Ejecta Layer Research Articles

Groundwater geochemistry of the Yucatan Peninsula, Mexico: Constraints on stratigraphy and hydrogeology

Summary We report 87Sr/86Sr and ion concentrations of sulfate, chloride, and strontium in the groundwater of the northern and central Yucatan Peninsula, Mexico. Correlation between these data indicates that ejecta from the 65.95 m.y. old Chicxulub impact crater have an important effect on hydrogeology, geomorphology, and soil development of the region. Ejecta are present at relatively shallow subsurface depths in north-central Yucatan and at the surface along the Rio Hondo escarpment in southeast Quintana Roo, where they are referred to as the Albion Formation. Anhydrite/gypsum (and by inference celestite) are common in impact ejecta clasts and in beds and cements of overlying Paleocene and Lower Eocene rocks cored around the margin of the crater. The sulfate-rich minerals that are found in rocks immediately overlying the impact ejecta blanket, may either be partially mobilized from the ejecta layer itself or may have been deposited after the K/T impact event in an extensive pre-Oligocene shallow sea. These deposits form a distinctive sedimentary package that can be easily traced by the Eocene–Cretaceous 87Sr/86Sr signal. A distinct Sr isotopic signature and high SO4/Cl ratios are observed in groundwater of northwestern and north-central Yucatan that interacts with these rocks. Moreover, the distribution of the gypsum-rich stratigraphic unit provides a solution-enhanced subsurface drainage pathway for a broad region characterized by dissolution features (poljes) extending from Chetumal, Quintana Roo to Campeche, Campeche. The presence of gypsum quarries in the area is also consistent with a sulfate-rich stratigraphic “package” that includes ejecta. The distinctive chemistry of groundwater that has been in contact with evaporite/ejecta can be used to trace flow directions and confirms a groundwater divide in the northern Peninsula. Information about groundwater flow directions and about deep subsurface zones of high permeability is useful for groundwater and liquid waste management in the area. Where it discharges at the coast, the unique chemistry of the groundwater that has interacted with the evaporite/ejecta strata may also have significant geomorphologic implications. While groundwater–seawater mixing at the coast has been shown to dissolve and erode limestone, PHREEQC modeling shows that mixing of water nearly saturated in CaSO4 with seawater has a less vigorous dissolution effect due to its high Ca content.

Layered ejecta craters and the early water/ice aquifer on Mars

Abstract— A model for emplacement of deposits of impact craters is presented that explains the size range of Martian layered ejecta craters between 5 km and 60 km in diameter in the low and middle latitudes. The impact model provides estimates of the water content of crater deposits relative to volatile content in the aquifer of Mars. These estimates together with the amount of water required to initiate fluid flow in terrestrial debris flows provide an estimate of 21% by volume (7.6 × 107km3) of water/ice that was stored between 0.27 and 2.5 km depth in the crust of Mars during Hesperian and Amazonian time. This would have been sufficient to supply the water for an ocean in the northern lowlands of Mars. The existence of fluidized craters smaller than 5 km diameter in some places on Mars suggests that volatiles were present locally at depths less than 0.27 km. Deposits of Martian craters may be ideal sites for searches for fossils of early organisms that may have existed in the water table if life originated on Mars.

A dual-layer Chicxulub ejecta sequence with shocked carbonates from the Cretaceous–Paleogene (K–Pg) boundary, Demerara Rise, western Atlantic

