Transition from the Hajir Formation to the Muaydin Formation: A facies change coinciding with extensional, syndepositional faulting (Ediacaran, Jabal Akhdar Dome, Central Oman Mountains)

https://doi.org/10.1016/j.jafrearsci.2019.02.016Get rights and content

Highlights

  • At the Hajir/Muaydin Fm. boundary a carbonate/siliciclastic facies change occurs.

  • We observed syndepositional extension at this Edicaran facies transition.

  • Extension produced a rollover syncline and anticline.

  • We conclude that extensional tectonics caused the facies change.

  • Extension ensued subparallel to the Semail Gap, indicating similar activity there.

Abstract

In the core of the Jabal Akhdar Dome, an irregular horst composed of carbonates of the Hajir Formation is bounded by syndepositional extensional faults on either side. The western, SSW-dipping fault is an extensional growth fault associated with rollover anticline and syncline within syndepositional siliciclastic strata of the Muaydin Formation. Syndepositional extension is indicated by increasing thickness of the involved strata of the Muaydin Formation towards the fault. The eastern, ESE-dipping fault, exhibits a larger displacement than the western fault. Both the western and the eastern fault are blanketed by postdeformational Muaydin deposits. Taking into account that the strike of both faults differs significantly (see above), we suggest that the eastern fault represents the main fault of a relay ramp system whereas the western one is the fault of a breached relay ramp with a dip angle of 65°. Compaction is able to explain some particular stratal patterns of the Muaydin Formation. The eastern fault may provide insights as to possible similar Ediacaran movements along the NNE-striking Proto-Semail Gap (i.e. the Proterozoic Semail Gap). Ediacaran vertical block movements at the latter may explain why the Muaydin Formation is present in the Jabal Akhdar Dome in the west but absent in the easterly located Saih Hatat Dome. The tectonic setting is characterized by WNW-ESE-oriented extension. Immediately west of the irregular horst we found a seemingly complete stratigraphic transition from the Hajir Formation to the Muaydin Formation. On top of this structure is a minor stratigraphic hiatus at the contact between both formations. Although the observed features are all outcrop-scale phenomena, we suggest that they provide insights into the general Ediacaran tectonic and depositional setting. This setting is characterized by complete and incomplete stratigraphies and by syndepositional extension which may be held responsible for the abrupt change from carbonate to siliciclastic facies as well as common slumps in the lower part of the Muaydin Formation.

Introduction

Widespread Neoproterozoic to Cambrian sedimentary rocks of eastern Oman display a facies division characterized by several alternations of siliciclastic and carbonate formations (Fig. 1; Gorin et al., 1982; review in Allen, 2007). Their facies (Gorin et al., 1982) show that they accumulated in a platform setting. The respective succession can be studied at the surface in the Huqf area and in the core of the Jabal Akhdar Dome (Fig. 2, Fig. 3).

We are exploring the facies transition between two Neoproterozoic formations – the Hajir Formation, consisting of dolomitic limestone and the overlying siliciclastic Muaydin Formation and its cause by syndepositional tectonism. In the following paragraphs we (1) introduce the stratigraphic position of these formations within the complex stratigraphic system which assigns different names for the same stratigraphic intervals, depending on their regional occurrence, (2) outline what has been published about the regional Ediacaran deformation, and (3) clarify the Ediacaran age of the transition interval. Previous works on the facies transition are summarized in section 1.2.

Our study area is located in the Central Oman (“Hajar”) Mountains (Fig. 1, Fig. 2b) within the core of the Jabal Akhdar Dome (Fig. 3). The lateral equivalents of the Hajir and Muaydin formations in the Huqf area are the Khufai and Shuram formations (Fig. 1), respectively (Gorin et al., 1982; Allen, 2007). The absence of both the Hajir and the Muaydin formations in the Saih Hatat Dome (Fig. 2, Fig. 3; Le Métour et al., 1986a, b; Villey et al., 1986) is not understood.

The stratigraphic contact between the two formations has been considered normal/regular (Beurrier et al., 1986a; Allen, 2007), characterized by a major unconformity (McCarron, 2000; Bowring et al., 2007; Forbes et al., 2010) or marked by significant flooding (Osburn et al., 2014). Both formations belong to the Nafun Group which in turn belongs to the Huqf Supergroup (Fig. 1). Their age is assigned to the Ediacaran (Fig. 1; Allen, 2007, his Fig. 4, Fig. 9). According to Walker et al. (2012), the lower limit (stratigraphic base) of the Ediacaran is defined at 635 Ma and its upper limit (stratigraphic top) at 541 Ma. Available descriptions of the Hajir-Muaydin transition for the Jabal Akhdar region reveal some variability, as the Hajir Formation may be missing and facies transitions can be more gradual or less gradual (section 1.2).

