SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 69, NUMBER 4 EARLY MESOZOIC PHYSIOGRAPHY OF THE SOUTHERN ROCKY MOUNTAINS (With 4 Plates) BY WILLIS T. LEE, PH. D., GEOLOGIST, United States Geological Survey (Publication 2497) CITY OF WASHINGTON PUBLISHED BY THE SMITHSONIAN INSTITUTION JULY, 1918 BALT1M0UK, MI)., V. S. A. » EARLY MESOZOIC PHYSIOGRAPHY OF THE SOUTHERN ROCKY MOUNTAINS By WILLIS T. LEE, Ph. D., Geologist united states geological survey (With 4 Plates) INTRODUCTION This paper results from an attempt to work out the ancient phys- iographic history of a part of the Rocky Mountain region which in Mesozoic time seems to have constituted a fairly well-defined phys- iographic unit. Although the history of neighboring provinces must be considered in connection with this one, it is in many ways one which may appropriately be considered independently and with which others may later be compared. The area to which attention is especially directed includes the mountains of Colorado, which extend southward into New Mexico and northward into Wyoming. This has been called the Southern Rocky Mountain Province by some geologists and the Park Range Province by others. From these mountains as a center, our study will lead us in all directions for data which help to interpret phenomena observed in this province. Stratigraphy has sometimes been called fossil physiography, and a knowledge of ancient physiographic history should be useful in solving some of the difficult stratigraphic problems of this western region. There is a certain uniformity in natural processes which may be relied upon. We may confidently assume that during the Mesozoic era the same laws were in operation that govern the present- day world. Then, as now, highlands were eroded, lowlands were built up by the debris washed onto them, and basins were filled with sediments. It seems clear that physiography might be used to better advantage than it has been used heretofore by the stratigrapher and the historic geologist. I am confident that a study of ancient geog- raphy and of the evolution of land forms will lead to conclusive results in correlation in certain places where other lines of investigation fail. 1 Published by permission of the Director of the United States Geological Survey. Smithsonian Miscellaneous Collections, Vol. 69, No. 4 1 2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 It is not easy to abandon inherited notions, even when it can be proved that they have no foundation in fact. I have found it neces- sary in this study, in order to harmonize relations described in a given locality, to take into account many factors, such as the date at which the description was published, the prevailing belief at the time, and the personality of the author—whether progressive or con- servative ; whether an independent thinker or one fearful lest he stand alone. I have attempted here to keep inherited notions in the back- ground and to carefully distinguish between the facts and their published interpretation. I have attempted to visualize the natural processes by which the observed relations were produced. I have endeavored to follow, step by step, the sequence of events as moun- tains were thrown up in the midst of the sea and have tried mentally to observe the evolution of the mountainous region as it was slowly molded under the forces of erosion, peneplained, base-leveled, and finally again submerged by the sea. I have attempted to follow the processes by which vast quantities of rock waste were transported from highlands to lowlands and spread out uniformly over hundreds of thousands of square miles. In brief, I have attempted to picture the physiographic processes which resulted in the stratigraphic rela- tions exhibited by the sedimentary rocks of Mesozoic age in the Southern Rocky Mountain region. One of the main objects of this study is to develop a logical grouping of the sedimentary rocks and to establish a method of correlation which may be applied in certain places where other methods fail. As correlation by physiographic criteria is somewhat unusual, opposition is anticipated. It is probably inevitable that the familiar arguments of established lines of thought will find wider acceptance than those of an untried line. The stratigraphic geologist has become so accustomed to relying on the paleontologist for corre- lations that he is apt to reject without due consideration any sugges- tion which seems to be at variance with that derived from the fossils. Chamberlin * recognized this attitude of mind when, in urging the merits of diastrophism, he said, " New criteria must not .... be judged solely by their concordance with established systems; certain divergences may be but signs of superiority." No claim is made that the investigation here described is a finished one. There are radical differences of opinion on some of the questions discussed 1 Chamberlin, T. C, Cong. geol. internal. Compte Rendu XII, Ges. Canada, p. 551, 1914. NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS—LEE 3 and there is seeming conflict of evidence. Different classes of data now seem to lead to contradictory conclusions. Conflict of evidence is only another expression for misinterpretation of evidence. There is no conflict when all facts are known, and I am convinced that physiographic principles can be used to great advantage in cor- relating some of the unfossiliferous sedimentary rocks in the moun- tain region. A PRELIMINARY SUMMARY The succession of events outlined in this paper begins with a time in the Carboniferous period when the sea covered the region where the Southern Rocky Mountains now stand. This sea was expelled and the ancestors of the present Rocky Mountains arose in its place. For long ages these mountains withstood the elements but were finally torn down and swept away. Before they had entirely disappeared other lands were elevated farther west and on them mountains were thrown up. Probably these new mountains were high, for a desert developed east of them, perhaps for the same reason that desert conditions prevail now east of the high mountains of California. The moisture from the Pacific was precipitated on these mountains and the streams carried the rock waste out into the desert, where the winds reworked it, piling the sand into dunes, which are now recognizable in their fossil state. A broad depression or valley somewhat similar to Mississippi Valley, except that it drained northward, developed between the new mountains and the ancestral Rockies. In the western part of this valley the dune sand accumulated to great depths and graded off toward the east, covering the lower parts of the older, deeply eroded mountain area, but leaving the hilly parts uncovered. After the sands had accumulated in eastern Utah and neighboring regions to a depth of nearly 3,000 feet, the sea advanced in late Jurassic time up the old valley, across British Columbia and the Mountain States as far south as northern New Mexico and Arizona. Much of the submerged area was nearly flat and the sea was shallow in most places. Around it, especially in Utah, New Mexico, and Colorado, were shallow, partly inclosed bays where gypsum was precipitated by evaporation of the sea water. Extensive beds of gypsum have been found in many places which mark the location of these ancient evaporation pans. The sea was short-lived, and as it retreated, sand drifted over the abandoned areas, covering in some places, but not in all, the gypsum, 4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 the fossiliferous limestone, and other beds which had formed in the sea and near it. The correlation of the fossiliferous marine beds with the gypsiferous strata is regarded as the chief contri- bution offered in this paper, for by this means a narrow zone of rocks, whose age is determined by means of fossils of marine organisms, may be followed far beyond the limits of the fossiliferous beds into unfossiliferous sedimentaries whose age has been in doubt. The area in which these beds are exposed is large and difficult to traverse. It may be many years before this tracing is done and until it is done physiographic data seem to furnish the best available means of correlating the unfossiliferous beds from place to place. The events following the withdrawal of the Jurassic sea are better known than the preceding events. In the epoch next following this withdrawal, the final stages of planation of the Rocky Mountain region were accomplished and on the extensive plain the sluggish streams formed bayous, swamps, and temporary lakes, and spread out the sediments of the Morrison formation, building up a plain which seems to have been almost perfectly graded from New Mexico to Montana, and from central Utah to eastern Kansas. Over this graded plain advanced the waters of the Lower Cretaceous sea and later those of the great submergence in Upper Cretaceous time, which covered the site of the Rocky Mountains and buried their roots beneath its sediments, where they remained dormant until stirred to life by the post-Cretaceous or post-Laramie movement, when the present Rocky Mountains began to rise. PRE-MESOZOIC PHYSIOGRAPHY PENNSYLVANIAN SUBMERGENCE It is not my purpose to consider the geographic conditions of the Rocky Mountain region, prior to the Mesozoic era, further than is necessary for a proper understanding of Mesozoic physiography. During much of the Carboniferous period sea water covered large portions of the area now occupied by the mountains. Marine lime- stone of Pennsylvanian age is abundant in central and northern New Mexico and in central and western Colorado. It has been the belief of many geologists that open sea conditions prevailed in western America during the time that the coal measures were forming in the eastern and central parts of the continent. Statements are frequently made that " in the western part of the United States there are no coal accumulations of this age (Pennsylvanian)." * There is unques- 1 Schuchert, Qias., Textbook, p.