED PEAK-RING CRATERS ration wells (all dry) into the structure. The only other known similarly sized sur- The Schrödinger basin near the south Petrologic analyses of polymict breccias viving impact structures, Sudbury and pole on the lunar far side is the youngest and melt rock in recovered core samples Vredefort, are tectonically deformed and and best preserved peak-ring crater on the revealed shock-metamorphic and shock- eroded. Recently, the International Ocean Moon (Fig. 2A). The ~320-km-diameter melted features diagnostic of impact Discovery Program (IODP) and ICDP crater contains an ~150-km-diameter peak cratering (Kring et al., 1991; Kring and drilled an offshore borehole into the crater ring that rises up to 2.5 km above the cra- Boynton, 1992; Swisher et al., 1992, (Fig. 1), recovering core from a depth of ter floor (Shoemaker et al., 1994). The Sharpton et al., 1992; Claeys et al., 2003), 505.7–1334.7 m below the sea floor (mbsf), peak ring is topographically complex, with GSA Today, v. 27, doi: 10.1130/GSATG352A.1. Copyright 2017, The Geological Society of America. Figure 1. IODP-ICDP Expedition 364 drilled into the subsurface Chicxulub peak ring at borehole M0077A (red dot), which was ~30 km northwest of Pro- greso and the north shore of the Yucatán Peninsula. The blue circle repre- sents the approximate diameter of the 180–200-km subsurface impact struc- ture. The gravity signature of the structure (from lows of -16 to highs of +30 mgal) and locations of other drilling sites are shown in the inset. The only two sites with continuous core are the ICDP Yaxcopoil-1 (Yax-1) and IODP-ICDP M0077A boreholes. Other boreholes are Yucatán-1 (Y1), Yucatán-2 (Y2), Yucatán-6 (Y6), Chicxulub-1 (C1), Sacapuc-1 (S1), and Ticul-1 (T1). Figure 2. (A) The morphology of a peak ring is evident in this view of the ~320-km-diameter Schrödinger basin on the Moon, looking from the north toward the south pole. NASA’s Scien- tific Visualization Studio. (B) A close-up view of a segment of the peak ring with rocks uplifted from mid- to lower-crustal levels by the impact event. The field of view is ~17 km wide through the center of the image. Lunar Reconnais- sance Orbiter Camera image M1192453566. steep cliffs and open chasms. Summit for future lunar sample return missions nearly doubling from 20 to 40 km from heights vary along the circumference of (Potts et al., 2015; Steenstra et al., 2016). the east to the west and producing bilat- the peak ring. On the Moon, where the Geologic mapping of those rock types eral asymmetry in the peak ring (Fig. 3). erosional processes familiar on Earth do and numerical modeling of peak-ring As shown below, those types of morpho- not occur, that differential topography is a emplacement (Kring et al., 2016) suggest logical effects, visible at the surface on primary feature, caused by shear and fault the rocks in the peak ring were derived the Moon, are mirrored in the subsurface displacement during the emplacement of from mid- to lower-crustal depths on the Chicxulub peak-ring basin on Earth. the peak ring (Kring et al., 2016). Moon (e.g., ~15–26 km deep). During the Spectral analyses of the lunar surface impact event, those rocks rose above the CHICXULUB captured from the orbiting Chandraayan-1 lunar surface and, without the strength to The subsurface morphological charac- spacecraft indicate the peak ring is com- maintain that elevation, collapsed out- ter of the peak ring of the Chicxulub cra- posed of anorthositic, noritic, and olivine- ward to form nappe-like structures in a ter is similar to that of Schrödinger, bearing (e.g., troctolite or dunite) rocks circumferential peak-ring. Pre-impact although the topography on the upper sur- from deep crustal or even upper mantle crustal strength seems to have affected face of Chicxulub’s peak ring is more depths (Kramer et al., 2013). Those rock that process. A gap in the peak ring subdued because of Earth’s greater grav- types occur in spectacular outcrops (Fig. occurs in the southeastern quadrant, ity. Thus, while Schrödinger’s peak ring 2B). Soon after Apollo, it was common to which is an area in the target that had rises up to 2.5 km above the basin floor, hear geologists lament that there are no been previously weakened by the seismic reflection data (Morgan et al., outcrops on the Moon because the surface Amundsen-Ganswindt basin-forming 2000) indicate Chicxulub’s peak ring had is covered with regolith. However, the event. There, the peak ring collapsed ~400 m of relief before being buried. rocks exposed in the peak ring constitute below the level filled by impact melts and Additional seismic data suggest the peak hectometer- to kilometer-size outcrops that impact breccias. Pre-impact crustal thick- ring varied in height circumferentially are now recognized as high-priority sites ness also varied across the target area, (Gulick et al., 2013), with reduced Figure 3. Pre-impact target conditions affected the formation of the peak ring in the ~320-km-diameter Schrödinger basin. In this view oriented with the south pole toward the top of the image, the peak ring collapsed below the level of the impact melt and breccia fill in the southeast quad- rant.