On the basis of field observations in the Huqf area in eastern Oman, Montenat et al. (2003) defined an extensional phase of faulting of Aptian age in the Arabian platform (see also Montenat and Barrier, 2002). We recently revisited the fault exposures of the Wadi Sha’bat al Tawraq from which this conclusion has been drawn (location in Figure 1a). Our observations contradict those of Montenat et al. (2003), and consequently our conclusions are different. Here we review the conflicting observations and discuss the different interpretations.
In an area of about 10 square kilometers encompassing the Wadi Sha’bat al Tawraq, two major faults referred to as F1 and F2 are exposed (Figures 1 and 2). Both are normal faults, striking northwest-southeast, dipping about 60° towards the southwest, and offsetting the limestones of the lower Aptian Qishn Formation (corresponding to the Shu’aiba Formation of northern Oman). Figure 2a shows a schematic cross-section of this area with the two faults. F1 is the fault depicted in Figures 6, 9, and 10 of Montenat et al. (2003), and F2 is the fault depicted in their figure 11 (see location of their figures in Figure 1c). Both faults are expressed in the field by fault scarps a few meters high (less than 5 m) and several hundred meters long (Figures 2b and 2c). The faulted blocks are slightly tilted and dip gently towards the northeast. Secondary faults are found in the vicinity of F1 and F2 (Figure 1c): two normal faults limit a small graben in the hanging-wall of F1, and several normal faults are observed in the footwall of F2.
Montenat et al. (2003) present a series of field observations and interpretations related to F1 and F2: (1) to assess the synsedimentary or syndiagenetic character of normal faulting; and (2) to infer a pre-Albian age of faulting. In the following, we review these observations and propose alternative interpretations.
Post-lithification Normal Faulting of the Aptian Qishn Limestone
Montenat et al. (2003) put forward sedimentological and tectonic observations to argue that faulting occurred prior to complete lithification of the carbonates of the Qishn Formation. They first describe the upper part of normal fault scarps as showing indications of near-surface, soft-sediment deformation with “small sinuous faults smoothed as a result of sediment creeping, boudinage and collapse of the cohesive muddy sediment” (their figures 9 and 10). This “jumbled deformation [is] due to the softness and plasticity of the unlithified sediment”. We disagree with this interpretation. The lithofacies of the Qishn limestone is not a mudstone but a coarse grainstone comprising rudist debris, i.e. a material that is non-cohesive before lithification and with no plastic rheological behavior. Moreover, the small sinuous fractures cut the rudist bioclasts and therefore postdated the lithification of the limestone.
Montenat et al. (2003) observed that the F2 fault plane is locally covered with patches of breccia with an irregular nodular aspect (their figure 11b). They indicate that the large breccia clasts “show prelithification deformations, such as stretching and pillowing, [and that] the carbonate mud between the clasts has a flow structure”. We do not confirm these observations. The breccia is a typical cemented fault breccia with rigid clasts. Its irregular nodular (not pillowing) aspect results from dissolution and weathering. We note that the same nodular aspect is observed in the limestone of the footwall of the fault, and we conclude that it is not related to the fault or to syndiagenetic deformation.
In their figure 11a, Montenat et al. (2003) show a so-called clockwise-rotated block at the foot of the F2 fault scarp. They state that a “part of the fault scarp collapsed and slid down with a clockwise rotating motion during or shortly after faulting”. The rotation is defined about an axis perpendicular to the fault plane and the clockwise sense is inferred from a series of fractures in the fallen block that Montenat et al. (2003) misinterpreted as the bedding plane. In fact, the bedding plane in the fallen block is sub-parallel to the bedding plane in the footwall of the fault. The fallen block is completely rigid and does not show any evidence of soft-sediment deformation. We cannot see any reason to assert that this block collapsed “during or shortly after faulting”.
