Dating the Hiawatha Impact Structure.

The Hiawatha Impact Structure is an approximately 31 km wide geomorphological structure beneath the Hiawatha Glacier in northwestern Greenland, which has been interpreted as an impact structure on the basis of the structure revealed by airborne radar surveys (a relatively flat, circular depression with an elevated rim and a subtle central uplift), structures in the bedrock along the ice margin, which strike tangentially to the subglacial rim, and the presence of shocked quartz and other impact-related grains in glaciofluvial sediments derived from the largest river draining the structure.

Dating this structure has proved to be difficult. It lies on the surface of the highly metamorphosed 1.95– to 1.75–billion-year-old Ellesmere-Inglefield Mobile Belt, and overlain by the Hiawatha Glacier, which is part of the 2.6 million-year-old Greenland Ice Shelf. This gives a maximum possible age of 1.75 billion years, but, due to the constantly moving nature of the glacier, no minimum age. Impact structures have a fairly constant width-to-depth ratio, so it can be predicted that when it formed the 31 km wide Hiawatha Structure would have had a depth of about 800 m when newly formed. Today it has a depth of about 320 m, implying a loss of about 480 m via erosion since the structure first formed. Estimates of the rate of erosion in subglacial environments vary between 10 m and 10 km per million years, giving the Hiawatha Structure an age of somewhere between 50 thousand and 50 million years. It has also been suggested that the anomalous radiostratigraphy of the ice of Hiawatha Glacier compared to the rest of the Greenland Ice Sheet may be a sign that the impact occurred after the formation of the glacier, making it less than 2.6 million years old.

In a paper published in the journal Science Advances on 9 March 2022, a team of scientists led by Gavin Kenny of the Department of Geosciences at the Swedish Museum of Natural History, William Hyde of the Globe Institute at the University of Copenhagen, Michael Storey of the Quadlab at the Natural History Museum of Denmark, and Adam Garde of the Geological Survey of Denmark and Greenland, present the results of a study which aimed to find a date for the Hiawatha Impact using argon-argon dating of impact-related glaciofluvial sands and uranium-lead analysis of shocked zircons from glaciofluvial clasts of impact melt rock.

Argon-argon dating relies on determining the ratio of radioactive argon⁴⁰ to non-radioactive argon³⁹ within minerals from igneous or metamorphic rock (in this case impact melts) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of argon⁴⁰ to argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further argon³⁹ will enter the mineral from this point, but argon⁴⁰ is produced by the decay of radioactive potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.

Zircons are minerals formed by the crystallisation of cooling igneous (or in this case, impact) melts. When they form, they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

Kenny et al. used a sample of well-sorted, fine-grained sand (HW21-2016) collected from a floodplain about 300 m from the terminus of the Hiawatha Glacier, and about 5 km from the Hiawatha Structure. Examination of satellite and aerial images shows that the section of floodplain material from which the sample was collected did not begin to build up until 2010, making Kenny et al. confident that it does contain material which has been washed along current sub-glacial waterways, and therefore does originate from the Hiawatha Structure.

 

Location and geomorphological setting of Hiawatha Glacier, northwest Greenland. (A) Regional view of northwest Greenland. (B) Bedrock topography mapshowing the Hiawatha structure, and sampling locations of glaciofluvial sediment for argon⁴⁰/argon³⁹ analysis (HW21-2016) and clasts of impact melt rock for zircon uranium/lead analysis (HW19-01 and HW19-05). Bed topography based on NASA and Alfred Wegener Institute airborne radar-sounding data. Samples HW19-01 and HW19-05 are from the same location on a wide riverbank 4 km downstream of the terminus of Hiawatha Glacier. White line represents the present-day margin of the Greenland Ice Sheet. Kenny et al. (2022).