To understand the transition of the Hajir Formation to the Muaydin Formation, it is of interest what their tectonic setting was during the Ediacaran (635–541 Ma). Thus, the effects of the younger Amar Collision (∼640–∼620 Ma) need to be considered (Al-Husseini, 2000). The collision followed a north-striking suture and was followed by a widespread extensional collapse of the Arabian-Nubian Shield between ∼620 and ∼530 Ma (Al-Husseini, 2000).

Somewhat differently, extensional collapse was envisaged by Blasband et al. (2000) for the entire Arabian-Nubian Shield within the time interval of 600–560 Ma. They also stressed involvement of igneous rocks and the extension of the Arabian Shield in NW-SE direction.

According to Loosveld et al. (1996), the lack of volcanic rocks and coarse-grained clastics implies that deposition of the Nafun Group ensued during tectonic quiescence. They suggested that sediment accumulation was controlled by thermal relaxation in the aftermath of the Abu Mahara rifting event which affected the eastern part of the Arabian Peninsula. Correspondingly, the Hajir and Muaydin formations as well as other formations have been interpreted as an extensive sheet of post-rift stratigraphy (Allen, 2007). The Abu Mahara Rift clearly predates deposition of the Nafun Group (e.g., Loosveld et al., 1996; Allen, 2007).

Stratigraphic and age data compiled by Al-Husseini (2014, his Table 1) indicate that different ages for the boundary between the Hajir and Muaydin formations have been assigned by different authors. These differences are due to a lack of direct geochronological markers and, thus, related to, e.g., different calculations of sedimentation rates within the formations. Our Fig. 1 follows the ages of Forbes et al. (2010; 590 Ma). These ages are slightly younger than those by Le Guerroué (2006) and Le Guerroué et al. (2006). According to Le Guerroué (2006) and Le Guerroué et al. (2006), who combined isotopic U/Pb ages with thermal subsidence modeling, the onset of a major negative δ13C excursion at the base of the Shuram Formation is constrained at ∼600 Ma. The ages of ∼600 and 590 Ma are compatible with the chronostratigraphic compilation by Allen (2007, his Fig. 8), which takes into account radiometric ages, according to which the Hajir (Khufai) Formation started to accumulate some time after 645 Ma and deposition of the Muaydin (Shuram) Formation ceased some time before 550 Ma. They are also compatible with the time constraint provided by Bowring et al. (2007). According to Bowring et al. (2007), the base of the Muaydin (Shuram) Formation has a maximum age of 620 Ma as revealed by respective detrital zircon ages.

The facies division of the Huqf Supergroup sedimentary rocks implies transitions whose causes are not well understood. We want to improve the causal understanding of this transition through new observations made at a superb new road outcrop.

We present field data of syndepositional faulting and related structural features at the Hajir-Muaydin transition. We can demonstrate that deformation is clearly linked to the transition because of the syndepositional nature of the observed faults. This, in turn, allows us to interpret the relevance of faulting to the facies change, determine the tectonic setting, and explain the absence of both the Hajir and the Muaydin formations in the Saih Hatat Dome.

The Oman Mountains formed during the Late Cretaceous in the course of SW-directed obduction of the Tethys-derived Semail Ophiolite onto the passive margin and platform of the Arabian Plate due to convergence of the Arabian and Eurasian plates (e.g., Glennie et al., 1973, 1974; Searle and Malpas, 1980; Lippard et al., 1986; Goffé et al., 1988; Hacker, 1994; Hacker et al., 1996; Searle and Cox, 1999; Searle, 2007). At the base of the ophiolite, Tethyan ocean floor sediments (Hawasina sediments) were thrust along.

In the eastern part of the Oman Mountains, the oldest rocks are exposed in the cores of two large anticlinal structures/culminations/domes (e.g., Beurrier et al., 1986a, b; Béchennec et al., 1992, 1993), namely the Jabal Akhdar Dome in the west and the Saih Hatat Dome in the east (Fig. 2, Fig. 3). Formation of both anticlinal structures is related to shortening in the course of ophiolite obduction (e.g., Searle, 2007). Final uplift and exhumation of the Jabal Akhdar Dome started during the Eocene (Hansman et al., 2017; Grobe et al., 2019).

