Coesite in Australasian microtektites.

Tektites and microtektites (i.e. very small tektites) are pieces of glass formed as ejecta from impact events. They are found in strewn fields which may extend thousands of kilometres from the original impact site. Current models of their formation suggest that they form from the material being impacted, as a spray of droplets of material melted by the rapid heating of the impact of an asteroid or comet fragment at speeds of over 3 km per second. However, while we have a general picture of how these events unfold, much of the detail is unclear. For example, many tektites from the Australasian Strewn Field contain inclusions of material, which have for a long time been accepted as fragments of the original rock trapped within a matrix of glass melt. This view has recently been challenged by an alternative hypothesis, that the matrix material may be a condensate from rock which was vapourised during the impact, and that the most distant microtektites, from Antarctica, may have formed from material which was vapourised by heat from the atmospheric shock wave before the impacting object touched down. This would help to explain why the Antarctic tektites lack inclusions, whereas in those from close to the presumed impact site, in Southeast Asia, they may make up as much as 5% of the mass of the tektite.

In a paper published in the journal Geology on 3 June 2025, Luigi Folco, Enrico Mugnaioli, and Matteo Masotta of the Dipartimento di Scienze della Terra and the Centro per la Integrazione della Strumentazione  at the Università di Pisa, and Billy Glass of the Department of Geosciences at the University of Delaware, present the results of a study of four microtektites from the Australasian Strewn Field.

The Australasian Strewn Field covers about 15% of the Earth's surface, and formed about 800 000 years ago through the hypervelocity impact of a chondritic body. It is the youngest and largest of five known Cainozoic strewn fields, with the others  being the North American Strewn Field, which is about 34.86 million years old (Eocene), the Central European Strewn Field, which is about 14.7 million years old (Miocene), the Côte d'Ivoire Strewn Field, which is 1.07 million years old (Pleistocene), and the Central American Strewn Field, which is about 820 000 years old (Pleistocene). No crater has been found which can be linked to the Australasian Strewn Field, although high pressure phases have been found in tektites and other ejecta which strongly indicate that the formation of the field was linked to a crater-forming event, probably in Southeast Asia or the surrounding seas, or possibly in northwest China.

Several previous studies have established that Australasian microtektites show Australasian microtektites with distance from the presumed impact site. For this reason Folco et al. selected three microtektites from deep-sea locations close to the putative impact site, SO95-17957-2,04 and ODP 1144A,01 from the South China Sea, and ODP 769A,15_26 from the Sulu Sea, and one, FRO 2.9-1, from a site in the Transantarctic Mountains. These samples were analysed using a combination of optical microscopy, microanalytical scanning electron microscopy, dual beam microscopy, and microanalytical transmission electron microscopy coupled with three-dimensional electron diffraction.

The Australasian Strewn Field microtektites from deep-sea settings are spheroid in shape and dark brown in colour, with a transluscent laustre and numerous inclusions. They range from 350 to 700 µm in maximum elongation, and dominated by silica phases, compositional bands (schlieren), and vesicles. SO95-17957-2,04 and ODP 1144A,01 have normal composition, whereas ODP 769A,15_26 has a high nickel content (232 µg/g). The Antarctic specimen is a pale-yellow transparent sphere 485 µm in diameter with normal composition,  devoid of vesicles, with only one microscopic silica-rich inclusion,  a few tens of micrometres across with diffuse boundaries. This is considered to be fairly typical of Australasian microtektites from Antarctica, although it is the only Antarctic Tektite known with a silica-rich inclusion.

Micrographs of sectioned Australasian microtektite SO95-17957-2,04. (A) Microtektite is pale brown with teardrop shape. It shows folded schlieren (Sch), microscopic vesicles (V), and mineral inclusions (arrowed). Thick white arrow points to inclusion studied in this work. Optical microscope image, plane polarized light. (B) Same petrographic features as in A in backscattered electron (BSE) image. White rectangle outlines field of view of image in panel C. (C) Close-up BSE image of a quartz (Qtz) + lechatelierite (L) + coesite (Coe) inclusion. Silica phases in the inclusion can be distinguished by their different electron density contrast, which increases from lechatelierite to quartz to coesite. A diffusive boundary layer (Dbl) discontinuously surrounds the inclusion. Dashed line traces location of the dual beam microscopy section. Folco et al. (2025).

Examined under the scanning electron microscope, the microtectites from deep-sea environments were found to contain a inclusions which comprise a matrix of vesiculated lechatelierite (shock-fused quartz glass), with variable proportions of microscopic quartz grains and submicroscopic coesite grains (coesite is a form of silica dioxide which only forms at very high pressures). These inclusions are surrounded by diffusive boundary layers a few microns thick. The quartz grains tend to be arranged around the edge of the inclusions, and themselves be surrounded by grains of coesite, while the interior of the inclusions tends to be dominated by lechatelierite. The quartz grains tend to be anhedral and heavily fractured, while the coesite grains comprise  polycrystalline aggregates of nanoscopic crystals set in silica glass. Towards the interior of the inclusions, the coesite grains become smaller, and comprise a higher proportion of silica-glass. The interior part of the inclusions, while dominated by vesiculated lechatelierite, contain many of these low-coesite 'coesite grains'. All three of these microtektites have essentially the same structure, although ODP 769A,15_26 and ODP 1144A,01 are more vesiculated than SO95-17957-2,04.

