Y. Lasemi A. H. Jalilian

Transcription

1 DOI /s ORIGINAL ARTICLE The Middle Jurassic basinal deposits of the Surmeh Formation in the Central Zagros Mountains, southwest Iran: facies, sequence stratigraphy, and controls Y. Lasemi A. H. Jalilian Accepted: 7 August 2010 Ó Springer-Verlag 2010 Abstract The lower part of the Lower to Upper Jurassic Surmeh Formation consists of a succession of shallow marine carbonates (Toarcian Aalenian) overlain by a deep marine basinal succession (Aalenian Bajocian) that grades upward to Middle to Upper Jurassic platform carbonates. The termination of shallow marine carbonate deposition of the lower part of the Surmeh Formation and the establishment of deep marine sedimentation indicate a change in the style of sedimentation in the Neotethys passive margin of southwest Iran during the Middle Jurassic. To evaluate the reasons for this change and to assess the basin configuration during the Middle Jurassic, this study focuses on facies analysis and sequence stratigraphy of the basinal deposits (pelagic and calciturbidite facies) of the Surmeh Formation, referred here as lower shaley unit in the Central Zagros region. The upper Aalenian Bajocian lower shaley unit overlies, with an abrupt contact, the Toarcian lower Aalenian platform carbonates. It consists of pelagic (calcareous shale and limestone) and calciturbidite facies grading to upper Bajocian Bathonian platform carbonates. Calciturbidite deposits in the lower shaley unit consist of various graded grainstone to lime mudstone facies containing mixed deep marine fauna and platform-derived material. These facies include quartz-bearing lithoclast/intraclast grainstone to lime mudstone, bioclast/ooid/peloid intraclast grainstone, Y. Lasemi (&) Oil and Gas Section, Illinois State Geological Survey, Institute of Natural Resource Sustainability, University of Illinois at Urbana-Champaign, Champaign, IL, USA ylasemi@isgs.illinois.edu A. H. Jalilian Department of Geology, Payame Noor University, Ahvaz, Iran jalilian@pnu.ac.ir ooid grainstone to packstone, and lime wackestone to mudstone. The calciturbidite layers are erosive-based and commonly exhibit graded bedding, incomplete Bouma turbidite sequence, flute casts, and load casts. They consist chiefly of platform-derived materials including ooids, intraclasts/lithoclasts, peloids, echinoderms, brachiopods, bivalves, and open-ocean biota, such as planktonic bivalves, crinoids, coccoliths, foraminifers, and sponge spicules. The lower shaley unit constitutes the late transgressive and the main part of the highstand systems tract of a depositional sequence and grades upward to platform margin and platform interior facies as a result of late highstand basinward progradation. The sedimentary record of the lower shaley unit in the Central Zagros region reveals the existence of a northwest southeast trending platform margin during the Middle Jurassic that faced a deep basin, the Pars intrashelf basin in the northeast. The thinning of calciturbidite layers towards the northeast and the widespread Middle Jurassic platform carbonates in the southern Persian Gulf states and in the Persian Gulf area support the existence of a southwest platform margin and platform interior source area. The platform margin was formed as a result of tectonic activity along the preexisting Mountain Front fault associated with Cimmerian continental rifting in northeast Gondwana. Flooding of the southwest platform margin during early to middle Bajocian resulted in the reestablishment of the carbonate sediment factory and overproduction of shallow marine carbonates associated with sea-level highstand, which led to vertical and lateral expansion of the platform and gradual infilling of the Pars intrashelf basin by late Bajocian time. Keywords Basinal facies Calciturbidite Sequence stratigraphy Pars intrashelf basin Middle Jurassic Zagros Mountains

2 Introduction The Lower to Upper Jurassic Surmeh Formation (James and Wynd 1965) consists mainly of limestone and dolomite and crops out in the High Zagros and Zagros Simple Fold Belt of the Zagros Mountains (Fig. 1) in southwest Iran (Zagros subdivision is according to Berberian 1995). The Surmeh Formation is up to 1,000 m thick and forms an important petroleum reservoir in a number of giant oil fields in the Persian Gulf area. Previous studies on the Surmeh Formation have focused on general lithostratigraphy (e.g., James and Wynd 1965; Kamen-Kaye 1970; Kheradpir 1975; Setudehnia 1978; Motiei 1993). The formation has been referred to as a shallow marine carbonate succession (James and Wynd 1965; Setudehnia 1978; Murris 1980; Alavi 2004), but to date, no studies concerning facies distribution, depositional environments, and controls have been presented. Detailed facies analysis has revealed that the Surmeh Formation is a dominantly carbonate succession composed of a basal shallow