The oldest rocks of the Jabal Akhdar and Saih Hatat domes are autochthonous and considered to be of Late Precambrian age (e.g., Beurrier et al., 1986b). These rocks received a gentle metamorphic overprint. The Muaydin Formation of Jabal Akhdar revealed corresponding chlorite K/Ar crystallization ages of 329 ± 11 and 321 ± 10 Ma (Beurrier et al., 1986b).

Between the Jabal Akhdar and Saih Hatat domes lies a ∼70 km long, NNE-striking, lineamentary feature known as the “Semail Gap” (Fig. 3). It borders the Jabal Akhdar Dome to the east. It is intriguing that its projected extension to the SSW coincides with the basin margins of the Ghaba and South Oman salt basins (Fig. 3) in which salt of the Ara Salt Formation of the latest Ediacaran to Cambrian (Fig. 1; e.g., Allen, 2007) accumulated. The modern Semail Gap is an extensional fault or fault zone (Searle, 2007), with the Semail Ophiolite on the eastern and downthrown side (Béchennec et al., 1992, 1993; Searle, 2007). The Semail Gap may have had a Precambrian/Cambrian history which could be related to the formation of the salt basins (above). However, its possible Ediacaran and Cambrian history is not well understood.

The thickness of the Hajir Formation varies. At the type locality, in the gorge of Wadi Hajir, the formation is 100 m thick (Beurrier et al., 1986a; Béchennec et al., 1992). In other places, the thickness may measure only 30 or 15 m (Béchennec et al., 1992). The formation may even be missing altogether (Béchennec et al., 1992) and, thus, also the transition between the Hajir and Muaydin formations. The Hajir Formation is mainly characterized by dark, fetid, dolomitic limestones (Beurrier et al., 1986a; Béchennec et al., 1992). At the type section, the lowest 15 m of the formation consist of dark, medium-bedded, dolomitic limestone which are overlain by 35 m of massive dolomitic limestone containing reworked quartz grains, angular fragments of beige dolomite measuring 1 mm to 3 cm, as well as larger intraclasts (up to 5 cm; Beurrier et al., 1986a). Further up, stromatolites occur (Beurrier et al., 1986a) which are associated with the cyanophyte microflora Palaeocryptidium cayeuxi (Béchennec et al., 1992). The top is marked by intercalation of silty layers towards the Muaydin Formation (Beurrier et al., 1986a). The carbonates of the formation have a high organic content of 2.5–3.5% (Cozzi and Al-Siyabi, 2004) indicating euxinic conditions (Béchennec et al., 1992). Their depositional environment has been characterized as “lagoonal” (Beurrier et al., 1986a; Rabu et al., 1986) or “littoral” (Béchennec et al., 1992), respectively.

In the Rustaq area (northern flank of the Jabal Akhdar Dome), the Muaydin Formation measures approximately 800 m in thickness, mainly comprising mauve to green siltstone with thin beds of sandstone and carbonate (Beurrier et al., 1986a). The basal 40–50 m consist of mauve siltstone with scarce rust-colored quartzose sandstone (Beurrier et al., 1986a). The formation's middle section measures a few hundred meters in thickness and is characterized by both laminated mauve and green siltstone exhibiting ripple and cross-lamination and by more sandy beds, a few millimeters to a few centimeters thick, composed of muscovite, chlorite, quartz, plagioclase and accessory carbonate in the matrix (Beurrier et al., 1986a). The upper ∼50 m show a transition to the overlying carbonates of the Kharous Formation, indicated by intervening carbonate lenses and millimeter-thick carbonate laminae. These intercalations become thicker (1–5 cm) upwards (Beurrier et al., 1986a). The formation reflects a littoral depositional environment (Beurrier et al., 1986a; but compare below with Gorin et al., 1982; Wright et al., 1990; McCarron, 2000; Le Guerroué et al., 2006!).

With respect to the subject of this study, it needs to be pointed out that the basal part of the Muaydin Formation in the southern area of the Jabal Akhdar Dome differs from that in the north. In the south, the formation starts with coarser clastic deposits, namely coarse-grained, carbonate-cemented quartz sandstone passing up into finely laminated, gray-green and purple, micaceous, shaly siltstones, including decimeter-thick beds of fine-grained sandstone (Béchennec et al., 1992). Evidently, the transition between the Hajir Formation and the Muaydin Formation is less gradual in the south than in the north within the Jabal Akhdar Dome.

