Shallow Burial Dolomitization of an Eocene Carbonate Platform, Southeast Zagros Basin, Iran
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GeoArabia, 2014, v. 19, no. 4, p. 17-54 Gulf PetroLink, Bahrain Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran Afshin Zohdi, Seyed Ali Moallemi, Reza Moussavi-Harami, Asadollah Mahboubi, Detlev K. Richter, Anna Geske, Abbas A. Nickandish and Adrian Immenhauser ABSTRACT Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite 18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while 13C (averaging -0.1‰) valuesδ are within the range of Eocene seawater values (averaging 0.5‰). Dolomite Type II and III 87Sr/86S values from 0.7079 to 0.7086δ are somewhat elevated with respect to Eocene seawater (0.7077 and 0.7078). Based on these data, it is suggested that moderately evaporated seawater, via shallow seepage reflux, acted as agent for the initial dolomitization process. Subsequently, early diagenetic dolomites were recrystallized during shallow burial to variable degrees. The absence of volumetrically significant evaporitic deposits indicates that the salinity of porewater during dolomitization was beneath the threshold limit for gypsum precipitation. In addition, ascending saline fluids from deep-seated salt diapirs might have affected dolomitizing fluids. INTRODUCTION Dolostone-capped shallow-water carbonate successions have been reported throughout the geologic record (Bosence et al., 2000; Jones, 2007; Rameil, 2008; Geske et al., 2012; Zhao and Jones, 2012; Meister et al., 2013; Corbella et al., 2014). Amongst these, pervasive secondary dolomitization of shallow-water carbonates is the most abundant dolostone type, but also one that is genetically the least understood (Budd, 1997; Coniglio et al., 2003; Frazer et al., 2014). This is because many dolostone bodies, formed under different depositional and diagenetic environments worldwide, are characterized by similar fabrics and geochemical features (Sass and Katz, 1982; Machel and Mountjoy, 1986). The ongoing interest in the topic of pervasive dolomitization is due to the fact that many hydrocarbon reservoirs worldwide are located in dolomitized successions (Purser et al., 1994; Braithwaite et al., 2004; Ronchi et al., 2011; Rott and Qing, 2013; Wen et al., 2014). Current understanding is that massive dolostones can be produced by fluids of various origin in different diagenetic environments. These include evaporitic marine brines in sabkha depositional settings (e.g., Geske et al., 2012; Bontognali et al., 2012; Wen et al., 2014), evaporated seawater in seepage reflux environments (e.g., Melim and Scholle, 2002; Al-Helal et al., 2012; Vandeginste et al., 2013), normal seawater in subtidal environments (e.g., Rameil, 2008; Maliva et al., 2011; Zhao and Jones, 2012), freshwater/seawater in mixing zones (e.g., Searl, 1988; Gaswirth et al., 2007; Azmy et al., 2009; Goldstein et al., 2012; El Ayyat, 2013; Li et al., 2013) and deep basinal fluids (e.g., Green and Mountjoy, 2005; Lonnee and Machel, 2006; Ronchi et al., 2011; Frazer et al., 2014). 17 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/19/4/17/4566022/zohdi.pdf by guest on 30 September 2021 Zohdi et al. One of the more often cited dolomitization models suggest that admixtures of evaporative brines with seawater acted as parent fluids for massive dolomite bodies in numerous shallow carbonate platforms (Mriheel and Anketell, 1995, 2000; Holail et al., 2005; Kirmaci, 2008; Salad-Hersi, 2011; Conliffe et al., 2012; Wen et al., 2014). This model, however, cannot be applied directly to extensive dolomitization of limestones that are not associated with important gypsum/anhydrite deposits. Simms (1984) has shown that the reflux of fluids of slightly elevated salinity on ancient shallow platforms during periods of hydrographic restriction and climatic aridity is favorable for dolomitization of thick carbonate sequences. According to Sibley (1991) and Vahrenkamp et al. (1991), the formation of massive dolomite requires a long residence time for the dolomitized body in a near sea-level position. Essentially, the most favorable setting for dolomitization is perhaps that of prolonged sea-level highstand under slow subsidence rates. On the other hand, Sun (1994) concluded that hydrographic restriction related to frequent pulses of sea-level fall under arid climate seem to be critical factors responsible for the massive dolomitization of Paleogene dolostones in several basins worldwide. Massive replacive dolostones have been reported from the Zagros Basin of Iran, the basin under study here, and several of these dolostone bodies are important regional hydrocarbon reservoirs (Sun, 1995; Warren, 2000; Zohdi et al., 2011). Understanding the genesis of dolomitized limestone might aid the prediction of the distribution of dolostone bodies and shed light on geochemical fluxes of fluids in the subsurface. Here, we document and discuss a case example of an Eocene, pervasively dolomitized ramp (Jahrum Formation) from the Zagros Basin in Iran. Published data on dolomitized ramp carbonates from this region are rather limited (Zohdi et al., 2013), in comparison to the better studied Permian and Triassic dolomitized ramps in the Zagros Basin (Moradpour et al., 2008; Rahimpour-Bonab et al., 2009, 2010; Tavakoli et al., 2011; Esrafili-Dizaji and Rahimpour-Bonab, 2013; Mohammadi Dehcheshmehi et al., 2013). In general and particularly so in Iran, dolomitization of Permian–Triassic platforms has been mainly ascribed to seepage-reflux and/or evaporative mechanisms (e.g., Moradpour et al., 2008; Rahimpour-Bonab et al., 2009, 2010; Geske et al., 2012; Meister et al., 2013; Jiang et al., 2013). In this paper we aim (1) to provide a comprehensive characterization of the petrography, geochemistry and spatial architecture of Jahrum Formation dolostones; and (2) to discuss the origin of this dolostone facies in its basin-wide context. GEOTECTONIC SETTING The Iranian plateau extends over a number of continental terranes welded together along suture zones of oceanic character (Berberian and King, 1981; Alavi, 2007). These terranes include the following provinces: (1) Zagros; (2) Alborz; (3) Central Iran; (4) Kopeh-Dagh; and (5) Makran sedimentary basin. The study area is located in the southeastern Zagros Basin (Figure 1). This basin constitutes a major structural feature within the Alpine-Himalayan Orogen, marking the transition between the Zagros Collision Belt to the west and the Makran and Oman Mountains to the east. Numerous salt diapirs characterize the southeastern Zagros Basin (Edgell, 1996; Jahani et al., 2009). These diapirs are composed principally of Upper Precambrian–Lower Cambrian Hormuz Salt. Where undeformed, the salt is overlain by more than four km of sedimentary rocks (Edgell, 1996; Jahani et al., 2009). The emergent diapirs provide an opportunity to study the diagenetic history of the affected units in association with the Hormuz Salt (Ghazban and Al-Aasm, 2010). The Hormuz Salt started to mobilize as early as Jurassic to Early Cretaceous based on geological evidence, but most diapirs did not reach the surface until the folding of the Zagros Mountains during the Paleogene (Ala, 1974; Jahani et al., 2009). At the initiation of the folding, diapirs had already been re-activated by earlier tectonic events and salt movement along faults resulting in evaporitic facies reaching the surface (Jahani et al., 2009). The overall geotectonic setting clearly affected the evolution of the Paleogene carbonate platforms in the southeast Zagros Basin (Zohdi et al., 2013). Hormuz salt diapirs are potentially of significance in the context of dolomitization processes as a source of saline ascending fluids (Ghazban and Al-Aasm, 2007, 2010). In the study area, anticlines are built most often by competent limestone rocks of the Eocene Jahrum and the Lower-Middle Miocene Gurpi formations (Figure 1). Here, large-scale anticlines and synclines 18 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/19/4/17/4566022/zohdi.pdf by guest on 30 September 2021 Dolomitization of Eocene platform, southeast Zagros Basin, Iran Faraghun Finu Khush Genow IRAN Anguru Strait of Hormuz Cenozoic Paleozoic to Mesozoic Aghajari Formation Jahrum Formation Bangestan Group Dalan Formation Mishan Formation Guri Member Pabdeh Formation Khami Group Faraghun Formation Razak Formation Jahrum Formation Neyriz-Khaneh Siahou and Sarchahan Gachsaran Formation Section Kat formations formations Asmari-Jahrum formations Road Khaneh kat Formation Hormuz Formation Figure 1: Geological map of the southeastern Zagros Basin, showing location of measured sections (blue stars). Modified after Fakhari (1994). mostly have an E-W trending orientation that differs from other regions