Re: Midnight ramblings about a Hell Creek Biostratigraphic geocartoon

[Phil Bigelow <bigelowp@juno.com>]

frank bliss <frank@blissnet.com> writes:
> Notes from the Bliss Ranch Hell Creek microsite digs. 
<snip> 
> It has been interesting lately down in the gullys.

A short note about gullies:  You may find that the gullies often form in
the coarser grained rock.  In the case of the HC, that usually means
medium- to (rarely) coarse-grained sandstone.  If you measure the
orientation of the foreset beds in these units, you may find that their
dip direction coincides with the azimuth of the gully.


<snip>
> I consistently fail to trace beds 
> laterally 
> over more that a few dozen feet only to find a very abrupt 
> transition 
> to a different rock type instantly.


Welcome to HC stratigraphy!  ;-)



> Gravelly sand turns to clay or 
> 
> barren sand abruptly and back again with no obvious transition 
> laterally as one would expect (I am used to gradual 
> environmental/sediment transitions) in a sedimentary environment.

In many localities, coarse-grained channel deposits laterally-abut
massive mudstone.  The units usually represent a fossil cut-bank, with
the mudstone representing the clay-rich river bank and the sandstone
representing the bottom of the river channel.  What you are looking at is
one side of a meander.  These contact relationships are interesting from
a fossil hunter's perspective, because whereever there was a cut-bank in
a stream, there was often a point bar on the other side of the stream. 
Point bars can accumulate large numbers of dead things.


> Therefore, it is my supposition that these sand, sand/gravel and 
> clay 
> lenses were probably shaped/reoriented by soft sediment flow 
> sometime 
> after deposition and mixing of what originally were superimposed 
> faunal 
> sequences occurs as a result.  (Certainly enrichment of the hard 
> fossil 
> parts occurred as a result of water scouring/flood concentration.)


If you have a Brunton compass, you should be able to map out the
paleo-stream flow direction(s) quite easily.  This will give you
invaluable information that you can use to better interpret your local
stratigraphy and the biota it contains.


 
> Any 
> interpretation biostratigraphically about succession of faunal 
> occurrence would be suspect if this observation holds true.


Demonstrating faunal succession in a convincing way is harder than it
sounds.  I wish you luck on that one.




> There 
> are 
> numerous other examples of soft sediment deformation on our ranch 
> (draped with Hell Creek) with abundant sand tubes (some up to 3 feet 
> 
> across and dozens of feet long.


Are you referring to the brown colored goertite/hematite concretions that
form log-like structures in the sandstone facies?



> There are also locally disrupted 
> sand 
> laminae from dunes which indicate the same phenomena.  It seems to 
> me 
> these Hell Creek sediments at this location were squeezed like silly 
> 
> putty by the weight of the accumulation above.  The mountains to the 
> 
> west must have really been dumping sand to cover this material 
> enough 
> to squeeze it big time before it hardened.  An additional 
> obfuscating 
> effect of any biofacies relationships would be the obvious cut and 
> fill 
> effects of flowing water and wind cutting penecontemporaneously with 
> 
> sediment deposition adjacent to the active water course.
> 
> You Cretaceous mammal junkies out there will have to wait for the 
> sieving to start later this summer before the really rare fossil 
> material starts showing up.  I am having too much fun finding new 
> dig 
> sites to sieve still moist sand. Mammal stuff rarely comes out of 
> digs. 
> More of it comes off the screen.
> 
> The main question that arises out of all this is, who else is doing 
> 
> biostratigraphic work on Hell Creek?



The Museum of the Rockies is doing a large-scale faunal inventory project
in the HC, but AFAIK, it isn't a rigorous biostratigraphic project, per
se.

It can be frustrating trying to understand the sedimentary structures
that one sees in the field.  The HC formation is no exception.  If you
don't already have them, get a copies of the following papers:

Fastovsky, D.E. 1987. Paleoenviroments of vertebrate-bearing strata
during the Cretaceus-Paleogene transition, eastern Montana and western
North Dakota. Palaios 2: 282-295.

Retallack, G.J., G.D. Leahy, and M.D. Spoon. 1987. Evidence from
paleosols for ecosystem changes across the Cretaceous-Tertiary boundary
in Montana. Geology 15: 1090-1093.


Can't distinguish a paleosol from a muddy overbank deposit?  (there IS a
difference).  Well, NO PROBLEM!  If you can get your hands on the
following textbook, you will be way ahead in your HC facies studies.

Retallack, G.J. 1990. Soils of the past. Unvin-Hyman, London. 520 p.


Thanks for the very informative post,

<pb>
--








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