Compaction history of Upper Cretaceous shale and related tectonic framework, Arabian Plate, Eastern Oman Mountains
Shale of the Upper Cretaceous Al-Khod Formation intruded younger conglomerates of the same formation. Intrusion followed a preexisting fault that had been widened by extension. The fissility of the shale mimics the contact contours of the conglomeratic host rocks. Sandstone and conglomerate clasts are “floating” in the shale. Vertical postintrusive calcite veins are ptygmatically folded by compaction. These ptygmatically folded compaction veins display horizontal to gently dipping axial planes. Shortening amounts to ~ 40%, indicating that shale intrusion ensued with high water content. It also shows that an estimated amount of 35 to 45% of water content was expelled by compaction after vein formation. Countless, randomly oriented calcite veins in the conglomerate at the shale contact point to fluid expulsion from the shale into the conglomerate. Shale intrusion postdates the late Cretaceous obduction of the Semail Ophiolite. Intrusion most likely occurred during the Oligocene for which extension of the nearby Frontal Range Fault was due to gravitational collapse, associated with isostatic/elastic rebound. Shale dike formation is related to a widened fault within a sinistral negative flower structure with a minimum width of 100 m. Intrusion ensued at an overburden of approximately 100 m of the upper part of the Al-Khod Formation and approximately 900 m of the Paleogene limestone. Folding of calcite veins and the significant water loss was caused by corresponding compaction. Nontronite is the red shale’s main clay mineral. It derived from a source area of exposed and weathered mafic to ultramafic rocks.
KeywordsShale compaction history Shale dike Ptygmatically folded compaction veins Al-Khod Formation Upper Cretaceous/Tertiary Negative flower structure Hajar Mountains
Associated with fault zones within well-cemented conglomerates of the Upper Cretaceous Al-Khod Formation, we encountered outcrops of red shale, one of them representing a shale dike. Field evidence establishes that during intrusion the red shale was still poorly compacted, containing high amounts of water. The field evidence also reveals that the shale lost a significant amount of water at some later stage.
The shale is superbly exposed. We describe and interpret these outcrops for the first time. At the same time, this is also the first report of a Cenozoic shale dike from Oman. We relate the formation of the studied shale dike to processes that postdate the obduction of the Semail Ophiolite. The shale’s unique geological setting is (1) on top of the obducted ophiolite, (2) in the area affected by late-stage obduction slab breakoff, (3) in the vicinity of sediments representing different loads of overburden over time, (4) near the Saih Hatat Dome, and (5) 2 km in the hanging wall of a major, gravity-driven (orogenic collapse), extensional fault (Frontal Range Fault, FRF; see Acknowledgments) which is critical for the regional geology (Mattern and Scharf 2018).
Shales of different color occur in the Al-Khod Formation. According to our field observations at and around the type locality of Al-Khod Village, beige, light greenish, red, and gray shale types of unknown composition occur. In this area, the main clast type is Hawasina-derived chert (almost monomictic conglomerates). In our study area, the upper part of the formation is characterized by only red shale and oligo- to polymictic conglomerates with dominant ophiolite clasts. The red shale’s composition had not been studied. Thus, the composition data of the Al-Khod shale represent a first step in understanding the formation’s differently colored shale types.
We provide a description of the shale outcrops with emphasis on the shale dike and its structural framework which includes the kinematic analysis of associated faults. We also relate the shale dike formation and compaction history to a particular geodynamic interval during gravitational collapse along the FRF.
The kinematic analyses of the faults in the field followed Petit (1987) and Angelier (1994), who outlined some deformation fabrics relevant for brittle fault analysis. These features are associated with brittle fault surfaces (mineral steps, Riedel shears, stylolitic peaks, tension gashes, etc.). Structural measurements were carried out, using the fabric compass after Clar (1954).