In addition to the sand samples, Kenny et al. selected on two pebble-sized clasts (HW19-01 and HW19-05) obtained from a wide riverbank roughly 4 km downstream from the terminus of Hiawatha Glacier and less than 10 km from the edge of the Hiawatha structure. Both are clast-rich impact melt rocks with a dark grey, aphanitic, hemicrystalline melt matrix dominated by lath-like plagioclase feldspar microlites, that are thought likely to have reached the location where they were found via subglacial and glaciofluvial transport. Portions of both these pebbles were crushed an zircons extracted for analysis.

 

Images of impact melt rocks from the Hiawatha structure. (A) Feldspathic microlitic matrix with clasts of toasted quartz (qtz) and checkerboard feldspars (fsp). (B) Lightly toasted quartz fragment with two sets of PDFs that are considered unequivocal evidence of shock metamorphism. (C) Checkerboard feldspar. (D) Petrographic context of a granular and porous zircon (zr) grain in the feldspathic (fsp) matrix of impact melt rock, with accessory biotite (bt), ilmenite (ilm), and altered cordierite (crd). In contrast to zircon grains like this one that were in direct contact with the impact melt, zircon grains within clasts in impact melt rock do not display porous and granular textures. BSE, backscattered electrons; PPL, plane-polarized light; XPL, cross-polarized light. Kenny et al. (2022).

The sand grains extracted from the floodplain close to the glacier edge were examined visually to look for signs of impact melting. Four types of grains were identified within the sample. The first, and most abundant group, making up 40% of the sample, have a greenish gray, yellow, or dark organic-rich matrix with feldspathic microspherulites about 10 to 50 μm across and fragments of quartz and feldspar. The second most abundant grain type, making up 20% of the sample, have a non-crystalline, glassy, or commonly schlieric matrix and mineral fragments. The third most abundant grain type, making up 12% of the sample have a hemicrystalline, presumably feldspathic matrix and numerous mineral fragments. Finally, 6% of the grains have a dark, hemicrystalline, presumably feldspathic matrix and microlites presumably of pyroxene and ilmenite. Another 20% of the sample have overlapping features between these groups or are dark without distinct features. Also included in the study was a grain of pale, ellipsoidal to spherical silica ooids with nuclei of quartz fragments.

Stepwise argon⁴⁰/argon³⁹ analysis of these sand grains produced a range of readings, which is consistent with minerals from older episodes of melting being included within an impact melt, with 29 of the samples producing more than one age (consistent with partial melting and recrystallization of a mineral grain), of which 23 produced a younger age of 58.5 million years. Since no younger age was produced by any grain within the sample, Kenny et al. take this as the most probable age of the impact melt, making the impact a Late (but not Terminal) Palaeocene event. 

Fifteen unshocked zircons were selected from the two pebble-sized clasts, and subjected to uranium/lead analysis, most of which produced ages clustering around 1915 million years old, with the youngest being about 1485 million years old and the oldest about 2300 million years old. This is consistent with the age of intrusive felsic rocks in the area, supporting the hypothesis that the melts are of local origin. The altered zircons within the sample provided a range of ages between 1915 and 57.99 million years old, with the majority clustered at the minimum end of this range.

Unshocked zircons from the two pebble clasts collected about 10 km downstream of the Hiawatha Glacier give uranium/lead ages consistent with those of intrusive felsic rocks which outcrop at a number of sites around the crater, and which are therefore likely also to outcrop beneath it. Shocked zircons from the same material produce uranium/lead ages of about 58 million years. Argon⁴⁰/argon³⁹ analysis of sand particles from closer to the glacier yield a similar age. All of these samples appear to have been washed out from beneath the Hiawatha Glacier by a river which cuts through the rim of the Hiawatha Impact Structure. The simplest explanation for this is that the impact which caused this structure occurred in the Late Palaeocene. 

