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Evolution of the Southern Kenya Rift from Miocene to Present with a Focus on the Magadi Area

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Evolution of the Southern Kenya Rift from Miocene to Present with a Focus on the Magadi Area Michigan Technological University Digital Commons @ Michigan Tech Dissertations, Master's Theses and Master's Dissertations, Master's Theses and Master's Reports - Open Reports 2007 Evolution of the Southern Kenya Rift from Miocene to present with a focus on the Magadi area Alexandria L. Guth Michigan Technological University Follow this and additional works at: https://digitalcommons.mtu.edu/etds Part of the Geology Commons Copyright 2007 Alexandria L. Guth Recommended Citation Guth, Alexandria L., "Evolution of the Southern Kenya Rift from Miocene to present with a focus on the Magadi area", Master's Thesis, Michigan Technological University, 2007. https://doi.org/10.37099/mtu.dc.etds/321 Follow this and additional works at: https://digitalcommons.mtu.edu/etds Part of the Geology Commons Evolution of the Southern Kenya Rift from Miocene to Present with a Focus on the Magadi Area By Alexandria L. Guth A THESIS submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Geology MICHIGAN TECHNOLOGICAL UNIVERSITY 2007 Keywords: Geology, Geochronology, Remote Sensing, Mapping, Lake Magadi, Soda Lakes, Chert, Kenya Rift Copyright Alexandria L. Guth ii This thesis, “Evolution of the Southern Kenya Rift from Miocene to Present with a Focus on the Magadi Area” is hereby approved in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in the field of GEOLOGY Department of Geological and Mining Engineering and Sciences Thesis Advisor: James R. Wood Department Chair: Wayne D. Pennington Committee Members: William I. Rose, Ph.D. William J. Gregg, Ph.D. Susan T. Bagley, Ph.D. Date: August 9, 2007 iii Alexandria L. Guth August 09, 2007 Abstract The Kenya (a.k.a., Gregory) Rift is a geologically active area located within the eastern branch of the larger East African Rift System (EARS). The study area is located in the southern Kenya Rift between 1° South and the Kenya-Tanzania border (covering approximately 1.5 square degrees, semi-centered on Lake Magadi) and is predominantly filled with extrusive igneous rocks (mostly basalts, phonolites and trachytes) of Miocene age or younger. Sediments are thin, less than 1.5Ma, and are confined to small grabens. The EARS can serve both as an analogue for ancient continental rifting and as a modern laboratory to observe the geologic processes responsible for rifting. This study demonstrates that vintage (as in older, quality maps published by the Kenya Geological Survey, that may be outdated based on newer findings) quarter-degree maps can be successfully combined with recently published data, and used to interpret satellite (mainly Landsat 7) images to produce versatile, updated digital maps. The study area has been remapped using this procedure and although it covers a large area, the mapping retains a quadrangle level of detail. Additionally, all geologic mapping elements (formations, faults, etc.) have been correlated across older map boundaries so that geologic units don't end artificially at degree boundaries within the study area. These elements have also been saved as individual digital files to facilitate future analysis. A series of maps showing the evolution of the southern Kenya rift from the Miocene to the present was created by combining the updated geologic map with age dates for geologic formations and fault displacements. Over 200 age dates covering the entire length of the Kenya Rift have been compiled for this study, and 6 paleo-maps were constructed to iv demonstrate the evolution of the area, starting with the eruption of the Kishalduga and Lisudwa melanephelinites onto the metamorphic basement around 15Ma. These eruptions occurred before the initial rift faulting and were followed by a massive eruption of phonolites between 13-10 Ma that covered most of the Kenya dome. This was followed by a period of relative quiescence, until the initial faulting defined the western boundary of the rift around 7Ma. The resulting graben was asymmetrical until corresponding faults to the east developed around 3Ma. The rift valley was flooded by basalts and trachytes between 3Ma and 700ka, after which the volcanic activity slowed to a near halt. Since 700ka most of the deposition has been comprised of sediments, mainly from lakes occupying the various basins in the area. The main results of this study are, in addition to a detailed interpretation of the rift development, a new geologic map that correlates dozens of formations across old map boundaries and a compilation of over 300 age dates. Specific products include paleo- maps, tables of fault timing and displacement, and volume estimates of volcanic formations. The study concludes with a generalization of the present