Four shale samples were dried, ground to a fine powder, and placed on a spinning single-crystal silicon sample holder. The X’ Pert PRO X-ray Diffraction machine (Panalytical), using Cu-Kα radiation (1.54060 Å) was set to 40 mA and 45 kV, and measured at a step size of 0.0167°2ϑ. The obtained mineral peaks were then evaluated with the help of the “High Score Plus” software. A Niton XL3t 950 Handheld XRF Analyzer with laboratory test stand (Thermo Scientific) and equipped with an SDD GOLDD+ detector was used to chemically analyze the samples for major, minor, and trace elements. The Ag-anode X-ray tube runs on an excitation voltage of 50 kV at 200 mA.
Northeastern Oman has been the site of a number of tectonic events during the late Cretaceous and Cenozoic that shaped the geology and landscape of the region. These include southwest-directed obduction of the Semail Ophiolite, deriving from the Tethys Ocean, onto the passive continental margin of the Arabian Plate during the late Cretaceous due to plate convergence between the Arabian and Eurasian plates (e.g., Glennie et al. 1974; Hacker 1994; Hacker et al. 1996; Searle and Cox 1999). At the base of the ophiolite ocean floor, sediments (Hawasina sediments) were thrust along. Obduction was accompanied by subduction of Oman’s continental margin to the northeast (Searle and Cox 1999; Searle et al. 2004; Saddiqi et al. 2006). In the course of subduction, the eclogitized root of the downgoing slab broke off which may have triggered rapid exhumation of the crustal slab (Searle et al. 2004).
A phase of overall stable shallow marine depositional conditions followed from the late Paleocene to the Oligocene (Searle 2007) as slow subsidence affected the area from the Eocene to the Oligocene (Poupeau et al. 1998). During this interval, an approximately 900-m-thick overall shallow marine limestone succession accumulated, unconformably overlying the Al-Khod Formation (Mann et al. 1990; Nolan et al. 1990; Béchennec et al. 1992; Dill et al. 2007; Hersi and Al-Harthi 2010). There is no stratigraphic record between the uppermost Cretaceous and the late Paleocene.
A second orogenic event related to the tectonic convergence between Arabia and Eurasia caused gentle folding and uplift of the Paleogene limestones since the late Oligocene-early Miocene (Searle 2007). Apatite fission-track analysis provides evidence for this compressional phase by weak heating up to 70 °C in the Saih Hatat Dome for the late Miocene-early Pliocene (Poupeau et al. 1998; Sadiqqi et al. 2006). This deformation phase finalized the structure and exhumation of the Saih Hatat Dome (Béchennec et al. 1992; Poupeau et al. 1998). Recent studies of Hansman et al. (2017, 2018) provided a late Eocene age for the doming of the Jabal Akhdar Dome (Fig. 1b).
Two intervals of tectonic activity have been identified along the FRF, one corresponds to stage I of Fournier et al. (2006) and a second one to stage IIa (Fig. 2) (Mattern and Scharf 2018). During the latter, the FRF fault segment between the Fanja Graben and the community of Sad was a dextral releasing bend (Mattern and Scharf 2018) which is close to our study area (Fig. 2). Dextral slip ensued in an overall extensional regime, but the orientation of the respective fault segment was suitably oriented for lateral shear (Mattern and Scharf 2018), (Fig. 2).
The Al-Khod Formation was first introduced by Nolan et al. (1990). Its thickness measures approximately 860 m at its type locality, which lies 25 km to the WNW of the study area (Pickford 2017). A lateral equivalent of the Al-Khod Formation is the Qahlah Formation (Nolan et al. 1990; Abbasi et al. 2014). In fact, “Qahlah Formation” may be considered a synonym for “Al-Khod Formation” in our study area (Abbasi et al. 2014). The Al-Khod Formation extends 25 km to the west from the northwestern margin of the Saih Hatat Dome. Besides the usual unconformable contact with the underlying Semail Ophiolite, the contact may at places also be a local fault (Nolan et al. 1990; Mattern et al. 2015; Scharf et al. 2016). The top of the Al-Khod Formation is marked by local normal faults or a low-angle unconformity with the overlying Paleogene rocks (Nolan et al. 1990; Mattern et al. 2015). The lithology varies within this terrestrial siliciclastic formation, ranging from interbedded, polymictic conglomerate via reddish and yellowish lithic sandstone to red shale with rare bands of microcrystalline limestone and dolomite (Nolan et al. 1990). The components of the poorly sorted conglomerate may reach cobble size and are sub-angular to rounded (Abbasi et al. 2014; Mattern et al. 2015).
