A study was made of the joint sets developed within two stratigraphic units of significantly different ages in the Sheep Mountain region, Bighorn County, Wyoming, in an attempt to determine if any pre-Laramide orogeny existed in the area. The field methods used and the relationships exhibited between joint sets are discussed. From the data presented, no significant support of the premise is concluded

Garside, Larry J.

Johnson, Gary D.

Warner, Albert J.

English

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 72 | Annual Issue Article 491965A Study of the Structure and Associated Features ofSheep Mountain Anticline Big Horn County,WyomingGary D. JohnsonLarry J. GarsideAlbert J. WarnerCopyright © Copyright 1965 by the Iowa Academy of Science, Inc.Follow this and additional works at: http://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academyof Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu.Recommended CitationJohnson, Gary D.; Garside, Larry J.; and Warner, Albert J. (1965) "A Study of the Structure and Associated Features of SheepMountain Anticline Big Horn County, Wyoming," Proceedings of the Iowa Academy of Science: Vol. 72: No. 1 , Article 49.Available at: http://scholarworks.uni.edu/pias/vol72/iss1/49332 IOWA ACADE.YIY OF SCIENCE [Vol. 72 Literature Cited Grantz, A., Plafker, G., and Kachadoorian, R., 1964, Alaska's Good Friday earthquake, March 27, 1964: U.S. Geol. Survey Circ. 491. Hodgson, J. H., 1964, Earthquakes and earth structure: Prentice-Hall, Inc., New Jersey, 166 p. Keenan, J. H., and Keys, F. G., 1936, Thermodynamic properties of steam including data for liquid and solid phases: John Wiley & Sons, New York. Richter, C. F., 1958, Elementary seismology: W. H. Freeman & Co., Inc., San Francisco, 768 p. U.S. Coast and Geodetic Survey, 1964, Preliminary report, Prince William Sound, Alaskan earthquakes, March-April 1964: U. S. Dept. Com-merce, 100 p. Vorhis, R. C., 1955, Interpretation of hydrologic data resulting from earth-quakes: Geologische Rundschau, v. 43, p. 47-52. Waller, R. M., Thomas, H. E., and Vorhis, R. C., 1965, Effects of the Good Friday earthquake on water supplies: Jour. Am. Water Works Assoc., v. 57, no. 2, p. 123-131. A Study of the Structure and Associated Features of Sheep Mountain Anticline Big Horn County, Wyoming' GARY D. JOHNSON, LARRY J. GARSIDE ,ALBERT J. WARNER Abstract. A study was made of the joint sets developed within two stratigraphic units of significantly different ages in the Sheep Mountain region, Bighorn County, Wyoming, in an attempt to determine if any pre-Laramide orogeny existed in the area. The field methods used and the re-lationships exhibited between joint sets are discussed. From the data presented, no significant support of the premise is concluded. INTRODUCTION The Problem It is generally accepted that a study of regional tectonic ele-ments such as folds, faults, and joints will suggest genetic con-nections with the standard stress-strain distribution patterns. If an area under study is not too complex, the type, relative age, and direction of diastrophic forces may be proposed. The goals of this study were two-fold: 1) ·To describe the tectonic and geologic setting of the area surrounding Sheep Mountain Anticline, Bighorn County, Wy-1 Funds for this study were made available through a grant from the National Science Foundation. 1Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 19651965] STRUCTURAL STUDIES 333 oming, by geologic and structural mapping, and the gathering of data on structural features (joints and folds). 2) To interpret such a tectonic study, and suggest the possible characteristics of the applied deforming stress in the area. The Area Lovell S. E. quadrangle is located entirely within the Bighorn Basin of the Middle Rocky Mountain physiographic province of Wyoming (fig. 1). The basin is underlain by up to 3000 feet of Paleozoic strata, 1500 feet of Triassic and Jurassic strata, 9000 feet of Cretaceous strata, and in the central and western parts by several thousand feet of Paleocene and Eocene strata. The · h:; sin region acted as a stable shelf until Cretaceous time when considerable subsidence occurred due to the active Cordilleran area to the west. Wyoming SCALE (mRn) Figure 1. Index Map of Wyoming showing area of study. Lovell S. E. quandrangle lies in the northeastern part of the basin. Sheep Mountain Anticline is the most prominent topo-graphic feature. Sheep Mountain rises some 900 feet above the surrounding basin and is a breached, plunging, asymmetrical anticline. It is roughly 15 miles long, m~ miles wide, and trends N45W along most of its length, plunging N28W and Sl4E. The dip of the western flank ranges from 30 degrees to. slightly over-turned, while on the western flank, the steepest dips are more than 40 degrees (fig. 2) . 2Proceedings of the Iowa Academy of Science, Vol. 72 [1965], No. 1, Art. 49http://scholarworks.uni.edu/pias/vol72/iss1/49334 IOWA ACADEMY OF SCIENCE [Vol. 72 ....... after RIOUX (1958) Figure 2. Cross sections at selected po:inits on Sheep Mountain (see Figure 3). The Bighorn River is superimposed upon the structure and cuts the anticlinal axis at one-third the distance from the north-ern end. Several other flexures, some quite sharp, are closely associated with the major anticline (fig.3). Its axial trend and those of the other structures roughly parallel the NW-SE trends of the typical fold structures of the Wyoming Rockies. (fig. 5A) The formations exposed on, and in the vicinity of Sheep Mountain range from the Madison Limestone of Mississippian age to the Fort Union Formation of Paleocene age. These are mostly marine sediments. (fig. 4) . Pru:NcIPLES INVOLVED AND PROCEDURES UsED Joints and Faults To better understand joints and faults, let us consider a sheet of rock being folded by compressive forces. Generally, two types of joints will be formed; shear-joints making an acute angle with the deformative stress and tension-joints parallel to this stress. Following this, a secondary stress condition could form as a result of the elastic bending of the sheet. In this stage a set of shear-joints may form which have their acute angle bisected by the fold axis. Also, a set of tension-joints may form parrallel to the fold axis. In addition, friction shear-joints may develop parallel to the fold axis. This latter system is formed due to op-3Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 19651965] STRUCTURAL STUDIES Rll4W STRUCTURE ·,_ '::t: ~~ .. O..!iillit-_.....". '"""'",. ~-0 1 2 Figure 3. Structural map of study area. RQ3W 335 T 5 5 N T 5 4 N T 5 3 N posing stresses directed on the upper and lower surfaces of the rock sheet. Finally, release tension-joints might form after the stress has dissipated. They will be either parallel or perpendicular to the fold axis, depending upon whether they release the main or secondary stresses ( deSitter, 1956). It is unlikely that all of the above sets of fractures and joints will be present in any one folded rock unit. Whichever sets are 4Proceedings of the Iowa Academy of Science, Vol. 72 [1965], No. 1, Art. 49http://scholarworks.uni.edu/pias/vol72/iss1/49336 IOWA ApADEMY OF SCIENCE [Vol. 72 GEOLOGIC AG" FO:l:.tATl0t1 SYMBOL THICKNESS {RIOUX) EOCENE: To!mo:i Tl Wi!IVl(!C;i Tw PALEOCENE Fort Union Tfo 3500' Lon ca Kl 600.t: UPPER Mcetc::is~ Kmo •co" CRETACEOUS Mesa Verde Kmv 12so:ot: Cody Keo 2230 Frontier Kf 600 Mowr~ Km 330 Li.)\;:::rt Thermopolis Kl 500 CRETACEOUS Clovctl'{ Kol 620 Morrison Jm JUR,'\SSIC Sundcnce "' 370 Gyp~um Spring Jgs 200 TRIASSIC Chugwater TRc 600 PC.Mi.iO ... TAIASS IC Embor l'-TRo 270 P.Er::~3YLVANIAN Tend:i!CP Pf 105 Amsden Pa 170 MISSISSIPPIAH Madison Mm 7BO DEVONIAN Jeffimon Dj IBO ORDOVICIAN Bighorn Ob 410 Gallatin Co 530 CAMElRIAN Gros V~nlre c;v "o Flot Hood (;fh 120 Pi\£ .. CAMBRIAN ...... Mainly 9ronllas, P'~motltes, tehl•h, ord o variety of dork colored dike rockt. Figure 4 Generalized stratigraphic chart of area surrounding Sheep Mountain (after Rioux). dominant are probably determined by lithology, as well as the position on the structure (the stress field) under consideration. Field Procedures The major concern of this study was to determine whether or not indentifiable joint patterns had been produced both by pre-Laramide and Laramide tectonic activity. It was assumed that, if two widely separated periods of orogeny took place, a system of fractures would occur in the older rock unit which would not appear in the younger beds. Since both units would have been subjected to the compressive forces of the Laramide orogeny, any significant difference in fracture patterns would suggest the possibility of some pre-Laramide deformation. Two competent units were selected for this study, uppermost sandstoRe member of the Tensleep Formation and the Greybull sandstone member of the Cloverly Formation, because of their mutual age relationships to two separate periods of crustal movement. The Tensleep is Pennsylvanian in age and the Cloverly is lower Cretaceous. The presence of a regional angular unconformity between the Tensleep and "Embar" Formations suggested the possibility of a post-Tensleep orogeny which 5Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 19651965] STRUCTURAL STUDIES 337 might be evidenced in a fracture orientation. The Laramide orogeny, during which the major folding in the area took place, is post-Cretaceous in age. Another determinant in the consideration of the rock units was their relatively equal properties of thickness and competency (fig. 5B and 5C). The reason for this consideration is quite obvious; rocks possessing different physical properties would necessarily behave differently under similar conditions of applied stress. The final factor in the consideration was the fairly uniform distribution of the units throughout the study area. In order to obtain an unbiased and representative sample of fractures, areas of observation were decided upon prior to field inspection. These areas were located at definite intervals periph-eral to the main axis of Sheep Mountain. The Tensleep forma-tion was observed at one-mile intervals along the flanks of the mountain. An attempt was made to completely encircle the mountain with accurate observations made at these intervals. The same procedure held true for the Cloverly. In the northern part of the area mapped exposures of the Cloverly were often found more closely associated with minor structures than with Sheep Mountain anticline. These exposures, however, were considered in the analysis. Method of Analysis Once an exposure was found, the analysis of fractures pro-ceeded as follows: A 15' x 15' grid was plotted on the outcrop face, and traverses were made across it at intervals of three feet. \i\'hen completed, the grid appeared to be composed of 25, nine-square-foot sections. Wherever a fracture or joint was noted to cross a grid line, its strike, length, and, if possible, dip and dip direction were recorded. The regional dip and strike were also recorded. Care was taken not to make multiple read-ings of the same joint. DISCUSSION To accomplish the first goal of the study, geologic and struc-tural base maps were prepared of the area, from considerable field reconnaissance and aerial photo interpretation (figs. 3 and 6). The strike and frequency of the various fracture and joint sets were plotted. Several interesting relationships were revealed (fig. 7). In most of the exposures studied, especially near the axial plunges of the main structure, there is a definite major east-6Proceedings of the Iowa Academy of Science, Vol. 72 [1965], No. 1, Art. 49http://scholarworks.uni.edu/pias/vol72/iss1/49338 IOWA ACADEMY OF SCIENCE [Vol. 72 A B Figure SA Looking southeast along axis of Sheep Mountain anticline from NE'4, NW'4, Sec. 18, TS4N, R94W. SB. Typical exposure of uppermost sandstone member of the Tensleep showing joint sets. 5C. Typical exposure of Greybull sandstone member of the C!overly showing joints sets. 7Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 19651965] STRUCTURAL STUDIES j39 west trend noted in the joint set. These joints intersect the stritc-tural axes at acute angles ranging from 35 to 50 degrees. Another major joint set is noted trending roughly parallel to the structural axes. These joints vary from N75W to N25W, and appear to be somewhat more prominent in the Tensleep over a larger portion of the area mapped. Occurrences of these joints within the Cloverly are relatively equally distributed over the entire area. A minor joint set which trends roughly north-south exists in the southeast portion of the area mapped. Here the southeast axial plunge of Sheep Mountain is intersected by two closely associated fold structures from the north (Crystal Creek Antic-line, etc.) .. In this area, joints of this set are about equally dis-tributed in exposures of both Tensleep and Cloverly formations. R94W ... =----.. ~---,.. ~ "J""" .,_.... .__ ____ ...__, """c:::J'--·'-·-~- ..... •c.. c:::l._ __ ~..,.. ""l.:::J___ ... c::::i ... - .... ft.= :::JCao!I% ... c:J~-·- .... b i'..."::::l"-k "ftoi. C::J~~ r.. C.:::J~.... ._,. C"-:J~ .. 11-.. i :J...,.~""' ""'c:_:l_....,.,.,._ "'-I "J..._..... ,.. c::::J a--...._ c. I "J,___ .._ C.:J __ ..._ Figure 6 Geologic map of study area. Ir: T j';/ ; 8Proceedings of the Iowa Academy of Science, Vol. 72 [1965], No. 1, Art. 49http://scholarworks.uni.edu/pias/vol72/iss1/49340 T 5 5 N T 5 4 N'-, T 5 3 N IOWA ACADEMY OF SCIENCE Joint STRUCTURE -Fr12quczncy w•lh rose dlagram1 0 R93W ' ' [Vol. 72 ' \ Figure 7 Map showing joint frequenCies at selected points of study with relation to structural axes. The acute angles formed by the joints with respect to the struc-tural axes are of the same magnitude as those noted for the east-west trending joints as before mentioned. Another set of joints, also minor in importance, is noted within the area. This particular set is approximately perpendicular to the strike of the structural axes, and trends from N25E to N65E in various exposures. Over the entire area, its occurrence is' rather uniform in the Cloverly, but sporadic in the Tensleep, occurring in appreciable numbers within the axial plunge areas of the major fold