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 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. ..............................