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The distribution of conventional subichnofacies in deep‐water fan systems (i.e. Ophiomorpha rudis–Paleodictyon–Nereites‐subichnofacies) depends on the presence of fully turbulent non‐cohesive flows such as low‐density turbidites and high‐density turbidites. However, changes in flow state between laminar and turbulent conditions can affect the distribution style of conventional subichnofacies. This study deals with the effects of environmental or ecological stressors as a result of hybrid sediment gravity flows (hybrid event beds) on distribution and composition of trace fossils in the deep‐marine depositional system of the Ordovician Ghelli Formation in the north‐eastern Alborz Range. The spatial distribution and sedimentological character of hybrid sediment gravity flows, in particular their abundance in the down‐dip parts of lobe‐related facies associations, are interpreted as products of longitudinal and lateral transformation from turbidity currents to debris flows promoted by incorporation of significant claystone and mudstone clasts into the flow by seafloor erosion and flow bulking. Seven trace‐fossil assemblages were recognized in the channel and lobe‐related facies associations. The observed spatial distribution of trace‐fossil assemblages in the interpreted frontal and lateral lobe fringes, somewhat contrasts with conventional subichnofacies distribution models. The development of cohesive flows and rapid emplacement of turbidite, banded and debrite beds could be accompanied by fluctuations in physico‐chemical conditions and, hence, the time available for colonization and the lifestyle of producers. This study suggests that, compared to the archetypal models of the subichnofacies in down‐dip parts of lobe‐related facies associations, ichnodiversity and density of bioturbation are drastically reduced, and trace fossils are limited to simple, shallow‐tier feeding strategies. Commonly, the presence of cohesive flows may have forced the producers in the Paleodictyon and Nereites‐subichnofacies to migrate into lateral lobe fringe with more participation of low‐density turbidites. This study shows that, in addition to sedimentological and architectural criteria, ichnological criteria can also be used to distinguish between frontal and lateral lobe‐fringe deposits in unconfined systems. [ABSTRACT FROM AUTHOR]

The Deh‐Molla Formation of the Alborz Zone in northern Iran is a key unit to understanding global sea‐level fluctuations, and likely climate changes during the Furongian Epoch. The sedimentological characteristics of the formation indicate that the succession was deposited on a gently sloping epeiric platform on the northern margin of Gondwana. The Paibian Member 1 of the Deh‐Molla Formation was deposited on a carbonate platform with stable environmental conditions, probably during a (semi)arid climate condition, as indicated by microbial–sponge patch reefs, thick‐bedded units formed in broad facies belts, and a thick dolomite unit formed under evaporative conditions. The overlying Member 2, considered to span the Jiangshanian–Age 10, includes thin‐bedded wavy, and rippled strata known as ribbon rocks. This sequence records considerable environmental fluctuations. Thin‐bedded strata with flat‐pebble conglomerates, hummocky cross‐stratification, lenticular, flaser, and wavy bedding, ripple cross‐lamination, and graded bedding signify that deposition of Member 2 took place in a depositional environment repeatedly affected by tidal currents, and storm waves. A comparison of the Deh‐Molla Formation, and particularly its ribbon rocks with coeval strata in Laurentia and North China reveals secular controls on their development. In contrast to the Furongian strata of the Great American Carbonate Bank, deposited under tropical conditions, the ribbon rocks of the Deh‐Molla Formation suggest deposition in a temperate latitude zone with dominant cool‐water conditions. However, the Deh‐Molla platform was seasonally subjected to monsoons that brought warm tropical waters to the region, and led to rapidly changing environments. Ooids, peloids, stromatolites, and other criteria suggestive of a tropical climate are frequent in Laurentian successions, but absent in the Deh‐Molla Member 2. However, both Laurentian and Gondwanan examples share characters that indicate comparable controls on their development. Significant environmental changes during the Furongian are indicated by the SPICE and HERB events, and the latter is taken as showing biotic and physico‐chemical changes in the ocean and atmosphere, possibly concurrent with the formation of ribbon rocks on different continental blocks. [ABSTRACT FROM AUTHOR]

