4.a. Stratigraphic section

A section (800 m thick) of the Hojedk Formation has been logged in the Kalshaneh area (Kalshaneh Coal Mine), c. 80 km NW of Tabas (Fig. 1b). Structurally, this section is located on the western limb of the Kalshaneh syncline, adjacent to the Kalmard Fault, the boundary area between the Kalmard sub-block and the northern Tabas Block (Fig. 4). The grey to brownish-grey siliciclastics of the Hojedk Formation are well-exposed in this section with a N–S trend and sub-vertical bedding dipping towards east with 45–75° (Figs 4, 5a, e). The boundary of the Hojedk Formation with the underlying Badamu Formation is conformable but abrupt. The Badamu Formation consists of dark, sandy oolithic limestones, thin-bedded calcareous sandstones and light green shales with a thickness of 175–275 m. At the top, the boundary to the overlying Parvadeh Formation is developed as a subaerial unconformity, indicated by a reddish, Fe-oxide-rich sandstone bed (Fig. 5b–d).

The basal 20 m of the Hojedk Formation in the Kalshaneh section consist of brown to light grey, fine-grained sandstone and light green to yellow shales (Fig. 6). Mud clasts are present at the base of the sandstones, and hummocky cross-stratification (HCS) was observed in some beds. Marine fossils (bivalves and oyster debris) and trace fossils (e.g. Thalassinoides) are also present. The basal unit is overlain by 65 m of alternating thick-bedded, trough cross-bedded sandstones, siltstones, dark grey to blackish coaly shales and workable coal seams with a thickness of 1–2 m, appearing c. 30 m above the base of the formation (Fig. 6). This alternation is followed by a 180 m thick succession of fining-upward cycles consisting of light grey, fine-grained sandstones, siltstones, silty shales and occasional well-developed light grey to coaly shales. The sandstones show sharp bases and are lenticular in shape with a mean width of 8–10 m and an average thickness of 1 m. When interbedded with fine-grained clastic rocks, the sandstone layers contain ball-and-pillow structures at the base. Fine-grained sandstone with sigmoidal shape and lateral accretion surfaces are common in this unit. Mud chips, plant debris and tree trunks are common components of the sandstones, mainly in their basal part. The following 130 m of the succession is more mud-dominated, showing overall fining-upward cycles grading from brown to light grey sandstones below to siltstones and light grey shales above; the fining-upward cycles are 5 to 10 m thick (Fig. 5e). Sandstone layers are laterally continuous, showing multiple lenticular bodies; tree trunks and plant debris are common. Common sedimentary structures in the sandstones include trough cross-bedding to low-angle lamination that grade upward to planar ripple laminations. Fine-grained rocks are dominant in this package, and the coaly shales reach a maximum thickness of 5–7 m. Up-section, 200 m thin-bedded, fine-grained sandstones, siltstones and shales with fining-upward pattern commonly contain tree trunks and plant debris. In this unit, macroflora is best preserved in siltstones. The uppermost 210 m of the measured section (from the 590 m level) is sand-dominated and characterized by a distinct increase in bed thickness and grain size of sandstones. This unit consists of brown to light grey, thick-bedded, medium-grained sandstones and laminated light grey to green, fine-grained siltstones and shales. Lenticular sandstones show sharp and erosional bases, commonly being 10–12 m wide and 2 m thick. At the top, the Hojedk Formation at the Kalshaneh section is capped by a 2 m thick, dark brown to red sandstone with iron oxide crusts and pedogenic overprint (Figs. 5c, d, 6). The unconformity is usually overlain by a thin, patchy basal conglomerate of the Parvadeh Formation, consisting of well-rounded quartz granules and pebbles, which is not developed in the measured section but preserved a few hundred metres towards the south (Fig. 5a, b).

Fig. 4. Geological map of the study area in the northern Tabas Block, simplified from the Shirgesht geological map (scale 1:100 000; Aghanabati, Reference Aghanabati1994). Location of the measured section and studied drill core (BH#8) are shown (A–B).

Fig. 5. Satellite image and field views of the Hojedk Formation at the Kalshaneh section. (a) Satellite image of the study area (Kalshaneh Coal Mine) showing lithostratigraphic boundaries between the Badamu, Hojedk and Parvadeh formations. The traverse along which the stratigraphic log (blue line) has been measured and its lateral shifts are shown. (b) The Hojedk Formation unconformably overlain along the mid-Cimmerian unconformity by the thin basal conglomerate of the Parvadeh Formation. The position of the field photo is indicated in (a). (c) The Hojedk Formation capped by reddish, Fe-oxide-rich sandstones indicating subaerial exposure associated with the mid-Cimmerian unconformity. The position of the field photo is indicated in (a). (d) Close-up view of the Fe-oxide-rich sandstone related to the mid-Cimmerian unconformity. (e) Typical fining-upward cycles grading from sandstones and siltstone to coaly shales in the middle part of the Hojedk Formation. For legend see Figure 6.

4.b. Core section BH#8

The borehole targeted for subsurface exploration of coal seams was drilled in the lowermost part of the Hojedk Formation at the Kalshaneh Coal Mine in 2017 (BH#8; 34° 10′ 56.43″ N, 56° 39′ 50.22″ E). The drill site is located ∼1.3 km to the south of the measured outcrop section (Fig. 4). The basal part of the drill core (from 167.4 m to the final depth at 187.2 m) represents the sandy bioclastic limestones of the uppermost Badamu Formation. At 167.4 m depth, the Hojedk Formation starts and includes several fining-upward cycles of light grey, medium- to fine-grained, trough cross-bedded sandstones (rarely pebbly at the base), dark grey, very fine-grained sandstones and siltstones, carbonaceous shales and coals. In total, the cored part of the lower Hojedk Formation contains seven coal seams of which only the thickest and workable seams are visible in the measured section due to the weathering.

