Stratigraphic application of Thalassinoides ichnofabric in delineating sequence stratigraphic surfaces (Mid-Cretaceous), Kopet-Dagh Basin, northeastern Iran

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Palaeoworld 21 (2012) 202–216

Research paper

Stratigraphic application of Thalassinoides ichnofabric in delineating sequence stratigraphic surfaces (Mid-Cretaceous), Kopet-Dagh Basin, northeastern Iran M. Sharafi, M. Ashuri, A. Mahboubi ∗ , R. Moussavi-Harami Department of Geology, Ferdowsi University of Mashhad, Iran Received 3 May 2011; received in revised form 6 May 2012; accepted 1 June 2012 Available online 12 June 2012

Abstract This study integrates ichnological and sedimentological data to refine depositional sequences and interpretations of sea-level dynamics for the shallow marine, Albian–Cenomanian Aitamir Formation in northeastern Iran. Three ichnofabrics are present in a succession of glauconitic mudstone and sandstone. This is a sequence that grades upward from a lower glauconitic sandstone unit with trough cross-stratification, hummocky and ripple cross-lamination into a fining-up unit of mudstone with intercalated sandstone beds. The lower unit contains an ichoassemblage of the Ophiomorpha–Palaeophycus ichnofabric (upper shoreface), whereas the upper unit bears ichnoassemblages of the Thalassinoides ichnofabric (in a distinctive level at the top cycle which demarcates the base of the next cycle) (middle shoreface) and the Chondrites–Planolites ichnofabric (lower shoreface). An upper shoreface–lower shoreface trend from the Ophiomorpha–Palaeophycus ichnofabric to the Chondrites–Planolites ichnofabric represents a deepening-upward sequence. An integrated sedimentological and ichnological approach has allowed the recognition of the internal organization of the sequence and the characterization of significant discontinuity surfaces at sequence scales. Thalassinoides ichnofabric reveals colonization of firmgrounds during prolonged times between erosion and deposition related to transgressive surfaces. Transgressive surfaces (sequence boundaries) are generally well-cemented and marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, unlined, and uncompacted burrows (omission suite) and are associated with rare highly abraded and fragmented shell remains. © 2012 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. Keywords: Mid-Cretaceous; Ichnofabric; Glossifungites; Firmground; Aitamir Formation; Kopet-Dagh

1. Introduction Ichnological analysis has become a valuable tool in basin research, and is of special interest for recognizing and interpreting genetically related sedimentary packages as well as bounding discontinuities (e.g., Tovar et al., 2007). Biogenic structures can be used in sequence stratigraphy into two principal ways: (a) to allow the identification of sequence stratigraphic discontinuities through the recognition of substrate-controlled ichnofabrics, and (b) to help on the interpretation of palaeoenvironmental changes through detailed characterization of vertical changes in softground ichnoassemblages (Savrda, 1991;

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Corresponding author. E-mail address: [email protected] (A. Mahboubi).

Pemberton and MacEachern, 1995; MacEachern et al., 1999; Pemberton et al., 2001, 2004; Tovar et al., 2007). In the most favourable situations, integration of data from substratecontrolled ichnofabrics with those from the vertical ichnological successions in softgrounds proves particularly informative in sequence stratigraphic analyses, even more so when these data are integrated with other sources of information. The use of trace fossils is particularly important when primary sedimentary structures are obliterated by bioturbation, and when other sedimentological features (for example grain size changes) are poorly expressed. This study shows how ichnofabrics analysis in conjunction with sedimentological evidence can be used both to document the internal organization of a depositional sequence and to define sequence boundaries in relatively homogeneous siliciclastic shoreface to shelf sediments. This research is a first

1871-174X/$ – see front matter © 2012 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. http://dx.doi.org/10.1016/j.palwor.2012.06.001

