Re: Bioturbation

From: Bill Payne (bpayne15@juno.com)
Date: Tue May 27 2003 - 23:30:08 EDT

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    On Tue, 27 May 2003 12:26:54 -0400 "bivalve"
    <bivalve@mail.davidson.alumlink.com> writes:

    I appreciate your thoughtful summary. I'll continue to chew on this, but
    my initial gut reaction is that you've listed a number of factors, most
    of which seem to be special circumstances that would occur in limited
    areas and times.

    > Fine layering suggests that bioturbation was very limited. This
    > could happen for any of several reasons:
    >
    > Bioturbators were globally rare or absent. This seems to be the
    > case for the Precambrian into the early Paleozoic, and immediately
    > after some mass extinctions.
    >
    > The environment was inimical to bioturbators. This includes anoxic
    > or dysoxic environments (the latter often anoxic in the sediment),
    > tidal flats, extremely unstable sediments (e.g., dunes or sand
    > waves), or hypersaline water.
    >
    > Net sedimentation was extremely slow, so that only a small amount
    > was deposited before the layer was lithified.
    >
    > Sedimentation was faster than bioturbation. This can happen in a
    > storm or a local flood, for example.

    That's the one I, of course, am leaning into. If it can happen in a
    local flood, it could also happen in the Flood.
     
    > Or, in environments that are
    > not especially conducive to high levels of bioturbation, this may
    > not require as much sedimentation. Deep lakes, even if they are not
    > anoxic, tend to have a relatively low number of large, active
    > bioturbators, and distinct annual layers easily accumulate.

    How deep are "deep lakes"? Do you have a reference for this claim? Does
    bioturbation occur in the deep ocean?

    > Bioturbation may not necessarily totally obliterate all layering.
    > If sediment is accumulating more or less continually, with some
    > variation in those sedmients that can be detected as layers,
    > bioturbation will mix things around a bit but eventually they get
    > buried deep enough to no longer get frequent mixing. Thus, although
    > the layering is smeared, it may not be totally gone.

    But most of the rocks I look at in north Alabama have clear distinct
    layers. Your distinction of "fine" layering can be extended to include a
    sharp contact between two massive beds.
     
    > There's a whole range in the degree of bioturbation observed in
    > sediments. You can look up various references. I think Sue Kidwell
    > and Mary Droser are some of the names to look for.
    >
    > Remember also that, although fine lamination is found here and there
    > throughout the geologic column, extensively bioturbated beds are
    > common. You can't honestly claim that the whole geologic column is
    > depauperate in bioturbation.

    No, and I am not claiming that. I have seen some evidence of what I
    suppose are burrows in interbedded Pennsylvanian sands and shales. But
    the bedding is still clearly evident. If these layers were subjected to
    bioturbation for a week, I would think they would be gone.

    > As I am generally looking for beds
    > with lots of shells, my collecting emphasizes well-bioturbated
    > facies, but I do see a lot of places with plenty of bioturbation. I
    > sometimes encounter layers with good lamination. Very many involve
    > dark layers, which indicates high organic content. High organic
    > content implies low oxygen levels.

    > Also, each lamination takes some amount of time to form. With about
    > 200,000 laminations in the Castille Formation in western Texas, you
    > would have to form one organic rich-organic poor couplet every two
    > to three minutes to fit it all into one year. That's just one
    > formation.

    The time required is a notion we bring from somewhere else. Somewhere I
    have a reference that says these interbeds can be formed spontaneously in
    the lab from a single sedimentary event.
     
    > Trying to explain any substantial portion of the geologic column as
    > resulting from the Flood requires extremely high sedimentation
    > rates, and the fact that organisms frequently had plenty of time to
    > burrow around poses a problem for this. On the other hand,
    > occasional deposits being deposited rapidly in no way poses a
    > problem for an old earth view. The young-earth view is at a
    > disadvantage, as anything taking a long period of time is
    > incompatible with it, whereas lots of things can happen quickly
    > within an old earth model.

    As I see it, the old-earth model is at a disadvantage. The geologic
    column is replete with bedded strata, and by comparison, very little
    evidence of bioturbation. The locally (vertical and/or horizontal)
    bioturbated zone in no way poses a problem for YEC.

    Can you name any marine and coastal swamp modern analogs, where bedded
    strata are being continuously formed with little bioturbation?

    Bill

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