Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2008 Long-term subsidence and compaction rates: a new model for the Michoud area, south Louisiana Clint H. Edrington Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Earth Sciences Commons Recommended Citation Edrington, Clint H., "Long-term subsidence and compaction rates: a new model for the Michoud area, south Louisiana" (2008). LSU Master's Theses. 356. https://digitalcommons.lsu.edu/gradschool_theses/356 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. LONG-TERM SUBSIDENCE AND COMPACTION RATES: A NEW MODEL FOR THE MICHOUD AREA, SOUTH LOUISIANA A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Master of Science In The Department of Geology By Clint H. Edrington B.S., University of New Orleans, 2005 May, 2008 ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Michael “Mike” Blum, for the many resources provided during my study, particularly for his influence in my choice of a thesis topic, which is such a critical issue facing south Louisiana, as well as the many stimulating ideas provided along the way. I would also like to thank committee members Dr. Jeffrey Nunn and Dr. Jeffrey Hanor for their advice and guidance, as they provided much insight from their respective areas of research. Of course, many others contributed to this thesis in many ways, both large and small: I would like to thank Dominion Exploration and Production for micropaleontological data, GEOMAP Company for structure maps, both the Louisiana Geological Survey and the Louisiana Department of Natural Resources for various subsurface data and guidance, both the Department of Geology and Geophysics at Louisiana State University and the National Science Foundation for financial support, and both faculty and fellow graduate students here in the Department, most notably Dr. Blum’s students, i.e. my good friends. Finally, and certainly not least, I would like to thank my family and friends who provided encouragement, support, and inspiration throughout my study, and an outlet when I needed to break away from it. ii TABLE OF CONTENTS ACKNOWLEDGMENTS ………………………………………………………………………. ii LIST OF TABLES ……………………………………………………………………………..... v LIST OF FIGURES …………………………………………………………………………….. vi ABSTRACT …………………………………………………………………………………… vii INTRODUCTION ……………………………………………………………………………..... 1 BACKGROUND ………………………………………………………………………………... 3 Study Area ………………………………………………………………………………. 3 Evolution of the Gulf of Mexico Basin ...………………………………………………... 3 Migration of Depocenters in South Louisiana and the Adjacent Continental Shelf: Miocene – Present ...……………………………………………………………………... 6 Salt Tectonic Controls: Emphasis on the Northern Gulf Margin ...……………………. 10 Compaction …………………………………………………………………………….. 14 Modern Subsidence …………………………………………………………………….. 15 METHODS …………………………………………………………………………………….. 22 Long-Term Subsidence Rate Calculations ……………………………………………... 22 Long-Term Compaction Rate Calculations ……………………………………………. 27 RESULTS ……………………………………………………………………………………… 37 Subsidence and Compaction Rates …………………………………………………….. 37 Long-Term Subsidence Rates ………………………………………………….. 37 Long-Term Compaction Rates ………………………………………………… 37 Structural Cross Section ……………………………………………………………….. 37 First-Order Interpretations .............................................................................................. 40 Differential Movement of Faults and Subsidence Rates ……………………… 40 A Revised Structural Cross Section …………………………………………… 41 Activation Order of Faults …………………………………………………….. 46 Differential Movement of Faults and Compaction Rates ……………………... 46 DISCUSSION ………………………………………………………………………………….. 48 The Michoud Fault ……………………………………………………………………... 48 Rate Comparison: Dokka (2006) and This Study ……………………………………… 48 Subsidence Rates: The Last 100 kyrs ………………………………………………….. 50 The Last 4,000 yrs ……………………………………………………………… 51 The Baton Rouge-Tepetate Fault System as a Model for the Michoud Fault …………. 54 CONCLUSIONS ………………………………………………………………………………. 56 REFERENCES ………………………………………………………………………………… 59 iii APPENDIX: SOURCES OF ERROR: ASSUMPTIONS AND METHODS …………………. 66 VITA …………………………………………………………………………………………… 70 iv LIST OF TABLES 1. Long-term subsidence and compaction rates for the Michoud study area ……..………. 38 v LIST OF FIGURES 1. LandSat image of southeast Louisiana and the Michoud study area …………….……… 4 2. Late Pliocene – Quaternary glacio-eustasy and the northern Gulf margin ……………… 7 3. The Miocene section of south Louisiana and the adjacent continental shelf .