structure. Other joints of varying strike wen;, ,IJ,Oted, but relationship to an important set seemed unlikely. These were regarded as unim-portant. In the northeast portion of the area mapped, three large very 9Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 19651965] STRUCTURAL STUDIES 341 good exposures of the Greybull sandstone were studied. The trends noted here belong to only two sets; north-south and east-west. In this area, due to the orientations of the fold axes of the minor structures with which they are associated, both were considered major in importance (fig. 7). The north-south joints corresponded roughly to the parallel set and the east-west joints corresponded roughly to the perpendfoular set as noted before. In one exposure a large set was noted which did intersect the fold axis of the anticline structure (Crystal Creek) at an acute angle. SUMMARY In general consideration of the various joint sets associated with both the Tensleep and Cloverly Formations, little distinction can be noted. Since there is little difference between those sets associated with the Tensleep and those associated with the Cloverly, and in view of the stratigraphy between these two formations, one conclusion may be readily drawn. As stated, there is no reason to believe that once a strain sys-tem has been set up within a rock, further strains, as a result of renewed stresses would not find their development along the previously existing strain patterns. This phenomenon could have an influence on all subsequently younger sediments and as a result, similar strain patterns or fracture systems would be set up. The authors believe that such was the case here. If some pre-Laramide orogeny did exist in the area, as is indicated by the presence of several unconformities, it was sufficient enough to stress the older, in this case, Pennsylvanian Tensleep sediments to some characteristic strain pattern. As subsequent, and eventual Laramide stresses affected the area, their release was found along these previously existing strain patterns. Hence, we see many similarities in the orientations of the fracture systems. Since we are dealing with an area of rather limited areal extent, it seems a bit futile to attempt to relate the joint sets exhibited in the area to regional stresses without first studying a wide variety of exposures over a much larger geographic area. It is interesting to note, however, that the deformation which came about in the area as a result of the Laramide orogeny was probably caused by compressional forces from the west thrusting the miogeosyncline of the Cordilleran onto the stable shelf in the latter phases of mountain-building. The authors would like to consider the strain system so devel-oped as a result of the secondary stress conditions which devel-oped with the folding of the local structures. Therefore, one can assume from the relationships presented that the system 10Proceedings of the Iowa Academy of Science, Vol. 72 [1965], No. 1, Art. 49http://scholarworks.uni.edu/pias/vol72/iss1/49342 IOWA ACADEMY OF SCIENCE [Vol. 72 represented suggests: 1) shear-joints developed at acute angles to the fold axis (north-south and east-west trending joints); 2) tension-joints developed parallel to the fold axis; 3) release tension-joints developed either perpendicular or parallel to the fold axis; and possibly 4) friction shear-joints on the flanks of the fold struchues oriented parallel to the axis. From the data gathered, it is not possible to support our earlier concept that a detailed mapping of fracture patterns in represen-tative, selected strata would prove more than one period of tec-tonic activity. ·we would not conclude that the concept has no merit, but rather that the scope of our study produced incon-clusive i;esults. ACKNOWLEDGMENTS Appreciation is extended to Dr. Keith M. Hussey, Professor and Chainnan, Department of Geology, Iowa State University, under whose supervision the study was planned and developed. Literature Cited deSitter, L. U. ( 1956) Structural Geology; McGraw-Hill Book Co., Inc., New York, pp. 122-134. Rioux, R. L. ( 1958) Geology of the Spence-Kane Area, Bighorn County, Wyoming; unpublished Ph.D. thesis, University of Illinois. Wyoming Geological Association ( 1947) Guidebook, Bighorn Basin Field Conference. 11Johnson et al.: A Study of the Structure and Associated Features of Sheep MountaiPublished by UNI ScholarWorks, 1965

A Study of the Structure and Associated Features of Sheep Mountain Anticline Big Horn County, Wyoming

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A Study of the Structure and Associated Features of Sheep Mountain Anticline Big Horn County, Wyoming

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