(Formerly Cretaceous–Tertiary Boundary)

Impact glass and spherules in carbonate matrix
Beloc, Haiti site
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Recognized in 1990
18° 22' 37" N., 72° 35' 22" E., approx.

A severe impact by a carbonaceous chondritic asteroid or long-period comet (e.g., Moore and Sharma, 2013; Goderis et al., 2013; Durand-Manterola and Cordero-Tercero, 2014) occurred 66.038 (±0.011) m.y. ago as determined by Ar–Ar chronometry, and is likely associated with a major extinction event. This period marks the Cretaceous–Paleogene (K–Pg; formerly K–T) boundary separating the Mesozoic Era (age of dinosaurs) and Cenozoic Era (age of mammals). The size of the impactor was calculated to be at least 10 km (~6 mi) in diameter and to have conveyed at least 1.3 yottajoules of energy (Durand-Manterola and Cordero-Tercero, 2014), equivalent to ~6 million 'Tsar Bomba' nuclear devices. The impactor had a trajectory angle of ~60° (Collins et al., 2017) which created a multi-ring crater ~180 km (112 mi) in diameter known as Chicxulub crater—now buried under ~1 km of Tertiary sediments in the Yucatán Peninsula, Mexico.

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Bouguer gravity anomaly map of Chicxulub crater
Image credit: Geological Survey of Canada
The white-dotted ring of cenotes (sinkholes) mark the subsurface structure of the crater rim. They were created by groundwater flow to the sea which dissolved ancient mineral deposits.

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3-D geomagnetic image of Chicxulub crater
Image credit: V. L. Sharpton, LPI

This impact produced lithologies such as polymict impact breccia and impact melt rock which were discovered in borehole samples obtained from the interior of the Chicxulub crater (David A. Kring, LPI, 'Chicxilub Impact Event'). In addition, the impact event is associated with shocked quartz with planar deformation features, as well as impact-generated melt products in the form of sub-mm- to mm-scale glass spherules (microtektites) and irregularly- to aerodynamically-shaped glass shards and fragments, all embedded in an iridium-enriched astrostratigraphic unit identified at various locations around the world (e.g., Hildebrand and Boynton, 1990; Kring and Boynton, 1991).

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click on photo for a magnified view with explanatory information
Global distribution map of key K–Pg boundary data locations
Image credit: Schulte et al. (2010)

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Western Interior Seaway during its retreat 65 million years ago
Image credit: Dr. Ron Blakey of Colorado Plateau Geosystems, Inc.

A fossil meteorite measuring 2.5 mm in diameter that was recovered from K–Pg boundary sediments in the western North Pacific by the Deep Sea Drilling Project was classified as a probable CV, CO, or CR carbonaceous chondrite, but a CM type was not excluded (Kyte, 1998). Results of a Cr-isotopic analyses conducted by Trinquier et al. (2006) indicate that a CM2 chondrite was the most likely impactor type. Goderis et al. (2013) analyzed highly and moderately siderophile element abundances and ratios from numerous K–Pg boundary ejecta layer sites at varying distances from the Chicxulub crater to help resolve the type of projectile involved in the impact event. They found Ir concentrations as high as 40 ppm, which is up to 2,000× that of mean continental crustal values; however, this is less than the maximum concentrations of 87 ppm reported previously (e.g., Claeys et al., 2002). Based on HSE concentrations and PGE inter-element ratios, and in consideration of other processes and factors which might have been involved (e.g., post-depositional remobilization, fractionation during vaporization/condensation, alteration and/or loss of PGE host phases, localized background contamination, loss of impactor component to space, etc.), a carbonaceous chondrite belonging to the CM–CO clan was determined to be the most likely parental source object for the Chicxulub impactor.

The impact spherules from the K–Pg boundary stratigraphic section in Haiti were identified by Bohor and Glass (1995) as splash-form melt droplets (spheroids, ovoids, teardrops, dumbbells, rods, etc.), which constitute the lowermost of two separate and compositionally different ejecta layers present at various sites within ~4,000 km of the crater. This thick, white-colored lower claystone layer consists of Type 1 spherules which are now mostly hollow due to dissolution, and are composed of secondary palagonite/smectite. Type 1 spherules were part of a ballistic ejecta curtain that was transported within the cold atmosphere at shallow angles and rapidly quenched. Acceleration of these spherules was aided by shock-volatilization of carbonates (and sulfates) composing the target rock, rapidly releasing CO2 and explosively converting water into steam. Consequently, these spherules are mostly restricted to sites within the Western Hemisphere. The thinner, black-colored upper claystone layer contains solid, crystalline Type 2 spherules along with Ni-rich spinel and abundant shocked quartz. Type 2 spherules are direct condensates from the plume of vaporized impactor and target rock, and are highly enriched in Ir compared to Type 1 spherules. Type 2 spherules are not only found at Type 1 sites proximate to the crater, but also at more distal sites worldwide (e.g., Umbria-Marche region in Italy). After the impact, the molten Type 1 glass spherules were quenched upon deposition into coastal waters (e.g., prior to tectonic uplift above sea level in Haiti) and shallow lakes or swamps (e.g., in the Western Interior of North America), while at some sites they were concentrated through megatsunami action (e.g., Shell Creek, Alabama). Although most of the glass spherules are now devitrified and have been altered to secondary clay minerals such as smectite, the glass spherules found most recently in a 2-cm-thick layer at Gorgonilla Island, Columbia, are virtually unaltered and represent the most pristine K–Pg boundary spherules known.

