Origin and Evolution of the Sebree Trough
Stig Bergström
(Department of Geological Sciences, Ohio State University, Columbus, Ohio, USA)
23 May 2000
Several
aulacogens and intracratonic basins are present in the American
midcontinent. One of these is the focus today, the Sebree Trough.
The Sebree Trough’s rocks have been known for a long time, but none are exposed
- they are known from oil and gas drilling. The Sebree Trough is floored
by Middle Ordovician limestones (with K-bentonites) and is filled with dark
shales. Dark shales replace the above-bentonite limestones in the area of
the Sebree Trough (i.e., southwestern Ohio and southeastern Indiana).
The
Sebree
Trough was recognized by Freeman et al. in the 1950s as a regional
feature. A Sebree
Trough distribution map (and see,
also
see) was published by Bergström in the early 1990s.
The
Lexington Platform occurs to the southeast of the Sebree Trough and the Galena
Platform occurs to the northwest of the Sebree Trough. The Trenton Shelf
occurs way to the northeast of the Sebree Trough. The Taconic Foreland
Basin occurs way to the southeast of the Sebree Trough (past the Lexington
Platform).
Drill
cores and seismic info. are available for the Sebree Trough, but the amount of
information is relatively limited.
The
New Point core of Indiana has a dark shale succession below the Kope
Shale. Bergström explored it by splitting for graptolites. The
trough sequence of dark shales had never been dated precisely before.
Bergstrom
got some money to make new cores along a Sebree Trough transect.
The
Elkhorn core is 300-400’ thick, and it was split into mm-thick pieces to look
for graptolites. This was a time-consuming job, done by Bergstrom and
Chuck Mitchell. But, it paid off. The graptolite zonation for the
dark shales in the Sebree Trough turned out to be the same as that seen in
surface rocks in the Mohawk Valley of New York State.
The
Middletown core (from the base of the Cincinnatian Series to the upper Middle
Ordovician) tied the conodont zonation to the graptolite zonation in this area.
The
Sebree Trough originated during the mid-M5 sequence (as defined by the first
dark shales). The Utica Shale in the Sebree Trough is coeval with the
Lexington Limestone and the Galena carbonates to the northwest and
southeast. The Sebree Trough seems to be gone by the ~beginning of the
Edenian Stage (~Clays Ferry Fm.).
Ohio
Geological Survey’s Schumacher plotted up Trenton tops and Trenton isopachs in
Indiana, Ohio, and Kentucky to show the Sebree Trough [can see cross-strike
discontinuities (CSDs) and the Coshocton Fault Zone on this map as well].
The
Sebree Trough extends up to Lake Erie and well into Kentucky, where we run out
of information. The Sebree Trough is ~50 miles wide and very long
(~400-500 miles), and it was presumably open to the ocean in the south during
late Middle Ordovician time. There is some indication that the Sebree
Trough basinal facies continues way south (into western Tennessee).
The
Sebree Trough has been reconstructed as a trough immediately adjacent to the
Cincinnati Arch trend. The Sebree Trough rocks are well-bedded shales
with no carbonate debris flows (laminated shales clearly deposited in
relatively deep water).
Uppermost
Middle Ordovician and lowermost Cincinnatian shallow-water (though shaley)
sediments cover the Sebree Trough - it became covered up by this time.
The
Sebree Trough is on a ~parallel trend with the Reelfoot Rift (further
SW). The Sebree Trough is not parallel to the Grenville Front.
The
formation of the Sebree Trough may have had something to do with reactivation
of old basement structures (part of a late Proterozoic-Early Cambrian graben
system)? The orientation of the Sebree Trough matches that expected for a
relationship with a reactivated old basement structure. There is parallel
and sub-parallel faulting in the area of the Sebree Trough. The Sebree
Trough correlation with basement structure (gravity-wise) is not great, but
there are hints.
If
faulting did affect the its formation, and there was a graben structure, we
would expect offset of the rocks below (older than the Sebree Trough), but maps
of Black River tops show no correlation to the Sebree Trough trend.
Well,
if there was a graben structure, it had to have reactivated after the Sebree
Trough to fit the top-of-the-Black River back to its original position.
This is not likely, according to Bergström. So, we don’t see an offset,
which is strange.
Well,
if it wasn’t a graben (which is the easiest explanation), what is the
Sebree Trough?
One
idea is that it was on one side of a peripheral bulge (a pre-bulge
basin). The bulge would be the Nashville and Jessamine Domes of Tennessee
and Kentucky, caused by nappe loading in the Taconic Orogen. This would
explain the absence of offsets in the subsurface - the Sebree Trough formed by
elastic deformation of the crust.
Is
there a more recent analog to this? The Andean Foreland Basin in Paraguay
and Uruguay is a good Cenozoic model that closely fits this model for eastern
North America during the Ordovician.
There
is a 3rd possible model for Sebree Trough formation (proposed by Kolata).
The carbonate factory in the area was shut off by cold water that came up the
Reelfoot Rift embayment, after phosphate deposition in the Nashville Dome -
Jessamine Dome area.
Was
there a combination of causes?
Need
more well control in western Tennessee to pursue this project further.
One
early author suggested that this was an erosive feature - a submarine
canyon. But, there is good evidence for interfingering, which wouldn’t
happen in a submarine canyon.
Transcript:
Today,
I will talk about some work we have been doing in the North American
midcontinent, and even in Ohio area. Several recent presentations have
been on geologic work done in other parts of the world, so it’s about time to
indicate that something else has been going on in this part of the world.
This is that may be something quite well known to most of you, mainly the major
cratonic basins, and aulacogens, and rifts, and things like this that we have
here in the North American midcontinent. You probably know about the
Oklahoma Basin, the Oklahoma Aulacogen, the Black Warrior Basin, the Reelfoot
Rift, the Illinois Basin, and the Rome Trough, and things like this. The
Michigan Basin isn’t even shown on this map - I don’t know why. But what
I will talk about is something that is also not shown, namely the Sebree
Trough, which is approximately there, which is an undoubtedly major feature in
the geology of the midcontinent.
Now,
the Sebree Trough has been, or at least the rocks, have been known for quite
some time, even if they are not exposed in anyplace I know of in its
area. They have been known from drillings for oil. And, here for
instance, you can see them. This is a portion of a transect here from
Kentucky into Indiana. And all this is a limestone sequence along here,
of Middle Ordovician age. And, up here, we have limestones again.
These lines here represent these major K-bentonite horizons that can be used as
timelines. But, above this limestone sequence in this area here, we have
a shale sequence, and it’s right down here. To the west over here are
limestones again. So, this is a peculiar type of lithology that comes in
in the sequence in the late Middle Ordovician in this area. And it is
perhaps more clearly shown right here. A trough. But, anyway, you
can see it here. And that’s, again, in this area right here.
Limestone here, limestone there, [shale in-between]. So, this is a rather
unique feature in terms of lithology. And it was recognized by Freeman
and others in the 1950s as a regional feature. And, we published in the
early 1990s this map - Chuck Mitchell and myself - on the distribution of the
Sebree Trough here. To the northwest, we have the Galena Shelf, and to
the southeast, the Lexington Platform, extending toward the Appalachian Basin,
and the Trenton Shelf to the northeast. I mentioned that the Sebree Trough
is not exposed anywhere. Its rocks are all covered up. It is only
known from the subsurface. The information we have that has been used is
from drillings, in order to examine the sequence. And, to decipher its
evolutionary history, we have to rely on drill cores. Unfortunately,
there is a relatively small number of drill cores available. You can use
seismic information as well, as we have been doing. Relatively few
drillers for oil would drill through this shale sequence and take continuous
cores.
Now,
in this map, we have 3 cores indicated, in the early part of this project,
probably about 10 years ago now, that were very critical. One was the New
Point core [Indiana] that is kept in the basement of Orton Hall. The
other one was a core in Indiana, the Cook Farm core. And the other, the
third, is the Middletown core - that is also kept here. In the early
1990s, I was involved in a project on the relations between graptolites and
conodont biostratigraphy in the Cincinnatian. In other words, how
graptolite zones in the Cincinnatian extended through the sequence and how
they’re correlated to the conodont zone sequence and the standard units.
