By Edwin D. McKee, Park Naturalist.
THE origin of chert, that flinty, siliceous rock which occurs so commonly as bands or as nodules in many sedimentary rooks, has long been a question of considerable controversy among geologists. Some have contended that cherts represent original deposits of silica, others that they are of secondary origin, formed through the work of underground waters at a time long after that when the encompassing sediments were deposited. Again, some geologists hold the direct or indirect action of plant and animal organisms to be the cause of chert formation, while others suggest that it is entirely the result of inorganic processes. It is today apparent that most if not all of these theories are correct for certain types of chert, and that each type must be considered a problem in itself.
Throughout most of the six hundred foot thickness of sand and limestone which makes up the Kaibab formation of Grand Canyon, silica occurring chiefly in the form of chert is a very conspicuous feature. Many of the locally abundant fossils have shells which are petrified showing that unquestionably in some cases lime has been replaced by silica as a secondary development. Furthermore, many of the abundant concretions and nodules, some of which are developed about sponges, suggest secondary origin. On the other hand, certain cherts which occur as bands and which are chalky white except where a brown iron rust has developed on the surface, are almost surely the result of the original deposition of silica. The prominent bands of this hard but brittle material actually form in the area of Grand Canyon Village nearly 50% of the great massive cliff that constitutes the upper part of the formation. It is this chert and its significance which the writer wishes to briefly discuss in this paper.
If we consider as a unit the massive, sandy limestone which forms the largest cliff of the Kaibab formation, we find that the sea whose deposits it represents varied considerably from east to west in the short distance represented in Grand Canyon National Park. Toward the west the chert bands are at a minimum and fossils of an open sea type are most abundant; toward the east, near the Kaibab trail, the reverse is true. As might be expected, the nature of those sediments continues to change eastward but as yet enough detailed study of them has not been made to justify at this time any statements concerning their character.
A second very noticeable and undoubtedly very significant relationship between chert and fossils is to be found in a vertical section of the formation. If one examines the great cliff of Kaibab limestone along the Bright Angel trail, for example, he finds layers of grey sandy limestone full of concretions, in places, alternating with bands of white chert containing tongues of yellow sandstone. In general chert and limestone are of about equal thickness-averaging about eight feet. The chert is barren of fossils and so also is the lower half of the limestone, for the most part. In contrast to this, the upper part of each limestone bed usually contains abundant fossils, thus forming very definite fossil zones throughout the formation.
From the testimony of the sediments as described above, it appears to the writer that of all the many theories which have been advanced to explain the origin of cherts, that offered by Tarr1 in connection with certain cherts of Missouri is also the most plausible in the case under consideration. Prof. Tarr attributed those beds of chert to silica derived from the land and transported to the sea by streams in the form of a colloid and he noted that areas of low-lying or peniplaned land would be particularly favorable for supplying the increased amounts of colloidal silica. This substance would be precipitated in the ocean, by the action of alkaline salts in the sea water. Fossils that fell into the colloidal mass would be perfectly preserved.
To return to the picture of the Grand Canyon region during the time of the Kaibab Sea (Permian), it is quite definitely known as a result of various studies that the land-areas of this region were at that time remarkably low and flat. This would make a very favorable condition for streams to gather colloidal silica from the rather pure sands and would account for the marked concentration of the chert beds, resulting from its precipitation, on the borders of the sea. This factor in turn suggests a reason for the lack in that area of the remains of lime-secreting animals whose shells are found so abundantly in the less siliceous deposits farther west. To be sure, here and there in the cherts are found marine fossils very beautifully preserved but they are uncommon and as suggested by Tarr, probably fell accidentally into the silica-gel.
The very definite zonation of fossils in the upper half of each limestone layer further demonstrates the plausibility of Tarr's theory. It suggests that the silica was poured in to the sea or concentrated there in rather regular cycles and that these were times when the waters were most unfavorable to the normal life of the ocean, thus driving away or killing all marine animals. As conditions changed to normal once more, lime began to be deposited but the sea life was not as quick to respond. In the course of time, however, the waters were repopulated. Some of the old types of animals and some new ones gathered over the floor of the sea, only to be banished in a similar manner with the next concentration of silica.2
A petrographic study of the white, banded cherts of the Kaibab formation helps to confirm the theory of their primary deposition since dolomite crystals which are quite numerous are seen in some cases to have developed around, hence after the cryptocrystalline silica. The chert as seen in thin-section appears to have roughly regular and intermingling zones of calcarious and siliceous material with a few clastic minerals throughout.
It is hoped that this brief sketch of certain conditions in the Kaibab sea will give something of a picture, and one of reasonable accuracy, of a small part of that most unusual and remarkable period, the Permian. It is realized that with present limited knowledge this description must necessarily be far from complete, nevertheless it is hoped that with continued studies and observations the facts will gradually accumulate to give us a better understanding of true conditions.
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