CRETACEOUS-TERTIARY BOUNDARY Ijst the ROCKY MOUNTAIN REGION1
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BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l..¿5, pp. 325-340 September 15, 1914 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY CRETACEOUS-TERTIARY BOUNDARY IjST THE ROCKY MOUNTAIN REGION1 BY P. H . KNOWLTON (Presented before the Paleontological Society December 31, 1913) CONTENTS Page Introduction........................................................................................................... 325 Stratigraphic evidence........................................................................................ 325 Paleobotanical evidence...................................................................................... 331 Diastrophic evidence........................................................................................... 334 The European time scale.................................................................................. 335 Vertebrate evidence............................................................................................ 337 Invertebrate evidence.......................................................................................... 339 Conclusions............................................................................................................ 340 I ntroduction The thesis of this paper is as follows: It is proposed to show that the dinosaur-bearing beds known as “Ceratops beds,” “Lance Creek bieds,” Lance formation, “Hell Creek beds,” “Somber beds,” “Lower Fort Union,”- Laramie of many writers, “Upper Laramie,” Arapahoe, Denver, Dawson, and their equivalents, are above a major unconformity and are Tertiary rather than Cretaceous in age. As this is essentially a stratigraphic problem and not, as some assume, an exclusively paleontologic one, a certain amount of structural data are necessary as a setting for the paleontology. It is proposed, therefore, first, briefly to present the evidence on which this unconformity is predi cated. S tratigraphic E v id e n c e The geological formations here involved are spread widely over the States of New Mexico, Colorado, Wyoming, the Dakotas, Montana, and 1 Manuscript received by the Secretary of the Geological Society June 14, 1914. Contribution to the symposium held at the Princeton meeting of the Society Decem ber 31, 1913, and January 1, 1914. Published with the permission of the Director of the U. S. Geological Survey. (325) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/25/1/325/3413597/BUL25-0325.pdf by guest on 01 October 2021 3 2 6 F. H. KNOWLTON----CRETACEOUS-TERTIARY BOUNDARY adjacent Canadian territory. It is held that the dinosaur-bearing beds above mentioned are separated from underlying beds by a major uncon formity which makes the logical line of separation between Cretaceous and Tertiary. In eastern Montana, eastern Wyoming, and the Dakotas, where far removed from the influence of the Rocky Mountain uplift, the forma tions involved arc approximately horizontal. In a majority of cases within this area the dinosaur-bearing Lance formation appears to rest conformably on the underlying beds, and it is this condition apparently which has led many observers to deny the possibility of the existence at these points of a time interval of any importance. It needs but a mo ment’s reflection, however, to show that because one formation lies in apparent conformity on another, this is not of necessity proof positive that the process of deposition continued uninterruptedly from the begin ning of the first to the close of the second. It may often happen that we must go outside the area where such ap parent conformity obtains for the evidence which shall not only prove the existence of the stratigraphic break, but also the value of the hiatus. It happens, however, that even in the flat-lying beds in the Dakotas there is some evidence of the measure of this time interval. The maximum thickness of the Fox Hills—the formation beneath the Lance—is given as 450 feet, yet in; many places it is 75 feet or less, and in exceptional cases appears to be entirely absent, and the Lance then rests on Pierre. An element of caution is necessary in interpreting this condition. Ine qualities in the thickness of a formation of the well known character of the Fox Hills may be due to erosion or to irregularity of original deposi tion. That this unequal thickness in the Fox Hills is actually due to erosion and not to irregularity of deposition is indicated in at least two ways: First, by the finding of actual erosion surfaces, as, for instance, the one described by Calvert on Grand River, South Dakota, where within a horizontal distance of 500 feet there is an observed vertical cut of at least 72 feet, and other similar occurrences in western North and South Dakota and eastern Montana, and, second, by the difference in the invertebrate fauna in the lower and upper portions of the full Fox Hills section, which is approximately 1,000 feet in thickness. In the type sec tion of Fox Hills at Fox Ridge, South Dakota, the beds show a thickness of only about 325 