And, as you probably know, graptolites are uncommon in the Cincinnatian rocks,
although they occur here and there. But, they’re uncommon. So, I
was looking for additional information, and one day, I got to think about the
New Point core that I had studied, that Dr. Sweet and myself had been studying
for conodonts in the Precambrian base. And it occurred to me that the
shale sequence, known as the Utica Shale in the upper Middle Ordovician of the
New Point core - black shale. And perhaps that was worth exploring for
graptolites. So, one evening, I split up a portion of this core, and sure
enough, there were graptolites in it - nice graptolites. And that was
really the beginning of the Sebree Trough project, because these rocks here in
the trough, these dark shales that constitute the trough sequence, had never
been dated previously. And, graptolites open up the possibility to get an
idea about the timing and the evolution of this trough sequence. Here is
an indication of the ranges of some of the graptolites we found in the sequence
subsequently. And, we developed this as a project in the early 1990s,
more or less, I would say, to gain a better understanding of what was going on
in the trough and how it evolved. Now, it turned out that 2 drill cores
were available. So, I got money from the Petroleum Research Fund to drill
a couple of drill cores, the Viser core and the Elkhorn core, which was done in
cooperation with the Ohio Division of Geological Survey and a couple of the
people there participated in this work. And the Survey had some cores up
here, too. The idea was to get a cross-section across the trough like
this, and see how the trough was in terms of time and lithology and fill
history and general sequence in an effort to try to gain a better understanding
of the evolution of this feature. So, we got a couple of long cores -
this is the Elkhorn core where we worked on the graptolites in the Utica Shale,
the trough sequence here. As you can see, it is 300-400 feet thick and
this was split into millimeter thick slices and bedding planes were searched
for graptolites. This was done by Chuck Mitchell and myself. And,
as you can imagine, it was a fairly time-consuming job, going through thousands
of feet of drilling, looking for these graptolites. But, it paid
off. Paid off well, I would say. Here are a couple of other diagrams
showing ranges of graptolites through the trough sequence of the Utica
Shale. And we could use the same zonation that had been worked out in
sequences in New York state in the Mohawk Valley there. So, the same
graptolites, or largely the same graptolites occurred in these rocks as in the
classical sequence of Mohawk Valley. So, the graptolite zonation that we
used is the same one as up in this area. The spiniferus zone, the pygmaeus
zone, and a couple of other zones that are slightly older. So, we have,
by and large, developed a fairly tight control over this clastic sequence in
the trough.
On
the borders of the trough, we have a little interfingering between the Utica
Shale and the carbonate rocks in the late Middle Ordovician. As shown
here, this is a diagram from the Middletown core in Ohio, and ranges of and
distribution of conodonts that occur in limestones and the graptolites to the
right. So here, we have the possibility to directly tie the graptolite
sequence zonation to the conodont record and the conodont zones that have been
recognized by Dr. Sweet and others in this area. So, with
biostratigraphic information, along with various types of other subsurface
information, like geophysical logs, which prove to be quite important, make it
possible to reconstruct the relations of the trough sequence - the Utica Shale
- to the surrounding carbonate sequences. This is just one illustration
that was published about 5 or 7 years ago on the relation between some of these
drill cores. And, here is another reconstruction, perhaps a little more
clear, of the mutual relations, as we see them, between lithic units here,
graptolite zones, and a little bit about the conodont zones. So, here we
have for the first time now, the time control of this dark shale sequence in the
trough. This is, as you can see here, [starting] above the big bentonites
here, and ends somewhere in the Edenian Stage in the early Upper Ordovician.
Now,
if we go - this is essentially the Cincinnati area, or southwestern Ohio - if
we go further to the south, into central Kentucky, we can, in an essentially
north-south cross-section, reconstruct the relations between the trough and the
carbonate sequence on the Jessamine Dome around Lexington and Frankfort,
Kentucky. And here, we have conodont data - here are mostly graptolite
and conodont data here. Of course, we have mostly graptolite data.
So we begin to get a feeling for facies relations between the trough sequence
in the Sebree Trough and the sequence on the Jessamine Dome in central Kentucky.
I
have here a reconstruction of the biostratigraphy between the trough sequence
here and the carbonate sequence on the Jessamine Dome. And I think the
biostratigraphic control is relatively tight here. We can also recognize
the various sequence stratigraphic units that Holland and Patzkowskiy have
recognized in the Jessamine Dome. From here, and up here, we have
Cincinnatian C1 and we can see on this diagram that the apparent start of the
formation of the Sebree Trough, as indicated by the sudden occurrence of dark
shales is about in the middle of the M5 Sequence of Holland and
Patzkowskiy. And that would correspond to the lanceolatus and
........ zones. Below here we have carbonate rocks belonging to the Black
River Group, or the Tyrone Formation, showing no indication of deep water
deposits. And, in some other areas, we can see that the trough sequence
actually rests directly on these carbonate rocks here. This is a little
futher to the south and we still don’t have a drill core showing this. But
it’s indicated by wire-line logs.
