STRATIGRAPHIC ARCHITECTURE OF A SHALOW-WATER DELTA DEPOSITED IN A COASTAL-PLAIN SETTING: NESLEN FORMATION, FLOY CANYON, UTAH by Kimber C. O’Brien A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date___________________________ Signed: _______________________________ Kimber O’Brien Signed: _______________________________ Dr. David Pyles Co-Advisor Signed: _______________________________ Dr. John Humphrey Co-Advisor Golden, Colorado Date __________________________ Signed: ____________________ Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering ii ABSTRACT Shallow-water, low-accommodation, coastal-plain deltas are deposited in wetlands detached from the coeval shoreline associated with broad, low-gradient settings and are an underappreciated architectural style in marginal marine systems. The goal of this study is to document the stratigraphic architecture and evolution of a shallow-water delta in the Neslen Formation, Floy Canyon, Utah and compare it to modern analogs deposited in a similar water depth. A further goal is to contrast the shallow-water delta documented to a comparatively deeper-water delta to test the implications of the numerical model developed by Edmonds et al., (2011). The delta analyzed in this study was deposited in a ~ 4 m deep wetland that was located about 65 kilometers landward of the coeval shoreline. The outcrop of the delta is highly rugose and exposes, in three dimensions, a large portion of the delta: from the feeder channel to the delta front. Upward and lateral facies variations of the delta stratigraphy are evident. The feeder channels contain fine-grained, cross-stratified sandstone. These channels transfer longitudinally to the axial parts of mouth bars that are composed of amalgamated fine-grained, upward- coarsening and thickening beds that contain predominantly ripple laminations. The beds de- amalgamate and become thinner and finer-grained towards the lateral and distal margins of the delta. Stratigraphically adjacent elements in the Neslen delta stack highly compensationally. Within the elements, stratigraphically adjacent stories stack laterally and rotate relative to one another to form a radially dispersive pattern. Younger stories stack progradationally down- current. The shallow-water delta documented in this study has similar longitudinal lithofacies trends and stacking patterns to modern analog deltas deposited in similar water depths (such as iii the Okavango and Wax Lake deltas). When compared to a relatively deeper-water delta, such as the delta documented by Matthew Andresen in the Iles Formation, the lithofacies trends and stacking patterns are different. For example, the shallow-water delta (Neslen delta) has bottomset aggradation only at the distal part of the system, the clinoform angles are shallow (1 to 3 degrees), the foresets were deposited primarily by tractional processes, and the mouth bars stack more compensationally. Conversely, the comparatively deeper-water delta has bottomset aggradation throughout the deposit, the clinoform angles are steeper, the foresets were deposited primarily by gravitational processes, and the mouth bars stack less compensationally. iv ORIENTATION This thesis is half of a paired study, the goal of which is to test the prediction that the initial water depth influences the stratigraphic architecture and lithofacies distribution of river- dominated deltas. Chapters 1 through 4 document and analyze a shallow-water, river-dominated delta deposited on a coastal plain in the Neslen Formation. A companion study by Matthew Andresen, another Masters student at the Colorado School of Mines, documents a river- dominated delta deposited in a deeper-water setting in the age-equivalent Iles Formation. Chapter 5 is co-written by Matthew Andresen and me. Chapter 5 summarizes and analyzes the properties of the Neslen and Iles deltas, thus accomplishing the goals of the paired study. v TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii ORIENTATION ............................................................................................................................. v LIST OF FIGURES ..................................................................................................................... viii LIST OF TABLES .......................................................................................................................... x ACKNOWLEDGEMENTS ........................................................................................................... xi CHAPTER 1 INTRODUCTION AND BACKGROUND ......................................................... 1 1.1 Introduction ........................................................................................................... 1 1.2 Geologic Setting ............................................................................................................. 3 1.3 Data ....................................................................................................................... 4 1.4 Heirarchical Framework for Deltaic Strata .......................................................... 5 1.5 Lithofacies ............................................................................................................ 7 CHAPTER 2 RESULTS .............................................................................................................. 8 2.1 Stories ................................................................................................................... 8 2.2 Elements .............................................................................................................. 10 2.3 Evolution of the Complex ................................................................................... 11 CHAPTER 3 DISCUSSION ...................................................................................................... 13 3.1 Compensational Stacking............................................................................................ 14 3.2 Water Depth ........................................................................................................ 15 3.3 Brackish-Water Influence ................................................................................... 15 CHAPTER 4 MODERN ANALOGS ....................................................................................... 17 4.1 Okavango Delta, Botswana ................................................................................. 17 4.2 Wax Lake Delta, Louisiana, USA ............................................................................. 18 vi CHAPTER 5 COMPARISON OF COEVAL DELTAS .......................................................... 21 CHAPTER 6 APPLICATION ................................................................................................. 25 CHAPTER 7 CONCLUSIONS ............................................................................................... 26 FIGURES ...................................................................................................................................... 28 TABLES ....................................................................................................................................... 50 REFERENCES ............................................................................................................................. 58 APPENDIX A ............................................................................................................................... 62 A-1 Stratigraphic Columns ....................................................................................... 62 vii LIST OF FIGURES Figure 1 Cross section of delta stratigraphy……………………………………………….28 Figure 2 Modified cross sections of delta stratigraphy……………………………………28 Figure 3 Aerial photographs of Okavango and Mississippi deltas………………………...29 Figure 4 Location and geologic maps of the study area……………………………….......30 Figure 5 Stratigraphy of the Book Cliffs…………………………………………………..31 Figure 6 Paleogeographic map of the Book Cliffs…………………………………….......32 Figure 7 Detailed Neslen Formation Stratigraphy……………………………………...…33 Figure 8 Hierarchical framework for deltaic stratigraphy…………………………………34 Figure 9 Photographic examples of lithofacies……………………………………………35 Figure 10 Interpreted photopanels…………………………………………………………..36 Figure 11 Common trace fossils…………………………………………………………….37 Figure 12 Diagram of ripple-laminated sandstone……………………………………….....38 Figure 13 Chart of amalgamation ratios…………………………………………………….39 Figure 14 Isopach map of sand thickness………………………………………………...…40 Figure 15 Paleogeographic reconstruction……………………………………………...…..41 Figure 16 Photograph of coastal-plain stratigraphy…………………………………...……42 Figure 17 Diagram summarizing story characteristics…………………………………..….43 Figure 18 Diagram of compensationally stacking mouth bars……………………………...44 Figure 19 Diagram of brackish-water trace fossils…………………………………………45 Figure 20 Diagram comparing deltas deposited in similar water depths………………..….46 Figure 21 Diagram comparing Neslen delta and Iles delta…………………………………47 Figure 22 Vertical transect comparison of Neslen delta and Iles delta……………………..48 viii Figure 23 Story-scale comparison of Neslen delta and Iles delta…………………………..49 ix LIST OF TABLES Table 1 Comparison of hierarchical classifications………………………………………50