Re: marine or eolian dunes?

Glenn Morton (grmorton@waymark.net)
Fri, 30 Jan 1998 21:47:21 -0600

At 04:21 PM 1/30/98 -0800, Arthur V. Chadwick wrote:
>At 09:56 PM 1/29/98 -0600, Glenn wrote:
>
>>I have found a major sedimentological difference between dunes deposited
>>beneath the sea and those which are found above. I am currently looking for
>>evidence either way from the Coconino and Navajo sands. I got a 400 article
>>literature search today on these two deposists.
>>
>>The difference between these two types of dunes involves the smallest levels
>>of crossbedding.
>>
>>"All the basic types of stratification found in dry windblown sand can also
>>be found in water-laid sands. As far as is known, eolian and subaqueous
>>planebed lamination cannot be distinguished by their structural
>>characteristics, nor can eolian and subaqueous grainfall lamination. Eolian
>>and subaqueous sandflow cross-strata of small slipfaces show some fairly
>>consistent difference, but the differences between eolian and subaqueous
>>climbing-ripple structures are even more distinct. The subcritically
>>climbing translatent strata produced by subaqueous current ripples generally
>>have distinct ripple-foreset crosslamination and are normally graded; both
>>of these featrues re evidently produced by miniature sandflows down the lee
>>slopes of the ripples. Other differences between eolian and subaqueous
>>climbing-ripple structures are related to differences in the
>>height-to-spacing ratios and plan forms of the ripples." ~Ralph E. Hunter,
>>"Basic Types of Stratification in Small Eolian Dunes," Sedimentology,
>>24(1977):361-387, p. 384-385
>
>I don't see anything in these paragraphs that can be cited as a"major
>sedimentological difference" unless by this you mean "smallest levels" or
>"cannot be distinguished by their structural characteristics" or "generally
>have" :-), but hyperbole aside (since I never am guilty of that myself),
>these subtle bedforms are extremely difficult to see, even in the modern
>environments, and I haven't seen them used to discriminate
>paleoenvironments, probably because they are so subtle and difficult to
>quantify.

The importance of that work is precisely that the smallest features of
marine dunes differ from eolian dunes. This allows it to be used on oil
well cores. It is a predictive difference which is observable and that also
is important.
>

>>
>>Other items which should be found if the dunes are subaqueous.
>>
>>1. Fish-droppings
>
>There is no organic material found in the Coconino at all, except in the
>marine interbeds. I think this rules out finding fish droppings.

I can understand why organic deposits like dung would be oxidized into
oblivion in an extremely arid desert. I have difficulty with the idea that
all organics would be destroyed in a subaqueous environment

>>
>>2. glauconite(which is generally accepted as being marine in spite of that
>>GSA article)
>
>I am glad to hear your accord on that point. We have been using the
>tremendous quantities of glauconite to argue for deep water deposition of
>the Tapeats, but have met considerable resistence to this from other
>researchers who have worked on Cambrian clastics. Of course their
>arguments are pretty much ad hoc, since they, like McKee have decided that
>the deposition is shallow on the basis of sed structures representative of
>shallow water deposition in the modern environment.

I am not just now concording with you on this point. I agree that glauconite
is marine. But it's lack in the Coconino and Navajo (as far as I have been
able to find) should also be significant!

>>
>>3. no caliche. I know of no way for caliche to be deposited beneath the sea
>>since its formation requries evaporation. (C. M. Rice Dictionary of
>>Geological Terms, p. 463) The mongolian dunes have multiple caliche horizons
>>
>Good. caliche is certainly not present in the Coconino, anyway.

Yes but it is in those "flood deposited" Mongolian dunes.

>
>Now back to the Nubian. A stable depositional environment for 260 years,
>across the Devonian, the Pennsylvanian, the Permian, the Great Permian
>extinction, the Triassic, Jurassic and Cretaceous, there was only
>continuous deposition of pretty much uniform sand, apparently uninterrupted
>by major events that characterize sediments everywhere else? And a few
>miles to the east, not a grain of sediment deposited during the same
>interval, so that Lower Devonian sand is in sedimentological conformity
>with Mid Cretaceous sand? Doesn't that beg for an explanation?

Yes, and I will admit that I don't have a good one. So, what is your
explanation?

glenn

Adam, Apes, and Anthropology: Finding the Soul of Fossil Man

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Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm