Re: marine or eolian dunes?

Arthur V. Chadwick (
Wed, 04 Feb 1998 15:21:27 -0800

At 08:50 PM 2/3/98 -0600, Glenn wrote:

>I don't think the suggestion is that the tracks were made in dry sand. The
>suggestion was that they were made in damp sand. I think most would agree
>that you can't make good tracks in absolutely powder dry sand.

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.

>And I do not stand as alone as you might think.
>"Glauconite is a mineral which is widely believed to form only in marine
>environments and to be so unstable that it cannot survive re-working. It
>is therefore held to be diagnostic of marine rocks. This criterion is not
>infallible. Detrital glauconite occurs in fluviatile Neogene red beds in
>the Dead Sea Valley. As a general rule however glauconite is a useful
>indicator of marine environments and studies of the present day distribution
>of glauconite suggest that it may be possible to narrow down the precise
>conditions which limit its formation."~R. C. Selley, "Ancient Sedimentary
>Environments" 2nd ed. 1978, p. 8

Note the logic: "Glauconite is widely recognized as a deposit formed on
the ocean floor (below 100 meters). However, we have these fluviatile
neogene beds (based on sed structures, etc.) that have glauconite in them,
so we will ignore the DATA from the modern environment and
this case, the present must not be the key to the past". ????

> "Glauconite is a complex mineral related to the clays and the micas;
>essentially it is an aluminosilicate containing magnesium, iron, and
>potassium. Glauconite occurs as a replacement of faecal pellets and as an
>internal mould of forams and other small voids. Detailed discussion of the
>genesis of glauconite in modern sediments will be found in Odin and Bjerkli
>and Ostmo-Saeter. Prrenga has revied the environmental parameters of
>glauconite formation.
> "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

And " Glauconite is formed only on the ocean floor today, but we have all
those Cambrian deposits that have been interpreted for years as shallow
water deposits, but they have glauconite, so, we will ignore the DATA from
the modern environment and suggest that at least in this case, ..."the
present must not be the key to the past".?????
Both these statements demonstrate the depths to which paradigms control
interpretations. Rather than challenge the depositional environment, the
attitude is, the modern environment must be wrong, since what we see
happening today doesn't agree with what we think happened in the past.

>Krauskopf, Introduction to Geochemistry, 1967 writes:
>"Under mildly oxidizing conditions in shallow seas, ferric iron may go into
>glauconite [approximately KMgFe(SiO3)3*3H2O], and near hot springs (perhaps
>sometimes also as a result of ordinary weathering) into jarosite[...]." p.

This statement is speaking of alterations to glauconite, not to its
formation, as ferric iron is not a normal dominant component of glauconite.
Glauconite is believed to form under reducing conditions when organic
matter is available in the sediments (thus the Fe++).

>None of these say anything about deep water. And Dot and Batten, 1988
>Evolution of the Earth, p. 290 merely say that it requires slow deposition.
>Some of the past epeiric seas where shallow and had slow deposition. And as
>you know depositional speed is not restricted only to deep waters.

Glauconite is not a product of slow deposition, but of anoxic alteration of
sediments containing high concentrations of organic matter. The
environment of formation is outer shelf to upper slope (sea floor) in water
depths exceeding 100 down to at least 500 meters (it is dredged off the
sea floor far from sources of sediment), where according to Weaver ("Clays
Muds and Shales") it is forming today off the continental margins of east
and west coasts of North America, along the southern areas of South
America, northern Spain, southern Japan, southwestern Australia, eastern
and southwestern Africa today. Deep is a relative term, but none of this
is "shallow", in the sense that it is all below storm wave base (my
personal distinction between shallow and deep).