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attempt to establish connection between ichnological approach and sequence stratigraphy in the Aitamir Formation. This paper: (1) documents the vertical distribution of the ichnofabrics of the Aitamir Formation, (2) displays the variations in ichnoassemblages along key stratal surfaces such as sequence boundaries, and (3) presents biogenic structures (particular Thalassinoides) that are useful for lateral correlation in shallow-marine successions. 2. Geological setting and stratigraphy The Sheikh and Sorkhezoo sections, on the limbs of the Sheikh syncline (northwest Shirvan), the Gadganloo and Passkooh sections, on the limbs of the Bibahreh syncline (northeast Shirvan), and the Tirgan and Robat sections (southeast Dargaz), are located in the Kopet-Dagh Basin of northeast Iran and southwest Turkmenistan (Fig. 1). The Kopet-Dagh petroliferous basin formed as a result of the southeastern extension of the South Caspian Basin by Neotethyan back-arc rifting after closure of the Palaeotethys and the early Cimmerian Orogeny (Berberian and King, 1981; Ruttner, 1993; Alavi et al., 1997; Garzanti and Gaetani, 2002; Brunet et al., 2003; Golonka, 2004; Wilmsen et al., 2009) (Fig. 2). According to Wilmsen et al. (2009), the timing of the Eo–Cimmerian event is Ladinian–Carnian. The continued northward drift of the Cimmerian continent and progressive subduction of the Palaeotethyan oceanic crust corresponded with the opening of the Neotethys. After a phase of Late Triassic–Early Jurassic compression, a rifting regime was re-established within the Scythian–Turan Platform and continued through to the Middle Jurassic times. In the western and central part of the platform, Early–Middle Jurassic rifting was concentrated primarily in the Greater Caucasus Basin and Caspian trough developed as a subsidiary rift of the Greater Caucasus rift system (Golonka, 2004; Poursoltani et al., 2007). During the Tithonian–Berriasian time, rifting commenced along the northern and eastern margins of the Lut block (Fig. 2). This rifting was followed by sea floor spreading during the Barremian–Hauterivian and the formation of the Sistan Ocean by Albian time (Golonka, 2004) (Fig. 2). Spreading continued in the Greater Caucasus proto-Caspian Ocean at the beginning of the Late Cretaceous and the Greater Caucasus Proto-Caspian Ocean was connected with the Sistan Ocean (Golonka, 2000, 2004) (Fig. 2). The Kopet-Dagh Basin covers an area of approximately 500 km2 in northeast Iran. More than 7000 m of carbonate, siliciclastic and evaporite sediments were deposited from the Jurassic through Miocene times in the eastern parts of the basin (Fig. 3; Afshar-Harb, 1979, 1994) that formed five major transgressive–regressive sequences (Moussavi-Harami and Brenner, 1992). During the Albian–early Cenomanian time, a relative sea level fall caused deposition of the glauconitic sandstone of the Aitamir Formation (Afshar-Harb, 1994). It is composed of glauconitic sandstone and green shale at the type locality (northeast of GonbadKavous). The thickness of this formation ranges from 240 m (in the west) to 440 m in the east, reaching its maximal thickness at the type locality (1000 m). The Aitamir Formation is divided into lower greenish glauconitic sandstone and upper

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green shale units (Afshar-Harb, 1994). The Aitamir Formation overlies the marine shales of the Sanganeh Formation with sharp surface and is unconformably overlain by the chalky limestones of the Abderaz Formation. According to AfsharHarb (1994), the upper boundary of the Aitamir Formation throughout the Kopet-Dagh Basin is disconformable, whereas Mokhtari et al. (1999) suggest that a correlative conformity in the Sarakhs area and Sadeghi and Froughi (2005) based on foraminiferal studies implies a stratigraphic gap between the latest Cenomanian–early middle Turonian. Hadavi and Poursmaiel (2007) suggest a monochronous boundary between the Aitamir and Abderaz formations based on nannoplankton assemblages. The age of the Aitamir Formation has been also a matter of dispute. According to Afshar-Harb (1994) the age of this formation is Albian–early Cenomanian, whereas Hadavi and Poursmaiel (2007) imply a Santonian to early Campanian age for this formation. Based on the recent studies, the age of this formation is late Albian–early middle Cenomanian and there exists a major unconformity at the base of the Abderaz Formation (Mosavinia et al., 2007; Mosavinia and Wilmsen, 2011). 3. Materials and methods This paper describes six measured sections across the KopetDagh Basin in NE Iran (Fig. 1). Two hundred and twenty thin sections and forty washed samples were examined to identify fine-scale physical characteristics, mineralogical composition, and fossils. Lithology, grain-size, and sedimentary structures were recorded. Both sedimentologic features and trace fossils were examined on fresh and weathered surface in the field. The ichnoassemblages in the Aitamir Formation are preserved mainly within sandstone and a few siltstone sediments. Most of the mudstone facies (fissile shale) have no identifiable biogenic structures. Percentages of glauconite were determined by visual comparison charts. Degree of bioturbation is assessed according to Taylor and Goldring (1993). In this scheme, a bioturbation index (BI), ranging from 0 (no bioturbation) to 6 (complete bioturbation), is defined. 4. Sedimentology 4.1. Description The study area has a tripartite division into eastern, central, and western sectors. The western sector includes the Sheikh and the Sorkhezoo sections. The Aitamir Formation at the Sheikh section is 240 m thick and at the Sorkhezoo section is 290 m thick. The Aitamir Formation is characterized by an alternating succession of sandstones and shale that is divided into two units (Fig. 4). At the Sheikh section, the lower unit is only 14 m thick and contains an alternation of siltstone and thin-bedded, principally homogenous, well-sorted, fine- to very fine-grained, glauconitic sandstone. Sparse bioclastic (mainly ammonites and bivalves) and glauconitic grains (5–10%) are present. Bioturbation is low (BI 1–2, mainly Ophiomorpha nodosa). Here, the upper unit consists of dominantly thick-bedded mudstone (200 m) with a few homogenous glauconitic sandstones. The

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Fig. 1. (A) Location map and studied sections of the Aitamir Formation in northeastern Iran. (B–D) Simplified geological maps of the Sheikh–Sorkhezoo area, the Gadganloo–Passkooh area, and the Tirgan–Robat area with location of the studied sections. Basemaps: sheets J-3 of 1:250,000 map of Bojnurd (Bolourchi and Afshar-Harb, 1987) and 7864–7865 of 1:100,000 map of Dargaz (Akrami, 2004).

