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Solander Basin is characterized by subduction initiation at the Pacific‐Australia plate boundary, where high biological productivity is found at the northern edge of the Antarctic Circumpolar Current. Sedimentary architecture results from tectonic influences on accommodation space, sediment supply and ocean currents (via physiography); and climate influence on ocean currents and biological productivity. We present the first seismic‐stratigraphic analysis of Solander Basin based on high‐fold seismic‐reflection data (voyage MGL1803, SISIE). Solander Trough physiography formed by Eocene rifting, but basinal strata are mostly younger than ca. 17 Ma, when we infer Puysegur Ridge formed and sheltered Solander Basin from bottom currents, and mountain growth onshore increased sediment supply. Initial inversion on the Tauru Fault started at ca. 15 Ma, but reverse faulting from 12 to ca. 8 Ma on both the Tauru and Parara Faults was likely associated with reorganization and formation of the subduction thrust. The new seabed topography forced sediment pathways to become channelized at low points or antecedent gorges. Since 5 Ma, southern Puysegur Ridge and Fiordland mountains spread out towards the east and Solander Anticline grew in response to ongoing subduction and growth of a slab. Solander Basin had high sedimentation rates because (1) it is sheltered from bottom currents by Puysegur Ridge; and (2) it has a mountainous land area that supplies sediment to its northern end. Sedimentary architecture is asymmetric due to the Subtropical Front, which moves pelagic and hemi‐pelagic sediment, including dilute parts of gravity flows, eastward and accretes contourites to the shelf south of Stewart Island. Levees, scours, drifts and ridges of folded sediment characterize western Solander Basin, whereas hemi‐pelagic drape and secondary gravity flows are found east of the meandering axial Solander Channel. The high‐resolution record of climate and tectonics that Solander Basin contains may yield excellent sites for future scientific ocean drilling. 

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.

 

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. 

We conducted a vertical seismic profile (VSP) in the borehole of International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 Site M0077 to better understand the nature of the seismic reflectivity and the in situ seismic properties associated with the Chicxulub impact structure peak ring. Extraction of the up‐going wavefield from the VSP shows that a strong seismic reflection event imaged in seismic reflection data results from discontinuities in the elastic impedanceZ(the product of density and wave speed) at the top and bottom of a zone of hydrothermally altered melt‐bearing polymict breccia (suevite) that are characterized by anomalously lowZ. Below this strong carbonate/suevite reflection event, the upgoing seismic wavefield is chaotic, indicating high levels of scattering from the suevites and underlying melt rocks and shocked granitoids of the peak ring, in contrast to the clear coherent reflections throughout the overlying Cenozoic sediments. We extract shear wave speeds, which, together with those provided from the complementary sonic log and densities from core scanning, allowed determination ofV_P/V_Sand Poisson's ratiov. These values are anomalously high relative to comparable terrestrial lithologies. We also calculate a variety of damage parameters for the disrupted peak ring granitoids. These values may assist in linking seismic observations to shock levels that are necessary to calibrate current impact models and may also be useful in assessing levels of fracturing within major fault zones.

 

The Chicxulub impact crater, 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) Expedition 364, Paleogene sediments 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 section that might include Eocene and Paleocene hyperthermals and/or the Paleocene/Eocene Thermal Maximum (PETM); the composition and character of the impact breccias, melt rocks, and peak-ring rocks; the sedimentology and stratigraphy of the Paleocene–Eocene Chicxulub impact basin infill; the chronology of the peak-ring rocks; and any observations from the core that may help us constrain the volume of dust and climatically active gases released into the stratosphere by this impact. Petrophysical property 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 impact crater models for crater formation and environmental effects. The proposed drilling directly contributes to IODP Science Plan goals: Climate and Ocean Change: How resilient is the ocean to chemical perturbations? The Chicxulub impact represents an external forcing event that caused a 75% 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 effects on biodiversity. Earth Connections/Earth in Motion: What are the composition, structure and dynamics of Earth’s upper mantle? What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami? Mantle uplift in response to impacts provides insight into dynamics that differ between Earth and other rocky planets. Impacts generate earthquakes, landslides, and tsunami, and scales that generally exceed plate tectonic processes yield insight into effects, the geologic record, and potential hazards. IODP Expedition 364 was a Mission Specific Platform expedition to obtain subseabed samples and downhole logging measurements from the sedimentary cover sequence and peak ring of the Chicxulub impact crater. A single borehole was drilled into the Chicxulub impact crater on the Yucatán continental shelf, recovering core from 505.7 to 1334.73 m below seafloor with ~99% core recovery and acquiring downhole logs for the entire depth. 

