Reconstruction of Sedimentary environment and sea-level fluctuations in the Cretaceous-Paleocene deposits, central Alborz, North of Iran

Alborz Mountain (with East-West trend) in North of Iran and South of Caspian Sea has been created by the collision of central Iran and South Caspian plates (Stocklin, 1974). Upper Cretaceous strata in the Alborz area are consist of marine carbonates and marls that are different in western and eastern regions. Tertiary conglomerates and sandstones strata disconformably overlie those. In the Northern Alborz, Cretaceous carbonate strata are continuously overlain by shallow marine lower Paleocene layers (Rezaeian, 2009). Cretaceous-Paleocene strata in the north flank of central Alborz are consist of 235 meters of carbonate, siltstone, sandstone, and evaporitic layers. For reconstruction of the sedimentary environment in this strata, one stratigraphic outcrop in the south of Sari city has been sampled and studied. In this study, benthic foraminifera, palynomorphs, and palynofacies have been examined.

Seventy rock samples have been processed using the standard palynological processing method (Traverse, 2007). In this method, carbonate and siliciclastic matters have been removed by HCL and HF (37%), then the residual matter has been sieved and centrifuged (using ZnCl2), and finally, palynological slides have been prepared. Also, seventy thin sections in order to the identification of benthic foraminifera have been prepared. Finally, all data have been plotted on diagrams using Corel Draw, Triplot, Adobe illustrator, and Excel software.

The studied section from base to top is consists of carbonate strata with siliciclastic intercalation layers, marl, argillaceous limestone, shale, and evaporate. To the identification of relative age and Cretaceous-Paleocene boundary in the studied section, fossils content of samples have been studied. Four species of dinoflagellates, ten species of spores, seven species of pollens, and two species of foraminifers have been identified. Based on stratigraphic distribution of Tricolpites phillipsii (Ocampo et al., 2006; Schulte et al., 2010; Vajda and Raine, 2003; Willumsen and Vajda, 2010), Palaeopridinium cretaceoum (Ogg et al., 2016) and Elphidiella multiscissurata, Campanian-early Paleocene age can be proposed for these strata. K-Pg boundary is identified in the sample (K-T 36) based on the first appearance of Tricolpites Phillipsii index species. Thin section studies lead to the identification of five sedimentary facies (evaporite, siltstone, litarenite, sublitarenite, and bioclast packstone/wackstone) in the studied samples that proposed shallow marine carbonate ramp as a sedimentary environment for these layers. Organic matter in palynological slides can be used as a paleontological factor to reconstruct the paleoecology and sea-level changes in the past (de Araujo Carvalho et al., 2006). High abundance of terrestrial palynomorphs (spore and pollens) and plants particles (phytoclasts) and low abundance of marine palynomorphs (dinoflagellate, acritarch, chitinozoans, …) and structureless organic matter (SOM) can prove landward environments and relative sea-level fall event and vice versa (de Araujo Carvalho et al., 2006; Handford and Loucks, 1993; Tyson, 1993). Palynological studies in this section show a relative sea-level rise trend from the base of the section to the middle parts, then a relative sea-level fall trend from the middle parts towards the end of the section. Moreover, there is a relative sea-level fall event across the K-T boundary the can be correlated with the global sea-level diagram (eustasy). Based on this fact, we can say that the Cretaceous-Paleocene basin in this area was connected to the open oceans.

Cretaceous-Paleocene strata in the South of Sari (Northern Iran) are composed of carbonate, evaporate, and siliciclastic sedimentary rocks. In this study, based on paleontological investigations and the appearance of Tricolpites phillipsii (index species of Paleocene age), Cretaceous-Paleocene boundary has been identified in K-T36 sample (133 meters from the base of section). Stratigraphical distribution of index fossils proposed a Campanian-early Paleocene age for the studied strata. Facies studies lead to suggest a shallow carbonate ramp as a sedimentary environment for these strata. Statistical analysis on palynological factors shows a deepening upward trend from the base of the section to the middle parts. Then there is a shallowing upward trend from the middle parts towards the end of the section. Moreover, one significant shallowing event can be detected across the K-T boundary that can be correlated with global sea-level fluctuations that show us the connection between this basins to the open oceans.AcknowledgmentThis study has been supported by the University of Zanjan and Khazar Exploration and Production Company.