Compaction of clay minerals/mudrocks
Upon deposition, muds contain 50–80 or 70–90 vol% of water (Müller 1967 and Tucker 2001, respectively). The initial water content depends on the percentage of the clay fraction (Müller 1967) and is also influenced by clay mineral composition, interstitial electrolyte solutions, exchangeable cations and the rate of deposition (Meade 1964).
Approximately 100 m north of our main outcrop, a second outcrop reveals the original depositional character of the two main lithologies within the upper part of the Al-Khod Formation (Fig. 4). The thick conglomerate overlies a several-meter-thick bed of red shale. At our study sites, the Al-Khod Formation dips gently/moderately towards the ESE.
Numerous faults dissect the main outcrop with different amounts of displacement ranging from millimeters to several meters. We identified steeply dipping, left-lateral strike-slip faults (strike ranging between 110 and 140° with lineations plunging subhorizonally to the SE or NW) and normal faults (strike ranging between 120 and 145°) as well as left-lateral transtensional faults (strike mostly SE with steep dip). We found no cross-cutting relationships between the different fault types. Drag folds (of bedding surfaces and veins), Riedel shears, and the displacement of marker beds are some of the more commonly used criteria for the shear sense analyses. Drag folds were not only used for kinematic fault analyses but also for the kinematic analyses of the shale movement relative to the conglomerate.
Within the Al-Khod conglomerate, three outcrops of red shale are exposed (Fig. 6). The contact relationships between the two lithologies locally range from conformable to discordant-intrusive. In the north-northeasternmost shale body (dike), the shale intrudes the conglomerate at a high angle and reaches the present surface (Fig. 6). The thickness of this shale dike amounts to a few meters in the lower part but narrows to 1 m towards the top (Fig. 6). The vertical exposure of the dike measures ~ 20 m (Fig. 6). The shale of the other two outcrops does not reach the surface but cuts the bedding plane of the conglomerate at a high angle and measure a few meters in thickness with a minimum height of 5 m (Fig. 6).
Whereas the contacts of at least one side of all shale outcrops are marked by a fault (Fig. 6), the non-faulted contacts are sharply jagged (Fig. 6). Several extensional faults are located within all three shale outcrops. Faults within the shale and the conglomerate are mainly filled with either calcite or, less often, chrysotile. The slickensides of the faults bounding the shale are commonly serpentinized. The mineralized faults within the shale are usually straight. However, some calcite veins within the shale resemble ptygmatic folds with horizontal or gently dipping axial planes with amplitudes and wavelengths at the millimeter/centimeter scale (Fig. 7b). The fissility of the red shale follows the outlines of the corners and edges of the country rock enclosing the shale units (Fig. 7c). In addition, we observed vertical to steeply dipping calcite veins that showed no ptygmatic folding, only some drag folding in some of the outcrops.
Between 15 and 25% of the host rock near the shale consists of calcite veins (Fig. 8). The ptygmatic folds of the shale dike do not cut into the conglomeratic host rock (Fig. 8). Within the host rock, no folded veins or ptygmatic calcite folds can be observed (Fig. 8).
Mineralogically, the four red shale samples of the upper section of the Al-Khod Formation display the same XRD results. They mainly consist of nontronite (Na0.3Fe2((Si,Al)4O10)(OH)2·nH2O) and clinochlore (Mg5Al(AlSi3O10)(OH)8). They further contain quartz (SiO2), lizardite (Mg3(Si2O5)(OH)4; serpentine group), traces of talc (Mg3Si4O10(OH)2), some pigment of hematite (Fe2O3), and scarce amounts of calcite (CaCO3). Fine secondary impregnations and/or crack fillings consist of calcite.
The aforementioned enumeration of minerals reflects the mineralogical contents of all four XRD samples in decreasing frequency. Differences in the mineralogical composition of all shale samples are marginal. The XRF results are in agreement with the XRD data. The element concentrations reveal only minor variations as well.