An up to ∼2-cm thick Chicxulub ejecta deposit marking the Cretaceous–Paleogene (K–Pg) boundary (the “K–T” boundary) was recovered in six holes drilled during ODP Leg 207 (Demerara Rise, tropical western Atlantic). Stunning features of this deposit are its uniformity over an area of 30 km 2 and the total absence of bioturbation, allowing documentation of the original sedimentary sequence. High-resolution mineralogical, petrological, elemental, isotopic (Sr–Nd), and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components. The deposit is normally graded and composed predominantly of rounded, 0.1- to max. 1-mm sized spherules. Spherules are altered to dioctahedral aluminous smectite, though occasionally relict Si–Al-rich hydrated glass is also present, suggesting acidic precursor lithologies. Spherule textures vary from hollow to vesicle-rich to massive; some show in situ collapse, others include distinct Fe–Mg–Ca–Ti-rich melt globules and lath-shaped Al-rich quench crystals. Both altered glass spherules and the clay matrix (Site 1259B) display strongly negative ε Nd T = 65 Ma values (−17) indicating uptake of Nd from contemporaneous ocean water during alteration. Finally, Fe–Mg-rich spherules, shocked quartz and feldspar grains, few lithic clasts, as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0.5–0.7 mm of the deposit. The carbonate clasts display in part very unusual textures, which are interpreted to be of shock-metamorphic origin. The preservation of delicate spherule textures, normal grading with lack of evidence for traction transport, and sub-millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air-fall deposit not affected by significant reworking. The ODP Leg 207 spherule deposit is the first known dual-layer K–Pg boundary in marine settings; it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites. Its stratigraphy strongly resembles the dual-layer K–Pg boundary deposits in the terrestrial Western Interior of North America (although there carbonate phases are not preserved). The occurrence of a dual ejecta layer in these quite different sedimentary environments – separated by several thousands of kilometers – provides additional evidence for an original sedimentary sequence. Therefore, the layered nature of the deposit may document compositional differences between ballistic Chicxulub ejecta forming the majority of the spherule deposit, and material falling out from the vapor (ejecta) plume, which is concentrated in the uppermost part.

Solar wind drivers of large geomagnetically induced currents during the solar cycle 23

In this paper we investigate the interplanetary drivers of the largest geomagnetically induced currents (GIC) during the solar cycle 23. Interplanetary coronal mass ejections (ICMEs) are the major causes of intense geomagnetic storms and large GIC. In particular, in this work we examine the effectiveness of different structures embedded in an ICME (namely sheath regions, ejecta, and boundary layers) in causing large GIC. It was found that when an ICME interacts with the Earth's magnetosphere the most intense GIC activity is likely to take place during the passage of the turbulent sheath region. The effectiveness of sheath regions in driving large GIC is possibly due to their capability to trigger substorms and to drive intense directly driven ionospheric activity. We also investigated the relationships between different solar wind parameters and the GIC amplitudes. The best correlation with the GIC amplitudes was found with the solar wind electric field and the epsilon parameter. Ejecta‐associated GIC seem to require an on‐going magnetospheric Dst storm while sheath regions and boundary layers can cause large GIC even when no activity is taking place in terms of Dst. Interestingly, four of the nine ejecta‐associated GIC took place during the recovery phases of a intense magnetic storm (but when Dst still was having storm time values below −50 nT).

Biotic effects of the Chicxulub impact, K–T catastrophe and sea level change in Texas

Biotic effects of the Chicxulub impact, the K–T event and sea level change upon planktic foraminifera were evaluated in a new core and outcrops along the Brazos River, Texas, about 1000 km from the Chicxulub impact crater on Yucatan, Mexico. Sediment deposition occurred in a middle neritic environment that shallowed to inner neritic depths near the end of the Maastrichtian. The sea level fall scoured submarine channels, which were infilled by a sandstone complex with reworked Chicxulub impact spherules and clasts with spherules near the base. The original Chicxulub impact ejecta layer was discovered 45–60 cm below the sandstone complex, and predates the K–T mass extinction by about 300,000 years. Results show that the Chicxulub impact caused no species extinctions or any other significant biotic effects. The subsequent sea level fall to inner neritic depth resulted in the disappearance of all larger (> 150 μm) deeper dwelling species creating a pseudo-mass extinction and a survivor assemblage of small surface dwellers and low oxygen tolerant taxa. The K–T boundary and mass extinction was identified 40–80 cm above the sandstone complex where all but some heterohelicids, hedbergellids and the disaster opportunistic guembelitrids went extinct, coincident with the evolution of first Danian species and the global δ 13C shift. These data reveal that sea level changes profoundly influenced marine assemblages in near shore environments, that the Chicxulub impact and K–T mass extinction are two separate and unrelated events, and that the biotic effects of this impact have been vastly overestimated.