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Fig. 2. (A) Map of Iran showing the nine geological–structural zones (modified from Stocklin, 1968). (B) Generalized palaeogeography of latest late Aptian–middle Cenomanian of the eastern Pangea and the Tethys, displaying northward drift of the Cimmerian continent, the subduction of the Neotethys under the Lut block, and the connection between the Proto-Caspian Ocean with the Sistan Ocean (modified from Golonka, 2004). AL: Alborz; He: Helmand block; KD: Kopet-Dagh; KI: Kirsehir; LC: Lesser Caucasus; LU: Lut block; Sa: Sakarya; SCM: South Caspian microcontinent; So: Sistan Ocean; SS: Sanandaj-Sirjan; TL: Talysh. Red line is a transform fault. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Fig. 3. Generalized stratigraphic chart for the eastern part of the Kopet-Dagh Basin. Modified after Kalantari (1987) and Immel et al. (1997).

thickness of the lower unit increases toward Sorkhezoo section and also eastern sections. It is about 60 m thick in Sorkhezoo and composed of siltstone, shale, glauconitic sandstone and limestone (shell concentration at the base of the formation with mainly ammonites, bivalves and gastropods). In the lower unit of this section, glauconitic grains decrease from 20% at the base to 7% at the top and bioturbation index is 1–4. The upper unit at the Sorkhezoo section is similar to the Sheikh section, only the thickness of glauconitic sandstone is greater (9 m) and a thin-bedded limestone is included in the upper unit (Fig. 5B). The central sector includes the Gadganloo and the Passkooh sections. The thickness of the Aitamir Formation is 250 m at the Gadganloo and 246 m at the Passkooh sections. The thickness of the lower unit increases toward the central sector, 63 m thick at the Gadganloo and 60 m thick at the Passkooh section. The lower unit in this sector consists dominantly of thick-bedded, fine- to medium-grain, glauconitic sandstone with a few sedimentary structures (trough and planar cross-bed) as well as some thin-bedded limestone (shell concentrations with ammonites, bivalves, and gastropods) (Sharafi et al., 2010). At the Passkooh section, the lower unit is onset by a lag concentration (fossiliferous glauconitic sandstone) (Fig. 5C), and continued with homogenous sandstone and shell concentration on the top of the unit (Sharafi et al., 2010) (Fig. 5D). Glauconitic grains and biogenic structures are more common in the lower unit of the central sections than in the western sector. The glauconitic grain content is between 10 (Gadganloo) and 14 (Passkooh) percent and bioturbation index is 2–6 (mainly with O. nodosa, Palaeophycus tubularis and Thalassinoides isp.). In these sections, the upper unit is composed of thick-bedded mudstone and alternation of glauconitic sandstone and shale. The content of glauconitic grains is 7–15% and bioturbation index is 1–2 in the western sections, whereas in the eastern sections, glauconitic grain content is 10–35% and bioturbation index is 2–4.

Fig. 4. (A) The lower thick-bedded glauconitic sandstone (at the Tirgan section). (B) The upper green mudstone with intercalated sandstone beds (at the Robat section). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

The eastern sector includes the Tirgan and the Robat sections. The thickness of the Aitamir Formation increases to 420 m thick at Tirgan and 430 m thick at Robat sections. The lower unit in these sections is characterized by mostly thick-bedded, fine- to medium-grain, glauconitic sandstone with a few sedimentary structures (trough and planar cross-bed and hummocky crossstratification) (Fig. 5A), as well as some thin-bedded limestone (ammonites-bivalves dominated shell concentrations) (Sharafi et al., 2010). Glauconitic grains and biogenic structures increase in the lower unit of the eastern sections. Glauconitic grain content is 15–40% and bioturbation index is 2–5 (mainly with O. nodosa, P. tubularis and Thalassinoides isp.). The upper unit in the eastern sector displays an increase in the thickness of the sandstone beds and is composed of thick-bedded mudstone and alternation of glauconitic sandstone and shale (Fig. 4B). The abundance of glauconitic grains and bioturbation structures in the upper unit of the eastern sections increases, so that the content of glauconitic grains is 8–30% and bioturbation index is 1–6 (mainly with Chondrites isp., Planolites isp. and Thalassinoides isp.). At the Robat section a shell concentration with well-preserved bivalve-dominated fauna is present in middle of this unit that is associated with network Thalassinoides isp. and high galuconite concentration (Fig. 6). In all, mudstone facies in the Aitamir Formation consists of low concentrations of glauconite grains (about 5–10%) and skeletal debris (bivalve and foraminifera Heterohelix), in addition to few trace fossils dominated by assemblages of O. nodosa.

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Fig. 5. (A) Trough cross-stratification in the lower unit sandstone of the Robat section at 7 m. (B) Lag concentration with erosive boundary in the upper unit of the Sorkhezoo section at 280 m. (C) Lag concentration at the base of the Aitamir Formation, Passkooh, overlies a transgressive surface interpreted as sequence boundary at the base of formation. (D) Ammonite-dominated concentration at the top of the transgressive system tract shows a maximal flooding surface (the Passkooh section at 7 m). Note well preserved ammonites (black arrows). White arrows indicate way up.

Compared with the upper unit, the lower unit is thinner and characterized by different facies and sedimentary structures. Furthermore, the lower unit displays a conspicuous and welldeveloped ichnoassemblage. In most of the sections (Sorkhezoo, Gadganloo, Passkooh and Tirgan), the base of the upper unit is marked by a well-cemented, high glauconitic, thin-bedded fossiliferous sandstone with strongly abraded and fragmented shell remains (as transgressive lag), and marked by intense bioturbation (Thalassinoides network) in the surface of bed, and displays a transgressive surface which can be traced out from proximal (eastern sections) to distal (western sections) settings over long distances. Bioclastic shell concentrations commonly form discontinuous deposits, but also with continuous horizons (Fig. 5B–D). 4.2. Interpretation Sedimentary structures such as trough and planar cross-bed and hummocky cross-stratification (mainly in the lower unit) in the sandstone sediments of the Aitamir Formation reveal deposition mainly above fair-weather wave-base (Walker and Plint, 1992; Uroza and Steel, 2008). Sporadic nature of trace fossil occurrences (mainly O. nodosa and P. tubularis) (Zonneveld et al., 2001) and abraded and fragmented shell remains (Fürsich and Pandey, 2003) indicates a stressed environment with high to moderate energy. The content of the shell concentration (ammonites, oyster, and abundant glauconite grains) indicates a normal marine environment (Fürsich and Pandey, 2003; Malpas

et al., 2005). Well-preserved fossils and abundant glauconite in the bivalve-dominated concentration show a low-energy marine environment with a low sedimentation rate (Parras and Casadio, 2005; Fürsich et al., 2009). The shelly and carbonate (rudstone) horizons probably result from reworking and winnowing by storms and currents (Jank et al., 2006; Sharafi et al., 2010). The grain-size, glauconitic grains, and bioclastic debris (bivalve and foraminifera Heterohelix) indicate that the green mudstone facies was deposited in a low-energy offshore setting. The upper unit of green mudstone with intercalated sandstone beds (Fig. 4) was deposited under low-energy conditions, occasionally influenced by storm and currents. In general, in the eastward direction, both grain-size and thickness of the lower glauconitic sandstone unit increase. In the same direction homogenous sandstone with sparse biogenic structures (BI 1–3) is replaced by glauconitic sandstone with primary sedimentary (trough and planar cross-bed and hummocky cross-stratification) and more biogenic structures (BI 2–6). Also, the upward decrease in average grain-size and lack of sedimentary structures indicate an overall deepening-upward sequence. 5. Results 5.1. Ichnofabric analysis The definitions of the ichnofabrics within this genetically related succession are based on previous studies (e.g., Taylor, 1991; Taylor and Goldring, 1993; Fürsich, 1998; Zonneveld

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Fig. 6. Sequence stratigraphy and distribution of trace fossils of the Aitamir Formation (Chalky limestone = Abderaz Formation, Black shale = Sanganeh Formation).

Table 1 Summary of the ichnofabrics in the Aitamir Formation. Description

Ophiomorpha nodosa–Palaeophycus tubularis

Predominantly vertical to 3–10 subvertical, unbranched O. nodosa and slightly inclined P. tubularis in buff to green glauconitic, sparse bioclastic, medium-grained sandstone Predominantly horizontal, 0.5–1 branched, large, positive hypo-relief and epirelief, box-work Thalassinoides suevicus and Thalassinoieds isp., with sparse-high abraded and rounded shell debris in buff to green glauconitic, fineto medium-grained sandstone

O. nodosa, P. 1–3 tubularis, Skolithos isp., O. annulata, Teichichnus isp., Planolites isp. Thalassinoides 4–5 suevicus and Thalassinoides isp., Rhizocorallium? isp., Planolites isp.

Trough cross bed, Predominantly fragmented and abraded hummocky cross bivalves, gastropods, bed, cross ripple ammonites, rare lamination planktonic foraminifers, echinoderm Predominantly Absent fragmented and abraded bivalves, gastropods, ammonite, rare planktonic foraminifers

Predominantly horizontal to 3–6 slightly inclined, branched to simple, burrow structures, in buff to green glauconitic, fine-(medium) grained sandstone

Planolites isp., Chondrites isp., Cylindrichnus concentricus, O. nodosa, P. tubularis, O. annulata, Thalassinoides isp.

