>At 01:25 AM 2/3/98 -0500, Steve wrote:
>>Geologists do indeed think that carbonate sediments are produced in their
>>environment of deposition (in situ). There are many good reasons for
>>this, not just a uniformitarian worldview. Furthermore, some
>>allochthonous (transported) carbonate deposits have been recognized in
>>the statigraphic record--they form under specific circumstances, such as
>>fore-reef talus deposits.
>
>I would have to take exception to this generalization. I think a grest
>deal of carbonate sediment is now recognized to represent transported
>materials. For example the entire thickness of the Devonian Denay
>Limestone in N. Central Nevada which is one Paleozoic carbonate I have
>studied in detail, is a lime mud turbidite sequence. The entire several km
>of the Jurassic/Cretaceous Great Valley Sequence in California, which I
>have also studied in detail, is lime mud carbonates, deposited as
>turbidites. These are not exceptions, as many limestone sequences
>represent rather uniform beds of transported material that cannot have been
>locally derived. While some limestones may be interpreted as forereef
>talus, many others cannot be, and for these, transport is generally a valid
>option, and is often the preferred interpretation.
I very much agree that carbonate and mixed-carbonate turbidite sequences,
such as the ones you mention, are transported deposits. And you're right,
these are not exceptions, since they occur all over the planet whenever
there was a carbonate platform or shelf adjacent to a deep marine
environment, such as the continental slope and rise. There are fantastic
examples of these in Mexico and Europe, too. But the carbonate sediments
had to be transported from somewhere by turbidity currents and other
types of gravity flows, and that was from the carbonate shelf or platform
where they were originally produced by marine organisms. Most carbonate
sediments remain on their shelf or platform and lithify in place.
However, you are right to emphasize that, in the last three decades, many
poorly understood deep-water carbonate rocks are now recognized to be
transported and deposited allochthonously. Some of these make excellent
oil reservoirs, since they developed secondary porosity and are bounded
above and below by fine-grained source rocks.
Steve
Steven Schafersman
schafesd@muohio.edu
http://www.muohio.edu/~schafesd/