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The trace fossil Rosselia with its dominant species Rosselia socialis is found with various densities in both open-marine and delta systems of the Lower Ordovician Lashkerak Formation (Alborz, Iran). The trace fossil shows two types of occurrence in these two depositional systems: solitary Rosselia and crowded Rosselia ichnofabric (CRI). On a proximal-to-distal transect, solitary Rosselia occurs during long-term stable conditions with enriched bottom currents in areas above storm-wave base together with a fair-weather community of dwellers and followed equilibrium strategies (K-selected). In shallower parts, in substrates around and below fair-weather wave base with high amount of deposition and erosion, dense populations of Rosselia (CRI) occur together with subordinate trace-fossil suites (such as Diplocraterion, Skolithos and Arenicolites) produced by deep-tier, suspension-feeding animals due to short-term stable conditions with low food flux. Solitary Rosselia, with lifelong activity due to sophisticated feeding strategies, are well developed in storm deposits, and are part of shallow tiers. Forms with concentric muddy laminae indicate long-term occupation and belong to the Cruziana ichnofacies. In contrast, the behavioural style of the studied CRI, which occurs in unstable, high stress, physically controlled substrates, is comparable to the Skolithos ichnofacies in the occurrence of equilibration traces with stacked segments. In terms of sequence stratigraphy, the transgressive system tract (TST) with lower amounts of organic detritus and low accumulation rates favours the development of solitary Rosselia. In this system tract, CRI is developed during times of ravinement when winnowing and erosion led to the early development of unstable substrates. However, most occurrences of CRI are observed within the progradational packages of the highstand system tract (HST). This study suggests that CRI in event beds can be a good signature of low accommodation space and prograding clinoforms. [ABSTRACT FROM AUTHOR]

The Tournaisian-Visean carbonate successions of the Esfahan-Sirjan Basin (ESB) from Sanandaj-Sirjan Zone, Iran, have been used to generate a sequence stratigraphic model that enhances facies characterization and improves paleoenvironmental interpretation of shallow marine successions deposited along the southern margin of the Paleotethys. Detailed facies analysis allowed to differentiate seven facies, which, in order of decreasing abundance, are: (1) shaly and marly, F1; (2) peloidal mudstones/wackestones, F2; (3) peloidal/bioclastic packstones, F3; (4) intraclastic/bioclastic packstones/grainstones, F4; (5) oolitic/bioclastic packstone/grainstone, F5; (6) sandy intraclastic/bioclastic grainstones, F6; (7) sandy oolitic/bioclastic grainstones, F7. The different facies can be grouped into three facies associations that correspond to different environments of a carbonate platform with ramp geometry (homoclinal), from outer ramp (F1 and F2), mid-ramp (F3, F4 and F6) to inner ramp areas (F5 and F7). Meter-scale cycles are the basic building blocks of shallow marine carbonate successions of the Tournaisian-Viséan ramp of the ESB. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. According to the recognized facies and the stacking pattern of high-frequency cycles across the ramp, five large-scale cycles in the southeastern outcrops (Tournaisian-Viséan) and three large-scale cycles in the northwest outcrops (Viséan) related to eustatic sea-level changes can be recognized. The overall retrogradational nature of the carbonate ramp, illustrated by both vertical facies relationships and the stacking patterns of high-frequency cycles within the third-order cycles, implies that the deposition of the Tournaisian-Viséan successions mainly took place under a long-term transgressive sea-level trend. The stratigraphic architectural style of the sequences, characterized by the lack of lowstand deposits and exposure surfaces, associated with the evidence of progressive increase in the proportion of backstepping of facies belts across bounding surfaces and predominant subtidal characteristics, is in accordance with the long-term transgressive sea-level trend and greenhouse conditions during the Tournaisian-Viséan. The continued transgression on this broad shelfal platform could lead to the shutdown of the shallow water carbonate factory, reduction in sediment supply or sediment transport towards the offshore setting and the development of give-up sequences. The association of transgressive events with the deposition of thick open-marine marls/shales is a common feature in Tournaisian to Viséan times of the southern margin of the Paleotethys. [ABSTRACT FROM AUTHOR]

The carbonate-evaporite mixed Asmari Formation was investigated on the three representative outcrops (these of Zarrin-abad, Abhar, and Darreh-shahr, corresponding to three sections) of the Lurestan Zone in NW Zagros, where the sedimentary facies of the Asmari Formation shows significant differences to those in other parts of Zagros. In the light of study results from the field observation and laboratory measurements, 16 facies have been recognized and differentiated, which are grouped into 5 facies associations, representing 6 sub-environments on a carbonate ramp. The sub-environments include peritidal area, lagoon, platform margin high-energy belts, middle ramp, outer ramp and evaporitic saltern. The evaporitic saltern points to such a condition, during which there occurred periodic change from carbonate to evaporitic settings. The alternation of carbonate with evaporitic settings appeared on a carbonate ramp connected with a hydrographically isolated intrashelf basin. The development of evaporite-dominated deposits of the Kalhur Member of Asmari Formation is related to the restriction of the basin during sea-level falls. This restriction led to succeeding evaporation of seawater and precipitation of evaporites. In times of sea-level rising, the basin was well reconnected to open ocean and thus the carbonate factory was reestablished over the basin. From sequence stratigraphic points of view, the formation is composed of two second-order regressive sequences, representing transitional conditions from deep-marine facies underlying Pabdeh Formation to shallow evaporitic facies overlying Gachsaran Formation. Each sequence is composed of a unit of evaporites at bottom displaying a falling-stage systems tract followed by thick carbonate strata, representing transgressive and highstand systems tracts. It is obvious that the major relative sea-level fall of the first second-order sequence was recorded by the facies development of the upper Kalhur gypsum of the Asmari Formation in the Zarrin-abad section, and there developed the paleokarst facies located at the sequence boundary in the Abhar, and Darreh-shahr sections. [ABSTRACT FROM AUTHOR]