J. D. Martín-Martín

Syn- to post-rift diapirism and minibasins of the Central High Atlas (Morocco): the changing face of a mountain belt

The Atlas Mountains are classically regarded as a failed Mesozoic rift arm subject to Alpine inversion, folding and thrusting. Here, we present new integrated structural and sedimentological studies that have revealed numerous Early–Middle Jurassic diapiric ridges and minibasins, characterized by distinctive halokinetic structures. Diachroneity in halokinesis is observed across the Central High Atlas, waning first in the SW during the Early–Middle Jurassic (Jbel Azourki and Tazoult ridges) and continuing to late Middle Jurassic towards the NE (Imilchil region). The halokinetic structures are readily differentiated from the effects of later Alpine deformation, allowing a new picture of the Central High Atlas to emerge. The most pervasive deformation in the Central High Atlas is associated with Early–Middle Jurassic diapirism, whereas the impact of Alpine inversion is mostly focused at the basin margins. This new understanding helps explain previously problematic aspects of the Atlas Mountains, which we now recognize as an exceptionally well exposed natural laboratory for understanding the interactions between halokinesis, tectonics and sedimentation.

Insights to controls on dolomitization by means of reactive transport models applied to the Benicàssim case study (Maestrat Basin, eastern Spain)

Partially dolomitized carbonate rocks of the Middle East and North America host large hydrocarbon reserves. The origin of some of these dolomites has been attributed to a hydrothermal mechanism. The Benicàssim area (Maestrat Basin, eastern Spain) constitutes an excellent field analogue for fault-controlled stratabound hydrothermal dolomitization: dolostone geobodies are well exposed and extend over several kilometres away from seismic-scale faults. This work investigates the main controls on the formation of stratabound versus massive dolomitization in carbonate sequences by means of two-dimensional (2D) reactive transport models applied to the Benicàssim case study. Simulation results suggest that the dolomitization capacity of Mg-rich fluids reaches a maximum at temperatures around 100 °C and a minimum at 25 °C (studied temperature range: 25–150 °C). It takes of the order of hundreds of thousands to millions of years to completely dolomitize kilometre-long limestone sections, with solutions flowing laterally through strata at velocities of metres per year (m/a). Permeability differences of two orders of magnitude between layers are required to form stratabound dolomitization. The kilometre-long stratabound dolostone geobodies of Benicàssim must have formed under a regime of lateral flux greater than metres per year over about a million years. As long-term dolomitization tends to produce massive dolostone bodies not seen at Benicàssim, the dolomitizing process there must have been limited by the availability of fluid volume or the flow-driving mechanism. Reactive transport simulations have proven a useful tool to quantify aspects of the Benicàssim genetic model of hydrothermal dolomitization.

Diapiric growth within an Early Jurassic rift basin: the Tazoult salt wall (Central High Atlas, Morocco)†

The Central High Atlas (Morocco) constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the Lower Jurassic rift of the Atlas Basin. This paper aims to study the structure and growth evolution of the Tazoult diapiric wall, located in the Central High Atlas, by means of structural and sedimentological fieldwork integrated with remote sensing mapping. The Tazoult salt wall is a 20 km long x 3 km wide NE-SW trending ridge that exposes Upper Triassic red beds and basalts along its core. The succession flanking the salt wall ranges from Hettangian to Bajocian ages displaying spectacular sedimentary wedges in the SE and NW flanks. The Hettangian-early Sinemurian carbonates mainly crop out as blocks embedded in the core rocks. The ~1-km thick Pliensbachian platform carbonates display large subvertical flap structures along the flanks of the Tazoult salt wall with unconformities bounding tapered composite halokinetic sequences. In contrast, the ~2.5-km thick late Pliensbachian-Aalenian mixed deposits form tabular composite halokinetic sequences displaying small-scale hook halokinetic sequences. Passive diapirism resulted in the lateral extrusion of the evaporite-bearing rocks to form an allochthonous salt sheet towards the adjacent SE Amezrai minibasin. The Bajocian platform carbonates partially fossilized the Tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from Alpine shortening. The Pliensbachian carbonate platform evolved as a long flap structure during the early growth of the Tazoult salt wall, well before the onset of the Alpine shortening.