When the Hiawatha Impact Structure was first discovered it was thought likely to be less than 2.6 million years old; i.e. younger than the ice sheet which covers it. It has even been proposed that it might be as young as 12 900 years old, linking the impact to the onset of the Younger Dryas glacial episode. Kenny et al.'s findings suggest that the impact structure is much older than this, long predating glacier formation in Greenland.

Modelling of the original shape of the Hiawatha Impact Structure suggests that it has suffered about 500 m of vertical erosion since it was formed 58 million years ago, a much lower rate of erosion than has been predicted for subglacial features. This potentially has profound implications for the interpretation of other features beneath the Greenland Ice Sheet, although Kenny et al. are cautious of placing to much emphasis on this result without drill-core data to confirm the current interpretation of the structure of the feature. However, if this is correct then it means that a number of other features beneath the ice sheet are likely to be much older than previously thought, including a substantial river system currently thought to be subglacial in origin, but which might instead represent a long-standing morphological feature.

 

Geological map of Inglefield Land and Prudhoe Land, northwest Greenland. Previously published zircon uranium/lead ages for bedrock samples are shown in black text, and the age of unshocked zircon in clasts of impact melt rock sampled 4 km downstream from the terminus of Hiawatha Glacier (present study) is shown in green text. The dominant age of unshocked zircon in the impact melt rock samples (1915 ± 8 million years) is indistinguishable from the zircon uranium/lead ages of three felsic igneous intrusions in the vicinity of Hiawatha Glacier (bold text), supporting a local origin for the clasts of impact melt rock. Kenny et al. (2022).

Numerous pebble-sized charcoal fragments, many with cellular structures indicative of Conifer wood, have been found in the outwash of the Hiawatha Glacier. These have previously been taken as evidence of an Early Pleistocene forest system in Greenland, but the new date for the Hiawatha Impact Crater suggests that, if these are related to the impact event, then they must also be Palaeocene in origin. This actually fits well with our understanding of the Palaeocene Arctic, with Conifer fossils known from several Arctic sites.

The anomalous radiostratigraphy of the ice of Hiawatha Glacier compared to the rest of the Greenland Ice Sheet has been invoked as evidence for a young age for the impact structure beneath the glacier. If the glacier is in fact much younger than the impact structure, then an alternative explanation for the radiostratigraphy is needed. Kenny et al. suggest that this might have been caused by water flowing into the crater beneath the ice sheet and then building up until it escaped catastrophically. Alternatively, a collapse of the Nares Strait Ice Bridge in the Early Pleistocene could have disrupted ice structures in northwest Greenland.

The boundary between the Palaeocene and the Eocene, 55.93 million years ago, is marked by a global carbon isotope excursion, and the onset of a period of rapid warming that led to the Palaeocene-Eocene Thermal Maximum. This is close to the age of the Hiawatha Impact Structure, but not identical, and is better explained by massive flood basalt volcanism associated with the opening of the northeast Atlantic about 56 million years ago. There was also a significant lava flow outburst in Greenland in the Palaeocene, but this has been dated to 62 million years ago, older than the Hiawatha Impact Structure, and therefore unrelated to it. A number of spherule beds have of Palaeocene age have previously been discovered in western Greenland and on the northeastern coast of the United States, but these are now thought to be of volcanic origin, rather than impact related.

However, the Marquez Impact Structure in Texas has been dated to 58.3 million years ago, which is a very close match with the Hiawatha Impact Structure, suggesting that a link between the two is quite possible. The coincident age of two large impact structures may imply that other impacts happened at the same time, and that evidence of these is either undiscovered or has been lost. The timing of the Hiawatha and Marquez impacts does coincide with the end of the Late Palaeocene Carbon Isotope Maximum, a sudden increase in the proportion of carbon¹³ and a concurrent episode of global cooling, which ended abruptly at about 58 million years ago, when carbon¹³ levels dropped sharply and the Earth began a long-term warming trend. The absence of a distinct ejecta layer associated with the Hiawatha Impact makes it impossible to date this event with sufficient precision to link it to this shift, but Kenny et al. do note that the shift in carbon isotope ratios was far more sudden than is usually observed. The Chicxulub Impact has been linked to a major shift in carbon isotope ratios, but this, much larger, event is also known to have caused major disruption to the biosphere, which is generally assumed to be the cause of the carbon isotope shift. No known shift in the biosphere has been recorded which can be associated with the Hiawatha Impact, and no impact other than the Chicxulub event is known to have had any measurable influence on the Earth's biosphere, but this does not rule out the possibility that an impact such as the Hiawatha event could have caused changes to the biosphere which have not been recorded.