Montenat et al. (2003) show one outcrop of supposedly synsedimentary faulting in the Qishn limestones in their figure 12 (reproduced in Figure 3). According to their interpretation, a continuous calcirudite bed seals the main fault of this outcrop. This observation is erroneous. Indeed, the fault crosscuts all strata and is never sealed by any bed on the outcrop. In this example, faulting clearly postdated the deposition and lithification of the Qishn Formation.
Lastly, the occurrence of slickolite striations (i.e. stylolites formed by pressure solution process along the fault plane) on normal fault planes (Figure 4) demonstrates that the Qishn limestones were indurated before deformation started. All this field evidence indicates that faulting was post-lithification of the Qishn limestone, and not syndiagenetic.
Normal Faults not Sealed by Top Qishn Formation Hardground
The top-Qishn Formation is capped by a ferrugineous hardground, which represents a hiatus of more than 5 my (Platel et al., 1992; Immenhauser et al., 2000, 2004). The top-Qishn Formation surface is non-angular with respect to the underlying strata at the outcrop scale, but it is an unconformity at the regional scale (Dubreuilh et al., 1992; Platel et al., 1992). The two fault planes F1 and F2 are partly covered by a thin (1 mm to 1 cm) ferrugineous crust. Montenat et al. (2003) claim that this crust is the same as the top Qishn hardground, and seals the extensional faulting. We disagree with these interpretations. First, the iron-stained top-Qishn Formation hardground has an irregular undulating surface perforated by borings that cut through the fabric of the underlying rocks, whereas the ferrugineous crust on the fault planes is totally devoid of perforation. The two ferrugineous surfaces are therefore different. Second, a fault scarp, even indurated by a ferrugineous crust, would not have resisted to post-Qishn Formation emersion and would have been eroded. Third, the stratigraphic sections of the base of the Nahr Umr Formation, on either side of the F1 fault, are perfectly identical. Had the fault scarp predated the deposition of the Nahr Umr Formation, the base of the formation would have displayed lateral variations. Fourth, striations on fault planes (including slickolites of Figure 3) are systematically found under the ferrugineous crust, implying that the crust postdated fault slip. The linear features borne by the so-called “striated” ferrugineous surface depicted in figure 10 of Montenat et al. (2003) are definitively not tectonic striations. These linear features, developed in the ferrugineous crust, result from erosion and weathering of the crust by meteoric water, and are not the result of mechanical damaging of the fault plane by hard objects. Fifth, most fault planes in the Huqf region, including fault planes cross-cutting Upper Cretaceous formations that are younger than the Qishn Formation, are covered by a ferrugineous crust similar to that observed on fault planes in the Wadi Sha’bat al Tawraq.
All these observations indicate that the ferrugineous crust on fault planes is not equivalent to the top Qishn hardground and that it postdates normal faulting.
Normal Faulting Affects the Albian Nahr Umr Marls
Figure 6 of Montenat et al. (2003) reproduced in our Figure 5 suggests that the Albian Nahr Umr marls seal the small graben affecting the Qishn limestones in the hanging-wall of F1. This interpretation is incorrect. Our observations show that Quaternary fluviatile deposits rest directly upon the Qishn limestones (Figures 2a and 5) and hide the contact between the limestones and the nearby Nahr Umr marls. Moreover, the dip of the Nahr Umr marls is in exact accordance with the dip of the underlying Qishn limestones, which implies that tilting postdated the Nahr Umr marls deposition. This observation strongly suggests that F1 normal faulting and associated tilting are younger than the Nahr Umr marls.
This interpretation is further confirmed by the existence of a normal fault cutting the Nahr Umr beds in the vicinity of F1 (Figure 6). The fault trends N110°E, dips toward the southwest, and cross-cuts the strata of the Qishn and Nahr Umr formations. This fault, which is of the same type and the same trend as F1 and F2, most probably formed during the same extensional episode after the deposition of the Nahr Umr marls.
Moreover, the 1:250,000 scale geological mapping of the Huqf area (Figure 1a; Platel et al., 1992) shows that F1 and F2, among other faults with the same strike, affect the Nahr Umr Formation and are sealed by Middle Miocene limestone (Naqd Formation).
