Internal Structure of Mushroom-Shaped Salt Diapirs

RI0181. Internal Structure of Mushroom-Shaped Salt Diapirs, by M. P. A. Jackson and C. J. Talbot. 35 p., 24 figs., 1989. ISSN: 0082335X: Print.


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ABSTRACT
Mushroom-shaped diapirs have an overhanging bulb fringed by one or more skirts (peripheral pendant lobes), which can curl inward to form vortices capable of entraining cover rocks to various degrees. The highly complex anatomy of mushroom diapirs, some of which have double and eventriple cores, is analyzed in centrifuged and natural diapirs. We conducted 8 centrifuge experiments, which produced more than 100 model diapirs under acceleration equivalent to 1,200 times that of gravity. The experiments were dynamically scaled to U.S. Gulf Coast salt domes, but the qualitative results are also relevant to salt diapirs in other provinces and to granitoid diapirs rising through metamorphic crust. The centrifuged domes grew under overburdens of constant thickness or under aggrading and prograding overburdens, a new experimental approach. Mushroom diapirs are readily recognized in vertical section but less easily in horizontal section. They share three identifying characteristics: the oldest buoyant unit occurs in a peripheral skirt as well as in the diapiric core; the younger evaporites or their immediate cover are folded into the diapir in crescentic patterns in plan view; and most of the internal folds are downward facing.

Toroidal circulation in rising diapirs leads to a range of bulb shapes that vary with their maturity and with the contrasts in effective viscosity between the diapir and its surroundings. The resulting shapes comprise external simple mushrooms, external vortex mushrooms, internal simple mushrooms, and internal vortex mushrooms. External simple mushroom diapirs have skirts that infold cover rocks. With increased maturity the skirts can curl inward to form an external vortex entraining cover rocks. In contrast, internal mushroom diapirs have skirts confined entirely within the intrusion. With greater maturity these confined skirts also may curl inward, but they entrain only diapiric material.

External mushroom structure results from toroidal circulation of buoyant source and immediate cover having similar effective viscosities. Entrainment of cover by toroidal circulation may be rare in salt diapirs because it appears to require conditions that are realized transiently at certain times and depths in compacting sediments or over long durations in basins where the intruded cover itself contains evaporites, as in Central Iran. Internal mushroom structure results from toroidal circulation confined within the diapir and is probably far more common than external mushroom structure because of isoviscosity within evaporite sequences; we describe natural examples in West Germany and the U.S. Gulf Coast. The internal structure of mushroom salt diapirs elucidates the mechanics of diapiric emplacement and indicates whether an external mushroom shape can be expected and sought by further exploration. Resolving this question is cost effective in exploiting potash ore and vital in the mining engineering of salt caverns. Screens of country rock inclusions entrained into the bulb of an external mushroom diapir could threaten the integrity of a cavern by creating a plumbing system of more permeable rock enclosed in evaporites. In general, engineering within salt structures is likely to be simplest where the evaporites are vertical linear tectonites and more complex where planar fabrics are present.

Keywords: diapirism, diapirs, flow folds, interference patterns, Iran, physical models, radioactive waste, salt domes, salt tectonics, West Germany

CONTENTS

Abstract

Introduction

Factors Controlling the Shape of Mushroom Diapirs

Experimental Results

Introduction

General Anatomy of Centrifuged Mushroom Diapirs

Anatomy of Specific Centrifuged Mushroom

Threefold Diapirs

Fivefold Diapirs

Sevenfold Double Diapirs

Elevenfold Double Diapirs

Vortex Structures

Evaporitic Mushroom Diapirs

Northwest German Plain

Canadian Arctic Islands

Central Iran

Diapiric Contact Strains and Viscosities in Nature

Nature of Diapiric Contacts

Effective Viscosity Contrast

Summary: Mushroom Diapirs and Their Implications to Engineering and Petroleum Exploration

Acknowledgments

References

Figures

1. Types of diapir bulbs

2. Schematic effects of viscosity contrast on shapes of domes based on physical and numerical modeling

3. Vertical sections showing the original configuration of eight models before acceleration in a centrifuge

4. Isometric block diagram of the upper surface of the source layer in model 840319-A

5. Four types of mushroom diapirs shown schematically in vertical half-section together with the terminology of second-order crescentic folds

6. Actual vertical sections through a variety of centrifuged diapirs

7. Nomenclature of schematic mushroom diapirs in vertical section

8. Isometric diagrams of schematic fivefold diapirs

9. Actual vertical and horizontal sections through two centrifuged threefold asymmetric diapirs

10. Actual vertical and constructed horizontal sections through a centrifuged fivefold asymmetric diapir

11. Actual vertical slice of a centrifuged sevenfold symmetric double-stalk diapir

12. Actual vertical and constructed horizontal sections through a centrifuged sevenfold asymmetric double diapir with a draped-arm structure

13. A centrifuged elevenfold double diapir combining a double stalk and a draped-arm structure

14. Vertical and horizontal sections through a hypothetical fivefold asymmetric diaper with vortex structure

15. Actual vertical sections through two centrifuged diapirs at different stages of development toward a sevenfold asymmetric vortex mushroom diapir

16. Vertical section through Asse Dome, a mushroom diapir southeast of Braunschweig, West Germany

17. Vertical section through Hanigsen Dome, a mushroom diapir northeast of Hanover, West Germany

18. Maps and vertical sections of Gorleben Dome, a mushroom diapir between Laase and Trebel, West Germany

19. Vertical sections of four West German salt domes characterized by internal mushroom structure

20. Vertical section of Palangana Dome, South Texas

21. Horizontal section through the northwest sector of Belle Isle Dome, Louisiana

22. Map and hypotheses of emplacement of Barrow Dome, Melville Island, Sverdrup Basin, Canadian Arctic

23. Vertical sections through two symmetric mushroom diapirs in the Great Kavir, Central Iran


Citation
Jackson, M. P. A., and Talbot, C. J., 1989, Internal Structure of Mushroom-Shaped Salt Diapirs: The University of Texas at Austin, Bureau of Economic geology, report of Investigations No. 181, 35 p.