Bivalves, gastropods, ammonites, echinoderm and rare planktonic foraminifers

Thalassinoides

Chondrites–Planolites

Thickness (m)

Trace fossils

BI

3–6

Body fossils

Sedimentary structures

Rare slightly inclined cross lamination and trough cross-lamination

Processes

Environmental interpretation

Shallow marine above High energy, high-sedimentation fair-weather wave base rate, high production, Upper shoreface softground substrate

High energy, high-sedimentation rate, high production, softground substrate

Above Ophiomorpha–Palaeophycus and Chondrites–Planolites ichnofabric and below Ophiomorpha–Palaeophycus ichnofabric Shallow marine moderately low energy between FWWB and SWWB lower–middle shoreface

Slow sedimentation rate, production related, high bioturbation rate, firmground developed from initial softground

Below Thalassinoides ichnofabric Shallow marine, low energy lower shoreface

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Ichnofabric

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Fig. 7. (A and B) Ophiomorpha–Palaeophycus ichnofabric. (A) Pt.: Palaeophycus tubularis; (B) O: Ophiomorpha nodosa. (C) Thalassinoides ichnofabric. Th.s.: Thalassinoides suevicus; Th.: Thalassinoides isp. The Tirgan section at 200 m (plan view). Note the sharp-walled burrows and difference between infilling sediments of the burrows and background sediments, displaying a firmground substrate. (D) Sparse abraded and rounded shell remains togther with Thalassinoides ichnofabric. The Passkooh section at 90 m (plan view). (E and F) Flooding surface associated with Thalassinoides ichnofabric in bed surface. The Passkooh section at 225 m (arrow indicates large burrow infilled by glauconitic sediment). White arrows indicate way up. Pen is 16 cm, hammer is 30 cm long.

et al., 2001; Buatois et al., 2002; Taylor et al., 2003; Tovar et al., 2007; McIlroy, 2007, 2008). Environmental interpretations are based on sedimentology, facies analysis, trace fossil diversity, and bioturbation index (BI) (Taylor and Goldring, 1993; Taylor et al., 2003). This study identifies three ichnofabrics (Table 1 and Fig. 6). Those ichnofabrics are spatially associated with marine flooding surfaces (Fig. 6), and include firmground indicators (Thalassinoides suevicus and Thalassinoides isp.). The vertical succession of ichnofabrics is interpreted to indicate environments within a shallow marine setting. 5.1.1. O. nodosa–P. tubularis ichnofabric (Fig. 7A and B) 5.1.1.1. Description. This ichnofabric consists predominantly of unbranched, vertical to sub-vertical, mud pellet-walled O.

nodosa, 0.5–3 cm in diameter, and simple, thinly-lined, horizontal to slightly inclined P. tubularis, 0.5–1.5 cm in diameter. It also has scarce Skolithos isp., Ophiomorpha annulata, Teichichnus isp. and Planolites isp. (Fig. 7A and B). Only one specimen of Helminthopsis isp. and Phycodes palmatus was found at the Gadganloo section. This ichnofabric occurs in units of 3–10 m thick, buff- to green-coloured, sparsely bioclastic, moderately well-sorted fine- to medium-grained glauconitic sandstone with trough and planar cross-stratification, hummocky and ripple cross-lamination and a bioturbation index of 1–3. Hummocky cross-stratification capped by ripple laminae is locally observed. Occasionally Ophiomorpha occurs in Palaeophycus mottled context. The bioclastic content of Ophiomorpha–Palaeophycus ichnofabric consists predominantly of fragmented and abraded

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Fig. 8. (A) Chondrites–Planolites ichnofabric at the Robat section at 370 m. Ch.: Chondrites, Pl.: Planolites, Cy.: Cylindrichnus. (B) Chondrites–Planolites ichnofabric at the Gadganloo section at 210 m. Poorly preserved and indistict the burrow margin indicates that these trace fossils were produced in a softground substrate (plane view). Pt.: Palaeophycus tubularis, Pl.: Planolites, Th.: Thalassinoides. (C) Cylindrichnus concentricus in muddy glauconitic sandstone at the Sorkhezoo section at 270 m. (D) Planolites in fine glauconitic sandstone at the Robat section at 80 m. (E) Whole and unabraded bivalves associated with Thalassinoides ichnofabric at the Robat section at 320 m. (F) Large, three-dimensional, positive hyporelief Thalassinoides isp. below glauconitic sandstone at the Tirgan section at 335 m. Arrows indicate way up. Hammer is 30 cm, pen is 16 cm long.

bivalves (oyster and others), gastropods, and ammonites as well as rare planktonic foraminifers. This ichnofabric is observed mainly in the lower unit (Fig. 6). 5.1.1.2. Environmental interpretation. This ichnofabric is a mix of suspension (O. nodosa) and deposit (P. tubularis) feeder traces. Trough to planar cross-stratification indicates an environment dominated by deposition from traction, frequent wave reworking, and intermittent strong currents in a highenergy, upper shoreface environment (Malpas et al., 2005; Uroza and Steel, 2008). This evidence, taken together with the low diversity, the low abundance of trace fossils, and abundant

vertical burrows in this ichnofabric, indicates a stressed environment with few taxa of organisms able to exploit nutrient resources (Zonneveld et al., 2001; Malpas et al., 2005; Gibert and Goldring, 2007; McIlroy, 2007). Abundant vertical burrows of suspension-feeders indicate relatively persistent highto moderate-energy (waves and currents) that kept organic particles in suspension in a mobile sandy substrate, most likely bars (Fürsich, 1998; Buatois et al., 2002). Moderately well-sorted quartz sand and the highly abraded nature of the bioclasts further support this interpretation (Zonneveld et al., 2001). Vertical burrows prevail as in the classic Skolithos ichnofacies model of Seilacher (1967).

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Fig. 9. Sequential development of the firmground at the Aitamir Formation. (A) Pre-omission suite (Ophiomorpha, Planolites, Chondrites, Palaeophycus, Thalassinoides; Ophiomorpha–Palaeophycus or Chondrites–Planolites ichnofabric) overprinted by large, box-work Thalassinoides ichnofabric (omission suite). (B) Scouring of the initial softground occurs (erosional exhumation), (C) shows the colonization by firmground dwellers and glauconite formation as a result of prolonged non-deposition and sediment starvation and in (D) die out with the influx of mud signalling the onset of the next sequences. Ch: Chondrites; Oph: Ophiomorpha; Pa: Palaeophycus; Pl: Planolites; Te: Teichichnus; Th: Thalassinoides. Stars are glauconite grains. See Fig. 6 for legend to symbols. Modified from Ruffell and Wach (1998).

5.1.2. Thalassinoides ichnofabric (Fig. 7C–F) 5.1.2.1. Description. The Thalassinoides ichnofabric consists of bed-parallel, glauconite and quartz sand-filled, greento bricky-green-stained T. suevicus and Thalassinoides isp. network (or patchy network). Thalassinoides has Y- and Tshaped branches, and is smooth-walled and unlined (i.e., do not have a pelleted burrow wall) with burrow diameters of 0.6–15 cm (Fig. 7C). Sharp-walled Planolites isp. with burrow diameters of 0.5–1 cm (at the Tirgan section) and Rhizocorallium? isp. (0.9 cm in diameter) (one specimen) are present. The Thalassinoides studied here is preserved as epireliefs and three-dimensional burrow systems, mainly horizontal in. The epireliefs of exhumed Thalassinoides are associated with glauconitic sandstone with strongly abraded and fragmented shells (bivalves, gastropods and ammonites) (Fig. 7D). The overall bioturbation index is 4–5 and generally the Thalassinoides ichnofabric overlies the Chondrites–Planolites or Ophiomorpha–Palaeophycus ichnofabrics and occasionally passes upwards into the Ophiomorpha–Palaeophycus ichnofabric. This ichnofabric is

observed mainly in the upper unit of the Aitamir Formation (Fig. 6). 5.1.2.2. Environmental interpretation. Shelly glauconitic sandstone with strongly abraded and fragmented skeletal debris is interpreted as a transgressive lag developed at the base of a transgressive system tract (Ghibaudo et al., 1996; Fürsich and Pandey, 2003; Mishra, 2009). The larger skeletal debris was sourced from shallower water or reflects a shallower, higher energy environment (Zonneveld et al., 2001; Malpas et al., 2005; Cantalamessaa et al., 2005). Strongly abraded and fragmented shell remains show high-energy currents that reworked bioclasts into low-energy environment, where trace markers produced intense biogenic structures. Furthermore, these features could reflect long time exposure on the taphonomic active zone (colonization window). Initially softground inhabiting organisms, probably crustaceans, produced Ophiomorpha and reinforced the burrow roof to prevent collapse. Over time, the original softground substrate developed into a firmground, and trace makers no longer needed to reinforce any part of

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the burrow system and so created a Thalassinoides network (Malpas et al., 2005). This ichnofabric is interpreted as representing the transition from softground (Chondrites–Planolites or Ophiomorpha–Palaeophycus ichnofabric) to firmground substrates. The overall increase in grain-size, reduced grain-size contrast between burrow-fill and background sediment and more complete bioturbation compared to lower ichnofabrics suggest a decrease in water depth and an increase in energy (Malpas et al., 2005). The increased energy, decreased water depth, and low diversity of trace fossils indicate a depositional environment between storm wave base and fair-weather wave base with an initially unconsolidated sediment (Martin and Pollard, 1996; Bromley and Ekdale, 1998). This ichnofabric corresponds to the intermediate Glossifungites ichnofacies (MacEachern and Burton, 2000). 5.1.3. Chondrites–Planolites ichnofabric (Fig. 8A–D) 5.1.3.1. Description. This ichnofabric consists of branched, meandering Chondrites isp. with diameters of 0.1–0.4 cm, unbranched, simple to meandering Planolites isp. with diameters of 0.2–1 cm, and Cylindrichnus concentricus which is a vertically oriented, bow-shaped burrow consisting of a central, sandy, passively-filled burrow (0.9 cm in diameter) surrounded by a concentrically laminated muddy lining (up to 4 mm thick) (Fig. 8A and C). The Chondrites–Planolites ichnofabric occurs in 3–6 m thick, buff- to green-coloured, weakly bioclastic, fine-grained glauconitic sandstone with rare crosslamination. The bioturbation index is 3–6. Burrow-fills of both Planolites and Chondrites are different in grain-size and composition with respect to the background sediment. Other trace fossils present in this ichnofabric are O. nodosa, O. annulata, P. tubularis, and Thalassinoides isp. The bioclastic content of Chondrites–Planolites ichnofabric consists of bivalves, gastropods, ammonites, echinoderms, and rare heterohelicid foraminifers. This ichnofabric is observed mainly in the upper unit of the Aitamir Formation (Fig. 6). 5.1.3.2. Environmental interpretation. The increase in ichnodiversity and bioturbation index suggests a low-energy, nutrient-rich, lower shoreface environment with a low sedimentation rate developed under normal marine conditions (Uchman and Kremnayr, 2004; Malpas et al., 2005; Aguirre et al., 2010). According to Buatois et al. (2002), the ichnoassemblage of the proximal lower shoreface combines elements of both the Skolithos (Ophiomorpha) and proximal Cruziana ichnofacies. The high bioturbation index results from the activity of the climax, resident infauna (Buatois et al., 2002). The ichnoassemblage is dominated by the feeding structures of deposit-feeders (fodinichnia), reflecting a Cruziana ichnofacies (Fürsich, 1998; Buatois et al., 2002). Cruziana ichnofacies is usually characterized by a wider variety of behavioural patterns, but the dominance of traces of deposit-feeders may be due to the generally low-energy regime, in which food particles tend to accumulate on the sea floor rather than being kept in suspension (Fürsich, 1998). This ichnoassemblage corresponds to the Cruziana ichnofacies.

