Repeating the cyclothem description (p.441 of Verhoogen et all):
quote:
A typical cyclothem in Illinois (see Figure 8-30) has sandstone and shale at the base, overlain by discontinuous layers of fresh-water limestone, which is overlain in turn by a coal bed. A meter or less of hard flinty clay beneath the coal, called underclay, is interpreted to be the soil beneath the coal-swamp vegetation. The coal is overlain by gray shale with a marine invertebrate fauna and scattered plant fossils (apparently floating vegetation). which grades upward into a variable alternating sequence of marine limestone, black marine shale, and gray marine shale. This marine sequence is overlain in turn by the basal sandstone and shale of the next cyclothem.
Note the phrase: "discontinuous layers of fresh-water limestone".
From TB (message 113):
quote:
When Verhoogen et al state that 'apart from the sandstones the layers are incredibly extensive' this clearly means that the other cyclothem layers, including freshwater limestones and shales are extensive. We already know this because if you dig up a cyclothem anywhere it will approximately have the cyclothem pattern including freshwater limestones and shales and obviously with or without freshwater sandstone.
Now, from Pettijohn's
Sedimentary Rocks, 3rd ed., p. 320:
quote:
Freshwater carbonates In some present-day freshwater lakes friable carbonate earths, designated marl, are forming (Davis, 1900). (...gap...) Marl beds also underlie many freshwater swamp peats, recording an earlier lacustrine stage.
TB continues:
quote:
I agree the paelocurrent SW measurements are from the sandstones. Regardless of the fact that they occur in bands it is clear that they are correlated half way across the continent. If you want to assume they are parallel rivers feel free but I think you must be using Mark's Occam's razor for shaving if that is the case.
I must repeat my Verhoogen et all quote, from message 112:
quote:
Also in the book is (p. 442-443):
The sandstone, on the other hand, occurs mainly in sinuous lenticular bodies, a few hundred meters to a few kilometers wide, and a few meters to 40 meters thick, although some of the sandstone is in thin sheets. The distribution of sandstone in Illinois has been mapped through study of data from thousands of wells, Typical results of this study are shown in Figure 8-32, which shows the sandstone bodies cutting out underlying beds and draped by and overlying bed. The lenticular sandstone bodies are interpreted as channel fillings - The bed load of streams flowing southwest from sources in eastern Canada, Pennsylvania, and New England, toward the sea in the southwestern United states. This interpretation is in agreement with the cross-bedding, which shows that the currents that transported and deposited the sandstone were flowing southwest.
Essentially, this does sound like "parallel rivers"! I'm not sure what your fantasy is.
TB continues:
quote:
It is clear that cyclothem coal seams sit on top of non-marine strata that are correlated half way across the continent. The bands of sandstone were brought in rapidly, presumably the fresh water limestone and shale formed as things settled down. And of course this whole thing is continuous and correlated across the continent with the coal and the upper marine layers.
I am not assigning the flood to these layers only but rather a phase of the flood.
It is clear that there exist vast non-marine beds some of which at least demonstrate correlated paleocurrents and some of which demonstrate vast sheet like extents across significant proportions of your continent. I suggest that these layers formed catastrophically and that this works in nicely with floating mat coal formation being in these cycles. The low relief across sub-continental areas is good evidence of this also.
By breaking things down into phases, it sounds to me, that as we examine the details of the "big picture", you're having uniformatarianism creep in. Instead of one big flood, you are having the seas transgress and regress repeatedly.
My simplified vision of these cyclothem formation is: You have a large low relief area, with a general slope to the southwest. Streams cut into this surface, and deposit the sandstones as channel fill. The non-marine shales are stream flood plain deposits, from when the streams overflow their channel banks. After this (for reasons unclear) large areas of freshwater lakes are ponded. This could perhaps be looked upon as being part of the stream flood plain environment. In these lakes, the freshwater limestone is sometimes deposited. These lakes fill with peat deposits, which are eventually covered by sediments of a sea transgression. The seas recede, and the cycle starts over.
Others have commented on some of your subsequent remarks. What I must again repeat, the only paleocurrents yet documented, are in the fluvial sandstones.