The Austrolasian microtektite ODP 769A,15_26. (A) Optical microscope, plain polarised image of the sectioned spherule, showing dark brown colour, prolate shape, schlieren, microscopic vesicles and transparent to partly opaque mineral inclusions. (B) The same textural and compositional features seen under scanning electron microscope, back scattered electron imaging mode. The white rectangle outlines the area of the next panel. (C) Back scattered electron view of a quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechaterlierite to quartz to coesite. The dashed line marks the position of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing tens of submicroscopic anhedral coesite grains dispensed in vesiculated lechatelierite. Inset: a close-up view of one coesite grain showing characteristic (010) polysnthetic twinning. The dashed line in panel (C) traces the location of the DB section seen in panel (D). Abbreviations: sch, schlieren; V, vesicle: Qtz, quartz; Coe, coesite; L,  lachatelierite. Thin white arrows indicate inclusions; the thick white arrow indicates the coesite bearing inclusion studied in detail. Folca et al. (2025). 

At the edge of the inclusion in microtektite SO95-17957-2,04 studied with dual beam microscopy, coesite could be seen forming euhedral crystals which overgrow the quartz grains. Towards the lechatelierite core of the inclusion, the coesite can be seen to be segmented in submicroscopict abular grains by a fine network of silica glass veinlets producing the polycrystalline aggregates. These coesite grains show a tartan-like texture similar to that seen in microcrystalline coesite aggregates in silica glass found in shocked porous sandstones. The proportion of silica glass between the segments of coesite within the grains increases towards the core of the inclusion, where anhedral nanoscopic grains of coesite can be found floating free within the lechatelierite core. 

Transmission electron microscopy images of electron transparent dual beam microscopy section of quartz + lechatelierite + coesite inclusions from Australasian deep-sea sediment microtektites. (A) Whole section of coesite-bearing inclusion in microtektite SO95-17957-2,04. White rectangle outlines area featured in panel (B). (B) Textural relationships between quartz (Qtz), coesite (Coe), and lechatelierite (L). Few microscopic quartz relicts at periphery of inclusion are overgrown by euhedral coesite grains with polysynthetic (010) twinning. Toward the core of the inclusion dominated by lechatelierite, coesite is segmented by a network of silica glass veinlets producing polycrystalline aggregates, which then disaggregate with increasing amount of silica glass. White rectangle traces area featured panel (C). (C) Close-up view of euhedral coesite (top) adjacent to polycrystalline aggregate with subhedral outline (bottom). (D) Reconstruction of reciprocal space sampled by three-dimensional electron diffraction from a twinned coesite grain. This picture displays a view of the diffraction volume along hh0 vector. Projections of 00l* and hh0* vectors are indicated. (E) Nanoscopic anhedral coesite grain with embayed crystal boundaries embedded in lechatelierite in microtektite ODP 769A,15_26. (F) Several nanoscopic anhedral coesite grains dispersed in an area of about 2 µm² of lechatelierite in microtektite ODP 1144A,01. Broken-apart grains are arrowed. Vesicle (V). Folco et al. (2025).

Inclusions in the Transantarctic Mountains tektite, FRO 2.9-1, could be seen to be featureless glass undr the scanning electron microscope, with diffuse contact with the glass matrix of the tektite. This host matrix was composed largely of silica, with smaller amounts of aluminium oxide, iron oxide, magnesium oxide, titanium oxide, calcium oxide, potassium oxide, and sodium oxide. The transmission electron microscope confirmed that this composition did not vary through the tektite.

The Australasian microtektite ODP1144A,01. (A) Optical microscope, plane polarized image of the sectioned particle. It is a brown glass broken tear drop. Few microscopic vesicles, and faint schlieren and few mineral inclusions are visible. (B) The same textural and compositional features seen under the scanning electron microscope, back scattered electron imaging mode. The white rectangle outline the area of the next panel. (C) Back scattered electron close-up view of a highly vesiculated quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechatelierite, to quartz to coesite. The dashed line marks the location of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing a polycrystalline aggregate of submicroscopіс anhedral coesite grains set in lechatelierite. The dashed line in panel (C) traces the location of the dual beam featured in panel (D). Abbreviations: Sch, schlieren; V, vesicle; Qtz, quartz; Coe, coesite; L, lechatelierite. Thin white arrows indicate inclusions; The thick white arrow indicates the coesite bearing inclusion studied in detail in this work. Folco et al. (2025).

Coesite is a fairly common mineral in settings where quartz-bearing rocks have been subjected to shock metamorphism. Whether it occurs as a metastable phase in shocked rocks that have experienced peak pressures and temperatures much beyond its stability field (i.e. pressures in excess of 10 gigapascals and temperatures in excess of 2700°C) has been debated since the 1960s. There are three current models of coesite formation. The first suggests that coesite may form in a silica melt as pressure decreases rapidly following an impact event. The second model suggests that coesite forms within silica glass at very high pressures, without any melting actually occurring. The third model also sees coesite forming within solid quartz, although this time in porous sandstones as the peak of the pressure wave passes through. 

In the Australasian microtektites, coesite appears to be overgrowing quartz crystals, which Folco et al. interpret as a sign that they formed while the matrix was under high pressure, but still in a solid state. However, the adjacent polycrystalline aggregates consisting of submicroscopic elongated grains of coesite pervaded by silica glass veinlets do indicate that some melting has occurred, possibly of the coesite itself, and the nanoscopic anhedral coesite grains dispersed in the surrounding lechatelierite as evidence to significant melting and dispersal of coesite aggregates. In this scenario the coesite nanocrystals re relicts formed by the melting and dispersal of larger, pre-existing coesite crystals during the shock-metamorphism process.

The presence of coesite in the Australasian microtektites provides information about the location of the putative Southeast Asian impact site. Coesite forms at very high temperatures, but is unstable unless cooled rapidly; it has been estimated that after 10 seconds at very high temperatures then all coesite will have transformed into cristobalite. Since there is no cristobalite in the Australasian microtektites, it can be inferred that quenching was much more rapid in this instance, possibly aided by reactions such as the transformation of quartz and coesite melt into lechatelierite, which is endothermic (absorbs heat).

