Thanks for the info on burrowing.
At 11:32 AM 12/3/97 -0800, Arthur V. Chadwick wrote:
> This raises another question: if modern rates of sediment accumulation on
>deltas are one meter per year, in 1000 years that should give one kilometer
>of sediment. What would happen in a million years? Yes, I know, the
>sediment is reworked into deeper water, etc, but we don't have all that
>many sequences in the fossil record that are deep water (something that I
>think is itself long overdue for correction, but that is for another day),
>so we invoke recycling, which is convenient, except that it eliminates the
>evidence for evolutionary sequences, which is also convenient, except that
>it just ain't so.
You left out the real liklihood that the delta will shift somewhere else.
The Mississippi River has not always emptied where it does today. 5000
years ago it was depositing sediment far to the west of its present location
and at one time it deposited its sediment eastward in the Chandeleur Sound.
Spreading the sediment out, allows for subsidence to accomodate the space.
I also did a calculation a few days ago in which I calculated that the
Mississippi spread its sediments over around 115,000 square miles and given
the rate of sediment influx observed today it would take 49 million years to
deposit the 40,000 + feet of sediment we see out there.
> Stratigraphic sequences I have studied in detail(mostly
>early Paleozoic) show very little evidence for reworking between beds, and
>certainly lack any substantial evidence for displacement of massive amounts
>of sediment to deeper water. Hypothesizing that sediment rates were far
>lower in the early Paleozoic doesn't cut it since generally they are
>believed to have been much greater, since there was presumably no
>vegetation to prevent erosion.
But since the Eustatic sealevel was higher, there was less land to erode and
less clastic input to the ocean. To me, the eustatic sealevel curve
explains this problem. It also explains why there was so much carbonate in
the Paleozoic.
In fact the whole problem of sedimentation
>rates through time has been the subject of a lot of study. The whole
>affair calls into question our current understanding of sedimentary
>processes. (Schindel, op.cit; Anders, M.h., S.W. Krueger and P.M. Sadler,
>J. Geol. 1987. 95:1) The conclusions (borrowed from a friend) are worth
>sharing:
>
>"Sedimentation rates [estimated by every conceivable technique] are
>alarmingly variable, spanning at least a dozen orders of magnitude. In
>every environment the rates decrease progressively as longer time spans are
>considered. The regression is partly a function of environment. There are
>several consequences of this pattern and they are not all intuitively obvious.
>
>1. Stratigraphic completeness is a function of time scale and can be
>meaningfully quoted only for a specified time resolution.
>
>2. Short term sedimentation patterns are a disasterous guide to long term
>accumulation and we should distance ourselves from the doctrine of
>uniformitarianism.
>
I might point out that one absolutely cannot believe that the rates of
geologic processes have remained
>3. If high resolution is required then stratigraphic sections must
>generally be expected to be disasterously incomplete.
>
>4. Even abyssal ooze accumulations may be quite incomplete.
>
>5. Beyond time spans of about 10^4 yr all sections at or above marine wave
>base seem to be controlled by tectonic and not sedimentological factors.
I am not sure that I can agree with this. What data can your friend cite?
>
glenn
Adam, Apes, and Anthropology: Finding the Soul of Fossil Man
and
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm