Search

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 Lower Carboniferous carbonate sediments of the Mobarak Formation have been analysed to evaluate the role of allogenic and autogenic factors on the development of a distally steepened carbonate platform. Thirteen facies are identified in the studied succession and are grouped into four facies associations: inner ramp FA‒FB; middle ramp FC; ramp slope FD; and deep ramp FE. The rifting event at the northern margin of the Gondwana supercontinent caused a remarkable change in the depositional system so that the precursor homoclinal ramp (Khosheilagh Formation, Devonian) turned into the distally steepened ramp of the Mobarak Formation. Six depositional sequences are identified in the succession, characterized by varieties the normal transgression‒regression cycles, the TST‒HST, or the transgression‒forced regression cycles consisting of the TST‒HST‒FSST. The main controlling elements in the stacking pattern of sedimentary packages, relative sea-level changes and the formation of depositional sequences were tectonic activity and eustatic sea-level changes. Development of the FSST package was caused by the increasing tectonic activity and the formation of turbiditic sediments, amplified by wave-induced currents as the autogenic factor on formation of the re-sedimented deposits. Development of the HST deposits displays a relative balance between sedimentation rate and the creation of accommodation space. Thick packages of deep ramp calcareous shale were developed during quiescence in the tectonic activity and increase in accommodation space, created by thermal subsidence and rise in eustatic sea-level. [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]