marine unit that is overlain by a deep marine shaley interval that in turn grades upward to shallow marine deposits. The shaley carbonate interval in the lower part of the Surmeh Formation is referred to herein as the lower shaley unit. The objective of this study is to describe facies and sequence stratigraphy of the lower shaley unit in the Surmeh and Khaneh-Kat surface sections and in a subsurface analog section, the Tabnak No. 21 well (Fig. 1), to establish sea level and tectonic controls on sedimentation and platform evolution in the study area during the Middle Jurassic. Methods This study is based on the field and laboratory investigation of the lower part of the Surmeh Formation in the Central Zagros region of southwest Iran. Two outcrop sections (Fig. 1) including Surmeh (type locality S, south of Shiraz) and Khaneh-Kat (locality K, northeast of Surmeh section) were studied to determine the facies types, depositional cycles, and their lateral and vertical variations. More than 200 samples were collected for petrographic studies to enhance the field descriptions. Whole rock samples of four horizons within the Khaneh-Kat section were analyzed for C and O isotope variations, and for Mn and Fe values. The facies stacking patterns recognized in the outcrop sections were correlated to the gamma ray log patterns of a subsurface analog section, the NIOC Tabnak No. 21 well (locality T, southeast of the Surmeh section along the Mountain Front fault in Fig. 1). Facies types and their depositional environments were determined based on the field and petrographic criteria and comparison with recent and ancient environments (e.g., Wilson 1975; Tucker and Wright 1990; Flugel 2010). Carbonate rocks are classified on the basis of Dunham (1962) textural classification scheme. Geological setting and stratigraphy The Zagros orogenic belt extends in a northwest southeast direction from southeast Turkey in the northwest to the north of the Strait of Hormuz in southern Iran. It is interpreted as the product of subduction of the Neo-Tethyan oceanic crust beneath the Central Iranian lithospheric plates, the Cimmerian Plate (Fig. 1), during Early to Late Cretaceous and subsequent collision of southwest Iran with central Iran in Late Cretaceous and later times (Alavi 1994, 2004). During the Late Permian through Late Mesozoic, the Zagros Mountain belt was a part of the Arabiansouthwest Iranian Plate of the northeast Gondwana margin, at equatorial location, at the Neotethys passive margin (e.g., Berberian and King 1981; Stampfli and Pillevuit 1993; Scotese and Langford 1995; Golonka 2004). Several thousands of meters of mainly carbonate sediments were deposited during the Mesozoic (James and Wynd 1965; Setudehnia 1978; Szabo and Kheradpir 1978; Motiei 1993; Kashfi 1992; Alavi 2004) in the extensive northeast facing platforms within the Neotethys passive margin of southwest Iran. The study area of Central Zagros is located in the Simple Fold Belt and the High Zagros thrust belt (Berberian 1995) between the Mountain Front fault () in the southwest and the Main Zagros reverse fault (MZRF: the Neotethys suture) in the northeast (Fig. 1). The faults were normal faults associated with the Carboniferous to Early Permian rifting of northeast Gondwana, which led to separation of the Cimmerian Plate and formation of the Neotethys passive margin in Late Permian (e.g., Stampfli and Pillevuit 1993; Sengor and Natalin 1996; Lasemi 2001). The Surmeh Formation (Fig. 2) is the lowermost lithostratigraphic unit of the Lower Jurassic to Lower Cretaceous Khami Group (James and Wynd 1965). James and Wynd named and measured the Surmeh type section in the Fars Province about 120 km south of Shiraz. In the type locality (Surmeh section) and in the Khaneh-Kat section (Fig. 1), the formation consists mainly of dolomite and limestone and is bounded, with unconformable contacts, by the lowermost Jurassic Neyriz Formation and the lowermost Cretaceous Fahliyan Formation (Fig. 2). In the southern part of the Fars Province, the Surmeh Formation is unconformably overlain by the Tithonian Hith Anhydrite (Fig. 2). Towards the northwest in the Lurestan sub-basin (Fig. 1, 2), a distance of over 300 km, the Surmeh Formation laterally changes to different rock units (Fig. 2) including, from base to top, Mus (Toarcian), Alan (Toarcian Aalenian), Sargelu

3 Fig. 1 Location map showing the structural features of the Zagros Mountains (index map) and the study area of Central Zagros region (based on Berberian 1995). BF Borazjan Fault, DE Dezful Embayment, HZ High Zagros, HZF High Zagros fault, K Khaneh-Kat surface section, KF Kazerun Fault, LB Lurestan Basin, MZRF Main Zagros reverse fault, Mountain Front fault, S Surmeh surface section, SFB Simple Fold Belt 30 LB Dezful Embayment KF Kazerun HZF Shiraz High Zagros LB Arabian-Persian Plate DE Caspian Sea MZRF Tehran HZ SFB IRAN Persian Gulf Cimmerian Plate HZF K Strait of Hormuz N 29 BF HZF Simple Fold Belt S 28 Persian Gulf Subsurface section Measured section Strike-slip fault Thrust fault T 25 Km (Aalenian Bathonian), and Najmeh (Callovian Kimmeridgian) Formations (James and Wynd 1965; Motiei 1993; Aghanabati 2004). In the study area, the Surmeh Formation is between 670 and 725 m thick and can be subdivided into five lithostratigraphic units including, from base to top, the lower carbonate, lower shaley unit (the subject of this study), middle carbonate, upper shaley unit, and upper carbonate (Fig. 2). The lower shaley unit overlies, with a sharp contact, the lower carbonate unit and grades upward into platform facies of the middle carbonate unit. Rocks of the Surmeh Formation, except for the lower shaley unit, record deposition in shallow marine environments related to a carbonate ramp platform (Jalilian 2010). Lithofacies and depositional environment Based on the field observations and petrographic investigations, the lower shaley unit of the Surmeh Formation consists of deep marine basinal facies and occurs within an entirely shallow marine carbonate deposit as follows. Bounding platform deposits The upper part of the lower carbonate unit consists of tidal flat and lagoonal facies and overlies, with an unconformable contact, the lower part of the lower carbonate unit (Fig. 2). In the Surmeh section (Fig. 3), this interval is

4 System Jurassic Cret. Series Surmeh Age (Ma) L U M L about 5 m thick and consists of thin-bedded stromatolite and fenestral dolomudstone facies containing evaporite casts, which suggest deposition in an arid tidal flat setting. Northeastward in the Khaneh-Kat section (Fig. 4), it consists of over 20 m of peloid oncoid wackestone packstone lagoonal facies. The lower shaley unit grades upward into open marine, shelf margin, and peritidal facies of the middle carbonate unit (Jalilian 2010). Lower shaley unit The lower shaley unit signifies a deep marine deposition and comprises well-bedded pelagic facies and intercalated calciturbidite deposits as described below. Pelagic facies Stage Berriasian Tithonian Kimmeridgian Oxfordian Callovian Bathonian Bajocian Aalenian Toarcian Pliensbachian sinemurian NW Lurest. Garu Gotnia Najmeh Sargelu Alan Formation Neyriz The pelagic facies include centimeter thick, dark gray pelagic lime mudstone/calcareous shale intercalated with numerous calciturbidite layers (Figs. 3, 4, 6a, b, 7a, b). Shale constitute the main portion of the lower part of the lower shaley unit in the Surmeh section (Figs. 3, 6a, b) Mus Adaiyeh Southern Fars Fahliyan Hith Upper carbonate Upper shaley unit Middle carbonate Lower carbonate SE Lower shaley unit Fig. 2 Stratigraphic classification of the Jurassic succession in the Zagros Mountains of southwest Iran (based on James and Wynd 1965; Motiei 1993). Time scale is according to Haq et al. (1988) and in the Tabnak No. 21 well (Fig. 5) along the Mountain Front fault. The calcareous shale changes to mainly pelagic limestone in the distal Khaneh-Kat section (Figs. 4, 7a, b). Pelagic limestone commonly consists of sponge spicules, thin-shelled bivalves, and planktonic crinoids (Saccocoma) (Fig. 7e, f) and records the low energy basinal (slope and basin plain) setting. Calciturbidite facies Close examination of the lower shaley unit has revealed the presence of numerous calciturbidite layers (3 cm 3 m thick) that alternate with pelagic facies (Figs. 3, 4, 6a, 8f). The calciturbidite layers are erosive-based and commonly exhibit graded bedding, incomplete Bouma turbidite sequence (Bouma 1962), flute casts, and load casts (Figs. 6c, d, 7c, d). These layers consist chiefly of platform-derived materials including ooids, intraclasts/lithoclasts, peloids, echinoderms, brachiopods, bivalves, and open-ocean biota, such as, planktonic bivalves, crinoids, coccoliths, foraminifers, and sponge spicules (Figs. 7c, d, 8a e). They comprise various graded grainstone to lime mudstone facies as follows: (1) Quartz-bearing graded lithoclast/intraclast grainstone to lime mudstone consisting of sand to gravel-sized micritic, red-stained lithoclasts/intraclasts and less than 10% sand-size quartz and bioclasts grains (Fig. 6d f). The grainstone is intercalated with pelagic calcareous shale in the lower part of the basinal succession in the Surmeh section (Fig. 3); in the Khaneh-Kat section, only a thin, centimeter thick lithoclast grainstone that is encased by pelagic limestone is recognized (Figs. 4, 7b). (2) Bioclast/ooid/peloid intraclast grainstone (Fig. 7c d); (3) Ooid grainstone to packstone that consist chiefly of ooids and rare bioclasts (Fig. 8a, b); (4) Lime wackestone to mudstone containing mixed deep marine fauna and platform-derived material that constitute the upper part of the graded turbidite cycles (Fig. 8c e). The calciturbidite layers become thinner and finergrained northeastward in the Khaneh-Kat section and their sedimentary structures display a northeast paleocurrent direction. Calciturbidite and pelagic facies are arranged into short-term fourth-order coarsening upward cycles that are superimposed on an overall thickening and coarsening upward cycle consisting of the lower shaley unit and the overlying middle carbonate unit (Figs. 3, 4, 5, 6b, 8f). Erosive base, normal grading, abundant incomplete Bouma turbidite cycles, mixture of platform-derived material with deep marine fauna, and intercalation with pelagic facies all suggest transportation from a carbonate platform by

5 Fig. 3 Facies column and sequence stratigraphy/cyclicity of the basinal lower shaley unit and its bounding platform carbonate ( lower shaley depositional sequence ) in the type locality of the Surmeh Formation. The sequence is a third-order cycle superimposed by coarsening and thickening upward fourth-order cycles. Note that the basinal facies overlie tidal flat facies in this proximal section. Note also that the pelagic facies in the lower part of the lower shaley unit is predominantly composed of calcareous shale (the uppermost part of the sequence is not shown) Series Middle Jurassic Stage Aalenian-Bajocian Unit Middle carb. Lower shaley unit Th. (m) Surmeh section Facies Lower shaley depositional sequence Seq. strat. Early HST Late HST Cyclicity Cycle order 3-rd 4-th Legend Basinal Facies Lithoclast grainst. to lime mudst. Bioclast/peloid/ooid intraclast grainst. Ooid grainst. to packst. Graded lime wackest. to mudst. Pelagic lime mudst. to wackest. Calcareous shale Platform facies Bioclast lime mudst. to packst. (open marine) Fenestral dolomudst. with evaporite casts (Tidal flat) Stromatolite boundst. (Tidal flat) Brachiopod Benthic foram Echinoid debris Gastropod Planktonic bivalves Planctonic foram Saccocoma 30 Coccolith Spone spicule mfs Intraclast Ooid Peloid Desiccation crack 20 Microbial lamination Fenestral Fabric Cross lamination Graded bedding Flute cast TST Erosional base 10 Unconformity Regressive Transgressive Lower carb. 0 m SB 1

6 Fig. 4 Facies column and sequence stratigraphy/cyclicity of the basinal lower shaley unit and its bounding platform carbonate ( lower shaley depositional sequence ) in the Khaneh-Kat section. Pelagic facies is predominantly composed of limestone and the intercalated calciturbidite layers are much thinner in this section. Note the sharp decrease in Mn and Fe values and the negative d 13 C excursion at the lower sequence boundary. Note also that the basinal facies overlie lagoonal facies in this distal section (the uppermost part of the sequence is not shown) Series Middle Jurassic Stage Aalenian-Bajocian Lower carbonate Lower shaley unit Middle carbonate Unit Th. (m) Khaneh Kat section Facies Seq. strat. Lower shaley depositional sequence Late HST TST Early HST mfs Cyclicity Mn (ppm) Fe (ppm) Cycle order 13 C ( PDB) 3-rd 4-th Legend Basinal Facies Lithoclast grainst. to lime mudst. Ooid grainst. to packst. Graded lime wackest. to mudst. Pelagic lime mudst. to wackest. Calcareous shale Platform facies Bioclast lime mudst. to packst. (open marine) Peloid oncoid lime wackest. to packst. (lagoon) Brachiopod Benthic foram Echinoid debris Gastropod Planktonic bivalves Planctonic foram Saccocoma Coccolith Spone spicule Intraclast Ooid Peloid Cross lamination Graded bedding Flute cast Erosional base Unconformity Regressive Transgressive 13 C Fe Mn 0 m SB 1 turbidity current and deposition in the proximal to distal areas of the basinal setting. Regional correlation and age of the lower shaley unit The lower shaley unit is up to 140 m thick and contains fossils of Aalenian Bajocian age (Wynd 1965). In the study area, it consists of interbedded dark gray calcareous shale and limestone in the lower part grading to mainly limestone facies upward (Figs. 3, 4, 5). Towards the northwest in the Lurestan Basin (Fig. 1), it is equivalent to basinal shale of the lower part of the Sargelu Formation (Setudehnia 1978) (Fig. 2). There is an excellent correlation between lower shaley unit of the Surmeh Formation and the Aalenian Bajocian Dhruma Formation of the southern Persian Gulf states and Saudi Arabia (Alsharhan and Nairn 1997; Sharland et al. 2001). The lower shaley unit overlies, with an abrupt contact, the basal lower carbonate unit that consists of thick-bedded limestone and dolomite containing abundant large skeletons of Lithiotis bivalves. This Lithiotis-bearing limestone (the Toarcian Lithiotis zone of Wynd 1965) is a widespread marker that is recognized everywhere in the Zagros region (Setudehnia 1978). Sharland et al. (2001) correlated their middle Toarcian maximum flooding surface (J 10) with the basal part of the Mus Formation, the Lithiotis limestone equivalent in the study area. An interregional unconformity

7 Series Stage Fm. Middle Jurassic Aalenian-Bajocian Surmeh Middle carbonate Unit Lower shaley unit LC NIOC Tabnak No. 21 GR (API) Unconformity Transgressive SB 1 Regressive is recognized near the top of the Lithiotis-bearing lower carbonate unit (see the following section). This unconformity coincides with the lower Aalenian unconformity of Golonka and Kiessling (2002) and is interpreted to correspond with the upper Toarcian-lower Aalenian unconformity between the Marrat and Dhruma Formations of Saudi Arabia (Alsharhan and Nairn 1997; Sharland et al. 2001; Haq and Al-Qahtani 2005). The lower shaley unit grades upward into thin- to thick-bedded and massive platform carbonates of the middle carbonate unit that encompass the Bathonian Pfenderina zone of Wynd (1965). The lower shaley unit and its equivalent, the lower part of the Sargelu Th. (m) Cyclicity Cycle order 3-rd Fig. 5 Gamma ray log of the subsurface analog section, the NIOC Tabnak No. 1 well (see Fig. 1 for the well location) showing sequence stratigraphy/cyclicity of the basinal lower shaley unit and its bounding platform deposits ( lower shaley depositional sequence ). The high gamma ray at the depth of 1,886 m is interpreted to be the early Bajocian maximum flooding surface corresponding to the J 20 mfs of Sharland et al. (2001). Note the sharp contact of the lower shaley unit with the upper limestone interval of the lower carbonate unit. Note also that the stacking pattern of high-frequency fifth-order cycles is retrogradational in the transgressive tract, followed by progradational- aggradational- progradational patterns in the highstand package Lower shaley depositional sequence Seq. strat. TST Early HST Late HST mfs 4-th Formation in the Lurestan sub-basin, contain species of Posidonia, a thin-shelled planktonic bivalve (Wynd 1965; Setudehnia 1978). The Bajocian Posidonia zone occurs within the lower shaley unit between the Bathonian Pfenderina zone and the Toarcian Lithiotis zone (Wynd 1965). Sharland et al. (2001) correlated their early Bajocian maximum flooding surface (J 20) with the Posidonia-bearing shale of the lower part of the Sargelu Formation. The presence of the Posidonia zone within the lower shaley unit and the occurrence of a thin and highly radioactive bed in the lower part of the unit in the Tabnak- 21 well (Fig. 5), which could be equivalent to the J 20 maximum flooding surface of Sharland et al. (2001) suggest a late Aalenian Bajocian age for the lower shaley unit. Sequence stratigraphy The lower part of the Surmeh Formation including the upper part of the lower carbonate unit, the lower shaley unit and the lower part of the middle carbonate unit (Figs. 2) encompass a third-order cycle (depositional sequence of Mitchum et al. 1977; Haq et al. 1988), referred here as the lower shaley depositional sequence (Figs. 3, 4, 5). The sequence consists of transgressive and highstand deposits superimposed by short-term high-frequency cycles (Figs. 3, 4, 5, 8f). Its lower boundary coincides with a reddish brown-colored paleosol horizon (Fig. 6a) near the top of the Lithiotis-bearing Toarcian Aalenian lower carbonate unit, which is characterized by a very negative d 13 C excursion and low Mn and Fe values (Fig. 4) indicating prolonged exposure. This unconformity corresponds with the lower Aalenian unconformity (Golonka and Kiessling 2002) that coincides with the boundary of the Absaroka and Zuni sequences of Sloss (1963). The unconformity is recognized in the platform areas of the Neotethys passive margin of the Zagros, Persian Gulf, and the surrounding areas (e.g., James and Wynd 1965; Steineke et al. 1958; Kheradpir 1975; Setudehnia 1978; Alsharhan and Nairn 1997; Sharland et al. 2001; Haq and Al-Qahtani 2005). Lowstand deposits are absent above the unconformity that is interpreted as both a type 1 sequence boundary and a transgressive surface. The peritidal deposit of the upper interval of the lower carbonate unit at the base of the lower shaley unit (Figs. 3, 4) is interpreted as the early transgressive tract. This interpretation is supported by the presence of a rather thin peritidal facies over the Aalenian unconformable boundary/transgressive surface and an abrupt facies change from peritidal to deep marine facies with no evidence of subaerial unconformity. The sharp contact of the early transgressive tract with the overlying basinal deposits of the lower shaley unit (Figs. 3, 4, 5)

8 Fig. 6 a View to north of the lower shaley unit and the underlying lower carbonate unit. The upper interval of the lower carbonate unit overlies, with an unconformable contact (white dash line), the lower part of the lower carbonate and underlies with a sharp contact, the lower shaley unit. The lower shaley unit grades to a massive middle carbonate in the upper right of the photograph (the encircled tree just to the right of the lower boundary is 5 m tall). b Close up view of the upper part of the lower shaley unit showing calcareous shale in the lower part and well-bedded pelagic limestone with intercalated calciturbidite layers in the upper part. c Hand sample of a calciturbidite layer showing lamination and cross-lamination (incomplete Bouma Tb-c cycle); coin diameter 2.5 cm. d Base of a calciturbidite layer in the lower part of the basinal facies in the Surmeh section showing graded bedding and micritic lithoclasts (coin diameter 2.5 cm). e Photomicrograph of the matrix in d composed of compacted lithoclast/intraclast grainstone. Note abundant lithoclast/intraclast fragments and presence of echinoderm (E) and detrital quartz (Q) grains (scale bars 1mm long). f Photomicrograph showing the sandy wackestone from the upper part of the turbidite layer shown in d (scale bar 1 mm long) suggests reactivation of the preexisting northwest southeast trending Mountain Front fault (Fig. 1) in southwest Iran and formation of a relatively deep intrashelf basin (see the following section). As the gamma ray log signatures of the Tabnak No. 21 well indicate, the lower part of the lower shaley unit (depth 1,902 1,886 m) is built by highfrequency fifth-order cycles that display a retrogradational stacking pattern (Fig. 5). Therefore, the lower part of the basinal succession is interpreted as the late transgressive tract. The most transgressive facies, the maximum flooding surface, is inferred to be within a dark gray deep marine pelagic calcareous shale interval in the Surmeh section (Fig. 3) and its equivalent interval in the Khaneh-Kat section (Fig. 4), which is characterized by a very positive d 13 C excursion and high Mn and Fe values. In the subsurface analog section, the maximum flooding surface is on top of the transgressive tract within the highly radioactive shale at a depth of 1,886 m (Fig. 5). This surface is interpreted to be equivalent to the J 20 maximum flooding surface of Sharland et al. (2001). The highstand systems tract comprises the middle and upper parts of the lower shaley unit and the platform facies of the lower part of the middle carbonate unit (Figs. 3, 4, 5, 8f). The upper boundary of the sequence is conformable with the overlying late Bajocian Bathonian sequence (Jalilian 2010). Therefore, it is interpreted as a SB 2 sequence boundary. The lower part of the highstand package in the Surmeh section is composed mainly of pelagic calcareous shale and calciturbidite intercalations (3 60 cm thick) that grade upward to interbedded pelagic limestone and calciturbidite layers up to 3 m thick (Figs. 3,

9 Fig. 7 a, b Field photographs of pelagic limestone in the Khaneh-Kat section. Note the graded lithoclast grainstone by the scale (pen) in the middle of the photograph in b. c Photomicrograph of intraclast grainstone calciturbidite facies containing peloids, ooids, and echinoderm fragments overlying a pelagic lime mudstone facies (scale bar 1 mm long). Note load cast and flame structures in the underside of the calciturbidite bed. d Photomicrograph of graded bioclast/ooid/peloid intraclast grainstone from the upper part of the calciturbidite layer shown in c (scale bar 0.5 mm long). e Photomicrograph of pelagic bioclast wackestone containing thin-shelled bivalve and sponge spicules (scale bar 0.05 mm long). f Photomicrograph of pelagic bioclastic mudstone including planktonic crinoids and sponge spicules (length of the scale bar 0.25 mm long) 8f). The lower part of the highstand package in the Khaneh-Kat section consists mainly of pelagic limestone and much thinner calciturbidite layers (Fig. 4). The lower shaley unit grades to late highstand open marine, shelf margin, and lagoonal facies of the middle carbonate unit as a result of basinward progradation (Figs. 3, 4, 8f). In the Tabnak No. 21 well (Fig. 5), the highstand systems tract consists of high-frequency fourth-fifth-order cycles that exhibit progradational (depth 1,886 1,810 m), aggradational (depth 1,803 1,786 m), and progradational (depth 1,786 1,750 m) stacking patterns. The lower shaley depositional sequence correlates with the Aalenian Bajocian sequence in the Jurassic global cycle chart of Haq et al. (1988) and is equivalent to the late Aalenian early Bajocian sequence recognized in the Arabian platform deposits (Haq and Al-Qahtani 2005). Controls on sedimentation and platform evolution The vertical and lateral facies changes within the lower part of the Surmeh Formation in the study area are interpreted to be the consequence of relative sea-level changes and syndepositional fault movement at the Aalenian Bajocian boundary. The calciturbidites and the associated pelagic facies of the lower shaley unit record the existence of a relatively deep basin in the study area, referred here as Pars intrashelf basin (Fig. 9). The basinal succession of the lower shaley unit occurs in the Zagros Simple Fold Belt and the High Zagros (Fig. 1) between the Mountain Front fault and the Main Zagros reverse fault. The abrupt contact of the basinal facies with the underlying peritidal platform facies of the Toarcian Aalenian lower carbonate unit suggests reactivation of the preexisting

10 Fig. 8 Photomicrographs of calciturbidite facies under polarized light. a Ooid grainstone showing imbrication; this facies grades upward to ooid packstone in b (scale bar 0.25 mm long). b Ooid packstone (scale bar 0.5 mm long). c Bioclastic ooid wackestone (scale bar 0.5 mm long). This facies grades upward to lime mudstone in d. Note the planktonic foraminifers in the center of photograph. d Bioclast and ooid-bearing mudstone. Note the presence of ooids, ostracods, coccoliths (white arrow) and other bioclast debris (scale bar 1 mm long). e Bioclastic wackestone consisting of mixed opensea and platform derived biota, which occur in the upper part of calciturbidite cycles (scale bar 1 mm long). Note the presence of sponge spicules, planktonic crinoids (upper left and lower right), and a brachiopod fragment. f Field photograph (view to the north) of the Surmeh section showing the main part of the lower shaley depositional sequence. Note that the lower shaley unit basinal facies grade upward to massive shelf margin and platform facies of the lower part of the middle carbonate unit. Note also the intercalation of pelagic shale and calciturbidite/pelagic limestone layers in the lower part of the lower shaley unit that grade upward to interbedded calciturbidite and pelagic limestone facies. The sequence is a thirdorder thickening upward cycle built by five fourth-order thickening and coarsening upward cycles northwest southeast trending Mountain Front fault (Fig. 1) in southwest Iran during late Aalenian early Bajocian and formation of the Pars intrashelf basin. The sedimentary record of the Surmeh Formation indicates the presence of a northwest southeast platform margin along the foot-wall edge of the Mountain Front fault that was facing the Pars intrashelf basin in the northeast (Fig. 9). The thinning of pelagic calcareous shale and calciturbidite layers towards

11 the northeast in the distal Khaneh-Kat section and the widespread Middle Jurassic ooid, peloid, and bioclast bearing Dhruma Formation (e.g. Murris 1980), the lower shaley unit equivalent in the southern Persian Gulf states and Saudi Arabia, support the existence of a southwest platform margin and platform interior source area (Figs. 9, 10). As shown in the depositional sequence model (Fig. 10), global sea-level rise during the middle Aalenian (e.g., Haq et al. 1988; Sharland et al. 2001; Golonka and Kiessling 2002; Haq and Al-Qahtani 2005) resulted in transgression and deposition of the upper interval of the lower carbonate unit (Fig. 10a). This early transgressive peritidal facies was deposited on a preexisting gently sloping ramp (e.g., Ahr 1973) capping the highstand deposits of the underlying Toarcian lower Aalenian sequence (Jalilian 2010). Reactivation of the Mountain Front fault during the late Aalenian early Bajocian resulted in the formation of a northeast facing platform margin (Fig. 10b) and configuration of a N S Carbonate platform 25 Km 52 Shiraz HZF Pars intrashelf basin T High Zagros K HZF distally steepened ramp (e.g. Read 1985). On the seaward edge of the platform, deposition of the late transgressive basinal facies of the lower part of the lower shaley unit occurred and covered, with a sharp contact, the previously deposited peritidal facies (Fig. 10b). Calcareous shale facies and graded red-stained lithoclast and quartz-bearing turbidite beds in the lower part of the basinal facies (Figs. 3, 6d f) record erosion of the uplifted southwestern margin before it was completely flooded. Flooding of the southwest margin during the early to middle Bajocian early highstand (Fig. 10c) resulted in the establishment of the carbonate sediment factory and overproduction of shallow marine carbonates leading to vertical and lateral expansion of the platform (e.g., Read 1985; Bosellini 1989; Eberli and Ginsburg 1989). During late Bajocian time, late highstand progradation of platform facies (the lower part of the middle carbonate unit) over basinal deposits of the lower shaley unit (Fig. 10d) resulted in gradual infilling of the Pars intrashelf basin and reestablishment of the ramp platform. In the northwest of the study area, west of the Kazerun Fault (Fig. 1), the basinal shale of the Sargelu Formation (Setudehnia 1978), the lower shaley unit equivalent, records the existence of a relatively deep basin. This basin, the Lurestan intrashelf basin (Murris 1980), which was in existence until the end of Cretaceous, could have formed as the result of tectonic activity along the Mountain Front fault. High carbonate production during highstand exceeded the accommodation space, which led to export of excess loose carbonates to the basinal setting