New Data on the Isostatic Deformation of Lake Bonneville By MAX D. CRITTENDEN, JR. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 454-E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Possible causes of deformation Continued Abstract._____________________________ El Isostatic deformation Continued Page Introduction _ _________________________ 1 Coincidence of uplift and load._______________ El6 Acknowledgments _____________________ 3 Degree of isostatic compensation____________ 17 Purpose and methods-_________________ 3 Rate of response_______--__--_-_----_--_---_ 19 Extent of lake____-.-__---________. 3 Geologic chronology___________________ 19 Nomenclature of Lake Bonneville events. 7 Relation between geologic chronology and Identity of the highest shoreline.________ 8 isostatic deformation__________________ 21 Observed deformation._________________ 8 Value of Tr for Lake Bonneville __________ 21 Water load.__________________________ 8 Present rate of uplift_--------_-----_--_- 24 Possible causes of deformation...._______ Comparison with Scandinavia____________ 24 Superficial versus deep-seated effects. Geologic consequences._____-__-.______-___-_____-___ 26 Elastic compression of the crust. 13 Vertical spacing of Alpine and Bonneville shorelines. 26 Epeirogenic deformation.___________ 13 Vertical spacing of Provo shoreline._______________ 27 Regional tilting______________ 13 Recent warping of basin.________________________ 27 Regional doming._____________ 14 Absolute movement on the Wasatch fault________ 27 Local warping_______________ 15 Relation of isostatic uplift to Basin and Range struc- Isostatic deformation_____________ 15 tures_____---___--__--------------_----_---_- 29 Mode of response._____________ 15 Conclusions ________________________________________ 29 Crustal model... ______________ 16 References cited-______--___-----_-_-----__--------- 30 ILLUSTRATIONS Page FIGURE 1. Map of Lake Bonneville-----_------___________________________________-_______-----------__---------- E2 2. Generalized history and nomenclature of Lake Bonneville used in this report______________________----_---_- 7 3. Map showing deformation of the Bonneville shoreline___________________________________________________ 9 4, 5. Maps showing depth of water averaged over circles of 25- and 40-mile radius, respectively__________________ 10, 11 6. Profiles showing observed deformation and average depth of water_-__________-_____------_----__--- 12 7. Comparison of observed deflection with depth of water averaged over various radii___ _ ______________ _. 18 8. Graphs showing interpretations of Lake Bonneville history.-_____-_--__-_____--_--_------_--_--_--- 20 9, 10. Calculated deflection based on 9. Morrison's curves ________________________________ _ _______________________________________ 22 10. Chronology of Searles Lake----__---_-----__---_____--_____--___------------------_--------- 23 11. Graph showing difference between first-order levels across part of the Bonneville basin in 1911 and 1953. 25 12. Diagrammatic profile showing Wasatch fault displacement at the eastern margin of Lake Bonneville- _. 28 TABLE Page TABLE 1. Bonneville shoreline elevations. E4 in SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY NEW DATA ON THE ISOTATIC DEFORMATION OF LAKE BONNEVILLE BY MAX D. CKITTENDEN, JR. ABSTRACT Lake City, and Provo, to the Toana Range, 10 miles Domical upwarping of the area formerly occupied by Pleisto­ west of the Utah-Nevada State line. Its maximum cene Lake Bonneville is verified by 75 new measurements of depth was a little more than 1,100 feet in the main elevation on the Bonneville shoreline. Gilbert's conclusion that northern body near the west edge of the present Great this uplift was an isostatic response to the removal of load is confirmed by maps which show that the deformation is closely Salt Lake. To the south, the lake extended through correlated with the former distribution and average depth of passes between the ranges into what is now the Sevier water. The area of maximum uplift is west of Great Salt Lake, Desert, where its average depth was about 500 feet. where the Bonneville shoreline reaches an elevation of 5,300 Although Gilbert's study (1890, p. 363) consisted pri­ feet, compared with 5,090 feet at the south end near Lund and marily of examining the bars, spits, and beach deposits 5,085 feet at the former outlet in Red Rock Pass in southern Idaho. The 210-foot difference is about 75 percent of that that formed within the ancient lake, he soon perceived theoretically possible if the lake had reached complete isostatic that the water surfaces delineated by these ancient fea­ compensation. tures were no longer level. More than 50 islands and Isostatic movements accompanying and following changes in mountain headlands stood above the surface of the an­ the depth of water would explain the observed differences in cient lake, and on each of these a record of the ancient elevation between the