This
is a reconstruction of the Sebree Trough sequence in Kentucky and in Indiana
and in Ohio. And of interest here, perhaps, is the fact that lower part
of the Jessamine Dome sequence, the isopach thicknesses are essentially
parallel to the trough. But, when we get higher up, the thickness
isopachs tend to be perpendicular to the trough sequence. And we
interpret these features here as indicating that there must have been some sort
of faulting, perpendicular to the trough sequence and the Jessamine Dome.
There is some other evidence of this now too. So, summarizing the trough
and its surrounding terranes in one diagram, you can have this picture.
Relatively, almost completely flat-lying, shallow-water carbonate rocks in the
bottom after these bentonites here, which represent time-lines. And then,
the trough formation - here is a limestone terrane of the Lexington
carbonate platform, and here is the limestone terrane of the Illinois
Basin. We have here shale, trough, and graptolites that is between the
Lexington Limestone and the carbonate terrane to the west. The whole
thing The whole thing is overlain by the Kope Formation. And the
differentiation of the trough rocks, the dark brown shales and so on, with the
Kope Formation is in the early Edenian, in the early Cincinnatian - somewhere
here. So, by that time, you have a fairly uniform lithology across, and
it appears that the trough was gone.
Audience question:
What was the thickness of this trough?
The
thickness, as shown previously, varies depending on where you are. I will
talk about that in a second. It’s several hundred feet, maybe 400-500
feet.
The
Ohio Division of Geological Survey has a lot of information of variable quality
from drillings. And, relatively few cores, but there’s lots of
wells. And in these wells, we do have information about the top of the
Trenton Limestone, because that was a target for much of the oil
drilling. So you can get good information about the top of the Trenton Limestone
in most of these wells. And Schumacher, who is working on this project,
looked over about 400 wells like this in order to establish where we have fill,
Trenton Limestone, and over ................. trough forms is shales. And
we got this picture. This is an isopach of the thickness of the Trenton
Limestone. So, this contour line here - the thickness is 30 meters here,
or less. ........... Yeah, right, less than 50 meters here.
So these are the deepest parts of the trough. And here is a .............
You could say, essentially, where is the area where shales occur. As you
can see, it can be traced almost from Lake Erie down into - well into Kentucky,
where we run out of information. So we are talking about a linear feature
here of widths of 50 miles here, or so, somewhat narrower in places, that
extends for a very long distance. Here is a more regional picture -
Indiana, Ohio, Kentucky, Tennessee - and you can see it extends like this, all
the way down towards Arkansas and Ouchitas down here - big feature. Maybe
400-500 miles long, something like that. And, presumably open to the
south during the late Middle Ordovician time. We don’t have a lot of
information from Tennessee, but this is a cross section across the Nashville
Dome in Tennessee and these are shallow-water rocks on top of the dome here in
the center going into somewhat deeper water rocks in eastern Tennessee -
eastern facies. And, to the west, we get into western facies, which is
definitely deeper in western Tennessee and which appears to represent Sebree
Trough rocks. So, there are some indications, but the drill core control
in western Tennessee is very poor at the moment. There is some indication
that it continues way south.
Now,
how about the evolution of the trough in general? I have here a little
diagram. Its showing in a very schematic fashion the growth outlines of
the formation of this very prominent structure. We’re in pre-bentonite
time, pre-........ bentonite time. We have the Black River limestones,
flat-lying like this, showing no indication of any trough development.
Early Chatfieldian time, or Trentonian time if you’re using the old
terminology, we begin to get this formation of black shales that contain no
bottom fauna. There are no fossils except graptolites. This
represents, apparently, a basinal facies - presumably deeper that what we have
up here on the Lexington Dome. This is well-bedded shale. And,
interestingly enough, it shows no debris flows or anything like this.
But, it’s laminated shale, clearly deposited in relatively deep water.
This continues here - this pattern in mid-Chatfieldian time in the latest
Middle Ordovician time - a sequence of considerable thickness here in the
trough and shallow-water limestones on the side. And when we get to very
latest Middle Ordovician time, and early Cincinnatian time, and you begin to
get typical Kope, shallower-water sediments, or shalier sediments, overlapping
the trough sequence. So, by then, it looks like the trough was filled up,
and not a depositional feature anymore, and rather uniform shales and
limestones are standing across this black shale. So, here is the basic
schemes of evolution of this feature.
Now,
the question comes, what is this? What kind of structure is this?