Fig. 6. Stratigraphic log of the Hojedk Formation in the Kalshaneh section, northern Tabas Block.

4.c. Sedimentary facies and facies associations

Based on lithology, texture and sedimentary structures as well as geometry and bedding contacts of the strata, ten characteristic sedimentary facies were identified in the field. Coarse-grained facies are absent in the Hojedk Formation at Kalshaneh, while six medium-grained (St, Se, Sp, Sr, Sh, Sl), two fine-grained (Fl, Fm), one interbedded sandstone–claystone (Sr/Fl) and one organic facies (C) are recognized. Major characteristics of the facies are summarized in Table 1 and Figure 7a–l.

Table 1. Lithofacies of the Hojedk Formation at Kalshaneh section, northern Tabas Block (lithofacies code modified after Miall, Reference Miall2014)

Fig. 7. Field aspects of the representative lithofacies of the Hojedk Formation in the Kalshaneh section. (a) Hummocky cross-stratified (HCS) sandstone of the shallow-marine transitional facies at the base of the section at c. 1 m. (b) Calcareous sandstone with bivalve shells (Ctenostreon sp.) from the shallow-marine transitional facies at the base of the section at c. 0 m. (c) Thalassinoides burrows in the shallow-marine sandstone at the base of the section at c. 15 m. (d) Mud chips at a basal erosional surface of a sandstone bed at c. 24 m. (e) Tree trunk and plant debris within the sandstone of a channel base from the lower part of the section at c. 79 m. (f) Medium-grained sandstones with coal clasts from a channel base of the upper part of the section at c. 615 m. (g) Cross-bedded sandstones (Sp) with unimodal palaeoflow directions from the lower part of the section at c. 166 m. (h) Rippled and planar-bedded sandstones (Sr, Sh) intercalated with laminated siltstones (Fl) from the middle part of the section at c. 362 m. (i, j) Current-rippled (Sr) surfaces on fine-grained sandstones from the lower and middle part of the section at c. 175 m and 185 m, respectively. (k) Interbedded rippled sandstones and mudstones (Fl/Sr) from the base of the section at c. 173 m. (l) Massive siltstone–claystone (Fm) grading to coal (C) from the middle part of the section at c. 353 m. Fe crusts and concretions are associated with these lithofacies.

The most common sedimentary facies within the succession is trough and planar cross-bedded sandstone (St, Sp). These facies are mostly very fine- to fine- and in places medium-grained quartz-rich, mature to super-mature sandstones. The succession most commonly consists of stacked, up to 10 m thick fining-upward cycles, which start at the base with facies Se, St and Sp and grade upwards into fine-grained facies, including ripple cross-laminated sandstone (Sr), planar laminated siltstone (Fl) and, finally, coaly shale (C) and mud rock (Fm). Coals (C) are also common in the cycles, especially in the lower part of the succession (Fig. 6).

Based on the lateral and vertical sedimentary facies distribution within the studied succession, three major facies associations (FA) have been identified that represent the main subenvironments of the overall depositional environment.

4.c.3. Facies association 3

FA3 consists mainly of laminated and structureless, fine-grained facies (Fl, Fm), commonly forming tabular and sheet-like depositional units. Facies Sh and Sr are subordinate in this FA. The commonly associated coals and carbonaceous shales contain rootlet structures and ferruginous crusts. Wedge-shaped, sharp-based, fine-grained, parallel laminated sandstones (Sh), a few metres wide, are rarely interbedded within the fine-grained facies.

The sedimentological character of the drill core was used to supplement the facies data recognized in the outcrop. The representative facies in the core, with well-developed internal sedimentary structures, are illustrated in Figure 8a–i. The three principal FAs recognized in the outcrop are observed in the drill core, too. An advantage of the fresh drill core is that it better displays the delicate sedimentary structures (e.g. interlaminated bedding, ripple cross-lamination) and the thin coal seams that are absent in the surface exposures.

Fig. 8. Overview of representative lithofacies of the Hojedk Formation in drill core BH#8 at the Kalshaneh Coal Mine. Width of the core is 6 cm and stratigraphic-up is towards the image top. The vertical scale bar is 2 cm long in all photographs. Facies associations (FA): FA1 = channels; FA2 = point bars; FA3 = floodplains. (a) Pebbly sandstones characteristic of the thalweg of a channel at 103 m core depth. (b) Light grey, medium-grained sandstones with intraformational dark grey and black mud and coal clasts at the base (white arrows) representing FA1, grading into trough cross-bedded sandstones (FA2) at 93.1 m core depth. (c) Slabbed core segment showing medium-grained, cross-bedded sandstones interbedded with darker streaks of organic-rich mudstones of FA2 at 77.1 m core depth. (d) Interbedded dark shale and light red, medium-grained sandstones representing upper part of FA2 at 133 m core depth. (e) Dark grey, cross-bedded sandstones at 128.90 m core depth. (f) Slabbed core segment showing fine-grained sandstones with cross- to climbing ripple-lamination and siltstones of FA3 at 47.5 m core depth. (g) Dark grey, planar-laminated siltstones and fine-grained sandstones at 49 m core depth. (h) Dark grey laminated mudstones and medium grey fine-grained siltstones (FA3) with intercalated reddish-brown ferruginous horizon (possibly a palaeosol) at 134 m core depth. (i) Dark grey, structureless mudstones and coal seams at 79.30 m core depth.

4.d. Palaeocurrent analysis

The palaeocurrent data were obtained from oriented sedimentary structures, including several stacks of trough- and planar-cross-bedded foresets (32 measurements) within lenticular sandstone beds throughout the succession. The results indicate two main transport directions perpendicular to the channel axes (Fig. 6), and imply sediment supply from north and south. The implication of these transport directions is addressed in the context of the inferred depositional environment and provenance analysis below.