first and second stands at the Provo shore­ line, the recent tilting of the basin floor, and several other fea­ water surfaces was carved. Many shore features were tures of lake history. close to the present Great Salt Lake, so that by using According to the most recent of several chronologies for Lake its surface as a plane of reference Gilbert was able to Bonneville, the anomaly created by removal of the lake water show (1890, pi. 46) that the ancient shorelines had been was reduced to - of its initial value in somewhere between warped upward as much as 180 feet and that the uplift e 4,000 and 10,000 years. On this basis, the calculated viscosity had been greatest near the west edge of the present lake of the subcrust in this area is 10a poises, compared with 1022 where the water of the ancient lake was deepest. Gil­ poises in Scandinavia. bert inferred from this that the earth's crust had been Recent displacements on the Wasatch fault are normal that domed upward in response to removal of load as the is, down on the west; this is opposite in direction to the effects water evaporated.1 But because he did not measure the of isostatic unloading, which tend to elevate the valley block elevations of any points around the west and southwest relative to the mountains. It is inferred that the two types of deformation are independent, one operating within the crust and sides of the basin and because the absence of geodetic the other mainly within the subcrust. control forced him to rely on barometric measurements for elevations in the southern part of the basin, the im­ INTRODUCTION pression has grown in recent years that many of his ele­ In the first monograph of the U.S. Geological Survey, vations were unreliable and that the evidence for iso­ G. K. Gilbert (1890) recorded in remarkable detail the static readjustment was inconclusive (Eardley and widespread traces left by Lake Bonneville, the largest others, 1957, p. 1164). At the same time, the extent of of the Pleistocene lakes of the Great Basin. He showed actual shoreline deformation has been further obscured that the lake at its maximum was some 325 miles long, because most modern studies of Lake Bonneville have 125 miles wide, and had a surface area of about 20,000 1 It has recently been pointed out to me by Franki Calkins that though square miles nearly a quarter as large as the State of the word "isostasy" appears in the index to Monograph 1, it is followed by only a single page reference; but the word is not used on that page, Utah (fig. 1). It extended from the site of Lund, near nor apparently on any other. Indeed, "isostasy" had first been clearly the southwest corner of the State, to Red Rock Pass in defined only 10 years before Gilbert's work was finished (1890), and the principle still was not widely accepted in this country. Gilbert's atten­ southern Idaho, and from the front of the Wasatch tion to shoreline elevations in the field and his awareness of their Range, where it flooded the sites of Logan, Ogden, Salt significance therefore seem all the more remarkable. El E2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 114 C 113 Outline of lake at Bonneville shoreline Surrounding area patterned Existing Lakes 10 0 10 20 30 MILES FIGUEB 1. Map of Lake Bonneville. NEW DATA ON THE ISOSTATIC DEFORMATION OF LAKE BONNEVILLE E3 been restricted to small areas along the Wasatch Range to locate salient lakeshore features on them by photo- front, where the shorelines are comparatively uniform geology. For present purposes, only those on the high­ in elevation. One result is that the figure 5,135 feet has est shoreline (Bonneville) have been used. These fea­ been somewhat loosely used as representing the approxi­ tures were then transferred to a topographic base by mate elevation of the entire Bonneville shoreline. Kail plotter, by projection, or by inspection, depending Today, however, when the topography of this region on the accuracy of the base available. The best results has been delineated by an abundance of aerial photo­ were obtained by using 1:20,000 photographs and base graphs and a rapidly increasing number of large-scale maps at 1:24,000. Where such work has been checked topographic maps, it is possible to establish the eleva­ in the field, as it has been along the front of the Wasatch tion of shoreline features in many parts of the basin. Range and in parts of the Confusion Range, the results The results fully confirm Gilbert's inferences regard­ proved to be nearly as accurate as any that could have ing isostatic response to unloading; they indicate, in been obtained by trigonometric measurement of eleva­ fact, that it was somewhat greater than he supposed. tions. In some places where the shorelines are very They also indicate that broad epeirogenic movements, faintly marked they can be located not only with great­ though probably not negligible, were small compared er ease but with more accuracy from photographs than with deformation of other types.