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K–Pg boundary at Gorgonilla Island, Columbia, with well-preserved glass spherules up to 1.1 mm in diameter
Photo courtesy of Bermúdez et al., 2015
'Discovery of a pristine Chicxulub impact glass spherule deposit on Gorgonilla Island, Colombia, in the Eastern Pacific Ocean'

The Haitian glass spherules typically have dacitic to andesitic compositions and are termed "black glass", while the much less common "yellow glass" spherules are high in Ca and S (Bohor and Glass, 1995). Yellow glass spherules were produced from an admixture of deep basement crystalline rock, from which the "black glass" derives, and surface carbonate limestone (with <10% anhydrite component). Other glass samples have been found which exhibit intermediate compositional ranges between these two end member rock types, and the differences are attributed to formation under a very large range of oxidation conditions (Giuli et al., 2008). Similar to Libyan Desert Glass, some K–Pg boundary glasses contain the highly-shocked silicate phase lechatelierite along with other mineral inclusions (Koeberl and Sigurdsson, 1992). Furthermore, a meteoritic component was identified in a glass sample through Re–Os-isotopic analyses (Koeberl et al., 1994), and a very low water content (up to 0.021 wt%) indicative of an impact origin was reported in other glass samples (Koeberl, 1992).

The extent of the known K–Pg exposure at the Shell Creek site in Wilcox County, Alabama is limited to ~200 m² near a narrow ford in the creek (King and Petruny, 2008). The stratigraphy of the K–Pg boundary section is 35–75 cm thick and consists of two distinct depositional layers. The basal gray–green-colored unit in direct contact with the upper Maastrichtian (Late Cretaceous) Prairie Bluff Chalk shelf is ~15–25 cm thick and is composed of Clayton Formation sand densely embedded with Type 1 impact spherules; hematite has stained the lowermost few cm of this sand unit a reddish color. This basal unit is overlain by a tan-colored, more fine-grained, hummocky cross-laminated sand unit ~30–50 cm thick, which lies below lower Danian (Paleogene) sandy marls. The impact spherules at this site occur in a variety of splash forms (spheres, spheroids, dumbbells, teardrops, and others) in a size range of ~0.5–3 mm, the majority measuring slightly less than 1 mm. Similar to most other spherule-bearing sites, the impact spherules at Shell Creek have been altered to a nontronitic smectite shell with hollow interiors often filled by calcite (Ferrell et al., 2011); however, they still exhibit many fine details intrinsic to the primary glass phase. The differences that exist between the two sandstone units present at this site have been attributed to energetic wave action involving tsunamis and/or strong cyclonic tropical storms.

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K–Pg boundary stratigraphic section at Shell Creek, Alabama (photos not shown to the same scale)
Diagram and photos courtesy of David T. King, Jr. and Lucille W. Petruny, 'The Sedimentary Record of Impacts', GSA Special Paper 437, pp. 179–188 (2008)

Excavations have been conducted near the northern-most extent of the alluvial plain of the Mississippi Embayment in southeastern Missouri. A K–Pg boundary stratigraphic section along the southeastern side of Crowley's Ridge in Stoddard County, Missouri was described by Campbell et al. (2008) as situated between Upper Cretaceous Owl Creek Formation clay and lower Paleogene Clayton Formation glauconitic sandy clay. Splash form impact spherules representing morphologies and sizes similar to those from other Gulf Coastal K–Pg boundary sites such as Shell Creek, Alabama were identified in Owl Creek rip-up clasts, embedded within a 30-cm-thick limestone coquinite zone marking the K–Pg boundary. The coquinite formed by chaotic mixing involving megatsunami movement into the embayment following deposition of hot, sintered ejecta from the Chicxulub impact. It was estimated by Campbell et al. (2008) that the waves could have reached a height of more than 200 m due to constructive interference within the narrowing embayment, eventually backwashing terrestrial material into the flood zone which gravitationally settled out to create a graded unit.

