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Various depositional and post-depositional factors interact to shape a reservoir, typically causing heterogenies in reservoir properties on different scales. This study investigates depositional and diagenetic factors controlling reservoir properties of the Asmari Formation as the most prolific reservoir rock in Iran. Data from six surface and 12 subsurface sections show that both primary and secondary factors governed reservoir quality. Lithology is the principal primary control, with major differences between siliciclastics and carbonates. Other primary controls include rock texture, facies, and depositional setting. Dolomitization, dissolution, and cementation are the main diagenetic factors that modify the original rock fabric. Dolomitization improved reservoir quality. Early diagenetic dissolution also enhanced porosity. However, the created spaces are separated vuggy and moldic pores with no effect on permeability. Cementation and compaction are major porosity-occluding processes. Anhydrite cementation degraded reservoir quality of oolites, one of the best productive facies of Asmari. Results show that tectonics, paleo-climate, and sea-level changes were allogenic controls on reservoir configuration. Lithology variations, as a principal control on reservoir quality, resulted from tectonic movements alongside sea-level oscillations. The diagenetic path--specifically dolomitization, dissolution, and cementation--was controlled by paleo-climate and sea-level changes. Hypersaline conditions prevailed during and after deposition, with major impacts on the reservoir quality (pervasive dolomitization and dissolution), were the consequence of combined paleo-climate and sea-level effects. Regional syn- and post-depositional tectonics created widespread fractured reservoirs with higher production rates than unfractured counterparts. The results of this study can assist in regional characterization of the Asmari reservoir throughout the Zagros area. [ABSTRACT FROM AUTHOR]

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]

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]

The Tremadocian‐to‐Floian siliciclastic deposits of the Shirgesht Formation in the Kalmard Block of central Iran show abundant occurrences of the mid‐tier Trichophycus venosus, a common ichnotaxon in the archetypical Cruziana ichnofacies. This trace fossil records a considerable increase in exploitation of offshore infaunal ecospace in comparison with older formations. Here, Trichophycus is relatively long and wide, with numerous and deep successively stacked causative burrows, which form dense burrow systems building the Crowded Trichophycus ichnofabric (CTI). Such development of CTI and the increase in depth of bioturbation had a negative effect on preservation of shallow‐tier traces, for example arthropod burrows and trackways. This resulted in decreasing abundance of trilobite‐produced trace fossils towards the top of the succession. The palaeobiological and palaeoecological interpretation of CTI points to stable habitats in muddy substrates above the storm wave base with high content of food and oxygen in pore waters, low‐energy hydrodynamic regimes and a long colonization window. Moreover, the architecture and the morphological features of Trichophycus suggest a combined dwelling‐feeding activity of the resident fair‐weather producer showing the K‐selected/climax strategy. The ethology of the producer, palaeoecological interpretations and taphonomic signatures revealed that it preferred dewatered, compacted muddy substrates (firmgrounds) in offshore settings, which might be related to minor hiatuses or short‐lived discontinuities. Although the appearance of Trichophycus is concomitant to the earliest Cambrian agronomic revolution, it was uncommon until the early Ordovician, when locally it records infaunalization related to the Great Ordovician Biodiversification Event. [ABSTRACT FROM AUTHOR]