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Looking for the eastern margin of the Palaeo-Tethys Ocean.

The Palaeo-Tethys Ocean ran separated the continent of Gondwana from the landmasses that would become Europe and Asia during the Palaeozoic Era, opening during the Middle Cambrian and eventually closing with the formation of the Supercontinent of Pangea during the Permian/Triassic. The western part of this ocean is reasonably well understood, but the eastern extent is less well known, as much of the geology of the region has been distorted and overwritten by the Himalayan Orogeny, as the Indian Plate has impacted Eurasia from the south. The ocean probably passed along the southern margin of the South China Block (also known as the Yangtze Plate) (i.e. the southeastern coast of China), producing an area of rifting similar to that seen beneath the Red Sea today, but there is little evidence to support this.

In a paper published in the journal Acta Geologica Sinica on 27 February 2018, Hu Lisha, of the Collage of Marine Geosciences at the Ocean University of China, and the Laboratory for Marine Geology at the Qingdao National Laboratory for Marine Science and Technology, Du Yuansheng of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, and Xu Yajun, Wang Zhiwan, and Wang Chenghao, also of the Collage of Marine Geosciences  at the Ocean University of China, describe the discovery of a volcanic tuff (ash) layer from Banchen in the Qinzhou area of the Guangxi Zhuang Autonomous Region of southeastern China, could provide evidence for subduction along the southern margin of the South China Block during the Palaeozoic.

Hu et al. report the discovery of a green tuff layer between layers of Devonian siliceous rocks, from which zircons were extracted for uranium/lead dating. Zircon is a mineral formed by the crystallisation of cooling magmas. When it forms it often contains trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead (which has a much lower precipitation temperature) will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

(a) Tectonic framework of the East Asia; (b) Simplified geological map of the Qinfang Trough and location of the study area; (c) Photo for the Late Devonian chert and tuff; (d) Concordia diagram and cathodoluminescence (CL) images of representative zircons for the tuff sample. Hu et al. (2018).

Eighteen zircons were subjected to this analysis. Seven of them produced ages older than 600 million years, whereas eleven produced ages of between 380 and 350 million years, consistent with a Devonian or Carboniferous age for the tuff layer. The presence of much older zircons in the sample does not undermine this, as zircons are extremely tough, and are known to be able to endure repeated cycles of subduction and volcanic eruption, nor is the broad spread of ages found in the Palaeozoic zircons, as ash deposits often contain mineral grains aggregated over a long period before being erupted.

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The Ya Ha Tinda Fossil Assemblage: An Early Jurassic Lagerstätten from North America.

Fossil Lagerstätten are sites of exceptional preservation, where organisms are preserved better and in greater numbers than in other parts of the fossil record. These sites provide unique insights into the history of life on Earth, preserving organisms that cannot be found in other places, or in more common organisms, tissues that are not otherwise seen. The Early Jurassic was a time of major ecological and environmental change, with a number of distinct events that influenced the change in the marine fauna that occurred during this time, most notably the Toarcian Oceanic Anoxic Event (about 183 million years ago). Sadly only three fossil Lagerstätten are known from the Early Jurassic, all of them from European deposits, so that we have almost no knowledge of many important groups outside of Europe during this period.