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Generalized map of the southeast coastal region at the time of the Chicxulub impact
Map courtesy of Campbell et al., 'The Sedimentary Record of Impacts', GSA Special Paper 437, pp. 189–198 (2008)

The K–Pg boundary site closest to the Chicxilub crater is located in the Orange Walk District in northern Belize, and this is the only known direct ejecta deposit, or fluidized debris flow (D.T. King, Jr, 1996). The outcrop at Albion Quarry has a thickness of 16 m and contains material ranging in size from finely-ground host matrix to boulders 8 m in diameter that were ejected from the crater during impact. The stratigraphy of this quarry includes a 1–2 m basal Type 2 spherule-bearing layer in direct contact with the Maastrichtian Barton Creek Dolomite bedrock, which is overlain by an ~14-m-thick, poorly sorted diamictite layer comprised of several roughly graded beds. Pebble-sized red and green clay clasts within the diamictite layer are likely altered glass fragments (Ocampo et al., 1996). In the basal layer, the spherules exhibit increased flattening towards the bottom of the bed. Pope and Ocampo (2000) described large, sub-rounded, extremely fine-grained limestone fragments (~4–23 cm in diameter) known as Pook's pebbles found at Pook's Hill in western Cayo District. These pebbles exhibit several features indicative of sub-orbital flight and ablation during atmospheric re-entry, including regmaglypts, impact and ablative pits, parallel gouges and striations, rounded facets, and thin, translucent, relict fusion crust (often truncated and located opposite a heavily pitted side, indicative of orientation); these pebbles are analogous to terrestrial meteorites (see photo below). Although to date no impact glass or shocked grains have been found at the western Cayo District site, Pope et al. (2005) did describe impact ejecta material such as spherules, altered glass shards, and shocked quartz from an outcrop at Armenia in the eastern Cayo District; notably, Armenia and Pook's Hill are equidistant from the crater, and therefore it could be inferred that both locations should contain similar deposits from the ejecta curtain and/or impact vapor plume.

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Pook's pebble with impact crater and thin fusion crust
Photo credit: A.C. Ocampo and K.O. Pope, Planetary Report, vol. 16, p. 9 © Planetary Society,
courtesy of David T. King, Jr, 1996—Cretaceous–Tertiary Boundary Stratigraphy Web Page

Impact spherules from the distal K–Pg boundary sections of the Umbria-Marche region in Italy were studied by Belza et al. (2017). It was ascertained that they are Type 2 spherules representing vaporized impactor (chondritic) and target (both sedimentary carbonate platform and crystalline granitic basement rock types) material which condensed out of the globally expanding impact vapor plume. They occur in relatively high concentrations (up to 200 per cm³) embedded in a thin layer (1–3 cm) of reddish clay demarking the K–Pg boundary. These Type 2 spherules measure 0.05–0.4 mm (ave. 0.25 mm) in size, which is significantly smaller than proximal Type 1 spherules. Like spherules from all locations, those from the Umbria-Marche region were initially emplaced into a marine environment at significant depth where alteration progressed for an extended period of time. The Umbria-Marche spherules have been categorized by Belza et al. (2017) into three distinct types based on texture and composition: 1) goethite, 2) K-feldspar, and 3) glauconite.
It was determined that the black goethite spherules are a product of primary olivine replacement under aqueous conditions. They contain very high abundances of refractory lithophile and siderophile elements consistent with early fractional condensation from a high-temperature region of the vapor plume. Belza et al. (2017) observed that these spherules preserve relict crystal textures analogous to several meteorite chondrule types, including porphyritic, radial/barred, relict/granular, skeletal, and barred. They concluded that the specific texture of the spherules was controlled by peak temperature, heating duration, cooling rate, and degree of undercooling, the latter related to the availability of nucleation sites within melt droplets as influenced by unmelted precursor grains or variable concentrations of dust in the vapor plume.

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Image credit: Belza et al., GCA, vol. 202, p. 251 (2017)
'Petrography and geochemistry of distal spherules from the K-Pg boundary in the Umbria-Marche region (Italy)
and their origin as fractional condensates and melts in the Chicxulub impact plume'

Moreover, Belza et al. (2017) ascertained that the white K-feldspar and calcite spherules are aqueous replacement products of primary pyroxene and plagioclase, respectively. These spherules are considered to have experienced varying degrees of supercooling and nucleation delay within a fractionated, cooler region of the vapor plume than the goethite spherules. In addition, they proposed that the green glauconite spherules probably formed through the transformation of smectite by K uptake in a marine environment associated with low pH and abundant organic matter, while the smectite could be the initial replacement product of quenched basaltic glass akin to the proximal Type 1 spherules. The source for the green glauconite spherules would be those melt droplets ejected at the steepest angles to become entrained in the expanding vapor plume.
Core samples from the central peak of the Chicxulub crater were obtained in 2016 and are now under analyses at the International Ocean Discovery Program (IODP) Bremen Core Repository in Germany. The K-Pg boundary sample shown above is from Beloc, Haiti and weighs 0.35 g. It consists of mm-scale spherules and black glass fragments in a limestone matrix.

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'The Day The Mesozoic Died', from Howard Hughes Medical Institute—on Vimeo