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1. Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization

   https://doi.org/10.1130/B36020.1  

   Kaskes, Pim ; de Graaff, Sietze J. ; Feignon, Jean-Guillaume ; Déhais, Thomas ; Goderis, Steven ; Ferrière, Ludovic ; Koeberl, Christian ; Smit, Jan ; Wittmann, Axel ; Gulick, Sean P.S. ; et al ( July 2021 , GSA Bulletin)

   null (Ed.)

   This study presents a new classification of a ∼100-m-thick crater suevite sequence in the recent International Ocean Discovery Program (IODP)-International Continental Scientific Drilling Program (ICDP) Expedition 364 Hole M0077A drill core to better understand the formation of suevite on top of the Chicxulub peak ring. We provide an extensive data set for this succession that consists of whole-rock major and trace element compositional data (n = 212) and petrographic data supported by digital image analysis. The suevite sequence is subdivided into three units that are distinct in their petrography, geochemistry, and sedimentology, from base to top: the ∼5.6-m-thick non-graded suevite unit, the ∼89-m-thick graded suevite unit, and the ∼3.5-m-thick bedded suevite unit. All of these suevite units have isolated Cretaceous planktic foraminifera within their clastic groundmass, which suggests that marine processes were responsible for the deposition of the entire M0077A suevite sequence. The most likely scenario describes that the first ocean water that reached the northern peak ring region entered through a N-NE gap in the Chicxulub outer rim. We estimate that this ocean water arrived at Site M0077 within 30 minutes after the impact and was relatively poor in rock debris. This water caused intense quench fragmentation when it interacted with the underlying hot impact melt rock, and this resulted in the emplacement of the ∼5.6-m-thick hyaloclastite-like, non-graded suevite unit. In the following hours, the impact structure was flooded by an ocean resurge rich in rock debris, which caused the phreatomagmatic processes to stop and the ∼89-m-thick graded suevite unit to be deposited. We interpret that after the energy of the resurge slowly dissipated, oscillating seiche waves took over the sedimentary regime and formed the ∼3.5-m-thick bedded suevite unit. The final stages of the formation of the impactite sequence (estimated to be <20 years after impact) were dominated by resuspension and slow atmospheric settling, including the final deposition of Chicxulub impactor debris. Cumulatively, the Site M0077 suevite sequence from the Chicxulub impact site preserved a high-resolution record that provides an unprecedented window for unravelling the dynamics and timing of proximal marine cratering processes in the direct aftermath of a large impact event. 

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2. Mapping the Chicxulub Impact Stratigraphy and Peak Ring Using Drilling and Seismic Data

   https://doi.org/10.1029/2021JE006938  

   Christeson, G. L. ; Morgan, J. V. ; Gulick, S. P. S. ( August 2021 , Journal of Geophysical Research: Planets)

   We integrate high‐resolution full‐waveform velocity models with seismic reflection images to map the peak ring and impactite stratigraphy at the Chicxulub structure. International Ocean Discovery Program/International Continental scientific Drilling Program Site M0077 provides ground truth for our interpretations. The peak ring is narrower (∼10 km width) where it is high relief (600–700 m below seafloor) and wider (∼15 km width) where it is lower relief (1,000–1,200 m below seafloor). Both target asymmetry and angle of impact could have contributed to observed differences in peak ring morphology. We interpret a layer of lowered velocities as a resurge layer formed from the ocean resurge, seiche, and returning tsunami flowing into the newly formed impact basin. This graded suevite layer has an average thickness of 187 ± 58 m with only local thickness differences within the annular trough, peak ring, and central basin. These observations suggest that the returning ocean was of substantial height and energetic enough to carry debris across the entire topographic peak ring. We map impact melt rock throughout the crater, with a thick impact melt sheet in the central basin (>500 m), thin intermittent melt rock capping the peak ring, and a ∼500‐m thick layer of melt rock in the annular trough near the peak ring that thins toward the crater rim. We estimate that ∼70%–75% of the melt rock volume is in the central basin. We image features above and adjacent to the central basin melt sheet that we interpret as upflow zones associated with a long‐lasting hydrothermal system.

    

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3. FACIES AND ICHNOFABRICS IN THE PALEOCENE OF CHICXULUB: A RECORD OF THE RECOVERY OF LIFE POST-IMPACT

   Whalen, M ; O’Malley, K ; Lowery, C ; Rodriguez-Tovar, F ; Morgan, J ; Gulick, S. ( January 2017 , Lunar and planetary science conference abstracts)

   Introduction: IODP/ICDP Expedition 364 recovered core from 505.7-1334.7 m below the seafloor (mbsf) at Site M0077A (21.45° N, 89.95° W) atop the peak ring in the Chicxulub impact structure. The core penetrated Paleogene sedimentary rocks, impactrelated suevite, melt rock, and granitic basement [1]. Approximately 110 m of post-impact, hemipelagic and pelagic sedimentary rocks were recovered, ranging from middle Eocene (Ypresian) to basal Paleocene (Danian) in age [1]. The transition between suevite and basal Paleocene sedimentary rocks is a remarkable succession of fining upward gravel to sand-sized suevite (Unit 2A) overlain by laminated carbonate-rich siltstone (Unit 1G, “impact boundary cocktail” [2]) that records the settling of fine-grained material postimpact [1]. This study concentrates on the carbonaterich Paleocene sedimentary rocks of overlying Unit 1F [1]. The degree of bioturbation, or ichnofabric index (II) [3, 4], provides a semiquantitative estimate of the density of burrowing within sedminentary facies. Collection of II data within the context of facies analysis thus yields insight into the initial and then continued disturbance of sediment by burrowing organisms recording the return of life to the crater (Fig. 1). Unit 1G: The unit extends from 616.58-617.33 mbsf (Fig. 1) and consists mainly of dark brown to dark grayish brown calcareous siltstone but is complex with several different lithologies and post-depositional pyrite nodules that disrupt bedding. The base of the unit is a sharp, stylolitized contact overlain by two ~1 cm thick, normally graded beds. Overlying, up to 617.17 mbsf, the siltstone contains internally finely laminated cm-scale beds that alternate between dark brown and grayish brown. Above, up to 616.97 mbsf is a package with mm bedded couplets of dark brown and grayish brown calcareous siltstone that grade upward into similarly colored cm bedded couplets that then thin upward into mm bedded couplets again. Above this interval bedding is indistinct and appears to be obscured by soft sediment deformation from 616.66- 616.97 mbsf. The upper part of the unit is slightly deformed with greenish marlstone and interbedded lighter gray siltstone displaying a distinct downwarp from 616.58-616.66 mbsf. Rare oval structures, that are potential individual burrows, occur down to 616.65 mbsf. Unit 1F: The unit records the remainder of the Paleocene and extends from 607.27-616.58 mbsf (Fig. 1). The base of the unit is a sharp contact at the base of a greenish claystone (II 2) that overlies Unit 1G [1]. It consists dominantly of interbedded light gray to light bluish gray wackestone and packstone (II 3-5) and light to dark bluish gray marlstone (II 2) at cm-dmscale. All lithologies contain wispy stylolites. The lower portion of the unit (616.58 and 607.74) is cyclic with cm-dm-scale bedding and light greenish-blue to bluish marlstone bases (II 2-3) that grade upward into light gray or light bluish gray wackestone and packstone (II 3-5). Contacts between lithologies are usually gradational due to burrowing. The upper portion of the unit from 610.25 to 607.74 mbsf is a light yellowish brown burrowed packstone (II 4) intercalated with gray marlstone (II 2). The uppermost 7.5 cm is calcite cemented with 1 cm wide burrows (II 3-4). Clasts are fine to coarse sand size and include foraminifera. The upper surface of this unit is a hardground and minor unconformity overlain by Eocene rocks [1]. Ichnofabric Index: II data provides a window onto the return of life post-impact (Fig. 1). Rare structures in the upper most sandy suevite (Unit 2A) and in Unit 1G (Core 40R-1) resemble bioturbation structures but may also represent fluid escape [1]. The first welldefined oval structures that appear to be burrows occur in the upper part of Unit 1G (Fig. 1, 616.58-616.65 mbsf). Unequivocal burrows (II 2) that disturb sedimentary facies occur just above, at 616.56 mbsf in Unit 1F (Fig. 1). II of 3-4 are reached 5-6 cm above indicating significant disruption of original sedimentary strutures. An II of 5 is first documented at 616.16 mbsf (Fig. 1). Above this level through the Paleocene succession II largely varies between 2 and 5 with rare laminated intervals (II 1). Bioturbation intensity correlates well with facies changes and more marly facies display lower levels of bioturbation than more carbonate- rich facies. This correlation implies a depth and/or paleoredox control on the distribution of bioturbating organisms. Discussion: II and the return of life: The II data indicate that burrowing organisms were likely reestablished in the crater before the end of deposition of Unit 1G. Biostratigraphic analyses document a mix of Late Cretaceous and earliest Danian taxa within Unit Lunar and Planetary Science XLVIII (2017) 1348.pdf 1G and lowermost Danian zone Pα documented in the lowermost part of Unit 1F down to 616.58 mbsf [1]. P1a taxa occur down to 616.29 mbsf with P1b-P4 recorded upward through 607.27 m [1]. Burrowing organisims were thus active by earliest Danian indicating a rapid return of life to the crater. Hydrocode modeling implies that much of the deformation and peak ring formation was completed within minutes of the impact [5]. Deposition and reworking of impact breccia by tsunami and seiches likely extended for several days [6]. More refined estimates for the return of life to the crater may be possible with more detailed analysis of the deposition of laminae within Unit 1G that records marine settling of fine-grained material that may have taken days to months. 

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4. Chicxulub: Drilling the K-Pg Impact Crater

   https://doi.org/10.14379/iodp.proc.364.2017  

   Morgan, J. ; Gulick, S. ; Mellett, C.L. ; Green, S.L. ( December 2017 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   The Chicxulub impact crater, on the Yucatán Peninsula of México, is unique. It is the only known terrestrial impact structure that has been directly linked to a mass extinction event and the only terrestrial impact with a global ejecta layer. Of the three largest impact structures on Earth, Chicxulub is the best preserved. Chicxulub is also the only known terrestrial impact structure with an intact, unequivocal topographic peak ring. Chicxulub’s role in the Cretaceous/Paleogene (K-Pg) mass extinction and its exceptional state of preservation make it an important natural laboratory for the study of both large impact crater formation on Earth and other planets and the effects of large impacts on the Earth’s environment and ecology. Our understanding of the impact process is far from complete, and despite more than 30 years of intense debate, we are still striving to answer the question as to why this impact was so catastrophic. During International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364, Paleogene sedimentary rocks and lithologies that make up the Chicxulub peak ring were cored to investigate (1) the nature and formational mechanism of peak rings, (2) how rocks are weakened during large impacts, (3) the nature and extent of post-impact hydrothermal circulation, (4) the deep biosphere and habitability of the peak ring, and (5) the recovery of life in a sterile zone. Other key targets included sampling the transition through a rare midlatitude Paleogene sedimentary succession that might include Eocene and Paleocene hyperthermals and/or the Paleocene/Eocene Thermal Maximum (PETM); the composition and character of suevite, impact melt rock, and basement rocks in the peak ring; the sedimentology and stratigraphy of the Paleocene–Eocene Chicxulub impact basin infill; the geo- and thermochronology of the rocks forming the peak ring; and any observations from the core that may help constrain the volume of dust and climatically active gases released into the stratosphere by this impact. Petrophysical properties measurements on the core and wireline logs acquired during Expedition 364 will be used to calibrate geophysical models, including seismic reflection and potential field data, and the integration of all the data will calibrate models for impact crater formation and environmental effects. The drilling directly contributes to IODP Science Plan goals: Climate and Ocean Change: How does Earth’s climate system respond to elevated levels of atmospheric CO2? How resilient is the ocean to chemical perturbations? The Chicxulub impact represents an external forcing event that caused a 75% species level mass extinction. The impact basin may also record key hyperthermals within the Paleogene. Biosphere Frontiers: What are the origin, composition, and global significance of subseafloor communities? What are the limits of life in the subseafloor? How sensitive are ecosystems and biodiversity to environmental change? Impact craters can create habitats for subsurface life, and Chicxulub may provide information on potential habitats for life, including extremophiles, on the early Earth and other planetary bodies. Paleontological and geochemical studies at ground zero will document how large impacts affect ecosystems and biodiversity. Earth Connections/Earth in Motion: What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami? Drilling into the uplifted rocks that form the peak ring will be used to groundtruth numerical simulations and model impact-generated tsunami, and deposits on top of the peak ring and around the Gulf of México will inform us about earthquakes, landslides, and tsunami generated by Chicxulub. These data will collectively help us understand how impact processes are recorded in the geologic record and their potential hazards. IODP Expedition 364 was a Mission Specific Platform expedition designed to obtain subseabed samples and downhole logging measurements from the post-impact sedimentary succession and the peak ring of the Chicxulub impact crater. A single borehole (Hole M0077A) was drilled into the Chicxulub impact crater on the Yucatán continental shelf, recovering core from 505.70 to 1334.69 meters below seafloor (mbsf) with ~99% core recovery. Downhole logs were acquired for the entire depth of the borehole. 

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5. Ocean resurge-induced impact melt dynamics on the peak-ring of the Chicxulub impact structure, Mexico

   https://doi.org/10.1007/s00531-021-02008-w  

   Schulte, Felix M. ; Wittmann, Axel ; Jung, Stefan ; Morgan, Joanna V. ; Gulick, Sean P. ; Kring, David A. ; Grieve, Richard A. ; Osinski, Gordon R. ; Riller, Ulrich ( March 2021 , International Journal of Earth Sciences)

   null (Ed.)

   Abstract Core from Hole M0077 from IODP/ICDP Expedition 364 provides unprecedented evidence for the physical processes in effect during the interaction of impact melt with rock-debris-laden seawater, following a large meteorite impact into waters of the Yucatán shelf. Evidence for this interaction is based on petrographic, microstructural and chemical examination of the 46.37-m-thick impact melt rock sequence, which overlies shocked granitoid target rock of the peak ring of the Chicxulub impact structure. The melt rock sequence consists of two visually distinct phases, one is black and the other is green in colour. The black phase is aphanitic and trachyandesitic in composition and similar to melt rock from other sites within the impact structure. The green phase consists chiefly of clay minerals and sparitic calcite, which likely formed from a solidified water–rock debris mixture under hydrothermal conditions. We suggest that the layering and internal structure of the melt rock sequence resulted from a single process, i.e., violent contact of initially superheated silicate impact melt with the ocean resurge-induced water–rock mixture overriding the impact melt. Differences in density, temperature, viscosity, and velocity of this mixture and impact melt triggered Kelvin–Helmholtz and Rayleigh–Taylor instabilities at their phase boundary. As a consequence, shearing at the boundary perturbed and, thus, mingled both immiscible phases, and was accompanied by phreatomagmatic processes. These processes led to the brecciation at the top of the impact melt rock sequence. Quenching of this breccia by the seawater prevented reworking of the solidified breccia layers upon subsequent deposition of suevite. Solid-state deformation, notably in the uppermost brecciated impact melt rock layers, attests to long-term gravitational settling of the peak ring. 

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