4.e. Petrography and modal analysis

Sandstone petrography can provide important information about the tectonic setting, source area and its exposed rock types and weathering intensity as well as the source-to-sink history of transport, deposition and diagenesis that would affect the texture and composition of the sandstones (Dickinson & Suczek, Reference Dickinson and Suczek1979; Dickinson et al. Reference Dickinson, Beard, Brakenridge, Erjavec, Ferguson, Inman, Knepp, Lindberg and Ryberg1983; Boggs, Reference Boggs2009). The grain-size analysis, based on a comparative grain-size scale in the field and on the subsequent examination of scanned petrographic thin-section images, indicates that the sandstones of the Hojedk Formation are mostly fine-grained and occasionally medium-grained; coarser-grained facies has not been observed. The sandstones are moderately well-sorted and silica-cemented (Fig. 9a, b). The main framework components comprise quartz (an average of 90 % total, which is mostly unstrained monocrystalline quartz), sedimentary and metamorphic rock fragments (on average, 8 % Lt), subordinate feldspar (on average, 2 %; typically altered potassium feldspar) and scarce, mostly resistant heavy minerals (Fig. 9c; Table 2). Stretched polycrystalline quartz grains constitute a subordinate but, as a provenance indicator, important quartz type in the sandstones (Fig. 9d). Chert fragments and, to a lesser extent, siltstone and shale clasts are common sedimentary rock fragments (Fig. 9c, d). Sand-sized coal fragments are another minor constituent in some of the samples (Fig. 9a). Some samples contain <5 % of pseudomatrix due to degradation of soft, unstable sedimentary rock fragments and/or alteration of minor feldspars grains. Hematite and Fe-rich carbonate cement is a common diagenetic feature in a few samples (Fig. 9e, f).

Fig. 9. Microscopic images of representative sandstone samples from the Hojedk Formation of the Kalshaneh section. (a) High-resolution photograph of a polished sandstone slab, showing well-sorted and silica-cement-rich sandstone with silt- to sand-sized coal fragments (sample K615). (b) Thin-section photomicrograph of well-sorted litho-quartzose sandstone (sample K615). (c) Polycrystalline quartz (Qp), chert, siltstone (Ls silt) and shale (Ls shale) sedimentary rock fragments (sample K666). (d) Stretched polycrystalline quartz (Qp), metamorphic (Lm) and chert rock fragments (sample K542). (e) Fe-oxide-rich carbonate cemented sandstone (sample K516). (f) μ-XRF element map of Fe-oxide-rich cemented sandstone. Si (blue) and Fe (red) represent quartz and Fe oxide cement, respectively (sample K516). For sample locations see Figure 6.

Table 2. Framework grains of 16 sandstone samples of the Hojedk Formation at Kalshaneh section, northern Tabas Block

Q = quartz; RF = rock fragments; F = feldspar.

Compositionally, most of the sandstones of the Hojedk Formation from the Kalshaneh section plot into the field of quartzarenite, while only a few samples fall into the litho-quartzose field of the newly proposed QFL plot of Garzanti (Reference Garzanti2019) or into the sublitharenite field (Folk, Reference Folk1980; Fig. 10). The latter compositional fields indicate mature quartz-rich sandstones with subordinate sedimentary and low-grade metamorphic grains.

Fig. 10. Classification of sandstone samples from the Hojedk Formation at Kalshaneh in the ternary QFL diagram of Garzanti (Reference Garzanti2019). Q = quartz, F = feldspar, L = lithic fragments. For the abbreviations of the six central fields, see Garzanti (Reference Garzanti2019).

The organic petrographic studies of representative coal samples from the main workable coal seams, stratigraphically equivalent to the lower part of the section at c. 32 m, indicate that the major macerals in the coal samples are vitrinite and inertinite (Fig. 11a–c). Clay minerals and pyrite are common associated minerals in the samples. The VR_r of the coal samples ranges from 1.81 % to 2.18 %, corresponding to low-volatile bitumen to semi-anthracite (Hartkopf-Fröder et al. Reference Hartkopf-Fröder, Königshof, Littke and Schwarzbauer2015; Fig. 12).

Fig. 11. Photomicrograph images of coal samples from the Hojedk Formation at the Kalshaneh coalfield (under oil immersion with reflected light). (a) Typical vitrinite (v) and inertinite (semifusinite) (I) macerals of coal sample JHK-3. Pyrite and framboidal pyrite present in the lower right. (b) Typical vitrinite (v) and inertinite (I) macerals of coal sample JHK-1. (c) Vitrinite macerals of coal sample JHK-2. The dark laminations are clay minerals and the bright band in the centre is inertinite (I). For sample locations see Figure 6.

Fig. 12. Vitrinite reflectance distribution of the three coal samples (JHK-1–3).

4.f. Heat flow and calibration

The calibration of the model is achieved based on measured vitrinite reflectance of three coal samples from the lower Middle Jurassic Hojedk Formation. No temperature data are available for present-day thermal calibration. A semi-constant heat flow of c. 46 mW m⁻² was assumed from the Silurian to the Late Triassic. From the Early Jurassic until the present day, excluding magmatic activity, a higher basal heat flow of 50 mW m⁻² was assigned in the model due to the proximity of the study area to the deep-seated block-bounding Kalmard Fault that was active following the Eo-Cimmerian Event (Salehi et al. Reference Salehi, Moussavi-Harami, Mahboubi, Fürsich, Wilmsen and Heubeck2018). Magmatic activity was modelled using the PetroMod ‘intrusion tool’ and replacement of the Eocene host layer by a sill- and/or dyke-like intrusion with andesitic composition during the earliest Eocene at c. 55.86 Ma. The ‘intrusion tools’ parameters were set to an intrusion temperature of 950 °C with a magma density of 2750 kg m⁻³, a magma heat capacity of 0.7 kcal kg⁻¹ K⁻¹ and a crystallization heat of 700 MJ m⁻³ (e.g. Kutovaya et al. Reference Kutovaya, Kroeger, Seebeck, Back and Littke2019).

