[meteorite-list] Ordovician Meteorites...was New or maybe old QUESTION???

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sun, 4 May 2008 20:34:13 -0700 (PDT)
Message-ID: <136242.76499.qm_at_web51010.mail.re2.yahoo.com>

I believe the paleo meteorites we are thinking about were found in a limestone quarry of Ordovician age in Kinnekulle, Sweden. If memory serves, only after the tiles containing the meteorites had been polished and installed in a building were they identified for what they were. Scientist went back to the quarry and found more in a narrow layer. There is also a citation for a 10cm meteorite found in Brunflo. "The first fossil meteorite found in ancient sediments was Brunflo, a heavily altered 10 cm chondrite found in Ordovician limestone (Thorslund et al., 1981)" Perhaps someone can reconcile this.

The Ordovician period was 488.3?1.7 to 443.7?1.5 mya. As explained below, a disruption of the L parent body around 500mya lead to a bombardment on Earth during Ordovician's early to mid epochs(470mya?). The bombardment is thought to be the cause of the great diversification of life forms during that time. It was also an ice age on earth and impact influence on the climate hasn't been ruled out.

The Cincinnati Arch and the Nashville Dome are Ordovician geological units, composed of very fossiliferous limestones as they were shallow seas during the time of deposition. The uniform shallow depth could be advantageous for preservation.

When collecting fossils from Indiana, Ohio, Kentucky and, Tennessee, one should keep an eye out for meteorite shaped rocks amongst all the brachiopods, bryozoans and trilobites. Fallback breccia from a blasted reef has been found from the Oneota Formation, Glover's Bluff, Wisconsin.

Who knows-- trilobites might have evolved eyes on stalks just to keep a better lookout given all the rocks falling out of the sky.
Elton

Here are some blurbs from Uncle Google:

Discovery of a second Ordovician meteorite using chromite as a tracer.

The small number of known fossil meteorites owe their discovery to the preservation of a characteristic composition or structure. These features are easily changed beyond recognition by diagenesis and other processes at low temperature after a meteorite becomes incorporated into sediments, because the meteoritic minerals are generally susceptible to even weak alteration. The mineral chromite, however, is an exception. Here we report the finding of a strongly altered fossil stony meteorite identified by the composition of its relic chromite, and suggest that chromite chemistry can be used as a basis for a systematic search for fossil meteorites.
 AND:
Abundant fossil meteorites in marine, condensed Lower Ordovician limestones from Kinnekulle, Sweden, indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the Early Ordovician than at present. Osmium isotope and iridium analyses of whole-rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust. Enhanced accretion of cosmic matter may be related to the disruption of the L chondrite parent body around 500 million years ago.

AND: From
<http://www.fossilmuseum.net/Fossil_Galleries/Stromatolites/DS612/Stromatolites61.htm>
This is unique stromatolite called impact fallback breccia. Coming from the Lower Ordovician, it is also very young, from a time when stromatolitic reefs no longer dominated the planet's marginal marine environments. Note the heterogeneous patterning with sharp and angular fragments of various size embedded in the reddish matrix. This beautiful pattern was formed during the so-called Glover's Bluff meteorite strike during the Ordovician; a fragment from the same meteor also struck in near Elm Rock Illinois, forming a mile-wide crater. The meteor that struck what is now the Oneota formation plowed through the stromatolite reef deep into the earth, spewing molten rock upward, with everything eventually falling back to earth, with one result being the "fallback impact breccia" seen here. It seems certain that the living stromatolite would have been decimated.
Received on Sun 04 May 2008 11:34:13 PM PDT