Rather prominent in an otherwise relatively structureless
.................... Here is the Sebree Trough, as we see it, extending
from northern Ohio, across Indiana, into Kentucky, and probably, as I
indicated, lining up with the Reelfoot Rift here in western Tennessee.
The Nashville Dome I talked about, and the Lexington, or Jessamine Dome is up
here, adjacent in a way to this deeper water trough. Now, if you look at
some of the other features, you can see the Rough Creek Graben, which is a
Proterozoic-Cambrian graben structure, and the Rome Trough, which is also a
graben structure as you probably know, of late Precambrian-Early Cambrian
age. The are certainly not oriented, except for part of the Rome Trough,
........................, but the Rough Creek Graben is at an angle to the
[Sebree Trough]. The Reelfoot Rift, on the other hand, has about the same
orientation. So it would seem like, perhaps, the formation of the Sebree
Trough had something to do with these old structures - that could be a reactivation
of the old basement structures. However, it’s not that easy.
Audience question:
How old is the Reelfoot?
It
is Proterozoic - late Precambrian. Here is a map showing the approximate
extent of the Sebree Trough. And there are the faults of the Reelfoot
Rift. And the Reelfoot Rift, of course, is supposed to be an expansion,
formed during the expansion of this area, and a number of..., a lot of faults
developed here. At least the orientation is such that you could expect
that there could be some sort of relationship. Now, we will go to Ohio
and to the structural geology of Ohio. .................. Here is the
trough. Here are various faults that are inferred, mostly based on
subsurface. Here is the Grenville Front. And you can see that there
are some faults that are roughly parallel, but we have some that are at a right
angle to the trough. So maybe there was some fault control.
However, if you look at the gravity anomalies, here is ........... milligal and
minor anomalies - here is the trough. You can see that the correlation
between the trough and the basement, gravity-wise, is not that great.
There may be a little bit of suggestion here, but by and large, if the trough
was associated with basement faulting, you would expect perhaps a better correlation
with the gravity data. Now, there are some other ways to look at
this. And that is to look at the structure contours of the top of the
sequence below the trough - the top of the Black River Group. And if
there was faulting that initiated the formation of the trough, you would
certainly expect there would be offset in the underlying rocks, right?
Well, here is the extension of the trough - the contour lines go right across
without offset, suggesting that the structure of the top of the Black River below
the trough shows no correlation to the orientation of the trough. How do
you explain that? Well, obviously if there was a graben structure during
the formation of the trough, then the faults must have been reactivated again
and the rocks moved up to their original position, which seems rather unlikely
- to have faulting and reactivation again and bringing up the rocks under the
trough to the original position. So the basement faulting idea has not
much support in structure contour maps of the underlying rocks. This is a
more extensive map showing the Sebree Trough here and the structure contours on
top of the Black River in western Ohio and adjacent Indiana. And again
you can see there’s no indication of fault offset in Black River rocks right
beneath the Sebree Trough sequence. Strange. You’d expect an
offset.
Now,
how about other explanations for the formation of this structure, if it was not
a graben structure, which seems to be the easiest explanation? Well, if
we look at it in a more regional context, here is a cross section going from
Indiana down towards eastern Tennessee, across the trough. We have the
trough here, the Lexington Limestone down here, and we get into the Martinsburg
clastic rocks the Appalachian Basin down here, as well as the Base [sp.?]
Formation, in the pre-trough rocks. An early phase of the Appalachian, or
Taconic, Orogeny, or whatever you would like to call it, the Blountian Phase,
is indicated down here in these areas where a lot of clastic material coming in
from the east. So there was orogenic activity down here in late Middle
Ordovician time. This is also indicated by the fact that around here, you
have a lot of bentonites that have been transported from the east - ash
beds. And they came from volcanoes near the plate margin, indicating that
there was some sort of active subduction going on at the plate margin at about
this time. So this is in good agreement with accepting the Base as
material that came from - clastic material that came from an uplifted areas, from
an early orogenic phase. From the late orogenic phase, or the Taconic
Orogeny, which we have up here, where, of course, we have again numerous
bentonites, but not so much up here in the midcontinent. Now, if we view
the events in the present Appalachian area and look for an explanation for the
formation of the trough, you can perhaps apply the migrating peripheral bulge
idea that has, as you know, been discussed very much in recent times, in New
York State and other places. It was loading, thrust-loading, of the
orogen and down-warping of these areas from the loading, and formation of the
foreland basin, and uplift of the other side - called a bulge, and the
formation of a pre-bulge basin. In this reconstruction, this would be the
Appalachian Basin, the Martinsburg Basin [foreland basin] - this would be the
Lexington Dome, or the Jessamine Dome, or the Nashville Dome [bulge], and this
would be the Sebree Trough [pre-bulge basin]. So, this is a possible
explanation to the formation. It would not be a graben formation, but
rather a bulge that was caused by thrust- or nappe-loading out in the orogen,
out in the eastern border of the plate. And this would also explain why
we don’t see any offsets in the subsurface below the trough - that would be
more elastic formation of the trough. Now, do we have any....
