Re: marine or eolian dunes?

Arthur V. Chadwick (
Thu, 05 Feb 1998 08:48:48 -0800

At 10:23 AM 2/5/98 -0400, David wrote:
>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?

Yeah, quartz sand, quartz cement. Your tracks may be clear, but they do
not resemble those in the fossil record, and unless you were walking up the
foreset slopes of dunes with angles near the angle of repose, they were not
made under similar circumstances (you can imagine what your tracks would
look like under those circumstances).
>>> "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.

True, the glauconite we see being formed today is offshore beyond the reach
of normal sediments. I have not seen any modern examples of shallow
occurrence, and while it is difficult to get maximum figures (it was first
found in bottom samples of the ocean floor at great depths), the ones I
have seen are the figure of Weaver of 400 meters and another source I
cannot recall at the moment of 1000 meters. I do not have Boggs, but will
try to see what modern examples he cites.