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.  ..............................

 


 

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