There’s another picture here illustrating this particular interpretation -
there would be the Blountian Highlands, presumably in the Carolinas, but I
don’t think anybody really knows - here would be the foreland basin, the
Martinsburg Basin, and here would be this bulge, extending from Tennessee and
Kentucky into Ohio, and here would be the Sebree Trough. And the whole
thing, the cross-section here would be on the order of 500 kilometers.
Now,
is there a more recent analog that fits the dimensions of this model?
Well, one possible analog that some of you may not be aware of is the Andean
Foreland Basin in Paraguay and Bolivia and Uruguay. Here are the Andes
thrust belt in front of the Andes here - here is the foreland basin right here
- here would be the bulge, the fore-bulge - and here would be the back-bulge,
or the Patana Wetlands, that would correspond to something like the Sebree
Trough. And the dimensions are comparable to the figure I showed you a
minute ago. So this is a Cenozoic model of the geology - a cross-section
essentially west-east, but in the opposite way to the Sebree Trough.
Whether it’s right or not, I don’t know, but it is interesting. So the
peripheral bulge model may be a useful one to explain the formation of this, at
least the start of the formation of this trough.
OK, now there is a third
possible explanation that has been proposed recently. So, we have the
faulting model, we have the peripheral bulge model, and here is another model,
and perhaps the most specific model. I am not quite sure that I believe
in this, but I think there may be something in it. It was developed by
Dennis Kolata of the Illinois Survey. Here is the Sebree Trough and the Reelfoot
Rift. And in this model, he feels that perhaps the early initiation of
the Sebree Trough had something to do with movements along the rift here.
And this opened up a canal or wide bay to the Iapetus Ocean, through which
cold, oxygen-poor, and phosphorus-rich water penetrated the midcontinent.
This is kind of interesting because to explain the graptolites in the Sebree
Trough here, we have to rely on some sort of connection in this direction,
because here you have limestone terranes, and it is very unlikely that the
graptolites would come from New York State across the limestone shelf here, but
rather in a deeper water connection to the Iapetus from the Ouchitas down here,
where we get some of the graptolites, migrating in this direction, so that
would provide some sort of support to this model. Now, if you look at the
center here, right here, he indicates a zone of upwelling, deposition of
phosphatic material ..............., and that would explain some features of
the trough, namely that we have phosphatic limestones in the Lexington
Limestone sequence here on the Jessamine Dome. Quite a bit of phosphate,
actually. And Kolata explains the lack of limestone deposition here as
caused by water that was too cold to support carbonate deposition, and oxygen-poor
as well. So, the carbonate factory would be shut off during the existence
of the Sebree Trough, and we would get deposition of fine clastics that were
long-transported. I don’t know if this is the whole story. There is
an interesting story here too, which I will not get into. But, there may
be something in this idea, certainly about the upwelling and deposition of
phosphatic limestones. And, interestingly enough, in the rocks here, in
the lower part of the Lexington Limestone, we have indication that they were
relatively cold-water limestones, whereas these rocks down here - the Black
River Group below the trough - are warm-water tropical limestones. These
would be cooler-water limestones. They don’t have the same
............... character as we found in the rocks below. So it looks
like there was cooler-water stuff here coming up. And actually, that
would also explain how we, in some cases, have cooler water conodonts in these
rocks that are completely missing in the Black River Group. So, Kolata’s
idea in the trough here, I think, has some support. But if this was the
only cause of the formation of the trough, I don’t believe. So, I would
say that at the moment, back to this map again, or a similar map, we really
don’t know why or how this structure was formed. If there is a tectonic
component in its formation, if there was not just graben formation, but
peripheral bulge ................ or if indeed there was something else -
transport of water into a shallow depression or relatively shallow depression,
we can’t say. Maybe it was a combination of all these things.
..............................