4.g. Burial, thermal and maturity history

The model comprises 12 layers ranging from the Early Ordovician to Miocene, covering a sedimentary thickness of more than 3500 m with a single erosional phase in the last 5 Ma (Table 3). According to the model results, the sedimentation in the Kalshaneh area is generally characterized by a low sedimentation rate of c. 10 m Ma⁻¹ during the Early Ordovician to Oligocene. Exceptions are periods of rapid sedimentation during the Early Jurassic to early Middle Jurassic (65 m Ma⁻¹), late Middle Jurassic to Late Jurassic (120 m Ma⁻¹) and Miocene (100 m Ma⁻¹) (Fig. 13). The sedimentation pattern controls the burial depth and therefore the temperature and maturity of different units through time. Accordingly, the Hojedk Formation was buried to c. 2300 m already by the Late Jurassic. Following low sedimentation rates during the Cretaceous to Palaeogene, the burial depth of the formation reached 3400 m in the Late Oligocene. However, rapid sedimentation in the Miocene led to a rather deep burial of up to 5000 m for this unit. The uplift and erosional event during the last 5 Ma shifted the Hojedk Formation to a shallower depth of c. 1200 m. During this time, the temperature experienced by the Hojedk Formation increased from 100 °C by the Late Jurassic to 136 and 185 °C in the Late Oligocene and Late Miocene, respectively. The late uplift in the region resulted in temperature cooling to c. 65 °C at the present day. In line with temperature variations, the maturity of the Hojedk Formation increased from 0.5 %R_o in the Late Jurassic to c. 1 %R_o in the Late Oligocene. Following the rapid sedimentation during the Miocene, the maturation reached c. 1.9 %R_o, which increased slowly up to the present day.

Table 3. Input data for maturation modelling in the Kalshaneh area, northern Tabas Block (timescale after Cohen et al. Reference Cohen, Finney, Gibbard and Fan2013)

LiSt = limestone; Ss = sandstone; Dol = dolostone; Cong = conglomerate; Gyp = gypsum; PWD = palaeowater-depth; Fm/fms = Formation/formations.

Fig. 13. Burial and thermal history in the Kalshaneh area (a), reconstructed palaeo-water depth, inferred sediment–water interface temperature (SWIT) and assumed basal heat flow through time (b) and model calibration (c).

The results indicate that the Eocene magmatic activity in the region did not directly affect the temperature and therefore the maturity of the Jurassic layers in the study region. This can be explained by the presence of more than 3000 m of sedimentary strata between the Eocene magmas and the deeply buried Jurassic strata, preventing any effects on the thermal maturation. According to the model, for a short period, the temperature reached up to more than 1000 °C in the early Eocene. This high temperature mainly affected the underlying Palaeocene strata and elevated their temperature up to 500 °C. Consequently, the maturity of the Palaeocene strata was elevated up to 3.8 %R_o. No prominent effect has been modelled for the deeper strata, e.g. Upper Cretaceous and underlying units (see discussion below).

5.a. Depositional environment

Sedimentary facies analyses of the Hojedk Formation led to the identification of a thin basal shallow marine facies, only reaching 20 m in thickness, including HCS, marine shell debris and trace fossils (Fig. 6). This basal marine unit shows a gradual transition with an overall regressive trend from the underlying deeper marine, ammonite-bearing Badamu Formation (Seyed-Emami et al. Reference Seyed-Emami, Schairer, Fürsich, Wilmsen and Majidifard2000, Reference Seyed-Emami, Fürsich, Wilmsen, Schairer and Majidifard2004 a, Reference Seyed-Emami, Wilmsen and Fürsich2020). However, based on sedimentary facies analysis, most of the Hojedk Formation at Kalshaneh was deposited in a continental, fluvial depositional environment that was characterized by widespread development of channel deposits as well as thick, well-developed siltstones and coal-bearing strata. The succession is characterized by three FAs that show several classical fluvial channel fills (FA1) and laterally accreted sandstones (FA2) that are associated with fine-grained inter-channel and abandoned channel deposits, represented by coaly shales and coal horizons (FA3, Fig. 14a). The basal erosional surface of sandstone beds with associated mud chips, coal fragments and plant materials as well as pebbles in a sandy matrix indicates the thalweg of the channels (e.g. Wang & Bhattacharya, Reference Wang and Bhattacharya2017; Fig. 14b). Channels are in places connected and superimposed onto each other and form sandstone layers of fairly constant thickness within the limits of the exposure. The sandstone facies that compose the channel fills show conspicuous stratal architectures such as sigmoidal-shaped bodies internally structured by lateral accretion surfaces, providing evidence for point bar deposition and its lateral migration patterns (FA2; e.g. Fustic et al. Reference Fustic, Hubbard, Spencer, Smith, Leckie, Bennett and Larter2012; Miall, Reference Miall2014; Fig. 14c). The facies distribution and the architecture of the depositional units within the succession indicate that, occasionally, the laterally accreted deposits were cut off and a narrow channel incised into the top of the point bars forming chute channels with over-bar flows, probably during flood events (Fig. 14d). The fine-grained facies, which is mainly composed of ripple cross-laminated sandstones, planar laminated siltstones, dark grey mudrocks and shales with abundant plant material and rootlets, indicate deposition from the lower flow regime or suspension in floodplain and overbank sub-environments, while coal was formed in associated swampy floodplains (FA3; Fig. 14e). The thin-bedded wedge-shaped sandstones with a restricted lateral distribution within floodplain strata are interpreted as crevasse splays that formed during flooding events (Fig. 14f).

