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THE CHICXULUB IMPACT CRATER: Producing a Cradle of Life in the Midst of a Global Calamity

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THE CHICXULUB IMPACT CRATER: Producing a Cradle of Life in the Midst of a Global Calamity THE CHICXULUB IMPACT CRATER: Producing a Cradle of Life in the Midst of a Global Calamity Featured Story | From the Desk of Lori Glaze | Meeting Highlights | News from Space | Spotlight on Education In Memoriam | Milestones | New and Noteworthy | Calendar LUNAR AND PLANETARY INFORMATION BULLETIN April 2021 Issue 164 FEATURED STORY THE CHICXULUB IMPACT CRATER: Producing a Cradle of Life in the Midst of a Global Calamity DAVID A. KRING, LUNAR AND PLANETARY INSTITUTE Expedition 364 mission patch Introduction when the International Ocean Discov- an area that had been a stable sediment ery Program (IODP) and International catchment for over 100 million years? Strategically located scientific drilling Continental Scientific Drilling Program Clues began to emerge when the core can be used to tap the Earth for evi- (ICDP) initiated a new campaign with was analyzed. Logging revealed chem- dence of evolutionary upheavals that the call sign Expedition 364. Drilling ical and petrological variations on the transformed the planet. A good example from a marine platform a few meters granitic theme, plus felsite and dolerite is the Yucatán-6 borehole in Mexico above the sea surface, the new borehole intrusions, in a granitoid rock sequence that recovered rock samples from 1.2 reached a depth of 1335 meters be- that represented continental crust that and 1.3 kilometers beneath Earth’s neath the sea floor (mbsf). The borehole had been assembled through a series of surface. I used those samples 30 years penetrated seafloor sediments that bury tectonic events over more than a billion ago to show that a buried, geophysical- the crater, finally reaching impactites at years. However, that crust in the core ly anomalous structure on the Yucatán a depth of 617 mbsf. Continuous core was crosscut by seams of impact melt Peninsula contained a polymict breccia was recovered from 506 mbsf, within rock and suevite. Moreover, quartz and with shock metamorphism and an impact Eocene sediments deposited 48 million other minerals in the granitoid rocks were melt rock, indicating the buried structure years ago, to the bottom of the borehole deformed, corresponding to shock pres- was an immense impact crater that was within the crater’s 66-million-year-old sures of 16 to 18 gigapascals, indicating excavated 66 million years ago. That peak ring. The core is a scientific marvel, the tectonic construction of the crust had structure, which we called Chicxulub, was exceeding the expedition’s highest hopes been superseded by an impact event. produced by a ~100-million-megaton of success. Here, I briefly summarize blast responsible for a global environ- the science party’s analyses of that core Those observations indicated the granit- mental calamity and mass extinction that and the insights they are gleaning about oid rocks were uplifted from the geologic defines the Cretaceous-Tertiary (K-T) peak ring formation and the biological basement of the Yucatán, far beneath a boundary in Earth’s evolution (see LPIB, communities that reoccupied the site after carbonate platform sequence of sedi- March 2016, for additional details of most life on Earth had been extinguished. mentary strata that covers the peninsula. that discovery). The impact provoked a Numerical simulations of the impact biological crisis that extinguished indicator Formation of the integrated with borehole observations species throughout the world, including suggest the crystalline rock was uplift- winged pterosaurs in the air, non-avian Crater’s Peak Ring ed from a depth of 8 to 10 kilometers. dinosaurs on land, and apex predator During the crater-forming process, the mosasaurs in the seas, along with 75% of Granite. Lots of spectacular-looking uplifted rock formed a transient central the total breadth of species that existed granite. That was a common observa- peak that collapsed outward to form on Earth at that time. Life was decimated. tion when meter after meter of core was a peak ring, overturning the granitoid pulled from the sea, bringing to light one rocks. A dramatic cycle of compres- Science has returned twice to probe the of the expedition’s key questions: Why sion, dilation, rotation, and shear all depths of Chicxulub, most recently in 2016 was granite so near Earth’s surface in occurred within minutes as the crust of 2 Issue 164 April 2021 © Copyright 2021 Lunar and Planetary Institute FEATURED STORY Earth flowed at speeds in excess of 100 being traced across the Chicxulub basin kilometers per hour, producing zones where the breccia blankets an ~3-kilo- of microcommunited rock (cataclasites), meter-thick central melt sheet. Ejected shear faults, and deformation bands that debris was also launched beyond the cross-cut shock metamorphic fabrics. crater rim, where some of it flew through Shearing is particularly intense in the the atmosphere faster than the speed of basal 100 meters of the core, produced sound, producing sonic booms like bil- when overlying granitoid rocks were