feet. The fauna in this type section is said to show more or less of a commingling of Pierre forms, and for this reason it was at one time the inclination to abandon the use of Fox Hills as a distinct formation and to regard it as merely a near-shore phase of the Pierre. Where the full 1,000-foot section of Fox Hills is present, as in the Denver Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/25/1/325/3413597/BUL25-0325.pdf by guest on 01 October 2021 STRATIGRAPHIC EVIDENCE 3 2 7 basin of Colorado, it appears, according to Dr. C. A. White, that the invertebrate fauna in the upper portion, while it contains some of the species of the lower portion, is on the whole distinct. This is taken to indicate that the full section of Fox Hills is nowhere present in the Da- kotas, and that the incomplete development of the formation here is to be ascribed to removal of the tipper part by erosion rather than to diminished rate of sedimentation or to stratigraphic overlap. In Worthless Creek Valley, South Dakota, the pre-Lance unconformity is angular as well as erosional, the Fox Hills dipping north at an angle of 4 degrees, whereas the overlying Lance is horizontal. On the Moreau River, near Govert post-office, South Dakota, the Lance is horizontal, while the underlying beds dip northwest at an angle of 10 degrees. At a number of points in the eastern part of Custer County, Montana, the Lance rests on a distinctly eroded surface of the Fox Hills. Likewise, according to Barnum Brown, the same condition obtains on Hell Creek, in Dawson County, Montana. Throughout much of Montana and Wyoming the conditions are the same as those above described, namely: The Lance is found resting on Fox Hills of different thicknesses, often with eroded surface, and in some cases, as at Forsyth, Columbus, etcetera, the Fox Hills is entirely absent and the Lance rests on Pierre and not always on its uppermost member. It has been suggested that in those cases where the Lance rests directly on Pierre the lower sandstones of the Lance may be the fresh-water phase of the Fox Hills. Proof of this contention would be the finding of an area in which there is evidence either of a transition laterally from the purely marine conditions of the Fox Hills through brackish water to the suggested fresh-water facies of the “Lance,” or a barrier separating two such areas of deposition on which neither facies was laid down. If the country within which the Lance is found resting on Pierre was one in which the stratigraphic relations were obscure on account of few or poor exposures, such transition or barrier might possibly have escaped detection; but on the contrary it is a region in which exposures are numerous and ample, and moreover is one in which investigation in recent years has been intensive, but no such condition has been observed. In the cases thus far considered the discordance between the Lance and underlying beds is not always evident, nor is it always conspicuous when present, and hence it is held by some geologists to be of no more importance than unconformities acknowledged to be present at various horizons in the Lance. The essential difference lies in the fact that the unconformities within the Lance are obviously local and can be traced only for short distances, whereas the evidence in support of the pre-Lance Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/25/1/325/3413597/BUL25-0325.pdf by guest on 01 October 2021 3 2 8 P. H. KNOWLTON----CRETACEOUS-TERTIARY BOUNDARY interval is cumulative, since it occurs at the same horizon throughout four great States. Of much greater significance, however, is the fact that this pre-Lance interval marks the boundary between the last of the under lying series of chiefly marine Cretaceous beds and the succeeding exclu sively continental deposits which prevailed thereafter in the Rocky Moun tain area. In other words, with a single exception this boundary marks the final retreat of the marine waters from the Rocky Mountain province. The most complete measure of this pre-Lance unconformity is to be found in the vicinity of the mouth of the Medicine Bow River, in Carbon County, Wyoming. This unconformity was first detected and studied by A. C. Veatch in the vicinity of the town of Carbon, which is about 25 miles south of the Medicine Bow River. Veatch holds that this time interval represents the removal of more than 20,000 feet of strata. The horizon below this unconformity was called “Lower Laramie” by Veatch; but it is now regarded by the writer as the true Laramie,, while the hori zon above was called “Upper Laramie,” now, in the writer’s opinion, proved to be the dinosaur-bearing Lance formation, since it contains the remains of Triceratops. The line then recognized as the boundary be tween these two formations was structurally correlated northwest to the vicinity of the mouth of the Medicine Bow River, and thence up that stream for a distance of some 25 miles above its junction with the North Platte. According to Ball, at a point about 20 miles above the junction the “Upper Laramie” and irLower Laramie” were found in contact with marked angular discordance.