Marine Impacts and Environmental Consequences—Drilling of the Mjølnir Structure, the Barents Sea

No abstract available. doi:10.2204/iodp.sd.6.09.2008

39Ar– 40Ar age and thermal history of martian dunite NWA 2737

We report an 39Ar– 40Ar age determination of a whole rock sample of the olivine-rich, martian meteorite Northwest Africa (NWA) 2737. Those extractions releasing 0–48% of the 39Ar define an 39Ar– 40Ar isochron age of 160–190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm–Nd age of 1.42 Ga. At least part of this excess 40Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar–Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that 40Ar was only partially degassed by an impact event ~ 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures > 600 °C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~ 11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~ 300–500 °C and buried in ejecta to a depth of ~ 1–20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~ 11 Ma ago probably heated NWA 2737 to no more than ~ 400 °C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.

Drilling of the Mjølnir Structure, the Barents Sea: Workshop on Marine Impacts and Environmental Consequences; Longyearbyen, Norway, 10–13 September 2007

In September 2007, thirty‐three scientists attended an international workshop in Norway to discuss impacts into marine targets and prepare the drilling of the 142‐million‐year‐old Mjølnir impact structure in the Barents Sea. The workshop focused on (1) mechanisms of marine impact cratering, including ejecta distribution, geothermal reactions, and tsunami production, and (2) environmental effects of marine impacts and potential links to the Jurassic/Cretaceous boundary (about 145 to 140 million years ago). A field trip visited the ejecta layer in Svalbard's Janusfjellet mountain.Impacts, particularly in marine environments, can significantly affect Earth's geological and biological evolution. However, detailed knowledge of the marine impact cratering process is still limited. Among the 170 terrestrial craters, Mjølnir and its proximal ejecta deposits are unique: It is one of few marine impacts, and since the proximal ejecta always remained under water in calm conditions, it is most likely very well preserved. Today, the crater's central uplift is buried under ∼50 meters of postimpact sediments and 350 meters of water.

New records of Ediacaran Acraman ejecta in drillholes from the Stuart Shelf and Officer Basin, South Australia

Abstract— New occurrences of the Acraman impact ejecta layer were recently discovered in two South Australian drillholes, SCYW‐79 1a (Stuart Shelf) and Munta 1 (Officer Basin) using lithostratigraphy, acritarch biostratigraphy, carbon isotope stratigraphy, and biomarker anomalies to predict the stratigraphic position. The ejecta layer is conspicuous because it consists of pink, sandsized, angular fragments of volcanic rock distributed along the bedding plane surface of green marine siltstone. In SCYW‐79 1a it forms a layer 5 mm thick; in Munta 1 the ejecta layer is thin and discontinuous because of its distance (˜550 km) from the impact structure. Palynological, biomarker, and carbon isotope anomalies can now be shown to coincide with the ejecta layer in SCYW‐79 1a and Munta 1 suggesting the Acraman impact event may have had far reaching influences on the rapidly evolving Ediacaran biological and geochemical cycles.