The shift of matground ecosystems to bioturbator-free settings was investigated in the Ordovician wave-dominated marine strata of the Shirgesht Formation in Central Iran. Ten ichnofabrics are recognized in shoreface–offshore and barrier-island sedimentary facies, representing a proximal-to-distal depositional trend along the studied profile. In the offshore settings, intensive burrowing on several tiers and bioirrigation, referred to the Thalassinoides (Th), Cruziana–Skolithos–Rosselia (CrSkRo), crowded Trichophycus (CT) and Cruziana–Helminthopsis (CrHe) ichnofabrics, prevented the development of matgrounds. As a result of the Ordovician radiation, progressive ecospace utilization by suspension-, deposit- and detritus-feeders, as represented by the Skolithos (Sk), Rosselia–Skolithos (RoSk), crowded Rosselia (CR) and Conichnus–Skolithos (CoSk) ichnofabrics, is regarded as a key factor for sediment mixing and reworking and thus disruption of the matgrounds in shoreface settings. However, the restriction on microbial growth was reduced in lower-offshore / shelf or tidal-flat and back-barrier settings, where Chondrites–Planolites (ChPl) and Planolites (Pl) ichnofabrics dominate. This restricted microbial growth resulted from physico-chemical stresses on infaunal communities, as shown by the low ichnodiversity, scattered burrowing, absence of complex tiering, and prevalence of simple feeding strategies. This study suggests that following the early Palaeozoic evolutionary radiations, a considerable increase in abundance of detritus-feeders, deposit-feeders, suspension-feeders, predators, vagile bilaterian metazoans and grazers in shallow-water benthic communities caused mat-building microorganisms to migrate into lower-offshore / shelf, tidal-flat and back-barrier settings, where colonization by burrowing organisms was delayed. [ABSTRACT FROM AUTHOR]

This study documents the distribution of matgrounds in a wide variety of environments recorded in the Ordovician Lashkerak and Ghelli Formations in the Alborz Mountains of northern Iran in order to evaluate controls on their distribution along the marine depositional profile. Detailed facies analysis allowed differentiating three groups of facies associations in the Lower to Upper Ordovician deposits of the Lashkerak formation: (i) estuarine system; (ii) wave‐dominated shoreface‐offshore complex; and (iii) mixed river‐ and wave‐influenced deltaic system. The Middle to Upper Ordovician deposits of the Ghelli formation are divided into two groups of facies associations: (i) tide‐influenced deltaic succession and (ii) deep‐water fan system. Microbially induced sedimentary structures (MISS) are present in deposits formed in the central estuarine basin (Lashkerak formation) and in proximal lobes and lobe fringes of deep–water turbidite fans (Ghelli formation). On the contrary, MISS are absent in deposits from the wave‐dominated shoreface‐offshore complex, river‐ and tide‐dominated deltas, and various subenvironments of the incised wave‐dominated estuary (i.e., bayhead delta and estuary mouth) and the deep‐marine turbidite fan system (i.e., turbidite channel, slope, and outer lobe). The lack of evidence of mat‐building microorganisms in the deltaic systems may have resulted from two factors: (1) high physico‐chemical stressors caused by river‐induced processes, and (2) increase in degree of sediment disturbance, biodiffusion, and bioirrigation by burrowing organisms. Formation of microbial mats in the wave‐dominated shoreface‐offshore complex was inhibited by the activity of an abundant and diverse infauna capable of reworking the sediment. Our analysis shows that the spatial distribution of microbial mats was controlled by an interplay of environmental factors and innovations in animal‐substrate interactions, mostly expressed by secular changes in bioturbation. This study supports the notion that the agronomic revolution was diachronic, with marginal‐marine and deep‐sea ecosystems lagging behind shallow‐marine settings. [ABSTRACT FROM AUTHOR]

Micro‐organisms producing microbially induced sedimentary structures, particularly epibenthic cyanobacteria, are not facies‐dependent and could flourish in any environment if appropriate ecological conditions were provided. Hence, the changes in environmental parameters are the controlling factors on ecological tolerance of the producers. This study on the lower Cambrian successions of the Lalun Formation in Central Iran shows that paralic environments reacted differently to changes in parameters such as river and tide energy, palaeo‐topography, the rate of sediment supply and fluctuations in sea‐level, even though all were characterized by sandy substrates suitable for the development of microbially induced sedimentary structures. Therefore, the abundance and preservation of microbially induced sedimentary structures varied in the different paralic environments. From a sequence stratigraphic viewpoint, this study demonstrates that erosional discontinuities lacked the conditions required for the substrate stabilization by microbial communities. The distribution, size and type of microbially induced sedimentary structures within high frequency cycles generally follow the trends of changes in vertical facies stacking patterns. Within systems tracts, the pattern, morphological diversity and size of microbially induced sedimentary structures are not dependent on the type of systems tract, but on the type of depositional system developed such as delta, incised valley, coastal plain, estuaries and shoreline to shelf systems. Generally, estuarine and peritidal carbonates record an increase in the development of mat colonization during the transgressive systems tract, owing to decreased sedimentation rate as well as extended shallow water habitats. In contrast, the existence of microbially induced sedimentary structures depends on the pattern of shoreline shift in depositional systems developed during the highstand systems tract, such as open coast tidal flat and delta environments. If a shoreline regression was continuous (depositional trend and stacking pattern are a set of high frequency cycles), a greater increase in the aggradational component than the progradational component would cause intensified destructive processes hindering the development of microbial communities. [ABSTRACT FROM AUTHOR]

Cyclic successions are one of the most important types of deposits in the carbonate platform of the Mississippian (Tournaisian-Viséan) in Gondwanan and Laurussian margins. The importance of climatically induced events impact on the amplitudes and frequencies of sea-level fluctuations in Mississippian of Iran is significant because the beginning, extent, duration, and significance of periods of climatically induced events in northern margin of Gondwana are not well recognized. A high-resolution sequence architecture in integration with paleoenvironmental reconstruction and foraminiferal biozonation from the Tournaisian-early Visean carbonate successions of the Mobarak Formation in Alborz basin along depositional strike (E-W- a distance of c. 340 km) suggests a mosaic-like pattern of deposition and compartmentalization of the depositional settings (homocline ramp to distally steepened ramp). According to the recognized facies, the stacking pattern of high-frequency cycles across the platform, and major accommodation events recognized on the correlated Fischer plots, six 3rd-order sequences (DS1–DS6) were identified. Detailed analysis on high-frequency cycles representing a highly variable number of shallowing-upward cycles, types, thicknesses, and facies heterogeneity along depositional strike. A combination of the episodic tectonic events along faults interacting with autocyclic processes, and inherited topographic irregularities are suggested as the driving mechanism for the cycle development. Six stages of Mobarak carbonate platform evolution based on understand the interplay of autocyclic and allocyclic controls are recognized in a hierarchical 3rd-order sequence stratigraphic framework and a chronological order: (1) early Hastarian corresponds to post-glacial transgressive episode with progressive backstepping of facies belts of early TST of DS1 throughout the region; (2) middle to late Hastarian corresponds to a major transgressional phase of DS1 following global temperature rise superimposed on different subsidence/uplift rates with the deformation of the platform along depositional strike; (3) Hastarian–Ivorian boundary corresponds to TST of DS2 with facies heterogeneity along depositional strike related to continuation of different subsidence/uplift rates. The absence of any systematic changes in the amplitude and period of cyclicity or lowstand deposits and subaerial surfaces during this phase shows the lack of influence of mid-Tournaisian glaciation and concurrent eustatic regression in northern margin of Gondwana. (4) early to middle Ivorian coincides with the RST of DS2 and TST of DS3 showing by remarkable topographic irregularities in the basin physiography and change in the position of the depocenter axis from a E–W trend to the center; (5) late Ivorian-early Moliniacian coincides with the RST of DS3 and DS4 showing by the Late Tournaisian thermal period and eustatic rise superimposed on uplift and subaerial exposure of the western part and subsidence in the central and eastern parts of the basin; (6) Moliniacian coincides with DS5-DS6 sequences showing by change from fault-related topography to major tilted fault-block topography accompanied by eustatic sea-level fall caused indirectly by the Viséan and Serpukhovian glaciations phases. • Analyze along-strike variations in stratigraphic architectural style of the Tournaisian-Viséan successions. • Synthesis of autocyclic and allocyclic controls on changes in accommodation space along depositional strike. • Access a framework for tectonic and climatically induced event histories in the northeastern margin of Gondwana. • The importance of climatically induced events impact on the amplitudes and frequencies of sea-level fluctuations. • Results are useful for correlation to Tournaisian-Viséan strata in other parts of Gondwana. [ABSTRACT FROM AUTHOR]

