Tuesday, October 21, 2014

Meteorite Impact May Have Triggered Largest Pulse Of Deccan Basalt Eruptions

What caused the mass extinction 65 million years ago?

a) It was a meteorite impact and the resulting environmental crises.
b) No,  it was the Deccan basalt eruptions and the resulting environmental crises.
c) It was both, the meteorite impact and the eruptions.

The two mechanisms were distinct. One, a calamity from space and the other a gigantic eruption whose cause was from deep within the earth.

Now, it seems they may be more intimately linked. The meteorite impact may have triggered the largest pulse of the Deccan volcanic eruptions. Be careful here. The impact did not initiate Deccan volcanism. That was caused by India rifting away from Madagascar (88 mya) and Seychelles (66 mya). The rifting and an unusually hot mantle underneath resulted in copious amounts of melt being generated in the mantle which found its way to the surface via the great tensional cracks formed when continents separate. The impact though may have resulted in increasing the permeability of the mantle underneath thus making it possible for larger amounts of magma to make its way to the surface.

Abstract- 2014 GSA Annual Meeting, Vancouver:

New constraints on the timing of the Cretaceous-Paleogene (K-Pg) mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental flood basalt province, suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically-plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was underway well before Chicxulub/K-Pg time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1-1.0 J/m3 throughout the upper ~200 km of the Earth’s mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai sub-group) lava flows that account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/K-Pg time a huge pulse of mantle plume-derived magma passed through the crust with little interaction, and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which in turn might help determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the K-Pg extinction.

Elsewhere in a GSA special issue on volcanism, impacts and mass extinctions more evidence that Deccan volcanism had a significant impact on the fauna and flora.

Chronology of the volcanic episodes is improving and pointing to a scenario wherein the volcanism coincided (or was causally connected) to within a hundred thousand years of the Chicxulub impact and overlapped the stratigraphic horizon defined as the Cretaceous -Paleogene boundary. Looks like everyone is going to be partially right on this one.

Multiple causes for the mass extinction.

Tuesday, October 14, 2014

A Small Note On Animal Fossils Before The Cambrian "Explosion"

Every now and then there appears a news story about metazoan fossil findings that expresses great astonishment and surprise that there is NOW... THIS TIME.. new evidence that multicellular animals evolved long before their celebrated preservation in the Chengjiang and Burgess shale Lagerstatte.

But we have known that for a long time.  The Neo-Proterozoic and early Cambrian fossil record is so much better and is improving and paleo-biologists and palaeontologists have recognized in it the gradual increase in complexity of metazoans over a 50-60 million year period before the exceptional preservation windows of Chengjiang and Burgess shale gives us a false impression of a sudden appearance of complex multicellular animals. This artifact has been exploited by creationists who claim that the fossil record actually supports their creation story of a sudden origin, under some intelligent guidance, of complex animals in the Cambrian, summarized in books like Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. The best rebuttal I have come across of the creationists many misunderstandings of early animal evolution is this excellent article by Nick Matzke.

Let me just post this invaluable figure below which summarizes the Neo-Protoerozic - Cambrian metazoan fossil record. This is from James Valentine's book On The Origin Of Phyla.  It shows clearly that metazoan complexity and diversity increased gradually over time. Molecular phylogeny which aims to reconstruct the last common ancestor of animals based on genetic similarities and differences also tells us that the origin of multicellular animals goes back at least 600 million years ago, maybe even more, a good 80-100 million years before the evolution of calcium carbonate skeletonization made their existence obvious in early Cambrian. Next time a news item appears that claims that somehow fossil embryos or fossil burrows from the Neo-Proterozoic times are some shocking new finding that will change our understanding of animal evolution - don't believe it.

Source: On The Origin Of Phyla

Thursday, October 2, 2014

How Are Diagenetic Studies Useful In Understanding Sedimentary Basin History

I dusted of my PhD dissertation last week for two reasons. A friend insisted that she wanted to see my research.. and then this paper in the Journal of Sedimentary Research (behind paywall):

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan by J. A. D. Dickson and J. A. M. Kenter

The work is eerily similar to what I did for my PhD which was carrying out a detailed study of cementation patterns in Middle and Late Ordovician carbonate parasequences from the southern Appalachians.

Dickson and Kenter use petrographic techniques along with cathodoluminescence to tease apart the cementation sequence and pore space modification of the carbonate rocks. Hydrocarbon reservoir quality depends in part on how reaction of sediment with water either dissolves material to create pore space or precipitates cements to modify pore space. So, understanding the timing of these events in the context of the burial history of the sediment pile on a basin wide scale can help geologists predict reservoir quality.

Ok, so what are Parasequences?

Tuesday, September 16, 2014

Cool! 3D Printing In The Geosciences

Geological  Fabrication Laboratory!.. Yes.. the future is already here.

and it is run by Franciszek Hasiuk of Iowa State University. He explains in a short note in GSA Today just why 3 dimensional  printing is so useful especially in the geosciences:

In the geosciences, we struggle with a fundamental problem—we love nature, but its aspects can be truly enormous or fantastically miniscule, very far away or exceedingly rare. Our burden is to overcome these conditions and communicate effectively about nature. With equal ease, 3-D printing can make hand-samples out of subduction zones and foraminifera, Martian topography, and seismic data.