In a paper published in the journal Geology on 9 January 2017, Rowan Martindale of the Department of Geological Sciences at The University of Texas at Austin and the Department of Organismic and Evolutionary Biology at Harvard University, Theodore Them of the Department of Geosciences at the Virginia Polytechnic Institute and State University and the Department of Earth, Ocean and Atmospheric Science & National High Magnetic Field Laboratory at Florida State University, Benjamin Gill, also of the Department of Geosciences at the Virginia Polytechnic Institute and State University, Selva Marroquín also of the Department of Geological Sciences at The University of Texas at Austin and of the Department of Geosciences at the Virginia Polytechnic Institute and State University, and Andrew Knoll, also of the Department of Organismic and Evolutionary Biology at Harvard University, describe a new fossil Lagerstätten from the Early Jurassic of southwest Alberta, Canada.

The Ya Ha Tinda Fossil Assemblage comprises several outcrops of the Fernie Formation outcropping on the Ya Ha Tinda Ranch. These deposits have been dated using Ammonites and Coccoliths (groups with very high species turnover often used to date Mesozoic strata), carbon isotope chemostratigraphy (the ratios of different carbon isotopes incorporated into sedimentary rocks varies in direct relation to global atmospheric temperature; this means that the proportion of these elements rises and falls at the same rate in rocks all over the world, creating a fingerprint that can be used to date rocks) and uranium-lead zircon dates from intercalated ash beds (zircon is a mineral formed by the crystallization of cooling lavas.; when it forms it often contains trace amounts of uranium, which decays into lead at a known rate - since lead, which has a much lower melting point, will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.), giving a very high confidence to the dates assigned to these beds; which are calculated to span the boundary between the Pliensbachian and Toarcian stages, including the Toarcian Oceanic Anoxic Event.

This site is the first Early Jurassic Lagerstätten known from outside Europe, and the third Lagerstätten to include the Toarcian Oceanic Anoxic Event. It has yielded Vampyropod Cephalopods (Vampire Squid), Lobsters, Shrimps, Leptolepiform and Saurichthyiform Fish, Ichthyosaurs, Ammonites, isolated Dinosaur bones, Brachiopods, Gastropods, Bivalves, wood, and Coccolithophores.

Exceptionally preserved fossils of Ya Ha Tinda Lagerstätte (Alberta, Canada). RDM—Red Deer Member; PCS—Poker Chip Shale Member. (A) Articulated Ichthyosaur vertebrae and ribs (RDM, late Pliensbachian). (B) Skull of small Teleost Fish (PCS, within Toarcian Oceanic Anoxic Event [T-OAE] carbon isotope excursion [CIE]); note preservation of gills (arrow). (C) Seirocrinus subangularis (Crinoid) calyx collected by Russell Hall (RDM, late Pliensbachian). (D) Vampyropod gladius with mantle muscle (white arrow) and ink sac (black arrow) (RDM, early Toarcian). (E) Loligosepiid Vampyropod gladius with ink sac (arrow) (RDM, early Toarcian). (F) Shrimp body fossil (PCS, within T-OAE CIE). (G) Complete body fossil of Uncina pacifica, Lobster, proximodistally flattened (RDM, late Pliensbachian). (H) Complete body fossil of Eryonid Lobster, dorsoventrally flattened, ventral view (RDM, late Pliensbachian). Martindale et al. (2017).

These specimens are preserved in finely laminated clays (shales) laid down on a gently sloping shelf and basin. The articulation of the skeletons preserved and high organic carbon content of the shales suggests that the specimens were preserved under anoxic (or at least very low oxygen) conditions, though the presence of large benthic (bottom dwelling) invertebrates and patches of bioturbation suggests normal oxygen levels were present for at least some of the time. Pyritization (the conversion of organic material to iron pyrites, which can occur under anoxic conditions) is rare, with most specimens preserved as carbonaceous impressions or calcareous shells with occasional replacement by apatite or clay minerals. Only hard, mineralize tissues such as shells and bones are preserved in three dimensions, suggesting that mineralization occurred after tissue collapse. 