Fig. 14. Field aspects of the representative facies associations (FA) of the Hojedk Formation in the Kalshaneh section. (a) Channel sandstone FA (CH) with fining-upward pattern from the lower part of the section at c. 55 m. (b) Erosional channel base with pebbles and/or mud chips in a sandy matrix, forming a basal lag in the middle part of the section at c. 448 m. (c) Sigmoidal-shaped channel sandstone (CH) showing low-angle lateral accretion surfaces indicating point bar migration, intercalated between soft-weathering, fine-grained floodplain deposits (FP) from the lower part of the section at c. 228 m. (d) Chute channel cutting into a point bar deposit (PB); note floodplain deposits (FP) above (lower part of the section at c.138 m). (e) Fining-upward channel sequence grading from channelized sandstone (CH) and accreted point bar deposits (PB) to fine-grained lithofacies and coal of the floodplain FA (FP) in the middle part of the section at c. 340 m. (f) Fine-grained sandstone (CS) with limited lateral extent and wedge-shaped geometry indicating crevasse splays within the floodplain environment (middle part of the section at c. 525 m).

The core of the lowermost part of the Hojedk Formation shows several well-preserved fining-upward cycles typical of a low-energy, high-sinuosity fluvial setting. The facies in the core consists of (occasionally pebbly) medium-grained channel sandstones at the base to trough cross-bedded fine-grained sandstones and siltstones of point bars. This succession grades into dark grey to black, planar-laminated siltstones and dark carbonaceous shales to coals of floodplains and swamps above (Fig. 8a–i).

In summary, based on the sedimentary facies distribution, including the lateral and vertical facies changes, the Hojedk Formation at Kalshaneh was deposited in a low-energy meandering fluvial environment with an extensive floodplain area that was well-vegetated and locally swampy (Fig. 15). This interpretation is supported by several lines of evidence such as common fining-upward cycles, high width/depth ratio of channels, limited or weak erosion at channel bases and rather thin lag deposits, extensive laterally accreted bedding patterns in the sandstones indicating point bar migration, and the preservation of coal, which suggests well-developed and densely vegetated swampy floodplains (e.g. Ghazi & Mountney, Reference Ghazi and Mountney2009; Miall, Reference Miall2014). Correspondingly, fossil swamps have recently been reported from the Hojedk Formation of the Kerman area, southern Tabas Block (Khalilizadeh et al. Reference Khalilizadeh, Ashouri, Ghaderi and Barbacka2022). In the lower to middle parts of the succession at Kalshaneh, the fluvial channels are mostly isolated within floodplain deposits, whereas in the upper part, the channels are more stacked and coalesced, suggesting a certain change in the fluvial regime that needs explaining (see section below). The common development of densely vegetated and temporally persistent floodplains, indicated by the preservation of thick units of fine-grained facies, shows the lack of extensive erosion and reworking and, thus, prolonged equilibrium phases during the deposition of the Hojedk Formation. The palaeocurrent measurements of laterally accreted, sigmodal-shaped sandstone bodies indicate two opposite north- and south-directed sediment transport trends within point bar deposits. These, together with palaeogeographic constraints (see below), indicate a west-to-east palaeoflow direction (present-day orientation) of the high-sinuosity meandering channels on the northern Tabas Block. The source area of the Hojedk Formation had to be located on the Yazd Block in the west and will be discussed below.

Fig. 15. Sedimentary model of the Hojedk Formation in the Kalshaneh area of the northern Tabas Block (not to scale). The swampy floodplains were the sites for accumulation of plant material and the formation of coals. For key to lithologies see Figure 6.