Such models are immediately useful because much of what we need to communicate concerns shape and form (Fig. 1). For these purposes, we can produce inexpensive teaching models on demand, saving acquisition costs while bringing unique specimens to broader audiences. Three-dimensional printing makes the natural specimen the starting point. Digital models can be transformed (e.g., scaled, mirrored, distorted) by an instructor or a student to explore concepts like morphology, vertical exaggeration, or strain. With a little CAD work, we can make flexible fossils to more effectively communicate how organisms, extinct and extant, locomote.

Students might more easily develop a sense of scale from a touchable topography—that they themselves choose and print—that combines local elevation data showing natural and human features. By printing in multiple colors, geological attributes (like geologic formations or geophysical measurements) can be printed over elevation data as a way to better understand a new field area or check field results.

There is more about the applications of 3D printing in understanding rock pore networks with applications in the oil industry this Science Daily article.


Tuesday, September 9, 2014

Remotely India # 7: Mesozoic Domes Of The Kachchh Basin

Remotely India # 7

A recent paper in Current Science (open access) on the geomorphology and tectonics of domal structures in Mesozoic strata prompted me to resurrect this old series titled Remotely India. In these posts I put up a satellite image(s) of an interesting geological feature somewhere in India followed by a brief explanation.

Today's post is on the Kachchh basin in western India. In Mesozoic times this region was a long lasting marine basin in which hundred's of feet of sediments accumulated. Occasionally the sea withdrew and fluvial and lacustrine deposits formed. But most the sedimentary sequence represents marine conditions. Over much of the region the sedimentary strata are horizontal to very gently dipping in disposition indicating very little tectonic disturbance of the basin since the Mesozoic.

At places though these Mesozoic sedimentary rock has been upwarped into domes. These appear as a linear series of blisters in the landscape. The observation that they occur in a linear arrangement is significant and geologists have understood that these blisters and domes occur along mostly east-west trending faults. Map below show the geomorphological and tectonic elements of the region. Red cross is the town of Bhuj.

Source: Kachchh Mesozoic Domes, Current Science 

And this image below is of the Habo dome near the town of Bhuj.

Observations reveal that these domes show an antiformal structure with diverging dips i.e. beds (strata) bending or dipping outwards from the core area.

Here is a series of  domes (Dudhai domes) east of the town of Bhuj.

How did these structures form? Geologists reasoning is as follows: a) The domes don't have corresponding basin like structures, so they did not form through compressional forces buckling the crust into swells and depressions b) the domes occur in Mesozoic rocks on the uplifted blocks of faults but they are not found on the other side of the fault blocks. This suggests that the domes are not "drape folds" formed by vertical movement and adjustment of the sedimentary blanket during faulting. Also supporting the view that faulting did not form the domes is the observation that at places the domes are truncated by faults indicating that this type of structural disturbance occurred much later than the formation of the domes.

Image below shows a truncated dome north of the town of Bhuj

c) there is a close association of magmatic bodies of mafic and ultra-mafic composition with the domes. In some domes plug like magmatic bodies occur in the core of the domes. At places small dykes and stringers of magma are seen to dart from the main magmatic body into the sedimentary beds. This indicates that the magmatic bodies intruded the sedimentary rocks. Based on this the best explanation geologists feel is that these domes formed when magma rose through the crust and impinged the base of the Mesozoic sediments causing bending and doming of the rocks.

When did this occur?  Since Mesozoic times as Gondwanaland split apart into fragments forming the western margin of India, the Kachchh region has been subjected to extensional forces which resulted in several rift type basins forming there. East West trending faults and fracture systems are typical structural elements of this region since the mid Mesozoic.  Radiometric dating says the magmatic intrusions into these sedimentary rocks took place about 68 mya to 64 mya (million years ago).  This was just before and overlapping the Deccan volcanism in the Late Cretaceous which suggests that during magmatic episodes molten material was channelized along older fracture and fault systems of the Kuchchh region resulting in a localization of the domes along faults.

Topographic features, the undulations and swells and mountains that wrinkle the earth's crust form in the variety of ways. Classic orogenic mountains like the Himalayas form when tectonic plates collide and deform and thicken and lift up the earth's crust along thrust faults. Prolonged explosive outpouring of lava can form the beautiful yet deadly mountains like Vesuvius, Mount St. Helens and Mount Fuji. Yet, quieter outpouring of lava like the one experienced during Deccan volcanism can form enormous piles of lava. This pile can later be affected by gentler vertical movements of the crust coupled with differential erosion, forming a landscape of cliffs and gorges as seen in the Western Ghats... and sometimes magma ascends and pushes the crust upwards to form blisters like the domes of Kachchh. Coincidently, David Bressen has a blog post on the history of the idea of plate tectonics. Before this theory explained the origin of large mountain chains, the prevailing view was that mountains are blisters and bumps formed by magma pushing the crust from underneath..!!

Interactive Map: