Evolution of a terminal Neoproterozoic carbonate ramp system (Buah Formation, Sultanate of Oman): Effects of basement paleotopography Andrea Cozzi† Institute of Geology, Department of Earth Sciences, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
John P. Grotzinger‡ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
Philip A. Allen§ Institute of Geology, Department of Earth Sciences, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
ABSTRACT The Buah Formation is a carbonate unit of late Neoproterozoic age, exposed in the Jabal Akhdar of northern Oman and the Huqf region of central Oman. The Buah Formation is interpreted as a distally steepened stormdominated ramp, containing inner ramp tidally influenced oolitic shoal and back-shoal lagoon facies and storm-dominated mid ramp facies. A slope break separates mid-ramp facies from outer ramp facies characterized by accumulation of redeposited carbonates on gently inclined slopes (1–2 degrees). Combined facies stacking patterns and outcrop correlation via δ13C profiles suggest that Buah carbonates formed as a highstand systems tract characterized by a general upward shallowing of facies and strong lateral progradation. On a regional scale, Buah facies distributions across Oman were controlled by the presence of basement highs defining areas of reduced subsidence, which promoted lateral progradation of the Buah carbonate ramp for tens of kilometers, while the faster-subsiding areas became sites of deep-water deposition. Keywords: carbonate ramp, carbon isotopes, correlation, basin analysis, Late Neoproterozoic, Oman. INTRODUCTION Carbonate ramps were common in the Proterozoic and particularly abundant during Neoproterozoic time (Grotzinger and James, 2000). †
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What makes them interesting for carbonate sedimentological studies is the relatively simple organization of carbonate production zones, consisting primarily of microbial communities that showed broad environmental distribution due to their lowered sensitivity to environmental variability. The comparative simplicity of Proterozoic depositional systems that results from the reduced sensitivity of biological communities to most environmental stresses, allows these older carbonates to preserve a geological record of platform-ramp growth in response to eustasy and basin subsidence that is easier to interpret than in their Phanerozoic counterparts (Grotzinger, 1989; Grotzinger and James, 2000). In addition, the absence of sediment bioturbators allowed the preservation of original chemically, mechanically, and microbially produced sedimentary features that would have been disrupted in equivalent Phanerozoic depositional settings, again helping to facilitate paleoenvironmental interpretation. In this paper we present the results of an integrated sedimentological, geochemical, outcrop, and subsurface study on a well-preserved Late Neoproterozoic carbonate ramp succession in Oman. The study was conducted to improve the understanding of the facies architecture of the Buah Formation, a gas reservoir in the Oman subsurface actively explored by Petroleum Development Oman (PDO) (Cozzi, 2000a; Tiley et al., 2000; Cozzi and Al-Siyabi, 2004). Basin compartmentalization as a result of basementinvolved deformation occurred at the end of Buah deposition (Cozzi et al., 2001; Grotzinger et al., 2002), making the Buah Formation an ideal research target for the investigation of tectonic and eustatic controls on carbonate ramp development. In addition, the presence of a large subsurface database made available by PDO and the
acquisition of new geochemical and geophysical data from outcrop allowed close integration of outcrop and subsurface data to produce a geological model for use in hydrocarbon exploration (Cozzi et al., 2002; Cozzi and Al-Siyabi, 2004). The study presented here also allows insight into the basic architecture and depositional processes of carbonates formed during a critical interval in earth history. Late Neoproterozoic shallow-water carbonates lack, for the most part, any significant influence provided by calcifying metazoans and metaphytes that is the hallmark of Phanerozoic carbonate systems. Yet, on the other hand, they differ from older Proterozoic and Archean platforms in lacking massive seafloor carbonate precipitates and containing ordinary marine evaporite minerals and successions. Thus, they represent the transition from an ancient, largely abioticmicrobial world of carbonate deposition to more modern modes of carbonate production and patterns of accumulation. GEOLOGICAL SETTING Precambrian rocks in Oman crop out in erosional windows formed within the tectonic thrust plates of Jabal Akhdar and Saih Hatat (northern Oman), in the Huqf uplift (east-central Oman), and in the Mirbat area of the southern Oman escarpment (Fig. 1). In addition, Precambrian rocks have been extensively drilled in the subsurface (Ghaba and South Oman Salt Basins) for hydrocarbon exploration by PDO. The Huqf Supergroup (Loosveld et al., 1996), formerly the Huqf Group of Gorin et al. (1982), comprises a Neoproterozoic sedimentary cover overlying older igneous and metamorphic basement rocks (ca. 800 Ma; Leather, 2001). The Huqf Supergroup is composed of the glaciogenic rift-related
GSA Bulletin; November/December 2004; v. 116; no. 11/12; p. 1367–1384; doi: 10.1130/B25387.1; 17 figures; 1 table; Data Repository item 2004170.
