GYPSUM DUNES AND EVAPORITE HISTORY OF THE GREAT SALT LAI(E DESERT Utah Geological and Mineralogical Survey Special Studies 2 UNIVERSITY OF UTAH A. Ray Olpin., Ph.D . ., President BOARD OF REGENTS Royden G. Derrick Chairman Spencer S. Eccles Vice Chairman Rulon L. Bradley Secretary George S. Eccles Treasurer Clarence Bamberger Member Reed W. Brinton Member Richard L. Evans Member George M. Fister Member Carvel Mattsson Member Wilford M. Burton Member Leland B. Flint Member Mitchell Melich Member Mrs. A. U. Miner Member A. Ray Olpin President I Uni v. of Utah I Ex -officio Member Lamont F. Toronto Secretary of State, Ex-officio Member Maurice L. Watts Alumni Assoc., Ex-officio Member UTAH GEOLOGICAL AND MINERALOGICAL SURVEY ADVISORY BOARD Mr. J. M. Ehr horn I Chairman U . S. Smelting, Refining, and Mining Co. Mr. J. W. Wade Retired Dean A. J. Eardley University of Utah Dr. C. J. Christensen Uni versi ty of Utah Dean J. S. Williams Utah State University Dean D. F. Petersen Utah Sta te University Dr. L. F. Hintze Brigham Young Uni versi ty Mr. M. P. Romney Utah Mining Association Mr. A. J. Thuli Kennecott Copper Corp., A. I. M. E. Mr. Wa lker Kennedy Liberty Fuel Co. , Utah-Wyo. Coal Oper. Assoc. Mr. L. S. Hilpert U . S. Geological Survey Mr. B. H. Clemmons U . S. Bureau of Mines Mr. J. C. Osmond Consulting Geologist, I.A.P.G. Mr. W. T. Nightingale Mountain Fuel Supply Co., R. M. O. G .A. Mr. LaVaun Cox Utah Petroleum Council Mr. E. 1. Lentz Western Phosphates Inc. Mr. E. C. Knowlton Utah Sand and Gravel Products Corp. Mr. R. S. Stone U. S. Steel Corporation STAFF William P. Hewitt Director Arthur L. Crawford Assistant Director Merriam F. Bleyl Office Manager Linda V.D. Robinett Bookkeeper Hellmut H. Doelling Chief Draftsman Kenneth C. Thorn s on Curator, Library of Samples Vicky Ann Goold Typist GYPSUM DUNES AND EVAPORITE HISTORY OF THE GREAT SALT LAI(E DESERT by Armand]. Eardley, Dean College of Mines and Mineral Industries University of Utah, Salt Lake City, Utah Coring Through the Bonneville Salt Flats, 1960 (A. 1. Eardley) Utah Geological and Mineralogical Survey affiliated with The College of Mines and Mineral Industries University of Utah, Salt Lake City, Utah Publication of this Study supported by the Uniform School Fund of the University of Utah SPECIAL STUDIES NO.2. PRICE $ .75 • DECEMBER,1962 TABLE OF CONTENTS ABS TRAC T 3 INTRODUC TION ••.•••••••••••••.••••••••••••••••••• 5 RESULTS OF CARBONATE RESEARCH. • • • • • • • • • • • • • • • • • • • 5 C 14 DATES ••.•••••••••••••••.•••••••••••••.•••••••• 7 EVIDENCE OF EROSION IN POST-LAKE TIME. • • • • • • • • • • • • 7 POST-BONNEVILLE ISOBASES AND TILTING. • • • • • • • • • • • • • 9 GYPSUM SAND DUNES ••••••••••••••••••••••••••••••• 10 GILBERT LEVEL RECONSIDERED. • • • • • • • • • • • • • • • • • • • • • • • 18 DEVELOPMENT OF CLOSURE IN THE DESERT FLOOR. • ••• • 18 SHIFTING OF SALT CRUS T • • • • • • • • • • • • • • • • • • • • • • • • • • • • 22 SALINE LAKE DURING LAST PLUVIAL (?)........ 23 BIBLIOG AAP IN . • • . • . • . • . • . • . • . 26 ILLUSTRATIONS Figure 1 Basin of the Great Salt Lake Desert •••••••••• 6 Figure 2 Section of the surface layers of sediments of the Great Salt Lake Desert basin. • • • • • • • • • • • • 8 Figure 3 Isostatic rebound of crust in Lake Bonne- ville basin. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 20 Figure 4 Chronology of isostatic rebound of crust in central part of Bonneville basin. • • • • • • • • • • • • • 21 Figure 5 East - west section of the Bonneville salt crust and calcareous sediments immediately beneath.. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 24 Plate 1 Photomicrographs of gypsum crystals in sur­ face clay and in sand dunes I 6 miles west of Krlolls. ......•.......................•...•. 12 Plate 2 Photomicrographs of gypsum crystals from be- low halite layer I Bonneville salt crust. •••••••• 14 -2- GYPSUM DUNES AND EVAPORITE HISTORY OF THE GREAT SALT LAKE DESERT by A. J. Eardley University of Utah ABSTRACT The Great Salt Lake Desert is a lake basin of about the same size and nature as that of Great Salt Lake, and the two are con­ nected by a spillway or low pass about 25 feet above the present level of the lake (4195 feet). Events in the last II, 000 years of the desert basin are inferred from a series of shallow cores taken across the basin, from a series of measured sections of the banks of the canals of the potash works on the west side, from a study of the gypsum sand dunes on the east side, from C 14 dates of the calcareous sediments, and from a consideration of istostatic adjustment and tilting incident to the disappearance of Lake Bonnevi lIe. It is concluded that Lake Bonneville had desiccated to the high­ salinity stage by at least 12,000 years ago, that the last carbona te sediments were deposited 11,000 to 