Earth’s sedimentary crust
Ages of Sediments
Sediments are classified chronologically by the fossils they contain. Strata at the very bottom of the geological column show no signs of life. Strata next above contain fossils of the very lowest life forms. Progressing upwardly, succeeding strata contain fossils of life forms growing increasingly advanced. It is evident that lowermost strata are oldest and that those above were laid down one after another as time went on and evolution of life progressed. It is equally apparent that accumulation world-wide took place in the same sequence. Everywhere throughout the world, Archeozoic sediments are topped by Pro-terozoic; Proterozoic are topped by Paleozoic; Paleozoic are covered by Mesozoic and the latter by Cenozoic. Subdividing the sediments according to shorter geological intervals, we find that uniformity of sequence continues to hold good. Thus we find Cambrian strata covered by Ordovician; Ordovician covered by Silurian; Silurian strata topped by Devonian; Devonian topped by Carboniferous; Carboniferous by Permian, etc., etc., on upward to deposits of recent period. Irregularities, unconformities and breaks in the sequence occur only where tectonic forces have created “overfolds” of older strata over younger ones, or where strata of certain periods have disappeared due to ablation.
Intermittent Deposition
The concept that sedimentary rocks were derived from highland sources of older, igneous rocks, inescapably means that sedimentation necessarily was regional and intermittent; that any one region received sediments during some intervals but not during others. Accordingly, one would suppose that an unconformity disclosing a break in continuity of deposition would be considered evidence that during the period of time represented by the unconformity, the area of the latter was not receiving sediment because it was uplifted above sea level. But such does not seem to be the interpretation. It is held that unconformity—the missing strata—was due to erosion. For instance, Dr. R. R. Shrock, in Sequence in Layered Rocks, says (p. 43) that
every unconformity, large or small, and regional or local, was once a surface, and most of the surfaces were once exposed to the action of subaerial weathering and erosion.
He continues to explain (p. 47) that an unconformity “usually implies that a long period of denudation preceded deposition of the materials covering the unconformity. . . .” This admission that all regions were once covered with the missing deposits, contradicts the concept that first one region and then another was covered with sediments derived and transported from highland areas situated elsewhere. The contradiction would not necessarily apply to trivial purely local unconformities; but it would apply to breaks in continuity of geological periods.
By far the greater volume of sedimentary rocks is buried and unavailable for examination. Hence the absence of strata of a particular period in outcrops, railroad cuts, etc., is not proof per se of a “lost interval.” It is probable, and in many instances clearly evident, that the rocks which contained the records of those intervals once existed but were subsequently removed by ablation. As to other missing records, it is probable that they have merely not yet been exposed and found.
However, deposits of so-called “derivative” rocks of successive ages, from Pre-Cambrian upward to recent, as proved by the fossils they contain, are so widely and generally distributed over the globe that simultaneous, planetary deposition, as contrasted with intermittent, regional deposition is strongly indicated. Exceptions only prove the rule. Exceptions are areas where erosion has removed strata of some periods or has continued until underlying basement-complex shields have become exposed or where subcrustal igneous rocks have been extruded up through sedimentary rocks by tectonic agencies.
Mountains and Geosynclines
It seems somewhat paradoxical that the prerequisite for the formation of a mountain range, could have been the prior existence of an elongated submarine trough or geosyncline and the filling of it by sediments transported from other mountains or highlands. Yet such is the basic premise of the popular theory. Daly, in Our Mobile Earth, elucidates the theory by saying: “A geosynclinal prism is the preliminary condition for the formation of any major mountain range.”
The theory, for instance, as to North American mountain ranges, is that the Appalachians, Rockies, Sierra Nevadas, Cascades, and Pacific Coastal ranges, originally were elongated submarine geosynclinal troughs; that the troughs were gradually filled during the ages by detrital sediments derived from elevated lands situated elsewhere; that some time, after filling of the troughs had been completed, the respective areas were uplifted into the present mountain ranges.
Daly says that the sediments accumulated to depths of from 10,000 to 50,000 feet. He explains that, as filling of the troughs progressed, subsidence of the synclines likewise progressed in order that sedimentation to the foregoing depths could result. He says that “as a rule, during the geological past, the sinking of the floor of each trough nicely kept pace with the loading.”
Glacier National Park
Dr. James L. Dyson, in The Geologic Story of Glacier National Park, says relative to the mountains of that Park:
During the Proterozoic Era of Earth history a long, narrow section of North America, extending from the Arctic Ocean southward, probably as far as Arizona and Southern California, slowly sank to form a large, shallow, se&-fiHed trough known as a geo-syncline. Inasmuch as thousands of feet of sediments were deposited, the geosyncline must have continued to sink throughout the period of sedimentation.
He says that the sediments are 20,000 feet thick. Hence he postulates a total subsidence of that amount. He says that after the trough was filled the land was uplifted to form the present mountains. The uplift obviously must have equalled the subsidence of 20,000 feet. The mountains of the Park are still some 13,000 feet above sea level after millions of years of denudation. Although sediments of some periods are missing in the mountains of the Park, sedimentation must have continued from Pre-Cambrian to as late as Cretaceous time, for rocks of the latter period are covered by “overthrust” of older rocks. Dr. Dyson suggests that the missing links are due to the fact that the Park area during the times represented by the missing strata was uplifted above sea level; that later it again subsided. Thus he contradicts his opening statement that subsidence was continuous throughout the total period.