environment at Magadi including discussions of lagoon chemistry, mantle gases in relation to the trona deposit, and biology of the hot springs. Several biologic samples were collected during the 2006 field season in an attempt to characterize the organisms that are commonly seen in the present Lake Magadi environment. Samples were selected to represent the different, distinctive forms that are found in the hotsprings. Each sample had it own distinctive growth habit, and analysis showed that each was formed by a different cyanobacterial. Actual algae was rare in the collected samples, and represented by a few scattered diatoms. Copyright Alexandria L. Guth v Acknowledgements This project was made possible through various awards granted to Dr. Wood by the United States Department of Energy and the ACS Petroleum Research Fund. I would like to thank my advisor Dr. Wood for providing me with this amazing research opportunity and for agreeing to hold our morning meetings at a nearby coffee shop. L. Kiema at the Magadi Soda Company deserves thanks for letting us borrow his copy of Baker’s 1958 “Geology of the Magadi Area” map when we were unable to attain it otherwise, and for providing us with company core data from the lake. Anthony Mbuthia, another employee of the Magadi Soda Company, also deserves recognition for his help in the field as a guide around Magadi. The list of in-field help would not be complete without gratefully mentioning Billy Wanzala who was our driver every year in Kenya, showed amazing skill in navigating Nairobi traffic, and, most importantly, kept us safe and out of trouble. As for local help, I wish to recognize my committee members, Bill Rose, Bill Gregg and Susan Bagley who provided numerous helpful comments and ideas in our meetings, in addition to Bill Everham who provided very useful feedback on my figure captions for this thesis. Thanks also go to Abass S. A., for his help in providing tips on biologic sample collection and preservation. Lastly, I would like to thank my family, friends and Fraggle for supporting me in a myriad of ways, not the least of being their ability to put up with me in kind and tolerant ways when I was sleep-deprived and a little crazy. vi Table of Contents Abstract _________________________________________________________________ iii Table of Contents __________________________________________________________vi List of Figures ___________________________________________________________ viii List of Files _____________________________________________________________ xiii List of abbreviations and acronyms ___________________________________________xiv Introduction_______________________________________________________________ 1 Purpose ___________________________________________________________________1 Regional Setting ____________________________________________________________4 Literature Review __________________________________________________________ 5 Geologic Introduction______________________________________________________ 13 Methods _________________________________________________________________ 17 Geologic Mapping __________________________________________________________17 Part B: ___________________________________________________________________21 Time Series_____________________________________________________________________ 21 Volume and Petrographic Analysis __________________________________________________ 22 Results and Discussion _____________________________________________________ 24 Geologic Maps_____________________________________________________________24 Rift Evolution and Time Series Images ________________________________________26 Events prior to 10Ma _____________________________________________________________ 26 10 to 6 Ma _____________________________________________________________________ 28 6Ma to 3Ma ____________________________________________________________________ 28 3Ma to 2Ma ____________________________________________________________________ 29 2Ma to 1.5 Ma __________________________________________________________________ 30 1.5Ma to 100ka__________________________________________________________________ 31 Fault Summary __________________________________________________________________ 32 Volume estimates __________________________________________________________35 Magadi-area Sediments ___________________________________________________________ 39 Part II: A Characterization of the Lake Magadi Area ____________________________ 46 Lagoon Water analysis ______________________________________________________46 Volcanic gas estimates ______________________________________________________50 Lake Magadi Biology _______________________________________________________53 Characterization of Lake Magadi Biology _____________________________________________ 58 Conclusions ______________________________________________________________
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