The abundant quartz and lechatelierite inclusions found in Australasian tektites are generally accepted to be indicative of a quartz-bearing target rock which underwent melting and fusion during the impact event. The boundary between these inclusions and the glass matrix of the microtektites shows varying levels of diffuseness, suggesting that these particles underwent varying levels of digestion into the matrix. The presence of coesite in tektites from deep-sea environments off Southeast Asia supports the idea that these tektites are from close to the impact site, and represent melt spherules formed by compression and depression of the impacted rock during crater formation. The absence of coesite from the Transantarctic Mountains tektite, FRO 2.9-1, could imply that this remained at higher temperatures for longer, allowing all coesite to be reabsorbed, possibly implying this tektite was exposed not just the heat from the origianal impact, but also from deceleration in ambient air or atmospheric re-entry. This would explain the near-total absence of inclusions in these tektites, and also the homogenous distribution of elements and isotopes observed. There is no structure in the microtektite indicative exposure of high pressure, which supports the idea that these more distant microtektites formed by rapid heating of the target rock prior to the actual impact.

The presence of pressure-related minerals and structures in microtektites from the South China and Sulu seas strengthens the argument for a Southeast Asian impact event.

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Large bolide impact detected on the surface of Jupiter.

Optical flashes caused by impact events on the surface of Jupiter have been within the detection capabilities of amateur astronomers for some time, with six such flashes reported between 2010 and 2020. These events have the potential to provide us with information about the abundance of small objects in the Outer Solar System, which are close to impossible to detect by direct observations, as well as the consequences of such impacts on planetary atmospheres, information which can be used to improve models of such impacts on Earth.

Models suggest that the majority of impacts on Jupiter are caused by Jupiter Family Comets (bodies with orbital periods of less than 20 years and relatively low orbital inclinations), while the majority of detections are of bodies with sizes calculated to be in the tens of metres range. However, these detections have all been essentially made by luck, when a professional or amateur telescope happened to be pointing in the right direction, rather than as a result of organised monitoring, which would provide statistically useful data on the frequency of such events.

In a paper published on the arXiv database at Cornell University and submitted to the Astrophysical Journal Letters, Ko Arimatsu of the Astronomical Observatory at Kyoto University, Kohji Tsumuru of the Department of Natural Science at Tokyo City University, Fumihiko Usui of the Department of Space Astronomy and Astrophysics at the Japan Aerospace Exploration Agency, and Jun-Ichi Watanabe of the Astronomy Data Center at the National Astronomical Observatory of Japan, report the first detection of an impact event on Jupiter by the Planetary ObservatioN Camera for Optical Transient Surveys observation system, which was set up to continuously monitor the surface of Jupiter for impact flashes, and commenced operating on 9 September 2021.

The event was detected at 1.24 pm GMT on 15 October 2021 (i.e. 36 days after the system began continuous observations of Jupiter), in both the visual and methane-absorbance bands of the spectrum, in the north tropical zone of Jupiter. The flash was also seen by two amateur astronomers in Japan, and one in Singapore; these observations provide a corroboration of the event which confirms it occurred on Jupiter, not in the Earth's atmosphere. No subsequent after-effects of the impact could be observed from Earth. An observation by the Juno Spacecraft made 28 hours after the impact showed some darkening at the site of the event, although it is impossible to determine if this was related.

Impact flash on Jupiter on 15 October 2021. Arimatsu et al. (2022).

The impact had a maximum apparent magnitude of 4.7 in the visual part of the spectrum (comparable to the Jovian moon Io at it's brightest), and lasted for about 5.5 seconds. This is brighter and longer lasting than any Jovian impact recorded since the Shoemaker Levy 9 impact observed by the Hubble Space Telescope in 1994.

The impact was significantly brighter at visible wavelengths than it was in the methane-absorbance part of the spectrum, which Arimatsu et al. attribute to reflection of light at these wavelengths by the Jovian clouds. Arimatsu et al. further calculate that the impact released an energy of about 740 000 000 megajoules, roughly equivalent to the detonation of 1.8 megatons of TNT. This is roughly equivalent to the amount of energy thought to have been released by the Tunguska explosion in 1908, and an order of magnitude greater than any Jovian impact previously recorded, with the exception of the Shoemaker Levy 9 impact.

Based upon this, Arimatsu et al. calculate that the impactor would have had a mass of about 4 100 000 kg, which would equate to a diameter of roughly 15-30 m, depending on the density and composition of the object.

By calculating the amount of time known to have been dedicated to the direct observation of Jupiter by telescopes capable of detecting large impacts since 2010, and the number of impacts observed in this time, Arimatsu et al. calculate that such impacts are 2-3 orders of magnitude more common on Jupiter than on Earth. This is an order of magnitude greater than previous estimates based upon observations of impact craters on the Jovian moons. 

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Determining the time of year when the Chicxulub Impactor fell.

The end-Cretaceous extinction event wiped out 76% of known species on Earth, but was strangely selective in the way it did so. The non-Avian Dinosaurs were wiped out, as were the Pterosaurs, most Marine Reptiles, Ammonites, Belemnites, and Rudists, amongst other groups. The extinction event is believed to have been caused by a bolide impacting the Gulf of Mexico near the Yucatan Peninsula, creating the Chicxulub Impact Crater. Evidence of the direct effects of this impact, including impact glass fallout, large-scale forest fires and tsunamis, have been found in areas of North America more than 3500 km from the impact site, and the subsequent events are thought to have included a global climatic breakdown which lasted for thousands of years.