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5.2. Discussion 5.2.1. Glossifungites ichnofacies and sequence stratigraphy application The ichnological approach is used here to characterize substrate-controlled ichnofabrics and to apply them to the recognition and interpretation of three orders of sequence stratigraphic discontinuities. This concept is reflected by many researchers with applying firmground Glossifungites ichnofacies in sequence stratigraphic studies (i.e., MacEachern et al., 1992; Pemberton et al., 1992; Ghibaudo et al., 1996; MacEachern and Burton, 2000; Buatois et al., 2002; Tovar et al., 2007). The Glossifungites ichnofacies is characteristic of firm but unlithified substrates, such as dewatered muds. It is also presented in incipiently cemented sands that are formed either by subaerial exposure or by burial and subsequent exhumation (e.g., MacEachern et al., 1992; Pemberton et al., 1992, 2001, 2004; Buatois et al., 2002; Malpas et al., 2005; Tovar et al., 2006). 5.2.2. Thalassinoides structures The Thalassinoides burrows are unlined, uncompacted and display sharp boundaries (Fig. 7C) and good preservation, presumably reflecting firmground conditions (early cementation and very slow or no sedimentation) (e.g., Ghibaudo et al., 1996; Ruffell and Wach, 1998; Gingras et al., 2001; Tovar et al., 2007). It can be expected that in firmground substrate, tracemakers do not need to protect their burrows against collapse by reinforcement of the burrow wall (e.g., Ghibaudo et al., 1996; MacEachern and Burton, 2000). Thus, these burrows should be discussed in the context of the Glossifungites ichnofacies. The producers of this form were probably crustaceans, which are known to be prone to very different adaptations (low- and highenergy environments, soft and firmground substrate) (Bromley, 1990). 5.2.3. Thalassinoides networks and transgressive surfaces The basal transgressive surface truncates older strata on a regional scale. Shelly sandstone facies directly overlies the transgressive surface and ranges in thickness from 3 to 7 cm. It consists of a sparsely bioclastic, fine-grained glauconitic sandstone. Shell remains (mainly bivalves, ammonites, and gastropods) are strongly abraded (rounded) and fragmented with chaotic orientation (Fig. 7D). The matrix is well-sorted glauconitic sandstone (20–40% glauconite). This coarse-grained, sparsely shelly, glauconitic basal facies is interpreted as a transgressive lag developed at the base of the transgressive system tract (Figs. 6 and 7D). Transgressive lags typically represent considerable time and multiple reworking by various processes, chiefly storm waves and currents or shoreface erosion within the early transgressive system tract that lead to sediment bypassing and winnowing (Fürsich and Oschmann, 1993; Fürsich et al., 2009). The unlined, passively infilled burrows and concentration of glauconite in the uppermost part of sandstone beds (Figs. 6 and 9) are thought to be produced during the omission stage, as a result of sediment starvation (Ghibaudo et al., 1996; MacEachern and Burton, 2000; Gingras et al., 2001). This sediment starvation is related with flooding events, and based on

