Re: marine or eolian dunes?
David Campbell (bivalve@mailserv0.isis.unc.edu)
Thu, 5 Feb 1998 10:23:43 -0400>The problem then is how do you preserve tracks in damp sand? If it rains
>again before they are covered, they are gone. If the sand dries enough to
>blow around, they will also dry out and be destroyed. Then there is the
>problem of how to get dunes wet without eroding them and leaving evidence
>of the rain. If there were much moisture, this should be enough to cause
>the growth of plants, which should be represented by fossil rhizoids all
>over the place. Generally, if you wet sand, you get no tracks at all,
>certainly not the clear precisely preserved trackways that Brand et al
>showed in the submarine trackways. And whatever the case for the arthropod
>trackways, the submarine vertebrate trackways produced in the laboratory
>are the only trackways that are anything like the fossil trackways.
When I walk on a sandy beach, I make clear tracks in the part with wet
sand. Is this quartz sand? How is it cemented?
>> "The consensus of opinion is that glauconite forms only as an
>>authigenic mineral during the very early diagenesis of marine sediment.
>>Penecontemporaneous reworking can concentrate glauconite in shoal sands and
>>transport it into deeper basinal sands." ibid p. 26
Low sedimentation rates were considered an important factor in the
formation of glauconite in what I've read. Boggs (1992, Petrology of
Sedimentary Rocks) cites 200m as a maximum depth for typical occurrence,
and several examples of shallow occurrence.