Interpretation and discussion
The Al-Khod Formation did not extend into the Paleogene (Nolan et al. 1990). This coincides exactly with the time at which exhumation of all high-pressure rocks was completed, including thinned continental crust of the Saih Hatat Dome which had been previously subducted to eclogite-facies conditions (e.g., Searle et al. 2004), (see high-pressure facies zones in Fig. 1b). Exhumation of the respective continental crust after the slab breakoff can be attributed to the end of the downward pull by the subducted slab and to buoyancy of the continental crust from which the oceanic slab broke off. The Al-Khod and Qahlah formations occur in a limited area northwest of the Saih Hatat Dome, in the vicinity of high-pressure rocks and extending only 30 km further to the west (Fig. 1b). Thus, formation and location of the studied shale in the Al-Khod Formation coincide also spatially with the slab breakoff.
Differences in the mineralogical composition of all shale samples are marginal and reflect the common origin. Nontronite, the Fe-rich end member of the smectite group, has been described from a number of geological environments, such as weathering (Dos Anjos et al. 2010), oxidation processes in gossans (Fernández-Caliani et al. 2004), relatively high-temperature hydrothermalism (Ellis 1967; Isphording 1975; Johnston 2001; Dekov et al. 2007), and microbial mineralization processes (Ueshima and Tazak 2001). The positive correlation of Al with Ti, P, and Cr, which is typical for weathering processes (Dos Anjos et al. 2010), has also been detected in the Al-Khod shales, although the chromium values are somehow ambiguous. However, the relatively high contents of chromium (Cr: 2730 ± 782 ppm) and nickel (Ni: 2108 ± 315 ppm), together with the contained lizardite and clinochlore (both deriving from the low-temperature alteration of olivines and pyroxenes), suggest a source area of mafic to ultramafic rocks of the Semail Ophiolite.
Our studies reveal two basic findings related to compaction of the shale. (1) The ptygmatically folded compaction veins (Fig. 7b) could only form due to dewatering of abundant fluid during compaction. (2) The multitude of randomly oriented calcite cracks (Fig. 8) is attributed to overpressured fluid that was expelled into the conglomerate, invading various, randomly oriented cracks near the dike precipitating calcite (see also below).
When sediment is deposited in an aquatic environment, it is saturated with water, and terrestrial sediments are commonly saturated by groundwater. Loading by more sediment leads to gradual expulsion of the pore water, except where the water gets trapped within a layer by an impermeable bed above (examples in Bruce 1973; Nicols 2009). What follows is twofold. (1) It is the impermeable red shale which trapped its own pore water and not the permeable conglomerates (as long as poorly cemented) that are overlying the shale. (2) There is evidence for a relatively high sedimentation rate. Firstly, gradual expulsion of pore water could not keep pace with sedimentation. Gradual expulsion would have otherwise precluded the state of overpressure. Secondly, the conglomerates are clear evidence for relatively rapid sedimentation.
Considering the accumulated thickness of the Al-Khod Formation within a short time (see the “Geological setting” section), deposition must have been relatively rapid. Taking into account the high shale content and swift deposition of the Al-Khod conglomerate, the water-rich shale may have been likely overpressured early on.
Due to the absence of cross-cutting relationships of the different main fault types, we assume that the faults formed during the same deformation event. Moreover, all faults strike similarly and converge downwards (Fig. 6). This and the fact that sinistral strike-slip and transtensional as well as normal faults are involved are strongly suggestive of a negative flower structure. In cross section, the width of the flower structure exceeds 100 m (Fig. 6). We refer to this fault zone as the “Sunub Fault Zone” (SFZ).
The SFZ strikes perpendicularly to the FRF and to the extension direction of stage IIa of Fournier et al. (2006), (Fig. 2). Post-Eocene dextral strike-slip along the FRF, including a releasing bend, was deduced by Mattern and Scharf (2018) close to the study area (Fig. 2). The geometric relationship between the dextral FRF and the sinistral SFZ is compatible with that of a dextral master fault and an antithetic sinistral fault (simple shear; Harding 1974) but also with that of a conjugate strike-slip fault system under pure shear conditions (Fig. 2). The latter would be in agreement with the extension direction of stage IIb of Fournier et al. (2006), (Fig. 2). Thus, the age of SFZ appears to be post-Eocene, and under consideration of Fournier et al. (2006; stage II) probably Oligocene (Fig. 2). A late Cretaceous to early Eocene deformation age sensu Fournier et al. (2006; stage I) seems to be unlikely due to the unfavorable orientation of the respective stress direction (Fig. 2).