Changes in stacking patterns of facies, stratal architecture and shoreline trajectory along depositional strike are mainly dependent on along-strike differences in factors such as physical processes (rivers, waves, tides), tectonics, eustatic changes, sediment supply, and local physiography. The lower-upper Ordovician successions (Lashkerak Formation) deposited in a Peri-Gondwana shallow continental shelf have been interpreted using the sequence stratigraphic concepts. This approach allowed to highlight main differences along-strike in the co-existing depositional systems (coastal plains, estuaries, deltas, and shoreline to shelf systems) in the Alborz basin along the northern edge of Gondwana. On the basis of sedimentary and ichnological characteristics, the siliciclastic successions are divided into four facies associations: (i) Wave-dominated estuarine, (ii) Wave-dominated shoreface-offshore complex, (iii) Mixed fluvial (F)- and wave (W)-influenced delta (Fw and Wf), and (iv) Tide-dominated, fluvial-influenced delta (Tf). Six third-order depositional sequences (DS1-DS6) were recognized, each of them are bounded by unconformities and pronounced shifts in the facies. Along depositional strike (NE/SW- a distance of c. 150 km), significant intrabasinal changes in sediment supply produced evident lateral changes in the shoreline trajectory and stratal architecture, of the DS1-DS3 (Tremadocian-Floian), may be attributed to differences in physical processes (rivers, waves, tides activity) and topographic irregularities derive probably by different rates of subsidence in the basin. During the deposition of the DS1-DS3, a significant along-strike differences in the composition of ichnological suites and the lifestyle of benthonic organisms, dependent on the relative supremacy of wave, storm, river, and tidal processes. Sedimentary supply and processes are the main controlling factors for the stratigraphic architecture rather than the tectonic activity between Tremadocian and Floian. While during the Darriwilian-Katian interval (DS4-DS6), the tectonic activity predominates with the beginning of extensional phases related to the rifting in the Alborz basin causing the activation of syn-depositional normal faults and the development of block-faulted craton-ward margin. Typically, differential subsidence due to extensional tectonic activity causes the uplift of tectonic blocks west of the Alborz Basin into the emerging areas, on one hand, and simultaneous deepening-upward stacking patterns (i.e., increased subsidence rates) in the hanging-wall depocenters of east of the Alborz Basin, on the other hand. This outcrop-based study suggests that the identification of genetically related depositional units in a larger paleogeographic context is fundamental for understanding along-strike variability in controlling mechanisms of the spatial distribution and stratigraphic architecture of coexisting and linked depositional systems. • Analyze along-strike variations in stratigraphic architecture of the Ordovician successions. • Synthesis of autocyclic and allocyclic controls on changes in accommodation space along depositional strike. • Interpreting along-strike variability in controlling mechanisms of stratigraphic architecture. • Unravelling the palaeogeography of continental shelf of the Prototethys margin along the northern edge of Gondwana. • Results are useful for correlation to Ordovician strata in other parts of Gondwana. [ABSTRACT FROM AUTHOR]

The stratigraphic architecture, changes of depositional systems in lateral and vertical extent, and 3D-stacking pattern of second- and third-order sequences of the Cambrian successions on the northern Gondwanan margin indicates a distinct sea-level fall in the Alborz Zone and Central Iran corresponding with global sea-level fluctuations. The peak of the sea-level fluctuations is located at the boundary between the Lalun Formation and the base of the overlying Mila Group. A vertical trend from gradual sea-level rise during the Cambrian Series 2 (recorded as thick transgressive deposits of the Lalun Formation) to transgressive valley-fill system at the base of the Fasham Formation and overlying highstand subtidal carbonates (carbonate platform of the Deh-Sufiyan Formation) can be observed. This trend is paralleled by a change from mixgrounds with metazoan body fossils to microbial matgrounds and microbial boundstones (thrombolites and stromatolites). The general increase in vagile bilaterian metazoans led to a distinct proliferation in ichnofossils in the Lalun Formation indicating a considerable increase in progressive ecospace utilization and tiering complexity. These are associated with biodiffusion, sediment reworking and bioirrigation so that matgrounds were disrupted and mixed layer substrates developed. A proliferation of microbes took place at the base of the Mila Group, which is regarded as a reaction to unfavourable environmental conditions and considerable challenges for metazoans. Consequently, a dramatic degradation of marine bottom communities followed the major eustatic sea-level fall at the Cambrian Series 2–Miaolingian boundary interval and led to the decrease in ichnodiversity, bioturbation intensity, depth of bioturbation and burrow size at the base of the Mila Group. The amount of biogenic mixing significantly decreased, and eventually sedimentary mixed layers disappeared. • Facies and sequence stratigraphic analyses of the Cambrian Series 2–Miaolingian boundary interval. • The impact of the eustatic sea-level fall on mixground and matground ecosystems. • The trace-fossil record of organism–matground interactions by changes in environmental conditions. • Document trend of change from mixgrounds to matgrounds and microbial boundstones. • Results are useful for correlation to Cambrian strata in other parts of Gondwana. [ABSTRACT FROM AUTHOR]