Crater Morphology; Some Characteristic Impact Structures; Impacts as causes of mass extinction of life - Remoe Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Crater Morphology; Some Characteristic Impact Structures; Impacts as causes of mass extinction of life Part-3

In May 2004 announcement was made that the Permian killer crater may indeed have been found. Marine geophysics surveys off the northwest coast of Australia turned up a distinct anomaly buried under shallow seas that was promising enough to drill two deep holes in search for oil (several impact craters have served as petroleum traps). Here is a seismic refraction survey map that shows the buried structure including its central peak (40 km [25 miles] diameter}.

Seismic multichannel profile across late Permian structure (blue line).

The structure has been identified by Professors Luann Becker (UCSB) and Robert Poreda and Asish Basu (University of Rochester) as an impact structure, and named the Bedout (pronounced "bidowe"; French word) crater. Most of the Bedout images shown on this page were extracted from their online paper (no longer available). The figure below shows the paleogeography at the close of the Paleozoic, in which most continents were grouped together in Pangaea. Bedout lies just off the soon to become Australian block, in shallow waters of the PaleoTethys Ocean.

See text next paragraph.

The yellow-centered circle is the Fraser Park, Australia locality containing what they (and others) interpret to be fallout debris - small fragments of rock and glass. The red dots are other localities - each containing a layer of highly probable impact ejecta containing such evidence as shocked meteoritic chips, glass spherules, Fe-Ni-Si grains, and fullerenes (so-called "buckyballs" made up of carbon arranged in a "geodesic" structure). Note the occurrence of extensive volcanism, the "Siberian Traps". (This outpouring of basaltic lava supports an alternative hypothesis for the demise of so much life, namely the accumulation of harmful gases and particulates in the atmosphere may have created adverse conditions [such as acidic seawater] that diminished life.) Basaltic volcanism also covered the Bedout structure on the sea floor; afterwards about 3 km (2 miles) of mid-Triassic through Cretaceous, and then Tertiary-Quaternary sediments were formed.

This montage of photos shows a land outcrop traceable to the Bedout event, a guide fossil, Glossopteris (a plant), which disappeared immediately afterward, and a grain of quartz which shows planar features - a key indicator of impact as discussed below and on the next page.
A field outcrop in Australia (left) that contains debris showing some evidence of a shock event; the Glossopteris guide fossil to the close of the Permian (upper right), and a shocked quartz fragment (lower right); all associated with the Bedout structure.

In the May 2004 paper, the above mentioned investigators claim to have now found actual pieces of the asteroidal impactor (estimated the size of Mt. Everest) itself. They note too that at the close of the Permian, Australia was part of Pangaea, so the asteroid might have struck on this supercontinent but the land containing the resulting crater has since detached and become buried by marine sediments and ocean water.

Core recovered from the structure, which presently has the dimensions of 200 km (125 miles) but may be even larger, produced fallback breccias in a zone more than 300 meters thick. This was initially interpreted as a volcanic breccia. One core segment shows the recovered breccia with a Chicxulub example along side.

Core from the Bedout and Chicxulub buried craters

Another group of Bedout cores and core from Chicxulub (bottom) at first glance looks almost like gray sediment of clay or fine-crystalline limestone nature:

Core from the Bedout structure (top) and Chicxulub structure (bottom).

When the Bedout core was examined in detail, the breccia clasts were determined to be mostly made up of largely devitrified glass, containing plagioclase, iron oxides, iron-titanium oxide, and recrystallized chlorite. This is a typical petrographic microscope view of a Bedout clast, which resembles impact melts seen at various accepted impact structures:

Photomicrograph of microcrystalline texture in a once glassy clast; yellow areas are glass not recrystallize; the larger light areas are plagioclase-rich; a calcite vein cuts across the chlorite-rich matrix; width of image equivalent to about 3 mm.