…………… 8 4. Locations of the depocenter during the Pliocene – Pleistocene …………………………. 9 5. The Cenozoic northern GOM margin divided into four structural provinces …………. 11 6. Evolution of allochthonous salt within the Eastern Province ………………………….. 12 7. Mechanisms, both anthropogenic and natural, contributing to modern land subsidence in south Louisiana ……………………………………………………………………… 16 8. Holocene RSL curve for the Mississippi Delta region ………………………………… 17 9. Subsidence model from Dokka (2006) for the Michoud area ………………………….. 20 10. LandSat images of the Michoud study area ……………………………………………. 23 11. Structure maps of the study area ……………………………………………………….. 28 12. Schematic representation of “decompacting” a sediment column ……………………... 31 13. Porosity versus depth profiles for south Louisiana …………………………………….. 33 14. Semi-log graph representations of the Dickinson (1953) porosity versus depth curve ... 34 15. Image of a 700 ft section taken from well log 146563 ………………………………… 35 16. Preliminary cross section of the Michoud study area ………………………………….. 39 17. Modification of Figure 11 (A) …………………………………………………………. 42 18. Final cross section of the Michoud study area …………………………………………. 44 19. Fault interpretations projected onto subsidence rates of the three Miocene horizons …. 45 20. Stratigraphic cross section of East New Orleans ………………………………………. 52 21. The New Orleans Barrier Complex ……………………………………………………. 53 22. Fault displacement history of the Baton Rouge-Tepetate Fault System ………………. 55 vi ABSTRACT This study examined the stratigraphic record of the Michoud area in East New Orleans, Louisiana to address questions concerning the magnitude of, and processes leading to land- surface subsidence. The hypotheses under review are based on recent geodetic studies, which challenge the widely held position that modern subsidence is primarily a function of shallow sediment compaction. Testing these hypotheses involved constructing a structural cross section of the Michoud area using well logs, chronostratigraphic data, and fault picks, so as to evaluate differential motion along specific faults through time. Employing ages and corrected depths for three key subsurface horizons, long-term (Middle Miocene to Present) time-averaged subsidence rates were calculated: rates range from -0.140 to -0.177 m/kyrs (-0.140 to -0.177 mm/yr). Long- term subsidence rates are incompatible with those derived from geodetic studies: geodetically derived subsidence rates (-14.2 to -23.0 mm/yr) for the Michoud area are two orders of magnitude greater than long-term subsidence rates. Considering the scale of resolution of respective techniques, caution is advised when comparing respective subsidence rates. Nevertheless, the new subsurface, structural model for the Michoud area suggests reactivation of local faults, including any recent movement of the Michoud Fault, is a transient phenomenon that is likely related to rapid Quaternary sediment loading. It is reasonable to compare mean long- term compaction rates, which is a component of total subsidence, derived from this research to geodetically derived compaction rates of pre-Holocene strata. Using a standard decompaction technique, mean long-term compaction rates for strata residing above the Middle Miocene Bigenerina Humblei horizon were calculated: rates range from -0.0704 to -0.0914 m/kyrs (-0.0704 to -0.0914 mm/yr), which are two orders of magnitude less than geodetically derived, pre-Holocene strata compaction rates (-4.6 mm/yr). The findings of this research, particularly the discrepancy between mean long-term compaction rates derived in this study and pre- vii Holocene compaction rates derived geodetically, raises questions into the interpretations and/or accuracy of the geodetic data for the Michoud area, and therefore, the subsidence rates determined from such data. viii INTRODUCTION Land-surface subsidence is the cumulative effect of processes that operate over different spatial and temporal scales, and at different depths. At its essence, subsidence creates accommodation. Accommodation leads to sediment accumulation, and over a large spatial and temporal scale, subsidence ultimately forms a sedimentary basin. The structural evolution of the Gulf of Mexico (GOM) basin, in particular, has been, and is currently affected by a number of subsidence mechanisms (Kulp, 2000). The scope of this thesis examines two such mechanisms in south Louisiana: faulting and compaction. Recent research on subsidence mechanisms has produced seemingly incompatible results. One view,