Mineralogical structure of the subsurface of Syrtis Major from OMEGA observations of lobate ejecta blankets

This paper focuses on the spectral characteristics of lobate ejecta across Syrtis Major volcano, a dark region of Mars presenting a mafic composition as revealed from the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument. Two spectrally distinct crater types are identified. Type I is enriched in high‐calcium pyroxene (HCP) relative to the volcanic rocks. Type II is similar to the lava flow mineralogy driven by HCP, compared to the Noachian crust dominated by low‐calcium pyroxene (LCP). Type I craters are systematically younger than type II. The type II mineralogy has been likely affected by a long‐term weathering in a cold environment for the last 2 Ga. The axisymmetric spectral signatures of type I ejecta appear dominated by rock‐forming minerals rather than by soils and reflect the composition of excavated materials offering a window for exploring the subsurface. A progressive change in the slope of the spectra around 1.5–2 μm is observed across the ejecta layers, pointing at a change in the HCP/LCP ratio. The deconvolution of the spectra by the Modified Gaussian Model unravels a maximum of HCP/LCP band strength ratio located between 1.2 and 3 crater radii for all type I craters. Using Z‐modeling, this observation translates into a maximum in the HCP/LCP abundances at a few hundreds of meters depth and suggests a homogeneous subsurface structure of the volcanic edifice. Below this horizon the HCP/LCP decrease may reveal a more ancient lava composition or the signature of the underlying Noachian crust.

Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

We propose new diagnostics for circumstellar interaction in Type IIP supernovae by the detection of high velocity (HV) absorption features in Halpha and He I 10830 A lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e. a depression in the blue absorption wing of Halpha and a pronounced absorption of He I 10830 A at a radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell (CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, ~60 d. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.

Combinations of processes responsible for Martian impact crater “layered ejecta structures” emplacement

We utilized images and stereo‐derived topographic data acquired by the High Resolution Stereo Camera (HRSC) and Thermal Emission Imaging System (THEMIS) images together with other data in order to study the geology of “layered ejecta structures” associated with relatively pristine Martian impact craters. The geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes. The studied (inner) layered ejecta structures often exhibit ground‐hugging characteristics, and many of them do not have topographic profiles expected from simple ballistic emplacement. Such profiles include ones that are plateau‐shaped or thickening outward. We think that water‐rich fluidized flows driven by the momentum due to the impact and by gravity, together with ballistic emplacement and vortex produced by the atmosphere‐ejecta curtain interaction, were essential to the (inner) layered ejecta structure formation. We hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave‐induced liquefaction of water‐rich near‐surface sediments, ballistic emplacement of ejecta‐entraining water, and strong winds (expanding vapor, vortex, base surge) related to the impact. The contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact.

Detonating Failed Deflagration Model of Thermonuclear Supernovae. I. Explosion Dynamics

We present a detonating failed deflagration model of Type Ia supernovae. In this model, the thermonuclear explosion of a massive white dwarf follows an off-center deflagration. We conduct a survey of asymmetric ignition configurations initiated at various distances from the stellar center. In all cases studied, we find that only a small amount of stellar fuel is consumed during deflagration phase, no explosion is obtained, and the released energy is mostly wasted on expanding the progenitor. Products of the failed deflagration quickly reach the stellar surface, polluting and strongly disturbing it. These disturbances eventually evolve into small and isolated shock-dominated regions which are rich in fuel. We consider these regions as seeds capable of forming self-sustained detonations that, ultimately, result in the thermonuclear supernova explosion. Preliminary nucleosynthesis results indicate the model supernova ejecta are typically composed of about 0.1-0.25 Msun of silicon group elements, 0.9-1.2 Msun of iron group elements, and are essentially carbon-free. The ejecta have a composite morphology, are chemically stratified, and display a modest amount of intrinsic asymmetry. The innermost layers are slightly egg-shaped with the axis ratio ~1.2-1.3 and dominated by the products of silicon burning. This central region is surrounded by a shell of silicon-group elements. The outermost layers of ejecta are highly inhomogeneous and contain products of incomplete oxygen burning with only small admixture of unburned stellar material. The explosion energies are ~1.3-1.5 10^51 erg.