The Tanis Event Deposits of North Dakota record a seiche event (tsunami-like wave within an enclosed environment such as a river valley or small lake) including a significant death assemblage of latest Cretaceous fauna, directly overlying the End Cretaceous Hell Creek Formation. This is thought to represent the very end of the Cretaceous, containing the fossils of organisms that died and to have been burried more-or-less instantly in a seiche event brought about by that impact. The reconstruction scenario is that within a few tens of minutes of the impact, a large volume of water and soil was forced upstream from the Tanis Estuary, pushing with it large volumes of marine, freshwater, and terrestrial organisms, whilst also accumulating impact spherules (spherical silica particles formed as melted droplets of rock recrystalise in mid air) which were falling from above. Within this deposit were large numbers of Acipenseriform Fish (Sturgeon and Paddlefish), which were buried alive in alignment with the flow of the seiche, and which had numerous impact spherules trapped within their gills.

In a paper published in the journal Nature on 23 February 2022, Melanie During of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and the Subdepartment of Evolution and Development at Uppsala University, Jan Smit, also of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, Dennis Voeten, also of the Subdepartment of Evolution and Development at Uppsala University, and of the European Synchrotron Radiation Facility, Camille Berruyer and Paul Tafforeau, also of the European Synchrotron Radiation Facility, Sophie Sanchez, again of the Subdepartment of Evolution and Development at Uppsala University, and the European Synchrotron Radiation Facility, Koen Stein of the Directorate ‘Earth and History of Life’ at the Royal Belgian Institute of Natural Sciences, and of Earth System Science at the Vrije Universiteit Brussel, Suzan Verdegaal-Warmerdam, again of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and Jeroen van der Lubbe, once again of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and of the School of Earth and Environmental Sciences at Cardiff University, attempt to determine the season in which the Fish of the Tanis Event Deposits died by examining cyclical bone growth patterns in their skeletons to see at which point growth ceased.

 

Reconstruction of a Paddlefish with impact spherules in the gill rakers. (a) Three-dimensional rendering of Paddlefish FAU.DGS.ND.161.4559.T in left lateral view with the location of a higher-resolution scan (depicted in (b)) indicated (white outline). (b) Three-dimensional rendering of the subopercular and gills in a with trapped impact spherules (yellow). Scale bars are 2 cm. During et al. (2022).

Tree-ring evidence of the Maastrichtian (latest Cretaceous, 72.1 to 66 million years ago) climate of North Dakota suggests a temperate climate with four distinct seasons. The Tanis site is reconstructed as having had annual temperature fluctuations that varied from an average of roughly 19 °C in summer, down to a winter average of 4–6 °C. As well as the rings in trees, these climate variations are recorded in the bones of Acipenseriform Fish, potentially providning a clock which could have recorded the season in which the Chicxulub Impactor fell. To this ende During et al. analysed three Paddlefish dentaries and three Sturgeon pectoral fin spines from the Tanis deposits.

 

PPC- SRμCT data of FAU.DGS.ND.161.4559.T, a partial Paddlefish from the Tanis locality. (a) Orthogonal virtual thin sections (100 μm thick, average-value projections) obtained in front, top, and right view. (b) Impact spherules in virtual thin sections of (a), indicated with yellow circles. Scale bars (a) 1 mm. (c) Three-dimensional rendering (in left lateral view) with virtual cross sections of (d) (blue), (e) (green), and (f) (red) indicated. (d) Coronal virtual slice. (e) Sagittal virtual slice. (f) Axial virtual slice, brain-enveloping tissues indicated with red arrows. (g) Three-dimensional rendering in right lateral view with anatomical labels. (h) Three-dimensional rendering in left lateral view with anatomical labels. During et al. (2022).

During et al. traced lines of arrested growth (marks left in growing bone by slowed growth at one time of year, typically winter in a temperate climate) in dermal bones from six Acipenseriform Fish, by preparing slices of bone as microscope slides. They corroborated this by creating a three-dimensional map of the skulls using propagation phase-contrast synchrotron radiation micro-computed tomography at the European Synchrotron Radiation Facility, which enabled optimal projection of the bone deposition pattern across multiple cross-sectional planes, enabling them to determine the relationship between growth lines and seasonality across a wider area of bone. In addition, During et al. carried out an investigation into the isotopic composition of the growth lines in one Paddlefish specimen.

 

Osteohistological thin sections of five Acipenseriform Fish. (a)–(e) Thin sections in transmitted light of VUA.GG.2017.MDX-3 (a), VUA. GG.2017.X-2743M (b), VUA.GG.2017.X-2744M (c), VUA.GG.2017.X-2733A (d) and VUA.GG.2017.X-2733B (e), showing congruent pacing of bone apposition during the final years of life, terminating in spring. Red arrows indicate lines of arrested growth. Scale bars are 0.5 mm. During et al. (2022).

The tomographic reconstructions also demonstrate that impact spherules are found only in the gill rakers of the Fish, and are absent elsewhere within their bodies, strongly supporting the idea that they were taken in during the last minutes of their lives, and did not have time to penetrate the oral cavity or further down the digestive tract before they died. This also rules out the possibility that they were introduced to the Fish post-mortem, by penetrating decomposing bodies, indicating the Fish were buried rapidly after their death, which strongly supports the idea that the death of the Fish was close to simultaneous with the arrival of the seiche wave, and therefore that these Fish were alive and active in the very last moments of the Cretaceous.

