The Ichnoassemblages and their environmental Importance in Geirud Formation, SW Shahroud and north Shahmirad, Central Alborz

The silisiclastic-carbonate sediments of the Geirud formation (Upper Devonian) are deposited in the Central Alborz Basin, northern Iran. Generally, these sediments display four depositional systems as follow: fluvial (alternation of red-purple shale, sandstone and conglomerate), estuary (thick-bedded, white sandstone with bi- to multiple-directional planar cross bed and herringbone), shoreface (alternation of thin to thick-bedded sandstone with trough and planar cross bed and HCS and gray-black, fissile shale) and shallow shelf or offshore (fossiliferous limestone with HCS and tempestite and black, fissil shale) that indicate a deepening up-ward trend. A prevailed stressful and unsuitable condition with respect to salinity led to preservation of the ichnoassemblages with low diversity and small size of the trace fossils in the studied sediments (Gingras et al., 2008). The studied sediments indicate remarkable changes in sedimentation rate, water circulation and salinity level during the Late Devonian time. Ichnological analysis has become a valuable tool in basin analysis, especially for recognizing and interpreting genetically related sedimentary packages (e.g., Tovar
et al., 2007; Sharafi et al., 2012 & 2014). The primary controls on the distribution of different burrowing behaviors and lifestyle of the existing fauna and the trace markers in the marine realm are nutrient supply, hydrodynamic energy, salinity, sedimentation rate, oxygen level, water turbidity (Seilacher,1967; Bromley & Ekdale, 1984; Pemberton & Wightman, 1992; MacEachern & Burton, 2000; Gingrass et al., 2008, 2012; Sharafi et al., 2014). The Alborz Mountains with a E-W trend is one of the structural zones in the northern Iran (Stöcklin, 1968). This structural zone is sub-divided into the east, central, and west parts and the study area is located in the central part. The Geirud Formation crops out at the Dehmolla, and the Shahmirzad sections, geographical coordinates are 36º 21' 53'' N and 54º 45' 28'' E and 35º 46' 12'' N and 53º 19' 25'' E, respectively. In the studied areas, the Geirud Formation disconformably overlies the marine shale of the Milla Formation (Ordovician) and is conformably overlain by the black limestone-shale of the Mobarak Formation (Lower Carboniferous). Various paleontological studies on brachiopods (Bozorgnia, 1964), Palynomorphs (Ghavidel-Syooki, 1995) and Goniatites (Dashtban, 1995) indicate the age of the Geirud Formation is Late Devonian (Frasnian–Famennian).
Tow stratigraphic sections measured in the Alborz mountain north of Iran for the purpose of this research. Tow hundreds and Eighty thin sections were examined to identify fine-scale sedimentological and textural characteristics such as grain size and sedimentary structures that are recorded in the studied successions. Both sedimentological and trace fossils features were examined on fresh and weathered surfaces in the field. Degree of bioturbation is assessed according to Taylor and Goldring (1993) and is done with using of comparative charts. The bioturbation index (BI) aims to relate the degree of bioturbation to the preservation of primary bedding features (Taylor & Gawthorpe, 1993). In this scheme, a BI is defined, ranging from 0 (no bioturbation) to 6 (complete bioturbation, total biogenic homogenization of sediments). Results and Conclusion: Nine ichnoassemblages are identified in the studied successions that indicate considerable environmental changes (e.g. salinity, nutrient supply, hydrodynamic energy, sedimentation rate, oxygen level) within a generally deepening up-ward sequence from fluvial-estuary depositional setting to open marine environment (shoreface, shallow shelf). Alternation of the red-purple, thick-bedded sandstone-conglomerate and red shale of the fluvial system in the lower part of the formation with low diversity of vertical trace fossils and low bioturbation index (B.I.= 0-1) (Skolithos-Arenicolites ichnoassemblage) indicate high sedimentation rate of the silisiclastic sediments in a high energy and agitated setting with unstable and mobile substrate. White, medium-thick bedded sandstone in the lower part of the successions with a few shell fragments and Skolithos-Arenicolites and Thalassinoides-Rhiocorallium ichnoassemblages displayed by mainly vertical burrow elements and low B.I. (1-4) indicate high energy, unstable substrate and periodically water circulation in a beach setting with high sedimentation rate. In the middle part of the studied successions thin-medium bedded fossiliferous sandstone with planar and trough cross beds and HCS and Thalassinoides-Rhizocorallium-Palaeophycus, Arenicolites-Thalassinoides and Arenicolites ichnoassemblages with medium B.I. (3-4) and a mixed vertical (suspension feeders) and horizontal (deposit feeders) burrow elements, display the various life style of the trace markers from suspension feeding during high energy phase with high sedimentation rate and deposit feeding during low energy periods with low sedimentation rate and more stable substrate, in a shoreface setting. Diversity and abundance of the trace fossils and shells in these sediments indicate a suitable marine condition with respect to salinity, light and water circulation. The High diversity and abundance of the trace fossils and the skeletal elements of the Arenicolites-Diplocraterion, Thalassinoides-Rhizocorallium-Palaeophycus, Arenicolites-Protovirgularia and Protovirgularia ichnoassemblages within the black, thin bedded fossiliferous limestone in the upper part of the formation display establishment a fully marine condition (e.g., light, water circulation, and salinity) along with a decrease in hydrodynamic level and turbulence in a shelf setting