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Analysis of Salt-Sediment Interaction

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Analysis of Salt-Sediment Interaction ANALYSIS OF SALT-SEDIMENT INTERACTION ASSOCIATED WITH STEEP DIAPIRS AND ALLOCHTHONOUS SALT: FLINDERS AND WILLOURAN RANGES, SOUTH AUSTRALIA, AND THE DEEPWATER NORTHERN GULF OF MEXICO by Thomas E. Hearon, IV Copyright by Thomas E. Hearon, IV 2013 All Rights Reserved 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 Doctor of Philosophy (Geology). Golden, Colorado Date Signed: Thomas E. Hearon, IV Signed: Dr. Bruce D. Trudgill Thesis Advisor Golden, Colorado Date Signed: Dr. Paul M. Santi Professor and Interim Department Head Department of Geology and Geological Engineering ii ABSTRACT The eastern Willouran Ranges and northern Flinders Ranges, South Australia contain Neoproterozoic and Cambrian outcrop exposures of diapiric breccia contained in salt diapirs, salt sheets and associated growth strata that provide a natural laboratory for testing and refining models of salt-sediment interaction, specifically allochthonous salt initiation and emplacement and halokinetic deformation. Allochthonous salt, which is defined as a sheet-like diapir of mobile evaporite emplaced at younger stratigraphic levels above the autochthonous source, is emplaced due to the interplay between the rate of salt supply to the front of the sheet and the sediment- accumulation rate, and may be flanked by low- to high-angle stratal truncations to halokinetic folds. Halokinetic sequences (HS) are localized (<1000 m) unconformity-bound successions of growth strata adjacent to salt diapirs that form as drape folds due to the interplay between salt rise rate (R) and sediment accumulation rate (A). HS stack to form tabular and tapered composite halokinetic sequences (CHS), which have narrow and broad zones of thinning, respectively. The concepts of CHS formation are derived from outcrops in shallow water to subaerial depositional environments in La Popa Basin, Mexico and the Flinders Ranges, South Australia. Current models for allochthonous salt emplacement, including surficial glacial flow, advance above subsalt shear zones and emplacement along tip thrusts, do not address how salt transitions from steep feeders to low-angle sheets and the model for the formation of halokinetic sequences has yet to be fully applied or tested in a deepwater setting. Thus, this study integrates field data from SouthAustralia with subsurface data from the northern Gulf of Mexico to test the following: (1) current models of allochthonous salt advance and subsalt deformation using structural analysis of stratal truncations adjacent to outcropping salt bodies, with a focus on the transition from steep diapirs to shallow salt sheets in South Australia; and (2) the outcrop-based halokinetic sequence model using seismic and well data from the Auger diapir, located in the deepwater northern Gulf of Mexico. Structural analysis of strata flanking steep diapirs and allochthonous salt in South Australia reveals the transition from steep diapirs to shallowly-dipping salt sheets to be abrupt and involves piston-like breakthrough of roof strata, freeing up salt to flow laterally. Two iii models explain this transition: 1) salt-top breakout, where salt rise occurs inboard of the salt flank, thereby preserving part of the roof beneath the sheet; and 2) salt-edge breakout, where rise occurs at the edge of the diapir with no roof preservation. Shear zones, fractured or mixed ‘rubble zones’ and thrust imbricates are absent in strata beneath allochthonous salt and adjacent to steep diapirs. Rather, halokinetic drape folds, truncated roof strata and low- and high-angle bedding intersections are among the variety of stratal truncations adjacent to salt bodies in South Australia. Interpretation and analysis of subsurface data around the Auger diapir reveals similar CHS geometries, stacking patterns and ratios of salt rise and sediment accumulation rates, all of which generally corroborate the halokinetic sequence model. The results of this study have important implications for salt-sediment interaction, but are also critical to understanding and predicting combined structural-stratigraphic trap geometry, reservoir prediction and hydrocarbon containment in diapir-flank settings. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES ....................................................................................................................... xiii ACKNOWLEDGMENTS ........................................................................................................... xiv CHAPTER 1 INTRODUCTION ...................................................................................................1 1.1 Halokinetic Growth Strata .......................................................................................3 1.2 Allochthonous Salt ...................................................................................................6 1.3 South Australia and the Gulf of Mexico ..................................................................9 1.3.1 Basin Types and Evolution ..................................................................14 1.3.2 Salt Composition and Deformation .....................................................19 1.3.3 Depositional Environments ..................................................................20 1.4 Summary of Chapters ............................................................................................20 1.4.1 Chapter 2: Geology and tectonics of Neoproterozoic salt diapirs and salt sheets in the eastern Willouran Ranges, South Australia ...............20 1.4.2 Chapter 3: Allochthonous salt initiation and advance in the Flinders and eastern Willouran Ranges, South Australia: Using outcrops to test subsurface-based models from the northern Gulf of Mexico ........21 1.4.3 Chapter 4: Testing the applicability of an outcrop-based halokinetic deformation model in a deepwater depositional setting: Auger diapir, Garden Banks 427, northern Gulf of Mexico ...........................22 1.5 References Cited ....................................................................................................22 CHAPTER 2 GEOLOGY AND TECTONICS OF NEOPROTEROZOIC SALT DIAPIRS AND SALT SHEETS IN THE EASTERN WILLOURAN RANGES, SOUTH AUSTRALIA ..........................................................................................28 2.1 Abstract ..................................................................................................................28 2.2 Introduction ............................................................................................................29 2.3 Geologic Setting of the Flinders and Willouran Ranges ........................................31 2.3.1 Regional Tectonic History ...................................................................32 2.3.2 Stratigraphy ..........................................................................................34 v 2.3.3 Callanna Group Breccia .......................................................................38 2.4 Observations ..........................................................................................................39 2.4.1 Structural Geometries ..........................................................................43 2.4.2 Stratal Geometries ................................................................................45 2.5 Interpretation ..........................................................................................................48 2.5.1 Autochthonous Salt Structures .............................................................48 2.5.2 Allochthonous Salt Structures ..............................................................51 2.5.3 Delamerian Orogeny ............................................................................54 2.5.4 Salt and Minibasin Evolution ...............................................................55 2.6 Discussion ..............................................................................................................59 2.7 Conclusions ............................................................................................................63 2.8 Acknowledgments ..................................................................................................64 2.9 References Cited ....................................................................................................65 CHAPTER 3 ALLOCHTHONOUS SALT INITIATION AND ADVANCE IN THE FLINDERS AND EASTERN WILLOURAN RANGES, SOUTH AUSTRALIA: USING OUTCROPS TO TEST SUBSURFACE-BASED MODELS FROM THE NORTHERN GULF OF MEXICO ........................................................................75 3.1 Abstract ..................................................................................................................75 3.2 Introduction ............................................................................................................76 3.3 Geologic Setting of the Flinders and Willouran Ranges ........................................79 3.4 Description of Stratal and Structural Geometries ..................................................83 3.4.1 Pinda Diapir ...............................................................................................83 3.4.2 Patawarta Diapir .........................................................................................88
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