5.b. Provenance and potential source area

Modal analysis shows that the sandstones of the Hojedk Formation at Kalshaneh comprise quartz-rich sandstones including quartz arenite and quartzo-lithic petrofacies. The modal data of these sandstones plot into the recycled orogenic field of the QmFLt and QtFL triangles for tectonic provenance (Dickinson & Suczek, Reference Dickinson and Suczek1979; Dickinson et al. Reference Dickinson, Beard, Brakenridge, Erjavec, Ferguson, Inman, Knepp, Lindberg and Ryberg1983; Fig. 16). This supports derivation of the sediment from a recycled-orogen provenance. Very few samples plot into the craton interior provenance which indicates derivation from relatively low-lying granitoid and gneissic source rocks (Dickinson et al. Reference Dickinson, Beard, Brakenridge, Erjavec, Ferguson, Inman, Knepp, Lindberg and Ryberg1983). However, using only detrital modes for reconstructing the plate-tectonic setting is the main point of criticism by Garzanti (Reference Garzanti2016) who recommends combining sandstone petrography with multiple techniques to infer the tectono-stratigraphic conditions of source areas. The high textural maturity of the sandstone samples also supports recycling rather than first-cycle sediments. However, recycling is a physical process and cannot solely produce mature sandstones (Garzanti, Reference Garzanti2017), though strong chemical weathering under humid Middle Jurassic climate, coupled with sedimentary recycling, definitely increased the maturity of the sandstones of the Hojedk Formation. The presence of (pseudo-)matrix in some of the sandstone samples is most probably due to squeezing of mud intraclasts eroded from floodplain deposits between rigid grains (e.g. Morad et al. Reference Morad, Al-Ramadan, Ketzer and De Ros2010). Palaeogeographic constraints (exposure of the Yazd Block as indicated by the widespread absence of Middle Jurassic strata), the distribution of facies associations and palaeocurrent data from the Hojedk Formation at the Kalshaneh suggest that one of the possible source areas for the Hojedk Basin in the northern Tabas Block is represented by the exposed rocks on the Yazd Block in the west. The source rock composition of the Yazd Block is rather diverse due to the existence of several basement rock units, including Precambrian and Ordovician–Triassic sedimentary, low- to upper-rank metamorphic as well as plutonic rocks (Bagheri & Stampfli, Reference Bagheri and Stampfli2008). The Posht-e-Badam complex of the eastern Yazd Block (Kargaranbafghi et al. Reference Kargaranbafghi, Neubauer, Genser, Faghih and Kusky2012, Reference Kargaranbafghi, Neubauer and Genser2015) had been considered as a potential source for the Ab-e-Haji Formation of the western part of the Tabas Block during the Early Jurassic (Salehi et al. Reference Salehi, Moussavi-Harami, Mahboubi, Wilmsen and Heubeck2014, Reference Salehi, Moussavi-Harami, Mahboubi, Fürsich, Wilmsen and Heubeck2018). However, based on the petrographic analysis, the sandstones of the Hojedk Formation are lithic-poor, though the presence of subordinate low-grade metamorphic rock fragments and stretched polycrystalline quartz could be evidence for a source area from the Yazd Block in the west. The quartz-rich sandstones of the Hojedk Formation might originate from erosion and recycling of older quartz-rich sediments or supracrustal sedimentary rocks of the Yazd Block, while erosion probably did not unroof the underlying plutonic and metamorphic basement rocks. The silt- to sand-sized organic matter (reworked coal fragments) in the sandstone samples also indicates erosion of coal-bearing strata within the Hojedk Basin or in the source area. Potential source rocks may be the siliciclastic, coal-bearing units in the lower Shemshak Group (Upper Triassic to ?Lower Jurassic) that are present in some parts of the Yazd Block (Wilmsen et al. Reference Wilmsen, Fürsich and Majidifard2015). Furthermore, a high degree of chemical weathering in the source area due to overall very humid climatic conditions during the deposition of the Hojedk Formation is evident by the presence of rich macrofloras (e.g. Vaez-Javadi, Reference Vaez-Javadi2016) and coals as well as the prevalence of grey colours and the lack of red beds (e.g. Sheldon, Reference Sheldon and Gornitz2009). The fossil-plant-rich floodplain deposits of the Hojedk Formation in the Kalshaneh area contain macrofloras (e.g. Nilssonia sp., Ginkgoales sp.; Vaez-Javadi, Reference Vaez-Javadi2016) typical of a humid subtropical to temperate climate, and Khalilizadeh et al. (Reference Khalilizadeh, Ashouri, Ghaderi and Barbacka2022) documented extensive swamps for the Hojedk Formation of the Kerman area. Such conditions might have removed unstable lithic grains, thus increasing the quartz content and maturity of sandstones (e.g. Weibel et al. Reference Weibel, Olivarius, Kjøller, Kristensen, Hjuler, Friis, Pedersen, Boyce, Andersen, Kamla, Boldreel, Mathiesen and Nielsen2017). Modal analysis of sandstones indicates a similar quartzose recycled provenance from a metamorphic source area with humid climatic conditions for the top part of the Hojedk Formation in the southern Tabas Block (Zamani-Pedram, Reference Zamani-Pedram2011). Likewise, this study considers the exposed rocks on the Yazd Block and Kalmard sub-block to the west as the source area for the clastic sediments deposited in the southern Tabas Block.

Fig. 16. QmFLt and QtFL ternary plots (Dickinson & Suczek, Reference Dickinson and Suczek1979; Dickinson et al. Reference Dickinson, Beard, Brakenridge, Erjavec, Ferguson, Inman, Knepp, Lindberg and Ryberg1983) for tectonic provenance reconstruction of the Hojedk Formation in the northern Tabas Block. Qt = total quartz; Qm = monocrystalline quartz; F = feldspar; Lt = total lithic fragments including polycrystalline quartz grains and chert fragments; L = unstable lithic fragments.

5.c. Thermal and maturation history

The VR_r measurements indicate advanced thermal maturity of the wet to dry gas zone for the analysed coal samples (Fig. 12). The maturity is comparable to that of Middle Jurassic coals from the adjacent Mazino coalfield to the south ranging from 2.08 % to 2.29 % (Zamansani et al. Reference Zamansani, Rajabzadeh, Littke, Zieger and Baniasad2019) and is far higher than the reported reflectance for the Upper Triassic coals in the Parvadeh coalfield ranging from 1.19 % to 1.33 %, corresponding to the upper oil window (see Fig. 1b for locations). In a recent study by Zamansani et al. (Reference Zamansani, Rajabzadeh, Littke, Zieger and Baniasad2019), the advanced maturation for the Jurassic coal in the Mazino area is attributed to a post-Jurassic overburden of 4500 to 6000 m according to applied basal heat flow. In the same study, a magmatic effect at 34 Ma and thick sediment deposition due to the Alpine orogeny at 20 Ma has been taken into account. However, the post-Jurassic strata on the Tabas Block are expected to be far thinner (e.g. Huckriede et al. Reference Huckriede, Ku Rsten and Venzlaff1962; Wilmsen et al. Reference Wilmsen, Wiese, Seyed-Emami and Fürsich2005; Zamani-Pedram, Reference Zamani-Pedram2011). All in all, the thickness of the strata overlying the Shemshak Group prior to erosion did not exceed c. 4350 m (Table 3).

After examining different scenarios in the model, the best calibration with the new kinetic model of Easy %R_oDL (Burnham, Reference Burnham2016) was achieved by considering a relatively deep burial depth which is consistent with the thickness of the lithostratigraphic units overlying the Shemshak Group in the type or reference sections on the northern Tabas Block before erosion. These strata include the upper Middle Jurassic to Palaeocene sedimentary succession, Eocene–Oligocene volcanic rocks, thick (c. 1500 m) Neogene red beds and conglomerates, equivalent to the Upper Red Formation, causing a maximum burial depth during the Miocene (Table 3; Fig. 13). In order to reach the calibration, a minimum of 3700 m of erosion is necessary in the model, which is in agreement with the present-day thickness of the overburden after a major uplift and erosion event during the Alpine orogeny sensu lato from 34 Ma to 0 Ma (Agard et al. Reference Agard, Omrani, Jolivet, Whitechurch, Vrielynck, Spakman, Monié, Meyer and Wortel2011); most of the exhumation in the northern CEIM occurred rather late from 5 Ma to 0 Ma (Konon et al. Reference Konon, Nadimi, Koprianiuk, Wysocka, Szaniawski, Wyglądała, Słaby, Beygi and Barski2016; Tadayon et al. Reference Tadayon, Rossetti, Zattin, Nozaem, Calzolari, Madanipour and Salvini2017) (Table 3).

In addition to the attributed maximum burial depth, the present-day thermal state of the coals from the Hojedk Formation at Kalshaneh may result from an advanced thermal maturation of the region caused by, for example, hydrothermal activities along the adjacent deep-seated block-bounding faults and/or intense magmatic activity during the Palaeogene (e.g. Kargaranbafghi et al. Reference Kargaranbafghi, Neubauer and Genser2015; Mehdipour Ghazi et al. Reference Mehdipour Ghazi, Moazzen, Rahgoshay and Wilde2020). The latter scenario is further supported by evidence of a regional magmatic event during the Eocene in the northern Tabas Block (Stöcklin et al. Reference Stöcklin, Eftekhar Nezhad and Hushmand-Zadeh1965; Ruttner et al. Reference Ruttner, Nabavi and Hajian1968; Konon et al. Reference Konon, Nadimi, Koprianiuk, Wysocka, Szaniawski, Wyglądała, Słaby, Beygi and Barski2016) and on the northern Yazd Block in the Khur area (Aistov et al. Reference Aistov, Melnikov, Krivyakin, Morozov and Kiristaev1984). Although the modelling did not show a direct effect of magmatic activity on the thermal maturity of the Middle Jurassic coals, the vicinity of the study area to the deep-seated Kalmard Fault (see Figs 1, 4 for location) implies a mixed scenario affecting the thermal maturation of the region by means of a slightly increased heat flow since the Early Jurassic when the fault became active (Salehi et al. Reference Salehi, Moussavi-Harami, Mahboubi, Fürsich, Wilmsen and Heubeck2018). There is also a possibility that the maturity of Middle Jurassic coal-bearing strata was affected by possible Eocene magmas that were emplaced in the subsurface and intruded the area in the form of sills or dykes. Although there are few documented occurrences of Eocene magmatic rocks in the northern Tabas Block, the factual record of volcanic or sub-volcanic rocks in the study area is sparse due to the later erosion. However, on a slightly larger regional scale there is widespread field evidence of Eocene to Oligocene sub-volcanic rocks with tens of metres-thick sills intruding Lower Cretaceous sedimentary strata in the Khur area of the northern Yazd Block (Aistov et al. Reference Aistov, Melnikov, Krivyakin, Morozov and Kiristaev1984, and unpublished data from the Rudkhaneh–Arusan area, south of Chupanan). Therefore, we cannot rule out a similar geological situation for the Kalshaneh area with sub-volcanic intrusions penetrating post-Shemshak strata and magnifying the thermal maturation of the coals within the Hojedk Formation. A detailed regional mapping survey would be needed in order to better understand the effects of Eocene to Oligocene magmatic activity on the thermal maturation of the Jurassic coals on the northern Tabas Bock. However, such investigation is beyond the scope of the present study.

5.d. Palaeogeographic implications

In this study, the stratigraphy and sedimentary facies analysis of the studied section, including palaeocurrent measurements and petrographic analysis of sandstones, are considered in order to reconstruct the palaeogeographic framework of the Hojedk Formation during the Middle Jurassic and the geological processes prior to the Mid-Cimmerian (Bajocian) tectonic event.

After the widespread transgression during the Toarcian–Aalenian, which resulted in the formation of the ammonite-rich limestones of the Badamu Formation in large parts of the CEIM (e.g. Seyed-Emami et al. Reference Seyed-Emami, Fürsich, Wilmsen, Schairer and Majidifard2004 a) and the deposition of deep-marine facies also in other parts of Iran (e.g. Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami, Cecca and Majidifard2005 b), the accumulation of the Hojedk Formation during the early Bajocian indicates rapid shallowing and progradation of facies belts. Thickness and facies variations over short distances have been reported as well (Wilmsen et al. Reference Wilmsen, Fürsich and Seyed-Emami2003, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b). This points to tectonic instability in the run-up to the Mid-Cimmerian tectonic event (Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 a; Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b). Based on the geodynamic models presented for the CEIM from the Early to early Late Jurassic (Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Taheri2009 a, Reference Wilmsen, Fürsich, Seyed-Emami and Majidifard2021; Mattei et al. Reference Mattei, Cifelli, Muttoni and Rashid2015), the three blocks of the CEIM (Yazd, Tabas and Lut) were in a pre-rotational position, their long axes approximately oriented W–E and the Lut Block facing the Neotethys in the south. Their bounding faults acted as normal faults, which explains the lithostratigraphic variation and facies patterns of the siliciclastic-dominated Shemshak Group and the carbonate-, mixed carbonate/clastic- and evaporite-dominated Magu Group Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Zamani-Pedram2010; Salehi et al. Reference Salehi, Moussavi-Harami, Mahboubi, Wilmsen and Heubeck2014, Reference Salehi, Moussavi-Harami, Mahboubi, Fürsich, Wilmsen and Heubeck2018; Fürsich et al. Reference Fürsich, Pan, Wilmsen and Majidifard2016; Seyed-Emami et al. Reference Seyed-Emami, Wilmsen and Fürsich2020). The extensional tectonic regime occurred in a continental retro-arc basin during phases of limited frictional stress in the subduction zone of the Neotethys Ocean to the south, the crust of which was subducted beneath the Iran Plate (Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Taheri2009 a; Salehi et al. Reference Salehi, Moussavi-Harami, Mahboubi, Fürsich, Wilmsen and Heubeck2018). However, several short-term tectonic events punctuated the Jurassic succession of the Tabas Block (Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami and Majidifard2003; Wilmsen et al. Reference Wilmsen, Fürsich and Seyed-Emami2003, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b, Reference Wilmsen, Fürsich, Seyed-Emami and Majidifard2021; Seyed-Emami et al. Reference Seyed-Emami, Fürsich and Wilmsen2004 b) that may have had their origin in subduction-related processes to the south of the CEIM (e.g. sudden changes in subduction rate and/or variations of frictional stress).