Widening of the fault (Fig. 9) triggered shale intrusion (Figs. 9.4 and 9.5). Intrusion may have been aided by overpressure. Calcite vein formation may have taken place as well (Fig. 9.6). The shale dike truncates the shallow dipping strata of the conglomeratic host rock at a high angle (Fig. 7).
Clay smear is typical for faults and is especially well studied for extensional faults with flat/even, parallel surfaces in which the clay can be dragged (“smeared”) into the fault (Vrolijk et al. 2016). This process becomes less obvious the more widely spaced the fault blocks are and the more irregular morphologies exist, as in the case of the studied dike. Nevertheless, some clay smear may have accompanied shale intrusion in the upper part of the dike. In the case of clay smear, the fissility of the shale is parallel to the fault. In the wide, lower part of the dike, we see evidence for clay adjusting to the contours (“corners”) of the dike (Fig. 7c). This could indicate that the shale was pressed and not smeared into the dike. The vertical ptygmatically folded calcite veins we depicted (Figs. 6 and 7b) are located in the lower part of the dike.
The “floating” cobbles and boulders in the shale dike (Fig. 7a) point to viscous/cohesive behavior of the shale during intrusion (Fig. 9.4). This type of behavior is also indicated by the fissility of the red shale (Fig. 7c) that follows the outlines of the corners and edges of the country rock surrounding the shale units (Fig. 9.4). It signifies that the shale’s clay minerals were mobile and under pressure and rotated with their flat crystal faces into parallelism to the contours of the country rock, driven by pressure perpendicular to their flat sides. Dike formation is related to transtension along the SFZ which was coeval with (1) gravitational collapse, (2) related extension along the FRF, (3) doming/exhumation of the Saih Hatat Dome, and (4) isostatic rebound.
The calcite veins in the shale dike indicate calcite vein formation following emplacement of the viscous shale. During intrusion, the shale must have had a high water content which was reduced afterwards during compaction (Fig. 9.7). As a result, the calcite veins were folded by postintrusive overburden (loading by younger sediments). We, therefore, refer to them as “folded compaction veins.” Quantification of the amount of vein shortening by folding revealed that the veins were shortened by ~ 40%. This provides evidence that shale intrusion ensued with high water content, and that after vein formation an estimated 35 to 45% of water content was expelled by later compaction in the course of loading. Today, the shale dike reaches the surface (Figs. 6 and 9.7). However, the fact that the shale dike narrows towards the top suggests that the dike was closed and did not reach the surface during compaction.
The steeply dipping calcite veins that show no ptygmatic folding are interpreted as late-stage compaction veins that postdate the ptygmatig folding (Fig. 9.7). The fact that some of these veins are dragged in antiforms is attributed to shale movements in the course of which shale was pressed into the dike from below (Fig. 9.7).
The compaction history seems to be relatively simple. In line with our structural interpretation, dike formation took place following the Eocene. Thus, the water/fluid content of the late Cretaceous shales must still have been high, allowing for overpressured conditions. Since the red shale was deposited, only a minor overburden of younger Al-Khod strata has to be considered (100–200 m? at the Cretaceous/Late Paleocene hiatus). Water/fluid may have been contained within the Al-Khod Formation not only because of the low permeability of the shale itself but possibly also because of the well-cemented conglomerate (in case of early cementation).
For the gap in the stratigraphic record (base Paleogene hiatus), we rule out an eustatic drop of sea-level as a cause for most of the Danian (see Haq et al. 1988). We tentatively suggest that this Paleocene gap was due to uplift (postobductional isostatic rebound, doming of Saih Hatat, slab breakoff) which began shortly after the final emplacement of the dense Semail Ophiolite and other allochthonous thrust units. This explains the unconformity between the Al-Khod Formation and the overlying Jafnayn Formation (Nolan et al. 1990). Tilting is attributed to differential denudation and uplift, resulting in the low-angle unconformity above the Al-Khod Formation.
