Nature and origin of the Moenkopi-Shinarump hiatus in Monument Valley, Arizona and Utah Item Type text; Dissertation-Reproduction (electronic); maps Authors Gray, Irving Bernard, 1921- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 23/09/2021 21:04:49 Link to Item http://hdl.handle.net/10150/565592 NATURE AND ORIGIN OF THE MOEMKOPI-SHINARUMP HIATUS IN MONUMENT VALLEY, ARIZONA AND UTAH Irving Biv Gray A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOLOGY In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY In the Graduate College THE UNIVERSITY OF ARIZONA 19 6 1 THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE I hereby recommend that this dissertation prepared under my direction by ______ Irving B. Gray_____________________________ _ entitled _____ Nature and Origin of the Moenkopi-Shinarump_______ Hiatus in Monument Valley, Arizona and Utah_______ be accepted as fulfilling the dissertation requirement of the degree of _________ Doctor of Philosophy_________________________ 7 /? / Dissertation Director After inspection of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:* 55? . 7 — '2rC/~~C> f *This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination. STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in The University Library to be made available to bor­ rowers under rules of the Library, Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: (3. APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: JOHN F. LANCE D>te Professor of Geology NATURE AND ORIGIN OF THE HOENKOPI-SHINARUMP HIATUS IN MONUMENT VALLEY, ARIZONA AND UTAH "" y * Irving B? Gray ABSTRACT The Triassic age Moenkopi Formation, prior to recent erosion, was less than 20 feet to about 450 feet thick in the Monument Valley region of southeastern Utah and northeastern Arizona. The top of this formation is a Triassic age surface of erosion; coarse-grained Shinarump deposits rest upon this surface across the beveled strata of the Moenkopi in much of this region. Each of four members which constitute the Moenkopi Formation in Monument Valley thins in a southeast direction, indicating that formational thinning in that direction may have been, in part at least, a feature of deposition. The surface of erosion is believed to have been formed prior to, and during, deposition of the Shinarump and certain other Lower Chinle deposits. The final form of the surface in Monument Valley probably re­ sulted from Shinarump age modification of a pre-Shinarump valley system, and by later Shinarump age pedimentation which produced a broad erosion surface of low relief across the low Moenkopi hills and older Shinarump valley-fill deposits. ii It is believed that the streams which deposited the latest Moenkopi were incised into the Moenkopi Formation daring a regional up­ lift which brought Moenkopi deposition to a close. These streams may have continued to erode the pre-Moenkopi outcrops of their headwater areas and to transport this material through their valleys much as they had done during Moenkopi time. After a period of time the downcutting must have ended and the coarsest sediments in transit came to rest in the valley bottoms as earliest Shinarump. Valley widening followed as the streams aggraded their valleys during Shinarump time. The valleys were progres­ sively opened upward during aggradation until the streams and their de­ posits coalesced across the interfluve areas and formed the Shinarump blanket deposit. The streams which deposited the Shinarump as valley fills were probably slowly aggrading streams near grade; i.e., a slight but gradual steepening of the longitudinal profile of the streams was required (and satisfied by aggradation) to enable them to handle their loads. As they filled their valleys they progressively approached, and then finally at­ tained graded conditions, when they achieved the vertical position at which the blanket-shaped Shinarump stratum was formed. The Shinarump blanket deposit was emplaced upon the older Shinarump valley fills and adjacent beveled Moenkopi strata by streams at grade; i.e., by streams whose gradients were such that they provided just the velocity required to transport all of the supplied load. The surface upon which this blanket rests is a pediment surface. The Shinarump valley iii fills and pediment mantle were deposited pari passu with the development of the erosion surface in those areas where Shinarump was deposited. Thus, the long lapse of time between the Moenkopi and Shinarump is repre­ sented by both the unconformity and the gravel sheet. iv TABLE OF CONTENTS Page INTRODUCTION.................. 1 Purpose, Approach and Field Methods 1 Geography . ........ ......... 4 Geology.......... .......... 6 Previous iBwestigatioB . ........ 8 Acknowledgments . .......... 8 ■■ ; ' ■ THE BOUNDING STRATA . ........... 10 Moenkopi Formation . ..... ....... 10 Shinarump Member of the Chinle Formation . ............... 17 THE NATURE OF THE EROSION SURFACE . ....................... 20 General Description . ................. 20 Detailed Description . ......................... ..... 21 THE ORIGIN OF THE EROSION SURFACE . ..... ................. 27 General . ....... ...... .... 27 Historical Background ........ 27 Stokes* Ancient Pediment Concept 30 Evaluation of Stokes* Concept ... 34 Moenkopi-Shinarump Ligature ..... 36 THE CHANNELS AND CHANNEL PATTERN ... 41 Channel Morphology ..... 41 Channel Pattern Development . ...... ...... ..... 49 DEPOSITION OF THE SHINARUMP.............. ....... 52 Shinarump Streams...... 52 Shinarump Deposits ................................. 59 SUMMARY AND CONCLUSIONS . ......................... .... 68 APPENDIX— ECONOMIC GEOLOGY . ..... 88 REFERENCES C I T E D ... ......... 93 V LIST OF ILLUSTRATIONS Figure Page 1* General location map . .......... 2 2. Locations of measured sections . ............. 5 3. Isopach map of the Hoskinnini Member of the Moenkopi Formation . ................. ...... 13 4. Isopach.map of the.Lower SiItstone Member of the Moenkopi Formation . ................... 14 5. Isopach map of the Middle Sandstone Member of the Moenkopi Formation . .................. 15 6. Isopach map of the Upper Mudstone Member of the Moenkopi Formation.................. 16 7. Channel segment,in upper Nokai Wash . ............. 23 8. Channel segment on Hoskinnini M e s a ................ 24 9. Channel segment near Comb Ridge ................... 25 10. Previous and present designations of the Shinarump . ................... 61 11. Map showing pinchout areas of some basal Chinle sands... .......... 62 12. Location and direction of transport of the Monument Valley and White Canyon Shinarump ... ... 64 13. Schematic diagram of idealized White Canyon channel system . ......... 65 Plate Page 1. Isopach map of the Moenkopi Formation in Monument Valley . ......................... in pocket 2. Typical mesa exposure in central part of Monument Valley . ................ 12 vi Plate Page 3. Two examples of peneconteoporaneous deformation within the Shinaruap . ...... .......... ....... 18 4. Paleochannels of the Monument Valley Shinaruap ....... in pocket 5. Narrow, flaring D-shaped channel . .................. 43 6. Broad, flaring D-shaped channel . ........... 44 7. Subdued W-shaped channel .......................... 45 8. Broad, subdued ^-shaped channel ......................... 46 9; Broad, shallow swale . ..... 47 10. Two contrasting types of Shinaruap in Monument V a l l e y....... 67 Table Page 1. Field measurement data of the Moenkopi Formation ..... 73 2. Sedimentary rocks exposed in Monument Valley .... 7 vii INTRODUCTION Purpose, Approach, and Field Methods A study was made,of the Triassic age erosion surface at the top of the Moenkopi Formation in the Monument Valley region of Arizona and Utah (fig. 1). It was sponsored by the United States Atomic Energy Commission because of an apparent spatial relationship of certain paleo- topographic features of that surface to important uranium deposits. The purpose of the study was to learn more about the nature of the surface by reconstructing the shape of the erosion surface in greater de­ tail than had heretofore been done, and by recounting, if possible, the manner in which the surface was formed. Field studies were made by the writer from July 1954 to January 1958 while employed originally as a Project Geologist and later as an Area Geologist by