and deposition of calciturbidites. Calciturbidites record the existence of platform margin and the associated basinal setting and are good indicators of sea-level changes. They record deposition during sea-level highstand when platform sediments are transported into the adjacent basinal settings ( highstand shedding, Droxler and Schlager 1985; Bosellini 1989; Schlager et al. 1994; Lasemi 1995; Andresen et al. 2003). Long-term basinward progradation of platform facies over the basinal facies associated with a sea-level highstand resulted in an overall coarsening upward cycle consisting of the lower shaley unit basinal deposits and the platform facies of the lower part of the middle carbonate unit (Figs. 3, 4, 5, 8f). Conclusions Fig. 9 Paleogeographic reconstruction of the Fars Province in Central Zagros during the Middle Jurassic (late Aalenian late Bajocian) showing a carbonate platform that was facing the Pars intrashelf basin to the northeast within the Neotethys Ocean. The measured stratigraphic sections were much farther apart prior to folding and thrust faulting in the Zagros fold thrust belt. HZF High Zagros fault, K Khaneh-Kat surface section, Mountain Front fault, S Surmeh surface section, T Tabnak-21 subsurface section (1) The abrupt contact of the basinal succession of the lower shaley unit with the basal shallow marine carbonates suggests reactivation of the old Mountain Front normal fault and establishment of a distally steepened ramp during late Aalenian to early Bajocian times.

12 SW Mean sea level NE D Late Bajocian Mean sea level C Early to middle Bajocian B Late Aalenian-early Bajocian Mean sea level Mean sea level A Middle Aalenian Unconformity Early transgressive platform facies Late transgressive basinal facies Early highstand platform facies Late highstand platform facies Fig. 10 Depositional sequence model for platform evolution in the Fars region during the Middle Jurassic. a Sea-level rise during middle Aalenian and deposition of early transgressive peritidal facies on a gently sloping ramp. b Reactivation of the Mountain Front fault, uplift of the southwest margin, and configuration of a distally steepened ramp, the Pars intrashelf basin during late Aalenian early Bajocian times. Erosion of the southwest margin resulted in siliciclastic influx during the deposition of the lower part of the late (2) Calciturbidites in the lower shaley unit of the Surmeh Formation, except for a few basal layers, are composed of pure limestone containing ooids, peloids, intraclasts, and bioclasts derived from the platform suggesting overproduction and off-platform transport of carbonates during sea-level highstand. (3) Carbonate sediment overproduction and progradation of platform facies over basinal deposits of the lower shaley unit during late highstand led to gradual infilling of the Pars intrashelf basin and reestablishment of the carbonate ramp that was in existence in the study area until the end of Jurassic. (4) The lower shaley unit coarsens and thickens upward and grades to late highstand platform margin and platform interior facies indicating platform expansion and basinward progradation, which led to an overall coarsening upward third-order cycle, ( lower shaley depositional sequence ). Highstand basinal facies transgressive basinal facies. c Flooding of the southwest margin during early to middle Bajocian and reestablishment of a carbonate platform with a northeast facing platform margin. Carbonate overproduction during early highstand led to off-platform transport of platform sediments and deposition of calciturbidites in the basin. d Seaward progradation of platform carbonates associated with late highstand led to vertical and lateral expansion of the platform and gradual infilling of the Pars intrashelf basin by late Bajocian time Acknowledgments The authors thank the three anonymous journal reviewers for their thorough and critical reviews that significantly improved the manuscript and R.C. Berg of the Illinois State Geological Survey, University of Illinois at Urbana-Champaign, USA for reviewing the final version of the article. M.A. Kavosi at NIOC Exploration in Tehran provided some of the thin sections for this study and A. Arshad at NIOC Southwest District in Ahvaz supplied the log for Tabnak well. This work was partially supported by the Tarbiat Moallem University in Tehran. Publication was authorized by the Director, Illinois Geological Survey. References Aghanabati A (2004) Geology of Iran (in Persian). Geological Survey of Iran, p 586 Ahr WM (1973) The carbonate ramp: an alternative to the shelf model. Trans Gulf Coast Assoc Geol Sci 23: Alavi M (1994) Tectonics of the Zagros orogenic belt of Iran: new data and interpretations. Tectonophysics 229: Alavi M (2004) Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. Am J Sci 304:1 20

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