Modes of ejecta emplacement at Martian craters from laboratory experiments of an expanding vortex ring interacting with a particle layer

Ejecta morphologies of many Martian craters indicate fluidized emplacement which differs from ballistic emplacement in dry, airless environments. Double Layered Ejecta craters possess particularly interesting ejecta morphologies: two lobes and radial lineations on their surface, which probably result from gas‐dominated radial flow during the emplacement. To examine how a radial flow interacts with surface particles to generate some of the observed morphologies on Mars, we have conducted water tank experiments in which a vortex ring encounters a particle layer. The threshold of particle motion and three interaction modes are described by two dimensionless numbers: particle Shields' parameter and particle Reynolds number. Our results show that gas‐dominated flows are possible during cratering and could be used to constrain the ancient Martian environment from observations.

On the Origin of the Type Ia Supernova Width‐Luminosity Relation

Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining B-band light curves than dimmer SNe Ia. We study the physical origin of this width-luminosity relation (WLR) using detailed radiative transfer calculations of Chandrasekhar-mass SN Ia models. We find that the luminosity dependence of the diffusion time (emphasized in previous studies) is in fact of secondary relevance in understanding the model WLR. Instead, the essential physics involves the luminosity dependence of the spectroscopic/color evolution of SNe Ia. Following maximum light, the SN colors are increasingly affected by the development of numerous Fe II/Co II lines that blanket the B band and, at the same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia are generally cooler, they experience an earlier onset of Fe III to Fe II recombination in the iron group rich layers of ejecta, resulting in a more rapid evolution of the SN colors to the red. The faster B-band decline rate of dimmer SNe Ia thus reflects their faster ionization evolution.

Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas

Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K–T boundary event. The K–T boundary, as identified by the negative δ 13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K–T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with re-colonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K–T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1.

Impact and extinction in remarkably complete Cretaceous-Tertiary boundary sections from Demerara Rise, tropical western North Atlantic

Ocean Drilling Program (ODP) Leg 207, on the Demerara Rise in the western tropical North Atlantic, recovered multiple Cretaceous-Paleogene boundary sections containing an ejecta layer. Sedimentological, geochemical, and paleontological changes across the boundary closely match patterns expected for a mass extinction caused by a single impact. A normally graded, ~2-cmthick bed of spherules that is interpreted as a primary air-fall deposit of impact ejecta occurs between sediments of the highest Cretaceous Plummerita hantkeninoides foraminiferal zone and the lowest Paleogene P0 foraminiferal zone. There are no other spherule layers in the section. In addition to extinction of Cretaceous taxa, foraminiferal abundance drops from abundant to rare across the boundary. Ir concentrations reach a maximum of ~1.5 ppb at the top of the spherule bed, and the Ir anomaly is associated with enrichment in other siderophile elements. We attribute the unusually well-preserved and relatively simple stratigraphy to the fact that Demerara Rise was close enough (~4500 km) to the Chicxulub impact site to receive ~2 cm of ejecta, yet was far enough away (and perhaps sheltered by the curve of northern South America) to have been relatively unaffected by impact-induced waves.

Fleshing out the Ediacaran period

Abstract The Flinders Ranges Ediacaran succession preserves an apparently primary palaeomagnetic record, distinctive stratigraphic events such as the Acraman ejecta layer, and fossils of the Ediacara biota at three well-separated levels. The fossil assemblage of the Ediacara Member includes both trace fossils and body fossils, and shares at least 65% of its taxa with the White Sea assemblages in northern Russia, suggesting they were coeval. Globally, Ediacaran successions are diverse in their lithostratigraphy but share many characteristics of biota, isotope signatures and events which can be calibrated with radiometric dating. Available radiometric dating of Ediacaran successions supports the concept of distinct Avalon (575 Ma–565 Ma), White Sea (558 Ma–555 Ma) and Nama (549 Ma–543 Ma) associations as a temporal succession rather than being controlled by environment or palaeobiogeography. The challenge to document the tempo and pattern of evolution of early animals and the associated changes in global climate make the subdivision and calibration of the Ediacaran a vital task for the next decade.