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

Abstract This study investigates the Late Aptian–earliest Albian platform carbonates of the Benicàssim area (Maestrat Basin, Spain) in order to assess the relationship between bed-parallel stylolites and the flow of diagenetic fluids during dolomitization and subsequent hydrothermal alteration. Dolostones and burial dolomite and calcite cements were studied by a combination of field geology and standard petrographic and isotope analysis. Field data indicate that dolostones are closely associated with seismic-scale synsedimentary faults, preferentially replace grain-dominated facies and typically show wavy dolomitizing fronts that mostly correspond to bed-parallel stylolites. The dolostones are corroded and contain bed-parallel pores that are filled with hydrothermal saddle dolomite and blocky calcite cements. This late calcite cement frequently engulfs clasts of the host dolostones, suggesting that hydraulic brecciation likely associated with overpressured fluid occurred. Results indicate that stylolites play a key role in the distribution of dolostones and subsequent hydrothermal mineralization. During the replacement stage, stylolites acted as baffles for the dolomitzing fluids controlling lateral fluid flow and resulting in the stratabound dolostone distribution. During the post-dolomitization stage, stylolites became preferred pathways for overpressured hydrothermal corrosive and mineralizing fluids that likely came from the underlying basement, and increased bed-parallel stylolitic porosity and probably also permeability.

Reactivity of Dolomitizing Fluids and Evaluation of Mg Sources in the Benicassim Area (Maestrat Basin, E Spain)

Hydrothermal dolomitization is one of the most important processes in enhancing or degrading carbonate porosity and permeability. This type of dolomite forms due to the circulation and/or mixing of different types of solutions, mainly seawater-derived or deep brines. The Lower Cretaceous Benicassim dolomitized ramp is an excellent example to study and evaluate the impact of hydrothermal dolomitization on reservoir quality distribution. In this contribution, the possible magnesium sources for the Benicassim case study are evaluated, as well as the reactivity of different dolomitizing fluids at variable temperatures. The results show that dolomitization at Benicassim occurred due to the circulation of a high temperature (> 80oC) fluid that had its origin from evolved seawater that interacted with K-rich rocks, probably from the Paleozoic basement. Reactivity evaluation of four possible dolomitizing fluids, by means of geochemical modelling, reveals that evolved seawater can be considerably more reactive than high-salinity brines. Fluid mixing between seawater and saline brines at 100 oC does not seem to affect the volume of fluid required to dolomitize the whole rock, but it considerably alters the saturation index of calcite. The variation of saturation index is a strongly non-linear process when two fluids are mixed.

Flow Patterns of Dolomitizing Solutions in a Buried Carbonate Ramp – The Benicassim Case Study (Maestrat Basin, NE Spain)

Geometric, petrographic, and geochemical observations suggest that sub-stratiform dolomitization in a Lower Cretaceous Benicassim ramp (Maestrat Basin, E Spain) was due to the circulation of high temperature brines through faults and high permeability layers. In this study, fluid and heat flow numerical simulations are applied to investigate the controls on hydrothermal flow in this area, depending on the tectonic activity at the time of dolomitization. The results indicate that flow caused by rapid release of overpressured fluids below seals in recurrent pulses through large-scale faults may drive enough fluid for dolomitization, but not enough heat. Thermal conduction dominates advection over large-time scale. However, long-term fluid circulation, due to differences in pressure and temperature within the basin, can pump dolomitizing fluids at high temperature during long periods of time if the system is open. Moreover, a permeability contrast of two orders of magnitude is required to have lateral flow preferentially in some layers and to form relatively sharp, sub-stratiform dolomitization fronts.

Imprint of salt tectonics on subsidence patterns during rift to post-rift transition: The Central High Atlas case study

This study was part of a collaborative research project funded by Statoil Research Centre, Bergen (Norway). Additional funding by the CSIC-FSE 2007-2013 JAE-Doc postdoctoral research contract (E.S.), the projects Intramural Especial (CSIC 201330E030) and MITE (CGL 2014-59516). We are grateful to Statoil for its support and permission to publish this study.