Very strong proof of impact origin for Bedout is the shock metamorphic phenomenon of conversion of plagioclase crystals (as laths) into the glass known as Maskelynite. This is evident in this pair of images showing photomicrographs of a sample in which (in the top view) laths of plagioclase showing a brown tone are set in a crystalline albite (white) and titanite (black) matrix. In the bottom, the laths are now revealed to be totally isotropic (remain dark in crossed-polarized light when the microscope stage is rotated) as bespeaks of a glassy state.

A breccia clast seen in plane polarized light in a microscope view (top); same field of view under crossed-Nicols, showing that the plagioclase laths have converted to thetomorphs (glass but the crystal retains its shape, i.e., did not melt and flow.

Some Calcium-Magnesium Carbonate material in the core has been converted to glass (this is very rare but was first produced by the writer in an implosion tube experiment). Another indicator of shock at the Bedout site is the "toasted" quartz grains with single to multiple sets of planar deformation lamellae found rarely in the core but more commonly in the fallout layer tied to this event. Here are two examples from the Fraser Park, Sydney basin site:

Quartz fragments containing sets of PDFs; Fraser Park locality.

The Becker team and others have done age dating on the core materials from the crater and from fallout layers elsewhere. The Ar40/A39 dating yields ages of 250.1 +/- 1 million years, almost exactly the time assigned to the Permian/Triassic boundary (end of the Paleozoic).

At the moment, several impact specialists have disagreed with the interpretation of the evidence cited so far as proof of an impact origin for Bedout, so the inevitable dispute from multiple interpretations has commenced (the concept of "multiple hypotheses" was first promulgated by a geologist). But the study is just beginning. If proved to be a huge impact, Bedout probably is at least one, and maybe the exclusive, factor in the Great Dying.

As so often happens in Science, just when a definitive answer seems to have been found, something new is discovered that clouds the issue. A competing event, in some ways superior because of size, has been announced as a leading candidate of the Killer Crater. This - as yet unnamed - lies under 1.5 km of ice in Wilkes Land in the Antarctic. The buried structure may be as wide as 500 km (300 miles); indirect evidence suggests it may be about the same age as Bedout (but drilling to sample its rocks will be needed to confirm this). It shows up as a gravity anomaly (mascon; positive mass caused by upward movement of denser subcrustal rocks) and has a crude circular outline. Inspect these three maps:

Gravity map showing a Bouguer anomaly (circled) in Wilkes Land.
Radar map indicating a possible surface expression under the ice in the Antarctic that may be an indication of a buried impact structure.
More detailed gravity map of the Wilkes crater.

A buried crater under the Chesapeake Bay may have been large enough to kill off some life during the Tertiary. This structure has a diameter of at least 90 km (56 miles), with a central peak. It is located as shown below:

The Chesapeake Bay crater, located near the mouth of that estuary; inner white line outlines the central uplift; outer line the crater rim; USGS image.

Like some others, the structure was first found during a geophysical survey. It lies buried beneath both coastal and estuarine sedimentary materials. It has been drilled; another drilling is underway. Recovered core shows extensive breccia units. Age dating of these deposits places an age of 35 million years for the breccia matrix. Proof of an impact origin includes telltale planar deformation features in quartz within breccia clasts, as shown here:

PDFs in quartz grains within the Chesapeake Bay crater breccia; USGS source.

Chicxulub, Chesapeake, Bedout and Wilkes Land are among a growing number of impact structures that are buried and have been discovered during geophysical surveys. Another - almost a type case - is the Ames structure (~13 km diameter) in northern Oklahoma, found when it was picked up by gravity and magnetic surveys and then drilled in search for oil. Here is a cross-section prepared by Prof. Judson Ahern showing the crater, its distribution of materials of different densities, and survey results:

The Ames Structure: magnetic and gravity profiles and materials density distribution.

The above proposed impact events extend into geologic time, well before humans entered the evolutionary picture. But the threat of world-level catastrophes owing to impacts has now been accepted as real and concerning. Has there ever been such an event? The answer seems to be 'yes'.