Now, for the record, I quote a big chunk of Verhoogen, et all (pp. 443-449). I pick it up where my previous quote about the stream deposits left off (Not using the UBL code for a quotation, so that the material will be full page width:
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A crucial factor in any explanation of the cyclothems is the interpretation of the linear sand bodies. Many geologists interpret the sandstones as the aggradational bed-load fill of channels cut by streams on the abandoned sea floor of a retreating sea. Other geologists believe that they could have been deposited below sea level, possible as "bar-finger" sands at deltaic distributary mouths similar to those described by Fisk from the mouths of the Mississippi, or in channels scoured by tidal currents. The pattern of the linear sand bodies and the current directions indicated by cross-bedding within them strongly support an origin as the bed load of streams. One might suppose, in this case, that the channel sands indicate periods of lowered sea level, when the streams cut deep channels in the abandoned sea floor. These periods might have been followed by periods of rising sea level, during which the streams filled the channels. The rising seal level might then have let first to coal swamps and then to open-sea conditions as the shoreline advanced landward. The amount of sea-level change required by this interpretation is open to question. For example, at Natchez, Mississippi, about 324 airline km above the river mouth, the alluvial plain of the Mississippi River is 20 meters above sea level, and the river has scoured its bed to a depth of 17 meters below sea level. If the channel at Natches were filled with sand, the linear sand body could be more that 40 meters thick without representing any change in base level.
Number of cyclothems It is difficult to determine the precise number of cyclothems in any one basin, because different authorities group the beds differently. Alternations between limestone and shale, or between coal and sandstone, which one worker regards as the result of minor changes in sediment pattern during a single cyclothem are interpreted by another as truncated cyclothems and are correlated with entire cyclothems elsewhere. In general, however, the cyclothems appear to range from 2 to 20 meters, and to average 7 to 15 meters, in thickness. About 30 cyclothems have been recognized in the Pennsylvanian of Illinois, and at least 40 and possible 50 or more from Pennsylvania and eastern Ohio, where the section of Pennsylvanian rocks is more complete.
Significance of the cyclothems If the usual interpretation of the cyclothems is correct (that is, that the lower part of sandstone and shale overlain by coal was deposited in a continental environment and the upper part of limestone and shale in a marine environment), the alternation of thin beds of coal, limestone, and shale over extensive areas indicates widespread stability. Thus slight changes in sea level or other controlling conditions could affect large areas. A vast area must have been tectonically quite stable during many of the cyclothemic marine invasions to allow such lateral uniformity, along with vertical diversity of lithology. Some regions, of course, were subsiding more rapidly than others. In the regions of even the most rapid subsidence, sedimentation seems to have kept the basins filled to the general level; otherwise they would have had relict lakes during periods of marine withdrawal. The southern Illinois Basin seems to have been such an area.
Any interpretation of the cyclothems must take into account the fact that they occur not only in North America, but also widely throughout northern Europe, in fact wherever late-Paleozoic coal measures occur. This cyclic alternation of continental and marine sediments must, therefore, be due either to worldwide cyclic alternations of conditions of sedimentation peculiar to late-Paleozoic time or to some influence, perhaps of the late-Paleozoic vegetation, that leads to a cyclic accumulation of sediment in the transitional environment. In regard to the latter possibility, it is well to note that cyclic deposition is a characteristic of coal measures of any age, even those that appear to have been deposited in continental basins far from the sea. For example, the Mid-Tertiary Nenana coal field of central Alaska, which accumulated in such a basin, contains at least thirteen cyclic repetitions of sheets of pebbly sandstone alternating with coal or coal and shale.
Theories of origin of the cyclothems Many workers see in the cyclothems evidence of a cyclic alternation of controlling external conditions, either climatic or tectonic; others attempt to explain the cyclothems as the result of an inherent cyclic tendency of sedimentation in the transitional environment even though tectonic and climatic conditions may have been constant or altering at a uniform rate.
J.M. Weller, who first recognized the importance of the cyclothems of Illinois, attributed them to tectonic oscillations, centered primarily in the source areas, Each oscillation, in his view, consisted of a long, gradual subsidence followed by a short, sharp uplift. This tectonic control theory was based on the disconformities that he supposed existed at the base of the sandstone units, which he interpreted as requiring withdrawal of the sea.
Other cyclic tectonic hypotheses invoke variations in the rate of subsidence, with constant sedimentation, or variations in the rate of sedimentation (due to uplift in the source areas) with constant subsidence, to explain the alternation between continental and marine sediments. When sedimentation exceeded subsidence, the shoreline would be prograded and continental conditions would prevail. When subsidence exceeded sedimentation, the sea would flood the coal swamps, and marine conditions would prevail. In such theories, the channel sands are not evidence of widespread disconformities.
