Dear Bill, Jonathan, Keith, David and all those following the Paleokarst
thread.
I'd like to add some comments and fodder to this discussion by going back
to just one of the paleokarst localities mentioned by Jonathan in his first
reply to Bill. Just 100 km west of Denver lies the town of Leadville,
Colorado; the type locality of the Leadville Formation and the location of
one of the most extensively studied mining districts in North America.
The interesting thing about this area is that so many of the geological
problems for YEC's occur together within a relatively small area. Let me
illustrate.
The Mississippian Leadville Dolomite lies on top of 300-400 m of Paleozoic
quartz sandstones, carbonates, and minor shales. In a general sort of way,
one could almost imagine this layer cake sequence of sediments being laid
down in a flood (If it were not for the minor details that include nearly
pure quartz sandstones, salt crystal casts, and interbedded conglomerates
containing pieces of former sedimentary units.)
As described by Jonathan, the Leadville Formation (and to a lessor extent
some of the carbonate layers beneath) contains karst features including
abundant sinkholes, karst towers, solution/stream valleys with up to 30 m
relief, and large multilevel, integrated cave systems containing stratified
sediments and minor flowstone. These karst features are partially filled
by oxidized red mudstone of the overlying Molas Formation. The presence of
extensive karst and the oxidized regolith muds are taken as evidence for a
long period of subaerial exposure.
There is also fairly convincing evidence that during this period of
exposure, pore waters in the Paleozoic rocks down dip from the cave systems
and in deeper parts of the depositional basin were able to leach metals.
These salty, warm, metal-rich fluids migrated updip into the Leadville area
and deposited lead-zinc sulfides in the porous, rubble-filled caves.
(Similar to Mississippi Valley-Type deposits). Some of these sulfides were
also oxidized, which suggests that the caves were still partially open.
(Interested readers are referred to Economic Geology Monograph 7, 1990,
"Carbonate-hosted sulfide deposits of the central Colorado Mineral Belt")
On top of the Leadville-Molas in-filled karst surface are 100-500 m of
organic-rich marine shales. Then things get interesting. Large faults
developed and huge blocks of Colorado crust were uplifted (horsts) and a
central block dropped forming a large trough. Geologists refer to this
tectonic activity as the Pennsylvanian Ouachita-Marathon Orogeny. (Note to
David Campbell: In Colorado, we do put our karst before the horst!). Over
3000 m of red, oxidized, arkosic sandstones and conglomerates were
deposited into this Central Colorado Trough as the uplifts were eroded. In
a subbasin of this trough, about 50 km northwest of Leadville, 2700 m of
halite-anhydrite-gypsum evaporites were deposited. The combination of
oxidized arkosic sediments and thick evaporites suggest that we have
another extensive period of subaerial exposure. (Remember that these
evaporites are sitting on top of the marine shales which overlie the
Leadville Paleokarst surface.)
Southeast of Leadville in the South Park Basin, these arkosic redbeds, are
covered by 1200 to 1800 m of Mesozoic sediments. These sediments include
the Morrison Formation with dinosaur bones, the Dakota Sandstone Formation
with dinosaur footprints, the Pierre Formation with over 800 m of black
marine shales, and the Laramie Formation with coal beds and additional
dinosaur footprints.
Then things got exciting again. All of the aforementioned sediments were
uplifted or downdropped in a bunch of new blocks of crust. We call this
the Laramide Orogeny. Accompanying this mountain building event, over 60
stocks, plutons, or sills intruded into the rocks of central Colorado.
Many of these intrusions were accompanied by hot mineralizing fluids. In
the Leadville area, these new lead-silver-gold ore deposits developed in
more of those Leadville Formation caves, along with fault zones and around
intrusion margins. (As an aside, I'm still waiting for YEC flood advocates
to come up with some explanations for metallic ore deposits. With the
exception of some hilarious speculations by Carl Baugh, I have yet to see
any YEC treatment of this important economic subject.)
Following all of this, the continental crust around Leadville and all the
way south through New Mexico began to rift apart. Huge sections of crust
have downdropped as much as 7000 m in a rift zone that cuts right through
some of the highest mountains in Colorado. (Geologists call this the Rio
Grande Rift). These rift faults expose many of the intrusions.
Now the problem for YEC advocates is this. Someday, they are going to have
to get away from single issues like paleokarst and leave the relatively
simple geologic setting of the Grand Canyon - little more than a stratified
sequence of different sediments - and tackle a tough geologic section. How
can a single one year flood produce in one small area, a sequence of ...
(1) quartz sandstones and carbonates; topped by
(2) an extensive paleokarst surface; filled in with
(3) oxidized muds and sulfides; overlain by
(4) thick reduced marine shales
(5) all cut by large faults which demonstrate that the earlier sediments
were already consolidated and brittle; and then buried by
(6) thousands of meters of oxidized arkosic conglomeratic sandstones;
and/or
(7) thousands of meters of evaporites; all covered by
(8) shales and siltstones containing dinosaur bones; overlain by
(9) quartz sandstones containing dinosaur footprints; drowned under
(10) hundreds of meters of black marine shales; topped by more
(11) quartz sandstones, shales, and coal beds containing additional
dinosaur footprints;
(12) cut again by huge fault zones, again demonstrating that earlier
sediments were hard; and
(13) intruded by multiple stocks and plutons with associated
mineralization, alteration, and metamorphism; and then finally
(14) cut by a large rift zone, which demonstrates that all of the
intrusions were already cooled and solidified?
Steve
_____________
Steven M. Smith, Geologist Office: (303)236-1192
U.S. Geological Survey Fax: (303)236-3200
Box 25046, M.S. 973, DFC smsmith@usgs.gov
Denver, CO 80225
--USGS Nat'l Geochem. Database NURE HSSR Data Web Site--
http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-97-0492/
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