The replacement of marine carbonates of the Badamu Formation by the siliciclastic rocks of the Hojedk Formation was probably caused by uplift of the Yazd Block with extensive erosion that supplied large amounts of clastic sediments to the Hojedk Basin (Fig. 17). The great thickness (800 m) of fluvial strata of the Hojedk Formation at Kalshaneh points to a pronounced extension-related subsidence in the sedimentary basin of the northern Tabas Block. The rapid rate of differential subsidence could be explained by accelerated normal block-faulting in the back-arc extensional basin of the CEIM and contemporaneous N–S extension in northern Iran and the South Caspian Basin (Brunet et al. Reference Brunet, Korotaev, Ershov and Nikishin2003; Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami, Cecca and Majidifard2005 b, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Taheri2009 a; Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Taheri2009 a). This presence of two parallel zones of extension will form a domain of compression in between, explaining the uplift of the northern basin shoulder (Yazd Block) of the CEIM. However, the fault-bounded, rapidly subsiding Tabas Basin was rapidly filled with clastic sediments of a low-energy and low-relief meandering river system flowing from the basin margin in the north (Yazd Block) southward (Lut Block; in present-day block orientation W–E) (Fig. 17). This fluvial environment prevailed on the western Tabas Block (Kalmard sub-block) and was replaced by marginal and shallow marine environments toward to the east, evidenced by marine facies of the Hojedk Formation on the eastern Tabas and Lut blocks (Seyed-Emami et al. Reference Seyed-Emami, Fürsich, Wilmsen, Schairer and Majidifard2004 a; Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b). The presence of meandering-rivers sedimentary facies within the succession confirms the deposition in an extensional and relatively low-gradient area rather than compressional setting for the (northern part of the) Tabas Block. Moreover, the uniform stacking of fluvial fining-upward cycles in large parts of the succession indicates continuous basin subsidence and uplift of the source area during an equilibrium phase of differential subsidence. The pronounced increase in bed thickness and sandstone grain size in the upper part of the Hojedk Formation is probably a response of the sedimentary system to the Mid-Cimmerian (mid-Bajocian) compressional tectonic event and associated deformation and increased uplift in the source area (Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 a). This tectonic event temporarily decreased the rate of basin subsidence and resulted in the deposition of the sand-dominated succession towards the top of the Hojedk Formation. The compressional regime finally terminated the sedimentary megacycle of the Shemshak Group and led to subaerial exposure and erosion (formation of iron oxide crust, pedogenic overprint) at the top of the Hojedk Formation in the Kalshaneh area. The overlying limestones of the Upper Bajocian Parvadeh Formation with its basal quartz conglomerate indicate a renewed phase of subsidence and generation of accommodation space on the Tabas Block, which resulted in a pronounced relative sea-level rise (deposition of the Parvadeh and Baghamshah formations) and the establishment of the large-scale carbonate platform system of the Esfandiar Subgroup (Wilmsen et al. Reference Wilmsen, Fürsich and Seyed-Emami2003, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 b, Reference Wilmsen, Fürsich, Seyed-Emami, Majidifard and Zamani-Pedram2010). Similar marine transgressions following a mid Bajocian (i.e. Mid-Cimmerian) uplift event occurred during the Late Bajocian in other parts of Iran such as in the Alborz, Binalud and Kopet-Dagh basins (e.g. Fürsich et al. Reference Fürsich, Wilmsen, Seyed-Emami, Majidifard, Brunet, Wilmsen and Granath2009 a; Taheri et al. Reference Taheri, Fürsich, Wilmsen, Brunet, Wilmsen and Granath2009) as well as the neighboring Afghan–Tajik Basin, which suggests a largely synchronous tectonic activity (Fürsich et al. Reference Fürsich, Brunet, Auxiètre, Munsch, Brunet, McCann and Sobel2017). Furthermore, in situ Rb/Sr dating recently yielded precise and accurate cooling ages ranging from 170 to 140 Ma that document the exhumation of Central Iran from the Mid-Cimmerian Event onward (Gyomlai et al. Reference Gyomlai, Agard, Jolivet, Larvet, Bonnet, Omrani, Larson, Caron and Noël2022) and provided a temporal link to the succeeding Late Cimmerian tectonics (Wilmsen et al. Reference Wilmsen, Fürsich, Seyed-Emami and Majidifard2021).

Fig. 17. Palaeogeography of the northern Tabas Block in east-central Iran during the Middle Jurassic (early Bajocian). The meandering river system on the Kalmard sub-block and western part of the northern Tabas Block was sourced from the Yazd Block, grading into shallow-marine lithofacies eastwards. NF: Naini Fault; KF: Kalmard Fault.