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Abu Mahara Group (Leather, 2001; Leather et al., 2002), the overlying mixed carbonate-clastic marine sequences of the Nafun Group, and the upper evaporitic-carbonate Ara Group, which spans the Precambrian-Cambrian boundary (Amthor et al., 2003), but which is known only from the subsurface of Oman (Hughes Clarke, 1988; Loosveld et al., 1996). The Neoproterozoic basin evolution of Oman is still under investigation. The Nafun Group is found extensively in north-central Oman in many well penetrations, and seismic reflection data show that generally it tapers gently in thickness toward the W. This suggests that the Nafun Group was deposited at a time of extensive regional subsidence. A number of explanations for this regional subsidence exist. First, the Nafun Group may be a result of postrift thermal relaxation following rifting (Loosveld et al., 1996). However, the thermal maturity of Abu Mahara and Nafun Groups is generally lower than might be expected for postrift age sedimentary rocks (Terken et al., 2001). Alternatively, the Nafun Group may represent dynamic subsidence in a retro-arc setting related to westward subduction of an oceanic slab beneath the Arabian plate from the northeastern margin of Gondwanaland (central Iran) (Grotzinger et al., 2002). In this scenario, the volcanics and volcaniclastics of the overlying Ara Group would be evidence of subduction-related melting and the carbonate-evaporite cycles the effect of orographic retro-arc desiccation. In a third scenario, the Nafun Group may have accumulated in a wide basin flexed down between a magmatic arc (in the E) at the margin of Gondwanaland and a transpressive megasuture (to the W) representing the accretion of a number of oceanic and continental terranes onto the Arabian area. Ongoing research is evaluating these different geodynamic scenarios. THE BUAH FORMATION IN OMAN The Nafun Group, as defined by Cozzi and Al-Siyabi (2004) is made of the Hadash Formation at the base, which in turn is overlain by two major clastic-carbonate cycles. The first is composed of the Masirah Bay Formation, comprising shallow to deep marine clastics, and the overlying Khufai Formation, which consists of ramp carbonates. The second cycle is made up of shelf clastics of the Shuram Formation and ramp dolostones of the overlying Buah Formation (Gorin et al., 1982; Hughes Clarke, 1988). The Buah Formation crops out extensively in the Jabal Akhdar region and in the Huqf uplift. It was originally defined in the Mukhaibah Dome outcrops of the southern Huqf area (Gorin et al., 1982), and Tschopp (1967), Gorin et al. (1982),
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Figure 1. Summary of outcropping Neoproterozoic stratigraphy in Oman. Absolute age dates from Brasier et al. (2000) and Leather (2001). Geographic map of Oman (inset) shows location of Figures 2, 8, and 15.
Rabu (1988), and Mann and Hanna (1990) recognized its lithostratigraphic equivalence with the Kharus Formation (Glennie et al., 1974) of the Jabal Akhdar. Possible Buah equivalents are also found in the Saih Hatat erosional window (Fig. 1, inset), as suggested by Mattes and Conway Morris (1990) and Mann and Hanna (1990), where a dolomitic unit called the Hiyam Dolomite (Glennie et al., 1974), later renamed Hiyam Formation (Le Metour, 1988; Rabu et al., 1993), is found below the sub-Permian unconformity. The Buah Formation generally is pervasively dolomitized; however, it has a well-defined calcareous basal zone. In surface exposures Buah carbonates conformably overlie Shuram
Formation clastics; the contact is characterized by a gradational boundary a few meters thick. In the Oman mountains, the Buah Formation is overlain by the Fara Formation, which consists of cherty limestones and volcaniclastics and/or Permian carbonates in the Jabal Akhdar. However, in the Huqf area a thick succession of peritidal carbonates (