With the Late Paleocene to Eocene Jafnayn Formation, significant sediment overburden started to accumulate above the Al-Khod Formation. An approximately 900-m-thick late Paleocene to Eocene limestone succession was deposited during stable conditions (Searle 2007), largely coinciding with slow subsidence (Poupeau et al. 1998). This overburden led to overpressure conditions. Post-Eocene/probable Oligocene faulting along the SFZ and widening of a fault allowed for shale intrusion and dike formation.
The shale’s significant water loss, associated with the formation of the folded compaction veins, is attributed to the overburden. We suggest that the formation of the folded compaction veins was completed during the late Oligocene.
The fact that so many calcite veins are concentrated within the conglomerate host rock near the shale contacts is related to intense dewatering of the adjacent red shale. At this stage, the water that was expelled into the conglomerate was likely overpressured, as this would be the best explanation for the invasion of all the randomly oriented crack/microcrack near the dike and the precipitation of calcite therein. Evidently, there was no opportunity for the fluids to migrate to the surface (Fig. 9.6).
The timing and stratigraphic extent of the siliciclastic Al-Khod Formation is linked to tectonic processes. While the onset of deposition occurred after ophiolite emplacement, the end of the formation coincided with exhumation related to slab breakoff.
Our analysis of the compaction history of the Al-Khod Formation’s red shale is compatible with the known regional stratigraphic record and the region’s tectonic history. At first, compaction was only moderate and due to the load of the uppermost part of the late Cretaceous Al-Khod Formation. During this interval, the Saih Hatat area was exhumed due to deformation (Searle 2007); regional slab breakoff, associated with gravitational collapse; and isostatic/elastic rebound. More intense compaction is related to deposition of approximately 900-m-thick Cenozoic limestone (Wyns et al. 1992) which accumulated under stable tectonic conditions of slow and unspecified subsidence. The latest age of these limestones, which is Oligocene, marks the time as to when this stage of compaction was completed.
A new implication is that the studied shale was rich in water (and, thus, mobile) from the Maastrichtian to the mid-Tertiary, allowing for new causal interpretations of Maastrichtian to Cenozoic regional structures in terms of shale displacement in the subsurface due to loading. One example structure is the Sunub Basin (Fig. 3), an enigmatic round basin (as in “basin and dome structure”) which sits on top of the shale (Mann et al. 1990).
Other main outcomes are that the shale dike formed within a negative left-lateral flower structure (SFZ) and that the SFZ was coeval with the FRF. Thus, there is mounting evidence for Cenozoic strike-slip tectonics in the region.
We are thankful for the constructive review of the manuscript by Mohammed Al-Wardi (Sultan Qaboos University, Muscat) and three anonymous reviewers. The following acknowledgment is based on oral communication with Mohammed Al-Wardi. According to Mohammed Al-Wardi, the term “Frontal Range Fault” was coined by Samir S. Hanna (formerly of Sultan Qaboos University, Muscat) in discussions within the geoscientific community. He had identified the persistent occurrence of normal/extensional faults along the frontal range of the northern Oman Mountains, thus representing a system of faults rather than a single fault. He had prepared a manuscript on the FRF shortly before he withdrew from geology, but his manuscript was left unsubmitted.
The shale outcrops have been studied by Al Mamari (2016) and Al Hinaai (2016) as part of their Final Year Projects at Sultan Qaboos University (SQU, Muscat, Oman). Hamdan Al-Zidi’s (SQU, Earth Science Department) support in the sample preparation and Saif Al-Mamari’s (SQU, College of Science, CAARU center) X-ray diffraction work in the mineral analysis are thankfully acknowledged. We appreciate fruitful field discussions of the overall structural interpretation (flower structure) with Eugenio Carminati (University of Rome – La Sapienza, Italy), which greatly improved the manuscript. We thank the anonymous reviewers for their constructive comments.
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