 

Carbon isotope record alongside the incremental growth profiles. (a) Proportion of carbon¹³ expressed as ‰ on the Vienna Pee Dee Belemnite reference scale. The colour gradient highlights the theoretical range between maximum values during seasonal (summer) trophic increase of carbon¹³ (yellow) and minimum values during trophic decrease of carbon¹³ (winter) (blue). (b) Virtual thick section (average-value projection with 0.1 mm depth) showing growth zones during the favourable growth seasons and annuli and lines of arrested growth outside the favourable growth seasons. (c) Cell density map of a virtual thick section (minimum-value projection with 0.2 mm depth) showing fluctuating osteocyte lacunar densities and sizes, with higher densities and largest sizes recorded during the favourable growth seasons (orange) and lower densities and smaller sizes outside the favourable growth seasons (purple). (d) Microscopic thin section in transmitted light showing lines of arrested growth (red arrows) and a single growth mark indicated (bracket) spanning the distance between two subsequent lines of arrested growth and including a zone and an annulus, Scanning data visualized in (b) and (c) were obtained approximately 10 mm distal from the physically sectioned thin slice of (d), which itself was located directly proximal to the thick section sampled for (a). Scale bars are 1 mm. During et al. (2022).

During et al. used micro-X-ray fluorescence to search for signs of taphonomic alteration within the bones, finding that iron and manganese oxides were present both within the surrounding sediments and the sediments surrounding the bones, and the vascular canals within the bones, but had not penetrated into the bone tissue itself. Potassium and silicon were present in the sediment but not in the bones, while the bones themselves maintained a homogeneous distribution of phosphorus and calcium, which would be expected in unaltered bone. The fine detail of the fossils, including the preservation of non-ossified tissues that surrounded the brains of the Fish, also points towards there being almost no taphonomic alteration of the specimens.

 

Elemental distribution maps of Acipenseriform elements from the Tanis locality obtained with micro-X-ray fluorescence. (a) Calcium, phosphorus, and manganese distribution in Paddlefish dentary VUA.GG.2017.X-2724. (b) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.MDX-3. (c) Calcium, phosphorus, and manganese distribution in Paddlefish dentaries VUA.GG.2017.X-2733A, VUA.GG.2017.X-2733B, and the surrounding sediment matrix. (d) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2743M. (e) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2744M. (f) Potassium, silicon, and iron distribution in Paddlefish dentary VUA.GG.2017.X-2724. (g) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.MDX-3. (h) Potassium, silicon, and iron distribution in Paddlefish dentaries VUA.GG.2017.X-2733A, VUA. GG.2017.X-2733B and the surrounding sediment matrix. (i) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2743M. (j) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2744M. During et al. (2022).

The dentaries of Paddlefish form by the ossification of tissue around the Meckel’s cartilage, while the pectoral spines of Sturgeon form by the ossification of embryonic mesoderm tissue within the skin. Neither of them form by the direct ossification of cartilage. Instead they grow incrementally, with new bone tissue being secreted by rows of osteoblasts on the growing surface. This generates an annual growth pattern, with each year represented by a thick zone of bone laid down under favourable conditions, followed by a narrower area of bone laid down under less favourable growth conditions, then a line of arrested growth, representing a period when no bone was laid down. During et al.'s examination of Acipenseriform Fish remains from the terminal Cretaceous Tanis Event Deposits showed that in all cases growth had stopped for the final time shortly after the bones had begun to grow again following a line of arrested growth.

 

Osteohistology of Acipenseriform Fish from the Tanis locality. (a) Thin section of Paddlefish dentary VUA.GG.2017.X-2724 under transmitted light. (b) Detail of VUA.GG.2017.X-2724 thin section (white box in (a)), scale bar 100 μm. (c_ Detail of VUA.GG.2017.X-2724 thin section (white box in (b)), scale bar 100 μm. (d) Thin section of Sturgeon pectoral fin spine VUA.GG.2017. MDX-3 under transmitted light. (e) Detail of VUA.GG.2017.MDX-3 thin section (white box in (d)), scale bar 100 μm. (f) Detail of VUA.GG.2017.MDX-3 thin section (white box in (e)), scale bar 100 μm. (g) Thin section of Paddlefish dentary VUA. GG.2017.X-2733A under transmitted light. (h) Detail of VUA.GG.2017.X-2733A thin section (white box in (g)) with red arrows indicating lines of arrested growth, scale bar 100 μm. (i) Detail of VUA.GG.2017.X-2724 thin section (white box in (h)), scale bar 100 μm. During et al. (2022).

The visible growth lines within the bones were corroborated using stable isotope ratios, which showed seasonal variations in the proportion of carbon¹³, which is linked to diet, and a constant proportion of oxygen¹⁸, which is related to environment, and therefore indicates that the Fish had been living within the river basin their entire lives, rather than migrating to the sea (as some modern do, and which is likely to have been done by some of their Mesozoic forebears). This confirms that the the variation in carbon isotope values and growth rates reflect seasonal variations within that river basin, and not migratory behaviour on behalf of the Fish. This can be seen in both Paddlefish and Sturgeon, despite the different lifestyles of the two types of Fish.

The Paddlefish of the Late Cretaceous Tanis River Basin were filter-feeders (as are Paddlefish today), which are thought to have fed on zooplankton such as Copepods. Such a feeding strategy would have led to seasonal variations in the availability of food, with the maximum availability falling in the summer. This would have led to a summer season when the Fish was both growing faster, and absorbing a higher proportion of carbon¹³. Examination of the distribution of carbon¹³ within fossil Paddlefish bones from the Tanis deposits reveals that at the time of their deaths the proportion of carbon¹³ in their diets was rising, but had not reached its annual peak, supporting the hypothesis that when the Fish died it was spring in the northern hemisphere.