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the relative duration of sediment starvation, different types of flooding surfaces can be developed: from maximal flooding surfaces to parasequence flooding surfaces, with Thalassinoides as a component of the firmground ichnofauna (Ruffell and Wach, 1998; Tovar et al., 2006). The glauconitic sediment was subsequently piped downwards through large burrows extending several decimeters below the surface. Probably Thalassinoides producers were partly suspension feeding during the omission stage (Ghibaudo et al., 1996). During this stage the burrows at the top of cycles acted as traps for glauconite. Trace fossil generations of lower glauconitic sandstone are produced in a soft-substrate (pre-omission stage) and consequently are poorly preserved, and their margins are indistinct. The slowdown and halt in sedimentation at the top of cycles (omission stage) resulted in enhanced cohesiveness of the substrate and in development of a firmground as a result of dewatering and high cementation (Ghibaudo et al., 1996; Ruffell and Wach, 1998; Tovar et al., 2006). Increased cohesiveness of sediment probably eliminated smaller soft-bottom burrowers, such as the producers of Planolites, Ophiomorpha and Palaeophycus. The time over which seafloor exposure occurs is possibly the most important criterion in firmground development (Ruffell and Wach, 1998). High glauconite content (20–40%) and strong cementation indicate a prolonged period of very slow sedimentation or non-deposition combined with possible winnowing, glauconite formation, intense burrowing, and increased cohesiveness of the sediment (Ruffell and Wach, 1998; Buatois et al., 2002; Parras and Casadio, 2005). In summary, the firm-substrate related to transgressive surfaces in the Aitamir Formation is characterized by sharp-walled, unlined, passively infilled, distinctive network Thalassinoides burrow, within preferentially cemented horizons associated with high glauconite content as well as strongly abraded and fragmented shell remains. 5.2.4. Thalassinoides network and maximum flooding surfaces The Thalassinoides burrows in the Aitamir Formation also occur as three dimensional, positive hyporelief, large network burrow systems with horizontal, Y- to T-shaped branches (Fig. 8F). The Thalassinoides burrows are characterized by having smooth margins without pellets in the burrow wall. As those in transgressive surfaces, these Thalassinoides also are uncompacted with sharp boundaries, and good preservation displays firmground conditions (Ruffell and Wach, 1998; Tovar et al., 2007). The positive hyporelief of Thalassinoides records phases of stable substrate conditions (Fürsich, 1998). The burrows are preserved on lower surfaces of fine-medium-grained sandstone, preferentially highly cemented, with high glauconite content (20–35%), without quartz granules. Indeed, network Thalassinoides occurs in the top of the inferred transgressive systems tract with slightly fining- and thinning-upward trend, which then is reversed to a weak coarsening- and thickeningupward trend (Fig. 6). At the Robat section, this horizon contains whole, unabraded, well preserved and a high percentage of conjoined bivalves (Fig. 8E). This concentration may be defined as a hiatal concentration (Kidwell, 1991; Parras and Casadio, 2005). The omission suite, suggesting firmground conditions,

is typically represented by uncompacted, unlined Thalassinoides (Ghibaudo et al., 1996; Fürsich, 1998; Gingras et al., 2001), extending downward into the underlying sandstone and infilled with strongly glauconitic sand. Characteristics of this bed (high glauconite content, omission burrows, shell concentrations, strong cementation) indicate a prolonged period of very slow sedimentation or non-deposition combined with possible winnowing, glauconite formation, intense burrowing, and increased cohesiveness of the sediment (firmground). These characteristics, together with the mid-cycle position of this unit, are correlated with maximal flooding surfaces as defined by Loutit et al. (1988). Taylor et al. (2003) mentioned omission surface with change in substrate consistency but without significant erosion or change in depth that represents a maximal flooding surface or sediment by-pass. This surface is therefore interpreted to represent the maximal flooding surface, separating the underlying overall fining- and thinning-upwards transgressive deposits from the overlying thickening- and coarsening-upwards highstand deposits. Trace fossils in the underlying deposits (not immediately) include Ophiomorpha, Chondrites and Thalassinoides (Chondrites–Planolites ichnofabric), whereas the overlying deposits are characterized mainly by Ophiomorpha (Ophiomorpha–Palaeophycus ichnofabric) and Cylindrichnus, Chondrites, Planolites, Thalassinoides and Palaeophycus (Chondrites–Planolites ichnofabric). Both ichnofabrics suggest a soft substrate and high-energy shallow water conditions (Ophiomorpha–Palaeophycus ichnofabric), and low-energy, deeper water conditions (Chondrites–Planolites ichnofabric) within an onshore–offshore profile. 6. Conclusions The ichnological approach is shown to be a valuable tool for improving high-resolution sequence stratigraphic models and high-frequency sea-level changes in the Aitamir Formation (northeastern Iran). Integration of sedimentological and ichnological data leads to identifying three ichnofabrics, namely, the low diversity Ophiomorpha–Palaeophycus (upper shoreface), Thalassinoides (middle shoreface), and high diversity Chondrites–Planolites ichnofabric (lower shoreface). The transgressive surfaces coincide with the sequence boundary determined by Thalassinoides ichnofabric within preferentially cemented horizons associated with high glauconite content as well as strongly abraded and fragmented shell remains. Although sequences have no obvious physical expression, they can be identified on the basis of the preservational state of trace fossil assemblages (softground versus firmground conditions) and to a lesser extent by slight vertical changes in grain-size and glauconite content. Discontinuity surfaces bounding the sequences are marked by an increased amount of glauconite. Enhanced consistency of the substrate and sediment starvation during the omission stage are indicated by a distinctive suite of densely crowded burrows, which are predominantly horizontal or oblique, relatively large, very distinct, unlined, and uncompacted.

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