These lines of field evidence consistently indicate that normal faulting postdated the Nahr Umr Formation. Hence, our field observations lead to the following chronology: (1) deposition of the Qishn limestones and the Nahr Umr marls; (2) post-Albian normal faulting; and (3) formation of the ferrugineous crust.
The preceding discussion casts doubt on the field arguments used by Montenat et al. (2003) to define a late Aptian tectonic event in the Huqf region. According to our observations, the top-Qishn Formation hardground does not seal any tectonic event. We conclude that there is no late Aptian extensional tectonic phase in the Arabian platform documented in the Wadi Sha’bat al Tawraq. The exact age of normal faulting is not known, but it occurred between the late Albian and the Middle Miocene. It could be related to the NE-SW extensional event that was active during the Oligocene to Lower Miocene rifting of the Gulf of Aden, and whose effects are widespread in southern Oman (Lepvrier et al., 2002; Fournier et al., 2004).
This study was supported by the MEBE program devoted to the Middle-East Basins Evolution. We are grateful to Dr. Hilal Mohammed Al-Azri and Dr. Salim Al-Bu Saidi from the Omani Directorate General of Minerals, Ministry of Commerce and Industry of Oman, for their support.
ABOUT THE AUTHORS
Marc Fournier is Assistant Professor in the Tectonics Research Team of the Pierre and Marie Curie University, Paris, France. He holds a PhD in Earth Sciences from Ecole Normale Supérieure, Paris. His doctoral research involved field study in Sakhalin and the Japan arc, and experimental modeling of back-arc extension in relation to the tectonics of Asia. Marc spent two years in Japan at the University of Tokyo working with the French-Japanese Kaiko Program on subduction processes. His current research interests include extensional tectonics, plate kinematics, geodynamics, and tectonics of the Arabian Plate. He is involved in field surveys on the margins of the Gulf of Aden, in the northern Oman Mountains, and in the Alps. Marc has supervised three scientific cruises in the Gulf of Aden in order to investigate the deep structure of the conjugate margins and the kinematics of the Arabia-India-Somalia triple junction.
Philippe Razin is Professor of Geology at the University of Bordeaux, France. He received his Doctoral of Science degree from the University of Bordeaux in 1989 and joined the Bureau de Recherches Géologiques et Minières as an expert in sedimentology and basin synthesis. Philippe was involved in various projects (mapping, water and mineral exploration, geotechnics, 3-D modeling). He moved to the University of Bordeaux in 1997, where he teaches sedimentary and structural geology, geodynamics and field mapping. His research activities concern relations between tectonic and sedimentation, in collaboration with BRGM, IFP, IFREMER and oil companies.
Olivier Fabbri is Professor of Structural Geology at the University of Franche-Comte, Besancon, France. His PhD thesis, presented at Orleans University in 1989, focused on deciphering the stratigraphy and structure of a transect of the Cretaceous-Tertiary Shimanto accretionary prism of Japan. After his PhD, he worked for four years as a Geologist in a consulting company in Japan, and another four years as an Assistant Professor at Yamaguchi University, also in Japan, where he extended his research to the structure and evolution of the Japanese island arc. Olivier joined the University of Franche-Comte as an Associate Professor in 1996. Besides yearly field trips to Japan, he has been involved in structural analyses of other areas in the world, notably Oman and France.
Jean-Paul Breton is a Structural Geologist with the French Bureau de Recherches Géologiques et Minières (BRGM). He holds a PhD in Structural Geology from the University of Paris (1972). He joined BRGM in 1972, working on structural studies applied to geological mapping, mineral exploration, underground storage and geotechnics in France, Europe, Africa and, from 1981 to 1992, in Saudi Arabia. Jean-Paul made a significant contribution to the conception and implementation of the Tectonic Map of France at the 1:1,000,000 scale (1st Edition, 1981). Since 1993, he has been the local representative of BRGM in Oman and, since 2002, is in charge of a scientific research program related to the geodynamics of the southern edge of the Arabian Plate.