Some 12,900 years before the present, there was a sudden, abrupt disappearance from North America of at least 35 species of large mammals including the Wooly Mammoth, the Saber-toothed Tiger, and the Giant Sloth. Primitive Indians known as the Clovis people also seem to have vanished as a culture. The climate too became colder (about an average of 8 degrees Celsius) and glaciation, already covering most of Canada, expanded. What could have caused this?

Although several explanations have been advanced, the best fit seems to be a comet impact. The Clovis comet, as it has now been named, is proposed as the culprit. There is abundant evidence for a major explosion, either in the atmosphere or in the Canadian ice (no crater resulting from impact has yet been found). A layer, named the Younger Dryan, of black soot, about 4 cm. thick, is found widespread across North America and even into Europe. Here is an outcrop in Arizona with this layer:

The layer contains several hallmarks of impact, including iridium anomalies, nanometer diamonds, and fullarenes. The iridium spikes are not as pronounced as those associated with the K-T boundary /layers.

Iridium spikes associated with the black (soot) mat at six localities

The spherules are distinctive, as shown here:

The diamonds are typical from 10 to 100 nanometers in width. Their crystal structure is hexagonal, not the isometric diamonds found in intrusive pipes on Earth; hexagonal diamonds are known only from settings that involve shock pressures, such as impacts.

Nanodiamonds.

These diamonds have unique signatures when examined by transmission electron microscopy:

Signatures of nanodiamonds.

But as is usually the case in Science, hypotheses as "outrageous" as killer impacts have their challengers. Despite models that show how an impact crater hundreds of kilometers in diameter could affect the atmosphere and surfaces worldwide, there are those still skeptical of this mechanism. These scientists propose other reasons for mass extinction, chief of which is intense regional to worldwide volcanism which at its height could saturate the atmosphere with light-blocking ash and moisture.

So, where does the scientific community stand on the general topic of mass extinctions, both from the standpoint of impacts and the other possible causes of disappearance of entire classes of animals and plants. Comments above indicate that there is a difference of opinions about the actual reason why the dinosaurs did not survive past the K-T boundary. Whether this was due to one short-lived event or was the result of a set of circumstances spread over a gradual period of time is still being debated. Multiple explanations have been proposed. This chart is the plot of a survey among involved scientists as to the primary cause as well as possible secondary or multiple causes:

A pie chart showing various causes or factors responsible for the K-T extinction.

The impact hypothesis remains the currently favored cause of the extinction that prevailed at and after the K-T boundary layer was deposited. But, the mechanisms responsible for the extinction are still being debated. Some scientists claim they have found contradictory evidence pertaining to proposed causes. For instance, if global fires were instigated by the fallout from the impact, the atmosphere would have held huge quantities of soot that would lead to deposits of charcoal in the boundary layer. In fact, only traces of charcoal are present. Therefore, such extensive fires seem unlikely. Again, atmospheric chemistry would have been affected by the impact, with some models leading to acid rain, which would have severely stressed vegetation. But evidence for this is inconclusive. Various models result in the possibility of overall cooling and/or warming (in succession) of the atmosphere. This is to be expected from the vast amounts of dust particulates that an impact would eject into the atmosphere. So, once more the evidence is frought with uncertainty for either modification of temperatures. The presence of fern remnants just above the boundary layer is consistent with the postulate that, although vegetation was severely diminished by whatever was happening to the environment after the K-T event, plant life bounced back rapidly. Animals - mainly small mammals - also reappeared soon thereafter, as these apparently survived the adverse effects in the global environment. Needless to say, much more research must be done to resolve these anomalies.

