Abstract

Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Highlights

  • The “salt” of ancient global salt basins never comprises solely halite

  • We argue that there are fundamental differences in style that can be used to help distinguish between different modes of salt tectonics

  • The effective viscosity contrasts between these strata and ductile halite and bittern salts range from 1 to over 5 orders of magnitude

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Summary

Introduction

The “salt” of ancient global salt basins never comprises solely halite (or rocksalt). Rowan et al.: Deformation of intrasalt competent layers in different modes of salt tectonics (Fig. 1) Bittern salts such as sylvite and carnallite, which require a large degree of evaporation, are less common but occur in many salt basins. Note that we discuss only those non-halite layers that are thick and strong enough to form independent mechanical units; very thin beds of anhydrite or shale tend to behave mechanically with the encasing halite, as seen within mines in the Cardona diapir of Spain or the Wieliczka diapir of Poland This analysis of intrasalt deformation is relevant to anyone concerned with the interiors of salt bodies. We elaborate on some of the practical implications for seismic imaging, seismic interpretation, and drilling into and through salt

Halite
Other evaporites
LES rheology
Non-evaporite layers
Extension
Contraction
Differential loading
Passive diapirs
Discussion
Definition of LES
Competent-layer properties
Thickness and strength
Mechanical stratigraphy of multilayers
Density
Combined modes of salt tectonics
Santos Basin
Southern Permian Basin
Implications
Well data
Seismic data
Drilling hazards
Conclusions
Full Text
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