Another group of cyclic theories calls upon variations in climate to account for the sedimentary cycles. The most appealing of these is the glacial control theory of Shepard and Wanless. They attributed the cyclothems to eustatic shifts in sea level caused by waxing and waning of great continental glaciers in the Southern Hemisphere continent of Gondwanaland, where there is widespread evidence of repeated Permocarboniferous glaciation (Chapter 13). This theory has great appeal, for it explains sedimentary phenomena in one part of the globe as the necessary consequence of conditions existing in another part, a relation we observe in the Quaternary. If it is correct, we have a means for worldwide correlation in the late Paleozoic, for there should be the same number of cyclothems in all regions that show them. The period of cyclic sedimentation, however, seems much longer than that for which we have direct evidence of glaciation, and those who doubt the glacioeustatic theory emphasize a general difficulty in correlating cyclothems even through a single continent, let alone from one continent to another.
Other climatic hypothesis have appealed not to the indirect effects of glaciation, but rather to direct climatic effects in the source area. Changes to aridity in the source area, for example, were thought to bring on rapid erosion through destruction of protective vegetation cover. When the resulting sedimentary wedge reached sea level, a coal swam would grow on it. Later inundation by the sea during the slow subsidence of the basin would drown the swamp and lead to accumulation of marine shale and limestone.
Contrasted with the theories of cyclic control are the theories, favored mainly by European geologists, and summarized by Duff, Hallam, and Walton (
Cyclic Sedimentation, Elsevier, New York, 1967, pp 148-156), that invoke and inherent cyclic habit of the depositional process. These theories emphasize that conditions in the source area could have been constant and that the depositional basin could have been subsiding tectonically at a uniform rate. In effect, they imply that cyclic variation in tectonic or climatic controls is not proved by the cyclic alternation in sediments; neither is it disproved. A modern analogy might be the overlapping deltas of the Mississippi River. According to these theories, cyclothems would not be contemporaneous throughout the entire area of deposition of the Pennsylvania coal measures, but would consist of bodies of sediment overlapping in space and time.
Some theories call upon variations in the rate of compaction of sediments, which lead to marine inundations followed by progradation of the shore. The sedimentary wedge is built upward to sea level, and at that point is covered by a coal swamp. An abrupt increase in the rate of compaction of the sediment would lead to flooding of the swamp by the sea, and the cycle would then be repeated. Rapid compaction is thought to be triggered by a slight increase in superincumbent load over what the uncompacted sediment, such a peat, could normally support.
Other theories call for changes in the position of rivers to account for the cyclothems. A supply of fresh water from a nearby distributary would prevent a salt-water invasion of a coal swamp at sea level. A shift in the pattern of distributaries cutting off the fresh-water supply would allow seawater to invade the swamp, killing the vegetation. With continued slow subsidence, deposition of shale and limestone would take place on the shallow sea floor, and would be brought to a close only by the advance of deltaic distributaries bringing sand and clay deposition to sea level or above. Coal swamps would accumulate on the delta surface and the cycle would be repeated.
Another theory calls for change in the character of the swamp vegetation through the passage of time. After the se floor is built to above sea level by incursion of sand and clay, the initial coal swamp consists of dense arborial vegetation that keeps out seawater. As the peat beneath the coal swamp increases in thickness and the necessary mineral nutrients are rendered unavailable, the vegetation of the coal swamp deteriorates, and eventually is not able to retard the inflow of seawater. A marine inundation kills the swamp vegetation. With continued subsidence and compaction of peat to coal, water depth increases, leading to open-sea conditions and deposition of limestone and marine shale. These conditions would be brought to and end only by the return to this part of the basin of deltaic distributaries, which would carve deep channels through the previously deposited marine and swamp sediments as the prograde forward between natural levees. Overbank silt and floods of silt and sand through crevasses in the levees would raise the sea floor to sea level, and conditions for a coal swamp would be restored.
It is not yet possible to choose definitely among many of these theories on the basis of the evidence available. Possibly several causes are responsible. Glacially induced sea-level fluctuations may be responsible for some of the widespread cyclothems, whereas the more local cyclothems may be the result of the cyclic character of sedimentation.
The environment of accumulation of the thin, persistent, late-Paleozoic coal beds remains something of a mystery. The cyclothems in general lack barrier-island sands of the sort found in the Cretaceous of the Rocky Mountains. Whatever their origin, if the coal-forming environment had been a relatively narrow swampy belt between a sea on one side and an alluvial plain on the other, it is hard to understand what kept the sea from inundating the swamp. On the other hand, if coal-swamp conditions had succeeded fluviatile conditions over the entire region at the same time, and were replaced in turn at an instant in time by marine conditions, the flatness requited for such widespread changes would not provide the slope for the late-Paleozoic rivers.
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I know, it was a pretty long quote. It just seemed to be a good place to display a rather detailed exposition of the mainstream geologic thought process.
Moose
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BS degree, geology, '83
Professor, geology, Whatsamatta U
Old Earth evolution - Yes
Godly creation - Maybe
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