Here's the last one:
>
>>> 5. How does a global flood explain angular unconformities? I can, for
>>> example, direct you to an outcrop along a railroad bed in southeastern
>>> Pennsylvania showing one mile of east dipping turbidite deposits
>>> (with hundreds of graywacke/shale packages) abutting against vertical
>>> quartz arenite sandstone beds. Please explain how features like this
>>> form by rapid sedimentation in a global flood.
>>
>> First I have to ask you how a mile of turbidite deposits with graywake/shale
>> packages could form slowly? If these are anything like the Great Valley
>> Sequence (in NW California, which I studied during PhD research) they show
>> little evidence of time between turbidites -- so little that there is hardly
>> any bioturbation (but a few examples, showing that organisms were alive in
>> the area, and some fossilized, during the deposition), and no signs of
>>erosion
>> between layers. To me, this (and other observations) speaks of rapid
>> deposition of the turbidic sequence, which was later tilted (in the case of
>> the GVS the tilting was part of seafloor accretion to W. North America).
>
> Why would you expect erosion between layers from turbidites deposited in
>the deep ocean? Especially in the distal parts of the fans? That's not an
>environment where one would expect erosion.
If you are thinking of years, even hundreds or thousands of years between
turbidites, I would expect some indication of erosion, especially
considering the earthquakes and volcanic events that occur in the ocean, in
conjunction with seafloor spreading, etc., not to mention changes in ocean
currents which may sometimes reach deeper than we usually think. Also, I
would expect some bioturbation, even in areas where life is not
particularly abundant.
I might, however, expect erosion
>in a flood model origin of turbidites (where all turbidity flows occurred in
>the time span of a few days) since we're talking about a very different type
>of higher-energy enviroment.
And some places, this is seen -- where an incoming turbidite lifts some of
the fine material from the preceeding one, or scours some of it and
incorporates it into the overlying stratum. Like in the Grand Canyon where
there are flame structures and other indications of mixing between
formations of different colors (even with a supposed gap of millions of
years between them).
>
> Of course the individual graywacke beds within the sequence represent quick
>sedimentation (turbidity flows). The long time is partly an inference given
>how turbidites form (based on our study of modern day continental shelf
>environments where they occur) and partly from the deep-water fossils (e.g.
>graptolites) preserved in the shales (that's why people were confused about
>how flysch formed before turbidity currents were recognized) and the fact that
>it's not possible to deposit clay-sized particles quickly on the deep seafloor
>(Stoke's Law and all that).
Unless they are flocculated clays.
>
>> How much later? I don't know. Long enough to dewater and set the
>> limestone well enough to keep its integrity (hours, days?) , but not so
>> long that the layers were indurated so much that they would shatter upon
>> tilting or folding (i.e., not months or years). There are some places with
>> folded strata.
>
> All guesswork. Please direct me to the primary literature discussing the
>theoretical and experimental work regarding this hypothesized rock behavior.
>I teach structural geology and would be very interested in seeing a rigorous
>treatment of this type of deformation.
>
When I demonstrate folding in a geology lab, I use good modeling clay of
several colors. That is what works. Wet sand and mud layers, or dry
brittle layers don't work. That's my experimental experience.
Papers on it -- perhaps someone on the list has the references for work on
the Green River Shale regarding soft-sediment deformation -- showing
evidence of minor breakages in places where the layers slumped a little,
after the laminations had basically set, but before the whole sequence had
really indurated -- I saw it in the cliff face when I was out there.
>> Such tilted turbidites abutting against vertical quartz arenite sandstone
>> beds! Any evidence of overthrusting? Must have been some dramatic earth
>> movement there! How do you read it?
>
> I read it as Martinsburg Formation turbidites forming in a deep narrow basin
>(Taconic foredeep) between the eastern coast of North America and an
>Ordovician
>island arc (bentonites in the Ordovician rocks preserve the evidence of
>volcanism quite well). The collision of the island arc was the well-known
>Taconic Orogeny and resulting in the initial tilting of the turbidites.
>Later,
>the Silurian Tuscorora Sandstone (a quartz arenite) was deposited horizontally
>on the tilted turbidites and the later Alleghanian Orogeny (forming the
>Pennsylvania Valley & Ridge) tilted the whole package to its present-day
>orientation.
>
Ah! There was some tilting of the greywakes, then more of both them and
the sandstone beds. That makes sense. No overthrusting necessary. Still
very dynamic, since it tilted 90 degrees to make the horizontal sandstone
beds vertical. And the twice-tilted greywakes extend for a mile,
indicating that they were once a mile deep under the sandstone, and now a
mile wide beside it? -- that means mega-tilting!
I have enjoyed this exchange.
Bless you.
Karen