IODP/ICDP Expedition 364 recovered ~829 m of core at Site M0077 including ~110 m of post-impact, (hemi)pelagic Paleogene sedimentary rocks overlying the Chicxulub impact crater peak ring formed from suevite, melt rock, and granitic basement. The transition between suevite and Paleocene limestone (Unit 1F) is a remarkable fining upward package of gravel to sand-sized suevite (Unit 2A) overlain by the laminated carbonate-rich Unit 1G that records deposition of fine-grained material post-impact and contains a mix of Late Cretaceous and earliest Danian taxa. This study concentrates on the overlying Unit 1F. The ichnofabric index (ii, 1-6 indicating no bioturbation to complete homogenization), provides a semiquantitative estimate of burrow density to help assess the return of life to the crater. Unit 1F is ~10 m thick with a sharp contact at the base of a green claystone (ii 2) that overlies Unit 1G. It consists of cm-dm interbedded blue-gray marlstone (ii 2) grading upward into gray to blue-gray wacke/packstone (ii 3-5). Contacts between facies are mostly gradational due to burrowing. The upper 3 m of the unit is a yellow-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) and fine to coarse sand size clasts including foraminifera. The upper surface of the unit is a hardground with an ~2 Myr unconformity overlain by Eocene rocks. The first well-defined burrows occur in the upper 30 cm of Unit 1G. Unequivocal burrows (ii 2) that disturb sedimentary facies occur in overlying Unit 1F with values of 3-5 recorded in the overlying 10 cm indicating significant disruption of primary sedimentary structures. The iis in Unit 1F vary between 2 and 5 with rare laminated intervals without bioturbation (ii 1). Values of ii correlate well with facies changes, i.e. marlstones display lower iis than more carbonate-rich facies, implying a depth and/or redox control on burrower distribution. The ii data indicate that burrowers were re-established in the crater before the end of deposition of Unit 1G. The lowest Danian zone Pα is documented in the lowermost part of Unit 1F. Trace makers were thus active by the earliest Danian with an increase in abundance and diversity during the lower Danian, indicating a rapid and continuous return of benthic life to the crater. 

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. 

The Chicxulub impact crater in Mexico 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 Earth’s environment and ecology. Our understanding of the impact process is far from complete, and despite more than 30 y of intense debate, we are still striving to answer the question as to why this impact was so catastrophic. International Ocean Discovery Program (IODP) Expedition 364 proposes to core through the peak ring of the Chicxulub impact crater 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 postimpact hydrothermal circulation, (4) the deep biosphere and habitability of the peak ring, and (5) the recovery of life in a sterile zone. Of additional interest is the transition through a rare midlatitude record of the Paleocene/Eocene Thermal Maximum (PETM); the composition and character of impact breccias, melt rocks, and peak-ring rocks; the sedimentology and stratigraphy of the Cenozoic sequence; and any observations from the core that would help us constrain the volume of dust and climatically active gases released into the stratosphere by this impact. Petrophysical property measurements on the core and wireline logs will be used to calibrate geophysical models, including seismic reflection data. Proposed drilling directly contributes to the IODP science plan initiatives (1) Deep Biosphere and the Subseafloor Ocean and (2) Environmental Change, Processes and Effects, in particular the environmental and biological perturbations caused by the Chicxulub impact. Expedition 364 will be implemented as a mission-specific platform expedition to obtain subseabed samples and downhole logging measurements from the peak ring of the Chicxulub impact crater. The expedition aims to core a single borehole as deep as 1500 meters below seafloor (mbsf) to recover rock cores from above and into the Chicxulub impact crater preserved under the Yucatán continental shelf.