Research paperShale tectonic processes: Field evidence from the Parras Basin (north-eastern Mexico)
Introduction
In most of the tectonic fronts of convergent orogens (including accretionary prisms, fold and thrust systems in mountain belts) but also in thick deltaic systems controlled by gravity tectonics on passive margins, except when evaporates are present, decollement processes occur commonly in overpressured shale. During shale tectonics associated with this type of decollement, shale is deformed in the deepest parts close to the decollement generating poorly understood structures which are commonly named by a number of generic terms (such as mobile shale, shale diapirs, clay diapirs, mud diapirs, argillokinetic structures, etc.; Bruce, 1973; Brown, 1990; Bradshaw and Watkins, 1994; Cohen and McClay, 1996; Huh et al., 1996; Morley and Guerin, 1996; Morley, 2003; Van Rensbergen and Morley, 2000, Van Rensbergen and Morley, 2003, Van Rensbergen et al., 2003; Van Rensbergen et al., 2000, 2003a and b; Corredor et al., 2005; Briggs et al., 2006; Deville et al., 2006, 2010; Wiener et al., 2010; Elsley and Tieman, 2010). These terms are currently used to describe geometric features in the seismic data but with only geophysical data and without direct constraints provided by wells or field observations, it is difficult to define exactly what are the prevailing deformations processes which occur within these shale-rich bodies (see an example in Fig. 1). These volumes of sediments poorly imaged in seismic data are commonly considered as mobile shale bodies but generally, authors do not prejudge about the nature, the structure and the genetic processes at the origin of the deformation of these shale-rich bodies. Present-day understanding of subsurface clay-rich sediment deformation remains relatively low and commonly made by comparison with the larger literature on salt tectonics (Morley and Guerin, 1996). However, these processes clearly differ from salt mobilization, notably by the crucial role taken by the fluid dynamics that is able to induce sediment liquefaction and controls overpressured shale deformation. In most cases, on seismic lines, it is difficult to define if sediment deformation occurred as a brittle or a ductile process, even though this implies drastic differences in the rheology of the material involved and the modes of deformation (see discussion in Wood, 2010, 2012). When interpreting seismic data, what is usually designated as mobile shale corresponds to volumes of rocks whose global geometry suggests a ductile deformation (pillow shapes, deformed cores of large anticlines suggesting diapiric shapes; Fig. 1). It is generally difficult to define if this deformation occurred as liquefaction of sediments, or as flow of ductile but still stratified material or else as deformation of intensively fractured rocks at depth (Deville et al., 2003, 2006; 2010). With the improvement of seismic data, it progressively appeared that what was considered as shale diapir is much more restricted than previously thought (Van Rensbergen and Morley, 2003, Van Rensbergen et al., 2003)
The widespread occurrence of shale tectonic processes, their common association with hydrocarbon producing areas and their influence on the development of a wide range of sedimentary basins require new studies of these phenomena. For a better understanding of the deformation processes of what is commonly named mobile shale, we made field studies on an outcropping case. The objective of this work was to study outcrops associated with a major decollement zone in shale located at the base of a thick tectonic wedge. In active or recent thrust systems developed on top of a decollement in shale or on top of intensively deformed shale, because of the burial, it is usually impossible to study the shale deformation directly on wide surface outcrops. For this reason, we choose a case study in the Parras Basin in northern Mexico (Fig. 2), which corresponds to an area where exposures of a large decollement system can be studied thanks to a late uplift and erosion (Fig. 3, Fig. 4). The objective of this study was to better understand the deformation mechanisms of the so-called mobile shale notably close to the decollement level and in the cores of clay-rich anticlines, as well as the scale factors (microstructures vs. macrostructures). We also wanted to use this outcropping analog to better understand the interactions between deformation - migration of fluids - diagenesis in tectonic fronts characterized by decollement in shale.
Section snippets
Depositional setting
What is commonly named the ‘Parras Basin’ is a part of the Mexican Laramide tectonic front which includes the deltaic uppermost Cretaceous and Early Tertiary terms forming the detrital stratigraphic series of the Jurassic-Cretaceous of the Sierra Madre Oriental in the area of Parras and Saltillo (Fig. 2, Fig. 3). The Parras Basin develops south of the Popa and Sabinas basins and it is limited to the northwest by the Coahuila platform and to the south by the front of the Sierra Madre Oriental (
Material and methods
A sequence of different approaches has been used to characterize deformation, fluid migration and diagenesis: field structural geology, optical microscopy of veins, XRD of clays, study fluid of inclusions in calcite and quartz, stable isotope composition of carbonate cements, study of organic matter maturity (using Rock-Eval and Raman techniques) and thermal modeling. The objective of this work was to better understand the evolution of the deformation processes, their physical conditions, and
Deformation processes in and around the decollement zone
The Cretaceous carbonate formations of the Coahuila arch which are located below the decollement zone are affected by long wave-length folding processes contrary to the formations located above the decollement (Fig. 3, Fig. 4, Fig. 6, Fig. 7). In the area studied, they are simply tilted towards the east or south-east (Fig. 3, Fig. 4, Fig. 6, Fig. 7). They are simply tilted towards the east or south-east. They show evidence of intense fracturing associated with well-developed calcite cements (
Interpretation
This study of a wide scale tectonic wedge affected by a major decollement located in a shale-rich formation has shown that this system evolved through different steps as summarized in Fig. 29, Fig. 30. Combined studies including field geology, XRD, SEM, microthermometry and barometry on fluid inclusions, isotopic study of carbonates, Rock-Eval and RSCM measurements and thermal modeling made possible to identify the following major stages concerning the history of deformation - fluid migration -
Discussion about shale mobility
The tectonic evolution of this case study can be compared with mechanical experiments made on shale rocks. Increasing importance of shale of deep hydrocarbon exploration targets but also gas/oil shale plays exploration led to improve the knowledge about the rheological properties of shale at depth. Series of geomechanical experiments have shown that, in absence of significant overpressure, the strength of shale rocks increases with depth before reaching the transition from brittle to ductile
Conclusion
The area studied is a rare outcropping terrestrial analog illustrating deformation processes which occur at depth, in thick sedimentary thrust wedges associated with major decollement situated in overpressured shale. It offers outcropping conditions over large areas which made possible a series of different observations and analytical approaches which have shown notably the following points: Massive volumes of deformed shale with scaly fabric, disrupted stratification and boudinage of the
Individual credits
Eric Deville has initiated the project and organized the field work. He managed the structural geology study, part of the thin section study, the thermal model and he wrote a large part of the paper. Clement Dutrannoy managed the Fluid Inclusion study. Julien Schmitz managed sampling and referencing of the sample on the field and the realization of maps. Benoit Vincent managed the diagenetic study and interpretation of isotopic data. Eric Kohler managed the XRD and SEM study of shale. Abeltif
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Thanks are due to TOTAL SA (DBR project) for financial support. Thanks are also due to Nicole Guihlaumou for her help in the work of Clement Dutranoy concerning the fluid inclusions study and to Emily Albouy, Stéphane Raillard and Nadine Ellouz who took part in field-trips in the Parras Basin. Thanks are also due to Michael Joachimski from the University of Erlangen (Germany) for carbonate isotopic analyses. We thank also the reviewers for their constructive contributions to improve the form of
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