 

Previous studies on palaeobotanical material from Wyoming has suggested that the Chicxulub Impactor fell in June, although those studies are now considered unreliable for a variety of reasons, leaving us with no standing theory on the time of year when the impact event happened. During et al.'s study appears to show a direct record of the season in which Fish that died directly as a result of the impact died, giving a strong line of support for a springtime extinction event.

If correct, the finding that the Chicxulub Impactor fell in the Northern Hemisphere's spring season may go some way to explaining the selective nature of the extinction it caused. Spring is the breeding season for many Animals, which can potentially make them more vulnerable to environmental perturbations. In the aftermath of the impact the ecosystems of the Southern Hemisphere are known to have recovered much more quickly than those of the north, which could potentially have been linked to an event which hit Northern Hemisphere organisms in their breeding season. The event also impacted larger organisms, with longer breeding cycles, more than it did smaller ones with shorter breeding cycles, which could again be linked to the disruption of a breeding season. An event which happened shortly before the onset of winter in the Southern Hemisphere would have been favourable for the survival of organisms which hibernate in burrows (such as many small Mammals), and which might already have been entering a dormant phase when the bolide impacted, something which could potentially also apply to some Amphibians, Birds, and even Crocodilians. 

 

If this is the case then it goes some way towards decoupling the short- and long-term effects of the Chicxulub Impact, with the initial events being particularly harsh on organisms with a spring breeding cycle in the Northern Hemisphere, and relatively benign to organisms with a winter dormant phase in the Southern Hemisphere, while the longer term climatic and ecosystem breakdowns would favour organisms flexible in their environmental and dietary needs, which would be better able to survive amid collapsing food webs, in either hemisphere.

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Secondary cratering from the Early Permian of Wyoming.

Many Solar System bodies, including our Moon, are covered by enormous numbers of impact craters. On Earth, in contrast, the total number of such craters discovered stands at 208. This is largely due to the Earth's active surface, with the continents and continental shelves being subjected to constant erosion and deposition of sediments, and the ocean floors being constantly recycled through subduction and seafloor spreading. However, the situation is more complicated than it seems at first sight; many of the craters seen on the Moon and other Solar System bodies are in fact secondary craters, formed by debris ejected from larger impact events. Once understood, such craters have been relatively easy to identify, frequently being elliptical rather than circular in shape, shallower than primary craters, and arranged radially around the initial impact crater. The implications of this are debatable, with some planetary scientists arguing that as many as 95% of small craters on some bodies may be secondary in origin, whereas others see them as a insignificant proportion of the total number.

In a paper published in the journal GSA Bulletin on 11 February 2022, Thomas Kenkmann and Louis Müller of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, Independent Consultants Allan Fraser and Doug Cook, Kent Sundell of the School of Science at Casper College, and Auriol Rae, also of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, describe the discovery of a secondary impact field comprising at least 31 craters, and possibly as many as 60 more, in southeastern Wyoming, USA. 

The presence of a field of impact craters in the Rocky Mountains in Wyoming was first reported in 2018 by Thomas Kenkman, Kent Sundall, and Douglas Cook. At the time they reported about 40 circular-to-elliptical structures on a tilted Permian exposure on the northeast flank of the Sheep Mountain, eight of which showed sufficient grain deformation to be confirmed as impact structures. These were initially interpreted as the result of a single large object which broke up as it entered the atmosphere, resulting in a group of closely clustered craters.

The new paper by Kenkman et al. describes the presence of several other craters on Sheep Mountain Ridge, and other exposures of the same age at Wagonhound Ridge, Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road, as well as possible craters at several other locations. These range from 10 to 80 m in diameter, and while many are circular, some are as much as 1.7 times as long as they are wide. All of the structures which are firmly established as being craters lie at the top of the Casper Formation, which is immediately overlain by the Opeche Shale member of the Permian Goose Egg Formation, dating the impacts to about 280 million years ago, making them late Early Permian in origin.

 

Digital elevation model map of southeastern Wyoming, USA, and adjacent areas showing the exposure of Casper Formation and the locations of the secondary craters. Based on the intersection of trajectory fans, the proposed site of the possible primary crater is reconstructed. SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road; WR, Wagonhound Ridge; BE, Box Elder Canyon; MR, Manning Ridge. Kenkman et al. (2022).

The Casper Formation is made up primarily of aeolian sandstones (i.e. sands laid down in a terrestrial desert or dune environment), although the uppermost portion of the section, where the craters are preserved, represents a marine transgression into this environment, forming a lagoon or sabkha environment (a sabkha being a coastal saltpan regularly refilled by tidal waters and emptied by evaporation). The craters sometimes form pedestals standing above the eroded surface of this formation, resulting from lithification processes of associated with the impacts, such as shock fusion (welding together of particles by a sudden impact), and cementing by a glassy melt formed within the crater.

 

Remote sensing images of selected craters of the different crater fields. (A), (B), (D), and (E) are drone images; (C), (F), and (G) are Google Earth imagery. Crater locations: SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road. (A) Crater SM-1 has an elliptical outline, a pedestal morphology with a preserved proximal ejecta blanket, and a raised rim. The downrange (northwest) rim shows an overturned ejecta flap. The linear ejecta wall is interpreted as a herringbone pattern. (B) Craters SM-6-3-4-5 form a northwest-southeast–trending radial crater chain. The ovoid crater SM-2 shows an overturned ejecta flap downrange (northwest) and a linear ejecta wall. (C) Craters MC-1 and MC-2 represent eroded craters with very little topography but a concentric fracture pattern. (D) Strongly degraded craters FR-1 and FR-2 contain shock effects along their crater rims. (E) The deeply eroded crater SM-9 is circular and has a bright halo of quartzitic sandstone. (F) Crater FRX-20 is a strongly elliptical landform with a gently rising rim. (G) The western and northern rim of crater PCR-1 exposes steeply dipping rocks. PCR-2 shows an elliptical outline. Kenkman et al. (2022).