10, 000 years ago, and that the floor of the Great Salt Lake Desert basin hada gradient eastward through the spillway into Great Salt Lake until about 6,000 to 5 ,000 years ago. By this time the spillway site had risen sufficiently to develop closure in the Desert floor, and a few floodings from Great Salt Lake thereafter acros s the spi llway into the incipiently closed Desert basin resulted in the salt now making up the Bonne­ ville salt crust. The salt crust first collected in the central part of the desert floor and later was shifted westward by rain water solution to its present site on the west side of the basin because of con­ tinued uplift on the east. At the time that the salt reposed in the central part of the basin, the eastern side had risen to the point that it began to suffer wind ablation and lost a layer of sediments, 10 to 15 feet thick, so that now the deposits at the surface are 25,000 years old. In the process -3- of ablation, the gyps urn sand dunes formed. Evaporation . at the surface of the plastic clays resulted in a capillary draw-up of the moisture, bringing with it sulphate ions. Probably associated with the capillary advance of the clay water were gypsum - producing bacteria, and myriads of sand-size gypsum crystals grew and are still growing in the top 3/4 inch or so of the clays. By further drying of the clay, these crystals are released to the wind and drift into dunes. Likewise, while the salt crust was in the central part of the basin, the western side suffered ablation down to sediments having an age of 17, 000 years. Gypsum dunes formed there also, but as the salt crust shifted westward incident to tilting, part of the gypsum dunes were bedded down by the salt and form an irregular layer under the salt. It has pre­ viously been supposed that the gypsum was an early pre­ cipitate of a lake that finally dried up where the salt crust now is. -4- INTRODUCTION The present report is an attempt to chronicle the post-Bonneville history of the Great Salt Lake Desert. The desert is a basin of the size and character of that of the Great Salt Lake but differs from it inasmuch as any lake waters that may have existed have dried up, leaving an extremely flat expanse of moist saline clays and a salt crust in the place of lowest elevations. See map of Figure 1. The study concerns mostly the upper few feet of the clays that spread acros s the desert floor. A series of shallow cores was taken along U. S. Highway 40 across the desert, and these, togetherwith measured sections of several pits and canal banks, constitute the basic infor­ mation. A number of C 14 dates of the sediments illumin­ ated the problems of correlation, and revealed areas of probable erosion in post-lake time. Crittenden1s (1961) recent definition of uplift since Lake Bonneville began to disappear ha s made neces sary the reconsidera tion of sev­ eral previous postulates regarding, for instance, the spill­ way between Great Salt Lake and the Great Salt Lake Desert, the Gilbert beach, and the origin of the Bonneville salt crust. A theory is proposed for the origin of the gypsum sand dunes in the desert, and for the granular gypsum layer below the halite salt crust. The research was done by means of a National Science Foundation grant to the writer and Donald L. Graf. Graf is concerned with the mineralogy and sedimentary geochemistry of the sediments; the present report is part of the writer's study of the past lake cycles and the climatic history of the basin. RESULTS OF CARBONATE RESEARCH Dolomite and magnesite-bearing layers have been reported (Graf et al., 1961) in shallow cores from the Great Salt Lake Desert. These layers are indicated in Figure 2, and may be used for correIa tion purposes across the basin. Those that appear to correlate are connected by dashed lines. -5- Figure 1. Basin of the Great Salt Lake Desert. Isobases are on the Bonneville beach and are taken from Crittenden (1961). Gypsum sand dunes are from Nolan (1927). Other lithic characters are also used for correlation, such as oolitic sand and marly hard layers. CI4 DATES + Graf~al. (1961) report a C 14 date of 11,300.-250 years for the dolomi te layer 10 1/2 miles wes t of Knolls, the sample coming from one foot below the surface. From C 14 dates of the sediments of Great Salt Lake and also here in the Great Salt Lake Desert basin, about one foot of sediments has accumulated in 500 to 1500 years. Therefore, at this locality the surface clays would have been deposited approximately 10,000 years ago. Other C 14 dates from pits on each side of the basin were determined for the writer by the LamontGeological Laboratory, and are shown on Figure 2. Those at the east side indicate that the surface calcareous sediments are 25, 000 years old in contrast to 10, 000 years in the central part.