University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 7-1948 Mississippian-Pennsylvanian Boundary Problems In The Rocky Mountain Region James Steele Williams United States Geological Survey Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Williams, James Steele, "Mississippian-Pennsylvanian Boundary Problems In The Rocky Mountain Region" (1948). USGS Staff -- Published Research. 502. https://digitalcommons.unl.edu/usgsstaffpub/502 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. The Journal of Geology, Vol. 56, No. 4, Jul., 1948 MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY PROBLEMS IN THE ROCKY MOUNTAIN REGION' JAMES STEELE WILLIAMS United States Geological Survey ABSTRACT A variety of paleontologic and stratigraphic problems are presented by rocks near the Mississippian- Pennsylvanian boundary in the central and northern Rocky Mountains. Stratigraphic sections of these rocks show diverse interpretations of fundamental concepts of stratigraphy and paleontology. In many places where Upper Mississippian rocks directly underlie Pennsylvanian rocks it is difficult to determine the precise location of the boundary between these units. Formations that straddle the boundary are very useful and satisfactory over large areas. Most geologists use various types of lithologic criteria to distinguish forma- tions, but some appear to rely mainly on faunal data, unconformities, or attempts to trace prominent beds. More uniformity in criteria than now exists for the delimitation of formations is desirable. Surface and sub- surface formations should conform to the same definition. Critical paleontologic studies of several common species and genera, if based on a large number of specimens, might help solve the boundary problem. More correlations based on several lines of paleontologic evidence and less reliance on a few index fossils wouldalso help. Larger and more varied collections of well-preserved fossils stratigraphically located are needed from critical areas. Additional stratigraphic work in this region should be of a detailed nature and should prefer- ably be done in connection with detailed mapping. Ecologic and paleogeographic factors merit more atten- tion. The age significance of unconformities has perhaps been overestimated generally. INTRODUCTION comes well informed on a single small As in many other parts of the United problemor spends considerabletime on a States, the Mississippian and Pennsyl- large problem. There is, however, also vanian rocks of the Rocky Mountain re- roomfor broadlyinterpretive work. Con- gion present many unsolved problems. sidering the vast area of the Rocky These problemsrelate to all stratigraphic Mountains underlain by Carboniferous zones from the base of the Mississippian rocks and the difficultyof access of many to the top of the Pennsylvanian.A group of the exposures, a very creditable of problemsthat involve beds at or near amount of knowledgeof the stratigraphy the Mississippian-Pennsylvanianbound- and paleontology has existed for a long ary are especially interesting because time; but not all of it is published, and they not only show places at which the much that is published is in papers con- geological data are sadly deficient but cerned also with general and economic also involve interpretations and differ- geology,with which papersmany stratig- ences in viewpoints on fundamentalprin- raphers appear to be unfamiliar. This knowledgemust be consideredby anyone ciples of paleontology and stratigraphy. All students of Carboniferousprob- starting work in the Rocky Mountains. lems, especially those who have them- Problemsin the Rocky Mountain Car- selves worked in the Rocky Mountains, boniferous(not all of which will be solved will agree that much geologicwork needs or even reviewed in this paper!) range to be done there. The type of work most from the need for more and better fossils, carefully needed, in the writer's opinion, is not, collected with respect to their however, reconnaissancework but de- geographic locations and stratigraphic tailed work, wherebythe investigatorbe- horizons, to the need for reviews, and perhaps reappraisals,of some of the fun- ' Published by permission of the Director, U.S. damental hypotheses and Geological Survey. Manuscript received February definitions 24, 1948. used in stratigraphy and paleontology. 