Depositional environment and Sequence Stratigraphy of the Eocene successions in the south of Kerman region (Lut Block): significance of nummulite accumulations (or nummulite Banks)

Up to now, the few studies have been investigated on the shallow benthic Eocene successions from the eastern Lut Block. Here, the larger benthic foraminifera were practically one of the most important components for reconstructing of the paleoenvironmental conditions. In fact, they were abundant during the Paleogene, and are as a useful tool for sedimentary and paleoecology interpretations. The highest diversity and abundance of the Large Benthic Foraminifera (LBF) was during Eocene, where they were known as the Foraminiferal limestones within (neritic) carbonate succession. The LBF are generally recognized with calcareous algae (red algae) which can be used as paleoenvironment indicator.  However, the investigation of the facies data which is combined with paleontologic characteristics confirm paleoecologic condition during deposition of Eocene strata in the studied area. Eventually, vertical distribution of identified microfacies and sequence stratigraphy of the mentioned succession supports the depositional system and paleoenvironmental condition.

In this study, the Eocene carbonate marine successions were investigated in south of Kerman from the Khan Gazan and Khadagan sections with thickness of 75 m and 48 m, respectively. The identification of the microscopic data are provide by Embry and Kelovan (1971) and Dunham (1962). Field work and laboratory studies based on LBF stratigraphic distribution of the Eocene deposits led to the identification of seven facies includes: (FT1) Terrigenous bioclast packstone, (FT2) Nummulites rudstone, (FT3) Bioclast–coralline algal packstone/grainstone, (FT4) Coralline algal rudstone/bindstone, (FT5) Coral wackstone/boundstone, (FT6) Nummulithoclastic packstone/grainstone, (FT7) Bioclast packstone/ grainstone into a carbonate ramp model, which can be divided into four depositional environments: tidal flat, lagoon, shoal and open marine. The regional discrimination most likely reflects their position on separate tectonic blocks on which different facies conditions developed due to different tectonic movements. Based on Catuneanu (2006), the results of sequence stratigraphy studies were identified two third-order depositional sequences containing of four systems tracts.  The gradual vertical changes of the facies and the process of rising of sea-level from the shallow area to seaward direction are well illustrated with lateral facies changes such as the nummulitic accumulations or banks that represents of a high-energy conditions and tests transport to the deepest parts of the basin.

According to studies, the frequency of microspheric forms increases with increasing water depth, while megalospheric forms are frequently in shallower or relatively deep areas (Beavington- Penny and Racey, 2004). Quartz grains are characterized by well sorting, sub-rounded, and maturity, which represents a large transport distance to the deepest parts of the basin. The proximity of coralline red algae facies trough FT5 can be referred to the margins of coral reefs. Also, Field observation suggests the development of small reefal build-ups (Patch Reef) with limited lateral extensions resulting from coral and coralline red algae assemblages in the Khan Gazan section. The evidence of our study on the FT5 in the Kagan section is indicating a gradual deepening trend of the basin. Likewise, developing of the Nummulites banks show a decrease in seawater levels as major sea-level fluctuation, thereby, the Khan Gazan section is deposited in an environment with high energy conditions and near to the origin of sediments.In addition, the nummulite accumulations (nummulite banks) or low-relief buildups into the middle ramp area under influence of a moderate-high energy condition (e.g. currents and waves) were represented with predominance of Nummulites perforatus monospecific tests for the first time in this region. According to field work, the mentioned nummulites Bank is assigned to B form (reaching to 10 cm in diameter).  The Nummulites banks were not referred to original biocoenosis which were formed by wave activity as parautochtonous or residual assemblage (Papazzoni, 2008). Combination of LBF and coralline algal facies was previously recorded from northeastern Italy (Bassi, 2005) which is close to FT5 of studied sections. The predominant facies associations are shown an overall transgression-regression cycle of the Eocene successions in the east Lut Block.

In order to reconstruction of the paleoenvironment and geologic history of the Eocene succession of the Lut Block, the litho and biostratigraphic studies of other exposures are required. Actually, vertical distribution of facies of several stratigraphic sections and their correlation supports paleo-tectonic behavior of various location during deposition of Lut Block Eocene sediments.