Another "bone of contention" is whether a Chicxulub-sized crater tosses enough material into the atmosphere to have had a global influence. To go global would require efficient transportation and distribution by the atmospheric circulation system operating at the time. In today's world, the present circulation would likely not carry particulates across the globe; transportation into the southern hemisphere from a northern hemisphere event would be difficult. In the writer's opinion, this is a major stumbling block to the idea that the Chicxulub impact (for which the evidence is very solid) was the sole cause of the general extinction that did happen. It seems more likely that there were other contributing factors. One, often cited, is the likelihood of extensive atmospheric contamination by chemicals (including sulphur compounds) introduced from the vast outpourings of lava that formed the Deccan Traps (basaltic flows) of India. These lava deposits extend over much of the Indian subcontinent and the sea floor to the west, as depicted in this diagram:

The Deccan Traps of India.

Another problem with the Chicxulub model may have merged. Examine this diagram.

Plots of four data sets.

The bottom chart is our "talking point". A group of scientists who are studying the Chicxulub material obtained through drilling have acquired radioactive dates from the volcanic rocks that indicate the rocks are 300000 years older than the K-T boundary layer age. If this is valid (but the error range for the dating might overlap the two ages), then the implication is that there were two separate large impacts closely spaced in geologic time - not impossible, but not of high likelihood.

There is another, perhaps more imposing, objection to impact as the main reason for dinosaur (and other animals) extinction. This is the mounting evidence that the dinosaurs were faced with adverse conditions over millions of years prior to the K-T event which were making their environment increasingly unstable. Evidence in upper Cretaceous rocks show a general trend of aridity (desertlike conditions) across much of the world. If this indeed did happen, it could account for the decrease in the number of dinosaur species observed in Cretaceous rocks prior to the K-T event. So, the dinosaurs were being stressed and heading for extinction anyway, according to some views. The impact(s) at the close of the Cretaceous would have some (probably serious) influence on the dinosaur demise - they would have administered the "coup de grace", but were not the sole cause.

The most frequently cited alternative to impact as the primary cause of mass extinctions is significant volcanism. Although stratocone eruptions can introduce a wide variety of toxic gases into the upper atmosphere, these tend to be localized and of short duration - one at a time. Flood basalts cover a much wider expanse, and calculations show that the amount of gases from such long-lasting events will contaminate the atmosphere (probably globally) to a much higher degree. This next diagram plots extinction events and major flood basalt volcanism:

It is evident that there is some definitive correlation - at least some volcanism on a grand scale occurred during periods of mass extinction. This diagram shows the geographic location of major flood basalt outpourings, along with other extensive volcanic deposits:

Geographic distribution of extensive igneous rock deposits.

But keep in mind that the distribution of continents was much different in past times. For the calamitous Permian extinction event, the Siberian Trap basalts - which many believe were the prime cause of the extinction - were located in the northern part of the supercontinent Pangaea.

Location of the Siberian Trap basalts at the close of the Paleozoic.

Details of the Siberian Trap deposits are evident in this map:

Geological map of the Siberian Traps.

The Siberian Trap basalts cover a large area of north-central Siberia. We saw a Landsat image showing Trap terrain in Section 6. The image used there is repeated here:

Siberian Trap rock terrain.

Two websites bearing on the Siberian Traps are found at Wikipedia and University of Glouchester (be sure to click on "Volcanism").

So, in sum: Like the K-T extinction, the cause(s) of the other extinctions is still being debated. Large impacts certainly can be detrimental to the animal and plant species of the time. But volcanism, climate change, and other factors must be considered. However, the mechanism that is most likely to force the various environments at any given time into "catastrophe mode" is worldwide atmospheric contamination that brings about temperature and precipitaton modifications so drastic that a multitude of species cannot adapt. Impact and subcontinental scale volcanism are the two causitive leaders.

A good treatment of the K-T event, with links to web sites that consider mass extinctions is presented on the reliable Wikipedia K-T Extinction and Mass Extinction Internet connections.

Now that you are familiarized with the general appearance of impact craters and their possible role in influencing life on Earth, we next will consider the best evidence for proof that a circular structure has been caused by a cratering event - shock metamorphism.

Source: http://rst.gsfc.nasa.gov