Whereas the Hajir Formation is interpreted as a carbonate ramp that passes from its inner part in the Huqf area to its outer realm in the Jabal Akhdar Dome, the Muaydin Formation may represent a storm-dominated siliciclastic shelf in the Huqf area that passes gradually into a more distal environment in the Jabal Akhdar Dome (Gorin et al., 1982; Wright et al., 1990; McCarron, 2000; Le Guerroué et al., 2006).

The Hajir Formation is subdivided into four facies associations: (1) fetid carbonates, (2) ooidal–peloidal grainstones and stromatolites, (3) peritidal shallowing-upward, m-scale cycles of ooidal/peloidal grainstones (base of cycles), channel fills, mudstones with mm-scale siltstone and sandstone beds passing up into stromatolites (top of cycles) and (4) intermixed siltstones and sandstones (McCarron, 2000; Le Guerroué et al., 2006). The latter records input of siliciclastic detritus at the top of the carbonate ramp (Le Guerroué et al., 2006). The siliciclastic deposits occur as dispersed quartz grains in a carbonate matrix (forming discontinuous cm-thick layers or graded material in the carbonates), whereas at a stratigraphically higher level, coarser clastics are found in erosive channel-fills, eroded into facies association 2 and thinly bedded pinkish mudstones. Facies association 4 marks the demise of the carbonate ramp in the Jabal Akhdar Dome (Le Guerroué et al., 2006). The Muaydin Formation comprises three facies associations: (1) dolomitic mudstones, bleached siltstones and common slumps, (2) monotonous siltstones and shales and (3) clastic/carbonate parasequences attaining a maximum thickness of only 20 m (Le Guerroué et al., 2006).

The studied outcrop is situated in the center of the Jabal Akhdar Dome (Fig. 2, Fig. 3, Fig. 4). It is a road outcrop that occurs on the southern side of the paved new road that follows along Wadi Hajir from the community of Al Hajir in the north to the community of Halhal in the south. Wadi Hajir is a southwestern tributary to the great Wadi Bani Kharous (Fig. 2B). The outcrop is located directly south of Al Hajir, 600 m east of the canyon of Wadi Hajir, the type section of the Hajir Formation. Because the road is built on a steep slope it is not possible to produce a high-resolution photograph of the entire outcrop (Fig. 4). Since it is a new road outcrop, its condition is excellent. The outcrop's coordinates are 23°12′16.2″ N and 57°30′03.1″ E. The outcrop can be accessed from the north via Al Awabi and the paved road in Wadi Bani Kharous. The outcrop is ∼100 m long and ∼5 m high (Fig. 5).

Section snippets

Results

The outcrop exposes exactly the transition from the Hajir Formation to the overlying Muaydin Formation (Fig. 5, Fig. 6, Fig. 7). The general bedding attitude in Clar values (dip direction/dip angle; used in this study) is (355/15-20). The outcrop reveals a structure which, at the first glance, appears to be a horst. It is cored by carbonates of the Hajir Formation and flanked, as well as covered, by the fine-grained Muaydin siltstones. The Hajir carbonates are separated from the Muaydin

Interpretation

Despite the lack of the above mentioned features it is possible to further characterize the nature of the faults. The fact that both faults deformed Muaydin sediments and are overlain by Muaydin sediments (Fig. 6, Fig. 7) shows that both faults were active during the earliest history of the Muaydin Formation. They are syndepositional faults and were, therefore, active at around ∼600/590 Ma (section 1).

Conclusions

The literature review of the stratigraphic/facies transition from the Hajir to the Muaydin Formation (section 1.2) does not indicate regular, quiet depositional conditions. The reviewed data provide leeway for a “dynamic” interpretation of the tectonic setting compatible with our structural findings and interpretations. The literature review also indicates that the transition from the Hajir Formation to the Muaydin Formation occurred at ∼600 or 590 Ma which age constraints the observed

Acknowledgements

We are indebted to Katharina Scharf for her field assistance. We also thankfully acknowledge the outstanding constructive manuscript review by Barbara Tewksbury (Hamilton College, New York State, U.S.A.). She suggested the breached relay ramp model which simplified our interpretation. We also thank her for additional communication and advice. We also thank Eugenio Carminati (Sapienza Università di Roma, Italy) for a helpful field discussion of compaction above the eastern fault. An earlier

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