lions of simultaneously falling meteorites. thrust over impact melt that had already That curtain of debris hit the sea surface covered underlying basement rocks. with such high speeds it caused the sea The resulting impact crater looked very to boil with cavitation. The debris dis- much like the Schrödinger basin on the placed seawater, too, while cascading Moon, before being hidden from view to the seafloor and pummeling marine beneath Tertiary sediments. Asymmetries organisms caught in its path. The speed in Chicxulub’s peak ring and underlying of that debris hitting Earth’s surface grew mantle uplift were noted, however, and larger with distance from the crater and explored in numerical simulations of increasingly ploughed into the surface the crater-forming event. Those results it landed upon. Because the Chicxulub suggest the impactor had a trajectory impact occurred at sea (albeit above from the northeast to the southwest. The continental crust rather than oceanic transient central uplift, potentially rising crust), the ballistic sedimentation process more than 10 kilometers into the atmo- often mixed ejecta with water. In those sphere, was momentarily higher than cases, fluid target materials escaped the An 83-millimeter-diameter granitic core from the Mt. Everest and would have been visible final deposit, leaving behind a blanket Chicxulub peak ring that is crosscut with cataclastic halfway across the Gulf of Mexico if of wholly ejected rock and solidified and hydrothermal veins, and which also has been not obscured by >25 trillion metric tons impact melt. At greater distances, shock-metamorphosed, as illustrated with planar deformation features with ~5-micrometer spacing of ejecta lofted into the atmosphere. beyond the unit traditionally mapped in quartz (inset, with field of view 245 micrometers as proximally emplaced continuous wide). Photomicrograph of quartz by expedition Deposition of ejecta, impact melt spherules cascad- scientist Ludovic Ferrière. Previously published by D. ed through the atmosphere and seas A. Kring, Ph. Claeys, S. P. S. Gulick, J. V. Morgan, G. S. Collins, and the IODP-ICDP Expedition 364 Sci- Impactites throughout the region, forming blankets ence Party (2017) Chicxulub and the exploration of of glass that are still preserved in Beloc large peak-ring impact craters through scientific dril- Some of that ejecta fell back onto the (Haiti), Arroyo el Mimbral (Mexico), ling. GSA Today, 27, DOI: 10.1130/GSATG352A.1. granitoid peak ring, producing 130 and Gorgonilla Island (Colombia). meters of melt-bearing polymict brec- cia (suevite) and impact melt rock. The The impact also generated a vapor-rich Decimating the basal melt rock is a small portion of the ejecta plume that expanded from the 104 to 105 cubic kilometers of molten point of impact, accelerating through the Marine Environment rock generated by wholesale melting of atmosphere as it raced toward space. Earth’s crust by the impact. Overlying Superheated to temperatures on the order The concept of “ground zero” literally ex- breccia clast sizes grow smaller toward of 10,000°, that plume and other ejecta ploded into our lexicon with the 21-kilo- the top of the suevite, but do not form ignited vegetation on distant shores. Back- ton Trinity blast in the Jornada del Muerto a single (normally graded) unit going wash from impact-generated tsunamis desert valley of New Mexico in 1945. from large to small clast sizes. Rather, and/or strong atmospheric circulation The devastating effects of high-energy ex- there is at least one erosional contact carried charcoal from those fires back plosions were immediately obvious and in the lower portion of the breccias and to the crater, where it is found buried in began coloring descriptions of impacting several size-graded intervals toward the core on top of the peak ring. That asteroids like the collision that produced the top of the sequence, indicating high-energy ejecta plume also carried Barringer Meteorite Crater (aka Meteor reworking by marine currents, including vaporized components of the impacting Crater) in Arizona. The Chicxulub impact impact-generated seiches produced object. When it and other debris reaccret- blast was nearly five billion times more when tsunamis and other waves washed ed to Earth, they heated the atmosphere energetic than the Trinity test and seven to and fro across the ocean basin. and generated a firestorm over a broader million times more energetic than the area. Scorched woodland fragments from Meteor Crater event. The Chicxulub blast Impact melt and suevite sampled in the those fires were incorporated into peak- occurred in a thriving marine ecosystem borehole cover more than 100,000 ring sediments, too, with iridium rainout that was, with a flash of light, vaporized. square kilometers of the Gulf seafloor. over a longer period of time, producing The seismic properties of the suevite are a second peak in charcoal abundance. In the mid-1990s, I used the results of 3 Issue 164 April 2021 © Copyright 2021 Lunar and Planetary Institute FEATURED STORY pounds per square inch (or a few to tens of megapascals) and likely lethal out to distances of about 2000 kilometers in the open sea.
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