The Casper Formation, on the upper surface of which the craters are preserved, has very little surface exposure, 1% of in Converse County and only 4% in Albany County, suggesting that many more craters may be preserved buried beneath the centre. This area lies within the Laramide Mountains, an area within the wider Rocky Mountains where Late Cretaceous to early Eocene (~75-50 million years ago) reverse faulting and folding uplifted an area of Archean basement rocks and the material overlying it. Most of the known craters lie upon the Sheep Mountain flank of the Sheep Mountain anticline, an extended fold-ridge mountain running from northwest-to-southeast, where the Opeche Shale has eroded back revealing the upper surface of the Casper Formation; it has been calculated that about 2 km of overlying rocks have been eroded away here since the onset of the orogeny; strata that are still likely to be in place in other areas, covering up other craters associated with this field.

 

(a) Simplified geological map of the Wyoming state. (b) Aerial photograph of Sheep Mountain anticline (view from the NNW); (c) Geological cross-section through of Sheep Mountain anticline perpendicular to the strike of the average fold axis, and (d) Geological map of the Sheep Mountain anticline area. Amrouch et al. (2010).

Wagonhound Ridge is a similar, and associated, structure to the south of Sheep Mountain, showing slightly less uplift. The Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road exposures are found on the southwestern slope of the basement uplift of the Laramie Mountains in transition to the Shirley Basin, where the Casper Formation has largely been eroded away, but is exposed on several remaining buttes.

The largest cluster of impact-related structures is found on the northeastern flank of the Sheep Mountain, where a series of circular, irregular-shaped, and ellipsoidal have been confirmed as impact craters. These vary in their preservation quality from pristine to heavily eroded, with erosion apparently linked to the recent exposure of structures which were rapidly buried after their formation; i.e. the most eroded structures are located higher on the flanks of the mountain. The most pristine structures show steep crater walls, raised rims with overturned ejecta flaps, and remains of proximal ejecta blankets. These craters have floors are covered by soil and filled with muds derived from the overlying Opeche shales, making it hard to establish their depth-to-diameter ratio. Many of these craters are elliptical-to-ovoid in shape, with their long axes having fairly consistent orientations of 315–328°. The distribution of ejecta around the craters is uneven, with well-developed overturned flaps on their northwestern sides, suggesting they were caused by debris thrown from a primary impact to the southeast. Four of the craters form a chain, with a similar orientation. The more eroded structures further upslope tend also to be more rounded, with an internal ring structures. These often stand proud of the eroded surface, being more resilient to erosion due to the shock-fusion of the sandstone.

Ten possible craters have been found on the exposed surface of the Casper Formation at Wagonhound Ridge. Two of these have been confirmed as definite craters due to elevated rims. These are again filled with soil, and slightly elliptical. 

At Mule Creek a large elliptical crater measuring 56 m by 44 m has an orientation of 284±5°. A second structure, measuring 30 by 27 m is adjacent to this. Neither of these are elevated above the surrounding rock surface, and neither preserves any rim structure or surrounding breccia. It is thought that these represent the lower portions of larger craters that have been mostly eroded away; the larger of them appears to be surrounded by a larger ring at a distance of about 100 m, possibly representing underlying rocks that were consolidated by the impact. This area is cut through by a north-south and northwest-southeast–trending tectonic joint system visible in remote sensing images, which would have served to hasten erosion in this area. This joint system extends about 300 m to the northwest of the main crater, and contains at least nine irregular, soil-filled depressions which might represent further impacts. Another cluster of possible craters, one of which has been confirmed as an impact structure, is found about 2.5 km to the southeast of the main crater at Mule Creek. These structures have crater rims composed of sandstone breccia sealed with chert matrix.

At Fetterman Ridge a series of erosional buttes have exposures of the upper surface of the Casper Formation which have been eroded away from much of the surrounding landscape. One of these, a hill measuring roughly 200 m by 100 m, hosts three impact craters, measuring 30 m by 22 m, 28 m by 17 m, and 10 m by 10 m. The long axes of the two elliptical craters trend west-north-west to east-south-east, although their southern rims are more eroded. The 28 m by 17 m is distinctly elevated on its northwestern rim, with a visible ejecta flap on its western side. Again, the brecciated and fragmented rocks are sealed by microcrystalline silica, making them resistant to erosion. Other buttes to the northwest and southeast show possible additional impact craters, although these are more heavily eroded.

The Fetterman Road cluster comprises six possible craters 7–30 m in diameter, some of which are distinctly eliptical. The most distinct of these is 25 m by 15 m and has an orientation of 296°. The rim of this crater is elevated about a metre above its interior.

Eight possible craters are present at Palmer Canyon Road, about 10 km to the southeast of the Fetterman Road cluster. The two most conspicuous of these measure 42 m by 40 m, and 28 m by 19 m. Again, elevated rims are composed of quartzitic breccia with microcrystalline chert fill.

 

Outcrop-scale observations at the Wyoming crater field. (A) Panoramic view of crater WR-5 that shows very little relief. Note the outcrops along the rim. Persons for scale. (B) Brecciated ejecta. Fragments are partly angular and partly subrounded (SM-34). (C) The wind-scoured crater walls composed of quartzitic sandstone show abundant ventifacts (PCR-001). (D) Chert with flow textures and vesicles is very abundant at most of the craters (SM-36). (E) Breccia with quartzitic matrix (PCR-001). (F) The variegated contact of Casper sandstone and the Opeche Member of the Goose Egg Formation contains a few shocked quartz grains. Kenkman et al. (2022).