327 328 JAMES STEELE WILLIAMS Among these last-namedare such things an importanttime and time-rockbound- as definitions of various rock and time ary, as distinct from a lithologic bound- units and the applicationsof these defini- ary. It may (and does) happen to coin- tions in the field; hypotheses of, and fac- cide with distinct lithologic changes in tors in, the correlationof strata; and the- some places but not with important ories of speciesdefinition in paleontology. lithologic changes in others. It coincides Despite the two hundred and thirty or with an unconformity in some regions more years of the existence of the science and not with a recognizableunconform- of stratigraphyand stratigraphicpaleon- ity in others. It is a practical boundary tology, many disagreementsexist in the for mappingin some places, and in others application, if not in the definition, of it is not. Nevertheless, this boundary is many of the fundamentalor near-funda- one of the more important ones in the mental concepts upon which the daily United States. work of the stratigrapher and strati- graphic paleontologist is based. POSITION OF MISSISSIPPIAN-PENN- Whether one considers the Mississip- SYLVANIAN BOUNDARY pian-Pennsylvanian boundary a sys- All who are familiar with the general temic, subsystemic, or series boundary geology of the central and northern depends on the definitionsof a system, a Rocky Mountains know that, broadly subsystem, and a seriesto which one sub- speaking, the Mississippian rocks there scribes and on the applications(or inter- constitute a sequence mainly of lime- pretations) of these definitions in par- stones, whereasthe Pennsylvanianrocks ticular regions and with particular se- constitute a dominantly sandstone or quences of rocks; also involved are the "quartzite"sequence. Between the domi- uses or underlyingpurposes that one has nantly limestone sequence of the Missis- in mind for each of the units, the general sippian and the dominantly sandstone usage throughout the world, the degree sequence of the Pennsylvanianthere lies of relianceand degreeof finenessof inter- a series of thin and in many places alter- continental correlationsof the particular nating beds of sandstones, shales, thin units of rocks under consideration,and limestones, cherts, and other kinds of the breadth of experience one has with rock. In many places this series of rocks the rocksinvolved. All these are variable, contains red or purple beds, material and there is certainly adequate room for from which stains associated beds and at justified disagreements in the weights many exposures the whole series has a and interpretations given each of the reddishtinge. In many places the Missis- above factors and for disagreementin the sippian-Pennsylvanianboundary is with- rank assignedto the units called "Missis- in this series of rocks, some of the beds sippian" and "Pennsylvanian."A defi- being Mississippianand others Pennsyl- nite agreement is not necessary, and it vanian. In other places, however, the would be outside the scope of this paper Mississippian-Pennsylvanianboundary, to present argumentsfor or against any as determinedby fossils, appearsto coin- specific conclusion.The writer considers cide with a lithologic boundary. The that the Mississippian-Pennsylvanian Mississippian-Pennsylvanian boundary boundary is an important boundary in is placed within a series of alternating the United States (more so in some re- thin-beddedrocks-a nonresistantseries gions than in others) and believes it to be -not only in the area here discussedbut BOUNDARY PROBLEMS IN THE ROCKY MOUNTAIN REGION 329 in a far wider area in the western part of Mississippian fossils from beds that the United States. might be considered Lower Morgan. The name "Morgan" has been extended FORMATIONS INVOLVED into the Cottonwood-American Fork re- Early practice.-The variable beds be- gion of the Wasatch Mountains and to tween the Mississippian limestones and other areas in this part of the Wasatch the Pennsylvanian sandstones or quartz- Mountains and has also been used at sev- ites have, in the area under discussion, eral places in the Uinta Mountains. been placed in different formations in dif- In central-western Utah, the nonre- ferent parts of the area. In western and sistant unit of alternating shales, lime- central Montana and in northwestern stones, and sandstones in which the Mis- Wyoming they were generally assigned sissippian-Pennsylvanian boundary oc- to the lower part of the Quadrant forma- curs has little red material. At this place tion and widely, but not universally, con- the unit was called the Manning Canyon sidered Mississippian in age. In west- shale by Gilluly (1932, pp. 31-34). The central and central-northern Wyoming name Manning Canyon has been used and in parts of Montana contiguous to also for units of approximately the