From: jjh00@outs.ccc.amdahl.com (Joel J. Hanes)
Newsgroups: talk.origins,sci.geo.geology
Subject: Re: Creative speculation on outrageous chance
Summary: Evidence for impact-generated extinction in Upper Devonian
Keywords: bolide microtektite Devonian
Date: 26 Aug 92 18:37:17 GMT
Reply-To: jjh00@OUTS.ccc.amdahl.com (Joel J. Hanes)
Followup-To: talk.origins
Organization: Amdahl Corporation, Sunnyvale CA

>   jgacker@neptune.gsfc.nasa.gov (James G. Acker) writes:
>>
>>   (quotes Washington Post article about microtektites found
>>    in 365-Myear old sediments in Belgium)
>>
>>  Quick question:  is the earlier extinction the Permian?


I responded:
>
> No, it's in the upper Devonian.  I have the relevant issues
> of _Science_ at home -- I'll bring 'em in and provide
> citations, more exact dates, and suchlike technical stuff.


And here they are:


"Another Impact Extinction?"
news report, Richard A. Kerr, reporter
_Science_, Vol 256, 12 May 1992, p 1280

  Cites P. Claeys, S. Margolis (UC Davis) and F. Kyte (UCLA)
  _Science_ 29 March 1991, and P. Claeys, J. Casier, S. Margolis,
  "Microtektites and Mass Extinctions from the Late Devonian
   of Belgium", EOS Trans. AGU 73, 328 (1992)

  Microspherules found in Belgium by Casier are dated to
  367 years BP, roughly coincident with the boundary between
  the Frasnian and Famennian stages of the Upper Devonian.
  This boundary is traditionally established by a change in
  conodont associations, but conodonts are scarce in the formation
  containing the find; Claeys places the microtektite-bearing
  layer within a few hundred thousand years of the F-F
  boundary.  The shapes of the microspherules, their low water
  content, and their distinctive elemental composition are
  cited as evidence of their origin in a bolide impact.

    Goodfellow and Geldsetzer of GSC CAlgary have
  found a strong iridium anomaly right on the F-F boundary at
  Montagne Noir in southern France.

  I quote in full the last two paragraphs:

    "Meanwhile, Claeys and Margois are on the trail of a different
     F-F mystery: where the impact struck.  They ar pursuing
     the possibility that their microtektites were splashed
     from a known crater, possibly the Siljan crater in Sweden,
     which is about as old as the F-F boundary.
        Without an impact crater or an indisputable link to the
     extinction, the signs of impact at the F-F boudary fall in
     roughly the same category as the shocked minerals recently
     discovered at the 202-million-year-old Triassic-Jurassic
     mass extintinction (see report in the 24 January _Science_):
     suggestive, buy by no means the clear signature of a killer.
     But if the case for impact-caused extinction strengthens at
     either boundary, the long-discussed possiblility that
     repeated killer impacts shaped the history of life may
     look more compelling."



"Glassy Microspherules (Microtektites) from an Upper Devonian Limestone"
 Kun Wang, Dept of Geology, Universigy of Alberta, Edmonton
 _Science_, Vol 256, 12 June 1992, pp 1547-1550.

 Abstract:  The properties of microspherules recovered from an Upper
            Devonian marine limestone immediately overlain by a
            geochemical anomaly of siderophile and chalcophile
            elements are similar to those of impact-derived micro-
            tektites.  These microspherules are glass, have
            splash-form shapes, contain spherical vesicles and
            lechatelierite inclusions, and show oxide compositional
            variations similar to those in known microtektites.
            These characteristics suggest that these Upper Devonian
            microshperules have an impact origin.  A bolide impact
            may have occurred about 365 million years ago on the
            South China plate and caused a faunal extinction on
            eastern Gondwana.

    The author found the possible microtektites near Qidong, in
    south China.  The limestone is dated by fossil association
    with the lower levels of the conodont Palmatolepis crepida.
    A 20 X background iridium anomalies are reported within this
    zone in Australia by Nicoll and Playford.  The paper shows
    pictures of the microspherules, X-ray diffraction patterns,
    and section graphs of the geochemical anomalies.  The author
    suggests an impact site under Taihu Lake, based on shocked
    quartz finds.



I find these articles interesting because I myself own a
few acres of Upper Devonian marine sedimentary rock in Iowa.
I'm not sure of its exact provenance - perhaps later than
the Famennian.  Anyway, I fantasize about having the time and
money to someday do a bit of private research on my own land,
looking for iridium and microtektites at the F-F boundary
if I have it in my section.

---
Joel Hanes
Newsgroups: talk.origins