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

Research Article| September 01, 2007 Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan Peir K. Pufahl; Peir K. Pufahl 1Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Eric E. Hiatt; Eric E. Hiatt 2Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Search for other works by this author on: GSW Google Scholar Clifford R. Stanley; Clifford R. Stanley 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Jared R. Morrow; Jared R. Morrow 4Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Search for other works by this author on: GSW Google Scholar Gabriel J. Nelson; Gabriel J. Nelson 5Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Cole T. Edwards Cole T. Edwards 6Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Peir K. Pufahl 1Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Eric E. Hiatt 2Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Clifford R. Stanley 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Jared R. Morrow 4Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Gabriel J. Nelson 5Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Cole T. Edwards 6Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Publisher: Geological Society of America Received: 28 Jan 2007 Revision Received: 02 May 2007 Accepted: 02 May 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (9): 827–830. https://doi.org/10.1130/G23751A.1 Article history Received: 28 Jan 2007 Revision Received: 02 May 2007 Accepted: 02 May 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peir K. Pufahl, Eric E. Hiatt, Clifford R. Stanley, Jared R. Morrow, Gabriel J. Nelson, Cole T. Edwards; Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology 2007;; 35 (9): 827–830. doi: https://doi.org/10.1130/G23751A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Asteroid mega-impacts and Precambrian banded iron formations: 2.63 Ga and 2.56 Ga impact ejecta/fallout at the base of BIF/argillite units, Hamersley Basin, Pilbara Craton, Western Australia

Abstract The temporal association between late Archaean to earliest Proterozoic asteroid impact ejecta/fallout units and overlying banded iron formations suggests that, in some instances, these impacts were closely followed by significant transformation in the nature of source terrains of the sediments. The Jeerinah Impact Layer (JIL) [B.M. Simonson, D. Davies, S.W. Hassler, Discovery of a layer of probable impact melt spherules in the late Archean Jeerinah Formation, Fortescue Group, Western Australia. Aust. J. Earth Sci. 47 (2000) 315–325; B.M. Simonson, S.W. Hassler, Revised correlations in the early Precambrian Hamersley Basin based on a horizon of resedimented impact spherules. Aust. J. Earth Sci. 44 (1997) 37–48; B.M. Simonson, B.P. Glass, Spherule layers — records of ancient impacts. Ann. Rev. Earth Planet. Sci. 32 (2004) 329–361; A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772; B. Rasmussen, C. Koeberl, Iridium anomalies and shocked quartz in a late Archean spherule layer from the Pilbara Craton: new evidence for a major asteroid impact at 2.63 Ga. Geology 32 (2004) 1029–1032; B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.] overlies an argillite-dominated unit (Jeerinah Formation, 2684 ± 6 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and lies directly below a thin volcanic tuff (2629 ± 5 Ma, [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and banded iron formation (BIF) (upper part of Marra Mamba Iron Formation, 2597 ± 5 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]). The Spherule Marker Bed (SMB) [B.M. Simonson, Geological evidence for an early Precambrian microtektite strewn field in the Hamersley Basin of Western Australia. Geol. Soc. Am. Bull. 104 (1992) 829–839; B.M. Simonson, S.W. Hassler, K.A. Schubel, Lithology and proposed revisions in stratigraphic nomenclature of the Wittenoom Formation (Dolomite) and overlying formations, Hamersley Group, Western Australia. Geol. Surv. W. Aust. Rep. 345 (1993) 65–79; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772. [5] ], which includes two impact cycles [A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50.], is located at the top of a carbonate/calcareous siltstone-dominated sequence (Bee Gorge Member, Wittenoom Formation, 2565 ± 9 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and below a carbonate-poor siltstone–chert–BIF sequence (Mount Sylvia Formation, Bruno's Band [BIF], Mount McRae Shale, 2504 ± 5 Ma [B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.]). No ferruginous sediments overlie impact layers hosted by stromatolitic carbonates (