Sedimentary environments and stratigraphy of the carbonate-silicilastic deposits of the Shirgesht Formation: implications for eustasy and local tectonism in the Kalmard Block, Central Iran

Sedimentological and sequence stratigraphic analysis providing insight into the main relationships between sequence architecture and stacking pattern, syn/post-depositional tectonics, and eustatic sea-level fluctuations (Gawthorpe and Leeder 2000; Zecchin et al. 2003, 2004; Carpentier et al. 2007). Relative variations in sea level are due to tectonic activity and eustasy. The Shirgesht Formation in the Kalmard Block of Central Iran provides a useful case study for to determine the processes responsible on internal architecture and stacking pattern of depositional sequences in a half-graben basin. In the Shirgesht Formation, siliciclastic and carbonate successions of the Kalmard Basin, the cyclic stratigraphic record is the result of the complex interaction of regional uplift, eustasy, local tectonics, sediment supply, and sedimentary processes (Bayet-Goll 2009, 2014; Hosseini-Barzi and Bayet-Goll 2009). Lower Paleozoic successions in Tabas and Kalmard blocks from Central Iran share the faunal and floral characteristics with other Gondwana sectors such as south-western Europe and north Africa–Middle East (Ghaderi et al. 2009). The geology of these areas was outlined by Ruttner et al. (1968) and by Bruton et al. (2004). The Cambrian-Middle Triassic strata in the Kalmard Block were deposited in a shallow water platform that possesses lithologic dissimilarities with the Tabas area (Aghanabati 2004). The occurrence of two active faults indicates clearly that Kalmard basin formed a mobile zone throughout the Paleozoic so that lithostratigraphic units show considerably contrasting facies in comparison with Tabas basin (Hosseini-Barzi and Bayet-Goll 2009; Bayet-Goll 2014). The Shirgesht Formation in the Block Kalmard is mainly composed of carbonate-siliciclastic successions that disconformability overlain Kalmard Formation (attributed to Pre-Cambrian) and is underlain by Gachal (Carboniferous) or Rahdar (Devonian) Formations along the basement Kalmard Fault. In the present study, three stratigraphic sections (NW-SE transects) were measured, described and sampled in the Kalmard area. In these sections, detailed considerations have been given on the lithofacies variations; bed geometry and contacts, faunal content, the potential of trace fossils as tools for reconstructing depositional conditions, sedimentary textures and structures, bounding surfaces, vertical trends and stacking patterns and lateral/vertical variations in facies and thicknesses. The observed siliciclastic facies can readily be interpreted using existing shelf sedimentation and shoreline succession models (e.g. Walker and Plint 1992). Interpretation carbonate facies have been done on the basis the microfacies analysis (200 thin-sections), sedimentary textures and structures and faunal content (Wilson et al. 1975; Flügel 2010). In final, internal architecture, characteristics of sedimentary facies, the overall stacking pattern and nature of sequence-bounding unconformities have been investigated to evaluate the influence of regional uplift, local tectonics and eustasy on both along-strike variations in sequence architecture and genetic complexity of sequence boundaries. Discussion, Results and The Lower Ordovician Shirgesht Formation in central Iran is composed of siliciclastic and carbonate rocks deposited in diverse coastal and marine shelf environments (tidal flat, lagoon, shoreface, and offshore-shelf and carbonate ramp). Relying on the facies characteristics and stratal geometries, the siliciclastic succession are divided into five facies associations, FA1 (tidal flat), FA2 (lagoon/washoverfan), FA3 (upper shoreface-foreshore), FA4 (lower to middle shoreface), and FA5 (offshore-shelf).Carbonate succession of this formation based on lithology, sedimentary characteristics and textures divided into four facies belts, FA (tidal flat), FB (lagoon),FC (shoal/barrier island), and FD (open marine). These facies associations are arranged in small-scale sedimentary cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Systematic changes in stacking pattern of these cycles allow inferring long-term changes in sea-level (Bádenas et al. 2010; Bayet-Goll et al. 2014). The high-resolution sequence stratigraphic analysis of the shirgesht Formation displays the presence of four well-defined 3rd order depositional sequences (DS1–DS4). The stratigraphic architecture of the Shirgesht deposits is the result of the interplay between regional uplift and high amplitude, Ordovician glacio-eustatic sea level changes. The Shirgesht Formation is composed of transgressive and highstand systems tract couplets interpreted as reflecting fault-driven subsidence and the continuous creation of accommodation in the hangingwall to the Kalmard fault. Activity on the Kalmard fault led to marked spatial variability in stratal stacking patterns, systems tracts and key stratal surfaces. Constant and slow uplift of the NE/S basin around the Kalmard faults extends across the north and southeast portions of the study area explains the differential subsidence and the observed facies zonation of the mixed shelf siliciclastic-carbonate systems of the Shirgesht Formation. The correlation facies zonation also, suggests that the high bulk of the deeper siliciclastic deposits (DS1-2 and DS4) and the mid ramp limestone facies (DS3) in the NW basin representing enough accommodation space. Rapid subsidence typically leads to strong stratigraphic expansion and good preservation of thick 3rd order depositional sequences in the NW basin. The prominent lateral change in component units (systems tracts) and nature of bounding surfaces within the studied sequence seems to be directly related to the presence of faults and differential subsidence in along basin. Based on the above observations, such as set of high-angle faults and other basement structures, the Shirgesht Formation deposits in study area, was deposited on a half-graben sub-basin. This half-graben basin formed during Ordovician time by subsidence along the northwest-wards downthrowing and southeast-ward propagating Kalmard Fault. In final, it suggests that sequence architecture and nature of bounding surfaces reflects not only eustasy and sediment supply, but also local fault-controlled, short-term creation and loss of accommodation space.