All of the discovered craters are on the upper surface of the Casper Formation, and all are in sandstones, but the nature of these sandstones varies slightly from site-to-site, reflecting an environment which was fully sub-aerial in the southwest, passing through a braided-river system into a shallow marine environment with some carbonate deposition in the northeast. The presence of water in the sands in some environments does not appear to have led to degradation of the crater rims. In all cases the craters were buried beneath Opeche Shale Member red beds of the Goose Egg Formation rapidly after their formation.

Brittle deformation, indicative of sudden physical shocks, can be seen on both large and small scales. The crater rims and ejecta all show brecciation (breaking into angular fragments) and brittle deformation, while individual grains are often intensely fractured.

Microstructures related to impact. (A) Crosscutting {1013} and (0001) PDF lamellae in sample from crater SM-19 are decorated by fluid inclusions. (B) {1013} PDF lamellae in sample from crater MC-1. A + B show that shock effects are restricted to the detrital grains while the overgrowth is undeformed. (C) Relatively wide-spaced planar fracture lamellae in sample from crater WR-4. (D) Concussion fractures in adjacent quartz grains emanate from initial grain contacts. (E) Crater SM-28 contains abundant chert layers and chert lumps that are embedded in the sandstone. Some of the chert lumps contain spherical lapilli. For interpretation, see text. (F) Close-up of a spherical lapillus that is interpreted as an accretionary lapillus. The concentric rings are composed of microcrystalline quartz around a dark-colored center. All photomicrographs were taken with crossed polarizers. Kenkman et al. (2022).

However, shocked grains are somewhat rare in the crater sediments, with slides made up from samples taken in the field typically showing only two-or-three shocked grains, and these usually being surrounded by unshocked grains rather than clustered together. Shocked grains were found in all parts of the crater structures, and at a much lower fequency level, outside the craters in undeformed sands, probably indicating their having been blown from craters by winds shortly after their formation. The degree of fracturing implies that these grains were subjected to pressures in excess of 10 gigapascals. 

Cherty (amorphous) silica is present at all sites, often forming the matrix which binds the sand grains together. Investigation of one of the craters at Sheep Mountain found a variety of structures within this chert, including elongated shapes and wavy layers. Within the chert were spherical structures resembling accretionary lapilli; glassy grains which are typically associated with violent volcanic eruptions, formed by the accretion of glassy siliica layers onto grains suspended in hot, turbulent air. 

The Wyoming Crater Field shows a number of features that help the reconstruction of the direction being travelled by impactors. The most obvious of these are oval or elliptical shaped craters, and craters arranged into chains, which gives the orientation of the direction of travel, but not the actual direction. However, direction of travel can be determined by using the following lines of evidence: (1) a steeper crater wall uprange; (2) a preserved overturned ejecta flap downrange, with a preferred deposition of ejecta downrange; (3) an ovoid crater shape with the strongest curvature downrange, and (4) V-shaped herringbone patterns of ejecta pointing up-range. 

The area has been subjected to some deformation since the impact craters formed, but none appears to have had its orientation changed by more than about 5%, enabling the use of craters from different sites to attempt to relocate the site of the original impact. Based upon this, Kenkman et al. suggest that the original impact happened at a site with map co-ordinates close to 41°28′N, 103°59′W; all of the craters lie between 150 km and 200 km from this site.

Modelling of possible trajectories of objects thrown from a primary impact crater suggest that boulders with a diameter of 2 m would need an initial velocity of 3-4 km per second to reach 150-200 km from the initial impact, while 4 m objects could reach this distance with an initial speed of 2 km per second would also fall within this zone. Such objects would deliver energy in the range of 12 to 400 gigajoules when they impacted, arriving at angles of between 45° and 60°.

Kenkman et al. calculate that an object with a density of 2500 kg per cubic meter (typical for many rock types), with a 4 m radius impacting at 1 km per second would create an impact crater about 45 m in diameter. A similar object 2 m in radius would create a crater 25-30 m in diameter, depending on its angle of approach. A range of objects between 1 and 4 m in diameter, travelling at between 500 and 1500 m per second, would generate craters between 8 and 55 m in diameter, with larger impacts releasing more energy and creating more shocked material as a proportion of the impactors mass.

Kenkman et al.'s findings reveal a series of clusters of craters across a wide area of southeast Wyoming, all of which appear to have formed simultaneously about 280 million years ago. These are best explained as secondary craters caused by material thrown from a large primary impact crater. No obviously foreign material was found in any of the craters, making it likely that the impacting material was similar in composition to the rocks of the areas impacted, and reconstructions of the direction of travel suggest the primary impact was between 150 and 200 km from the discovered craters.

Based upon the reconstructed size and impact speeds of the ejecta material, Kenkman et al. predict that the original impactor would have been 2.0-2.7 km in diameter, and to have hit the ground at about 20 km per second, creating a crater 50-65 km in diameter. 

This would place the location of the original impact in Goshen or Laramie counties in Wyoming, or Kimball, Banner, or Cheyenne counties in Nebraska. These areas fall within the Denver Basin, and Permian strata are deeply buried beneath subsequent Mesozoic and Cainozoic deposits. Despite this apparent lack of accessibility, these deposits have been heavily boreholed by hydrocarbons exploration companies, which may enable reconstruction of the original impact site. To date, Kenkman et al. have not found evidence of distorted or missing sedimentary strata which might be associated with such an impact in any well log data examined, but one drill core, 1–35 Hawk Fee, does show a breccia layer at 3023–3066 m below the surface, and Kenkman et al. are hoping to visually inspect this core in the near future. 

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