[meetyeti] Fwd: Field Photos: Red Layers Within Deccan Lavas And Mass Extinction
- From: YETI <meet.yeti@xxxxxxxxx>
- To: meetyeti@xxxxxxxxxxxxx
- Date: Fri, 24 Feb 2012 07:14:18 +0530
For some years Suvrat Kher, a geologist by training, has been posting on his
blog, http://suvratk.blogspot.in/, interesting posts not just on geology but
also on biogeography, geology & society and so on. I highly recommend following
his blog! See below for a very interesting and most recent post.
Robin
Begin forwarded message:
> From: Rapid Uplift <suvrat_k@xxxxxxxxx>
> Date: February 23, 2012 11:36:32 PM GMT+05:30
> To: robinvvijayan@xxxxxxxxx
> Subject: Field Photos: Red Layers Within Deccan Lavas And Mass Extinction
>
> Field Photos: Red Layers Within Deccan Lavas And Mass Extinction
>
> Field Photos: Red Layers Within Deccan Lavas And Mass Extinction
> Posted: 23 Feb 2012 08:19 AM PST
> I'll lead you into the field photos in a minute, but I just want to comment
> on something else first.
>
> Prof. Gerta Keller and her collaborators are at it again. Two new papers in
> the Journal of the Geological Society of India and Earth and Planetary
> Science Letters have come out in recent months in support of the theory that
> the Deccan volcanism played a large role in the mass extinction that took
> place 65 mya. The mass extinction resulted in biologically new conditions on
> earth and we recognize that formally by naming post extinction geological
> layers to represent the beginning of the Cenozoic Era.
>
> I haven't read the papers but from following some early work of this group
> and the abstracts it is worth summarizing Prof. Keller's argument
>
> 1) The Chicxulub meteorite impact which is dated to around 65 mya and
> considered to be the main cause of the mass extinction occurred 300,000 years
> before the mass extinction and so couldn't have been the sole cause of the
> extinction. This conclusion is arrived at by studying Maastrichtian (latest
> Cretaceous) to Danian (earliest Cenozoic) sections near the impact crater in
> Mexico and Brazos river, Texas. The sections show a meter or so of sediment
> between the impact breccia and the Cretaceous -Cenozoic boundary layer
> which is recognizable by C-13 shifts and an irridium anomaly, marking a
> pronounced environmental shift. Sedimentological analysis of the meter of
> sediment suggest normal quiet conditions of deposition and biostratigraphy
> suggests a time lag of about 300,000 years between the impact breccia and the
> boundary layer. The irridium anomaly thus points to a second meteorite
> impact.
>
> 2) On the other hand, the main pulse of the Deccan volcanism accounting for
> some 80% of lava volume erupted in the latest Maastrichtian in magnetic
> polarity zone C29R. Planktic foraminifera and other ecological indicators
> from marine sediments spanning Maastrichtian to Danian deposited between lava
> flows in the Krishna Godavari basin show drastic drops in species richness,
> an indicator of extreme environmental stress. Formaminfera in Danian
> sediments between younger lava flows are represented by just a few species.
> These are survivor fauna. This then demonstrates a cause and effect
> relationship between the Deccan volcanism and environmental stress.
>
> The mass extinction according to Prof. Keller had multiple causes, Chicxulub
> impact, then Deccan volcanism and then a second meteorite impact.
>
> Most scientists working on this problem reject this scenario. This is not
> because they object to the premise that the Deccan volcanics would have been
> harmful. Their objection is to Prof. Keller's interpretation that the
> Chicxulub meteorite impact took place 300,000 years before the mass
> extinction. They interpret the meter or so sediment near the impact crater
> and the Brazos section as a tsunami deposit and not indicative of a long
> period of quiet conditions. And in many many sections spanning Maastrichtian
> to Danian around the world, there is no sediment between the impact layer
> (recognized by typical glass spherules and shocked quartz) and the boundary
> layer. No sediment therefore no time lag say the scientists and so Chicxulub
> meteorite impact coincides with the boundary layer and is the main cause of
> the mass extinction.
>
> Prof Keller argues that such sections wherein the impact layer coincides with
> the boundary layers are condensed sections i.e. there was no sedimentation at
> all in the interval between the impact and the mass extinction and therefore
> the two seem to appear the same! One problem has been the lack of high
> resolution dating which could separate these events and settle the issue.
>
> The main pulse of the Deccan volcanism lasted maybe half a million years or
> so and temporally does seem to coincide with the mass extinction. This would
> have resulted in at least severe regional environmental stress and
> extinction. The paper in Earth and Planetary Science Letters documents
> regional environmental effects. But would it have global impact?
>
> A couple of months ago I was driving west of Pune (location in image below)
> and came across a road section that exposed several red clayey layers (red
> arrows) between lava flows.
>
>
>
> These red layers locally known as red bole have been interpreted as
> weathering products formed during a hiatus in volcanic activity. Some are
> debris and soil that formed on top of a flow which then got fossilized due to
> burial by the subsequent flow. Others though are weathered tuffs or ash fall
> deposits. They have been found to contain basaltic shards (silicate glass
> that forms from sudden chilling of lava) diagnostic of an explosive
> pyroclastic origin. These explosive ash layers settled on lava flows and in a
> period of quiescence that followed got altered to a clay bearing layer.
>
> These pyroclastic layers occur at regular stratigraphic intervals throughout
> the main Deccan volcanic phase. Ideas about their origins are not new. There
> is work dating back to the late 1970's by geologists from Pune (Dr. L.K.
> Kshirsagar - Ph.D thesis) that had recognized the pyroclastic nature of these
> layers. But the environmental connection to the mass extinction was not made
> then. There has been more recent work on these red layers and the idea is
> being floated around that such type of explosive volcanism may have resulted
> in columns of ash being ejected tens of km up to stratospheric levels and
> that would have influenced global climate leading to environmental
> perturbations.
>
> okay.. so finally here are the photos of these red layers. I can't say
> whether these represent in situ soils or ash fall deposits.
>
> Long view of a red layer-
>
>
>
> Close up of a red layer -
>
>
>
> Red layers containing broken blocks of basalt -
>
>
>
> A rubbly top of a lava flow. You can see red material infiltrating through
> the cracks and opening between the basalt rubble.
>
>
>
> The Deccan volcanics are considered to be non explosive fissure type
> eruptions. Can explosive volcanic eruptions resulting in Plinian columns (
> ash columns reaching great heights) form in such a volcanic setting? I am not
> a volcanologist but I have always associated such eruptions with classic
> volcanic cone formation. Have such cones and eruptive vents been identified
> in the Deccan volcanics? Is this a problem of preservation, the cones being
> weathered away, or is it that cones need not always be associated with
> explosive volcanism? And were there enough of these episodes to have
> influenced global climate?
>
>
>
>
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