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Using zircon uranium-lead geochronology to understand the origins of the Miocene Sanya Formation beneath the South China Sea.

The Miocene Shanya Formation forms part of the geological sequence in the Yinggehai Basin, which underlies part of the northwestern South China Sea. The formation is 2950 m thick at its maximum, and is known to host a number of hydrocarbon reserves, making it of great interest to exploration geologists. The formation comprises shallow marine sedimentary deposits formed by the deposition of material washed from nearby terrestrial environments, however, like many offshore deposits, the processes that led to the formation of the Shanya Formation are poorly understood, making it hard to make predictions about where hydrocarbons might be found within these beds.

In a paper published in the journal Acta Geologica Sinica in February 2016, Wang Ce of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry and the University of the Chinese Academy of Sciences, Liang Xinquan, also of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, and Fu Jiangang, Jiang Ying and Dong Chaoge, again of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry and the University of the Chinese Academy of Sciences, describe the results of a study in which zircon uranium-lead geochronology was used to develop an understanding of the origins of sedimentary material in the Shanya Formation.

Zircon is a mineral formed by the crystallization of cooling lavas. When it forms it often contains trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead (which has a much lower melting point) will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

 A ziron crystal, about 250 µm in length. Wikipedia.

This has obvious uses for the dating of igneous rocks, but can also be useful in sedimentary geology, since it enables geochemists to link zircons found in sedimentary rocks to their sources (i.e. the igneous rocks from which they were eroded before being deposited in the sedimentary strata).

Wang et al. collected zircons from sandstones attributed to the Sanya Formation from drill cores in the northern, northeaster and eastern parts of the Yinggehai Basin. The first sample (from the north) yielded zircon uranium-lead dates with a very wide range of dates, though the most abundant were 274, 432 and 957 million years old, with smaller peaks in abundance at 793, 1966 and 2481. The second site (from the northeast) yielded a similar range of dates, with the commonest dates being 156 and 428 million years old and smaller peaks at 41, 239 and 733 million years old. The third site (the one in the east) produced zircons with a smaller range of dates, being much younger, with peaks at 99 and 238 million years old.

The youngest zircons present, the 41 million-year-old crystals in the second sample, are approximately the right age to be associated with melting and exhumation during a period of motion on the Red River Fault Zone, which runs through Yunnan Province and Vietnam, and which is a fault within the Yangtze Block that accommodates movement by the Indo-Australian Plate.

The zircons aged 99 and 156 million years old, found in the second and third drill cores, are consistent with the formation of the Yanshanian Granites on Hainan Island in the Late Jurassic-Cretaceous.

Zircons with dates of 230-250 million years old, again found in the second and third samples, are likely to have originated during the collision of the Indochina and Yangtze Blocks, and could have come from a wide range of sources in South China, Vietnam or Hainan Island.

Samples dating to about 430 million years ago, found in the first and second samples, are thought likely to be associated with the Caledonian Orogen, when the ancient continents of Laurentia, Baltica and Avalonia collided during the closure of the Iapetus Ocean; grains of this age are common in South China, but very rare on Hainan.

Grains 700-1000 million years old, again found in the first and second samples only, are likely to have formed during the Jinningian Movement, part of the breakup of the ancient supercontinent of Rodinia, and originated on the Yangtze Block.

Zircons 1966 and 2481 million years old, found only in the first sample, are likely to have formed during the origin of the Yangtze block.

Thus zircons in the sediments from the first and second drill cores are dominated by grains that originated from the Yangtze Block to the north (i.e. mainland China), while the third sample appears to contain mostly zircons that originated on Hainan Island, a source which also provided a significant minority of the material in the second sample.

Possible provenance directions of the Lower Miocene Sanya Formation in the Yinngehai Basin. Wang et al. (2016).

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