Depositional environment and sequence stratigraphy of the middle Jurassic deposits: case study of Navia section in the west Bojnurd, west of Kopet-Dagh

Introduction The siliciclastic sediments of Kashafrud Formation (Bajocian–Bathonian) in the Kopet-Dagh Basin (Madani, 1977; Afshar-Harb, 1979) are composed of conglomerate, sandstone and shale. It is reported that these sediments were mostly deposited in deep water environment in the eastern Kopet-Dagh basin (Poursoltani et al., 2007; Taheri et al 2009; Poursoltani and Gibling, 2011). The study area is located in west of Bojnurd in the western parts of the basin with a thickness of about 749 m and consists of three units. It covers unconformably the limestone of Triassic Elika Formation and overlaid by carbonate sediments of Chamanbid Formation. The purpose of this research is to interpret depositional environment and sequence stratigraphic analysis of the Kashafrud Formation in order to understand the paleogeography of Middle Jurassic time.
Methods This research is based on field observations of one measured stratigraphic section in Navia area and petrographic studies of 69 samples. Lithofacies are classified, using Miall codes (2000, 2006) and thin sections are used for determination of composition and texture of coarse grain rocks. 10 shale samples have also washed for micro- paleontological study and three samples, including shale and sandstone, have studied with SEM and EDX by LEO-1450 V model in Central Laboratory of Ferdowsi University of Mashhad. We also measured 44 directional structures azimuths for paleocurrent analysis.
Results Based on field studies, three lithofacies are recognized in the study area as follow.
Conglomerate lithofacies are observed in the basal part of the studied section with 7 m thickness and is composed of Gcm, Gt and Gmm lithofacies. They are mostly grain-supported with graded bedded, poorly sorted with rounded pebbles, low sandy matrix and trough cross-beds (Gt). Gmm is identified by increases in sandy matrix, moderate sorting and quartz pebbles content.
Sandstone lithofacies consist of St, Sp, Sm, Se, Sl, Sr, Sh, Sm and St lithofacies. St is the most abundance sandstone lithofacies in unit 1. In this lithofacies grain size decreases upward and changes to thin-bedded and fine grain sandstones. Sp with planar cross bedding has Ophiomorpha trace fossil in unit 2. It has low angle and planar cross bedding that vertically changes to Sr lithofacies. Sr (Rippled lamination and layered sandstone) is present in both units 1 and 2 but it has Palaeophycus and Thalassinoides trace fossils in second unit. Sh lithofacies is recognized by parallel or horizontal bedding and lamination. This lithofacies is less abundant in unit 1 and interbeded with mud rocks lithofacies, while in unit 2 it seen with Sl lithofacies. Sl (Low angle cross-bedded sandstone) and is the most abundant lithofacies in unit 2 and mostly fine grain and associated with Sh and Sr. Se (Erosional scours) is mostly observed at the base of sandstones and associated with Sp and Sm.
The Kashafrud sandstones are mostly sublitharenite and minor amounts of litharenite with mineralogically supermature and textural submature.
Mud rock lithofacies consist of laminated (Fl) and massive (Fm) fine grain rocks. The Fl lithofacies is mainly composed of silt-size grains with Palaeophycus, Planolites and Rhizocorallium trace fossils in unit 3.
Discussion Kashafrud Formation is interpreted to be deposited in fluvial and deltaic environments. Fluvial environmentis supported by fining upward cycles, abundance of sandstone, well-rounded grains, trough cross-bedded and Plant remnants in red shale. They have mostly sublitharenite composition. Calculated chemical weathering index indicated that the rocks at the source area have been highly affected by high weathering condition that can be related to warm and semi-humid climatic conditions (Sarbaz et al., 1395, in press). Paleocurrent analysis also revealed that the probable source of these sediments can be located in south of the Kopet-Dagh basin.
Deltaic sediments are composed of proximal and distal delta front as well as prodelta. Proximal deposits consist of several coarsening upward cycles including shale and fine to coarse grain sandstones with a thickness of about 7 m. Dominant sedimentary structures are horizontal and low angle cross-lamination, wavy ripple cross-lamination, massive beds, hummocky and trough cross-beds as well as plant fragments. These evidences show that sediments may have been affected by interaction of long-shore waves as well as storm events (e.g. Edwards et al., 2005; Hampson, 2010).
Distal delta front form the main parts of sediments including fine-grain black to olive green shale and siltstone with fine sandstones interbeds. Major lithofacies are Fl and Fm with Palaeophycus, Planolites and Rhizocorallium trace fossils. These intervals may have been deposited below wave base under low energy conditions (Einsele 2000; Alvan and Eynatten, 2014).
Prodelta sediments are mostly composed of clayey and silty shale. Based on vertical variations of these sediments to silty and fine to very fine grained sandstones of distal delta front, they may have been deposited in low energy and probably more reduction conditions (Hoir et al., 2002). The study of small foraminifera of deltaic shale led to identification of 14 different species of 12 genera. The community of these microfossils, which is reported for the first time, show that deposition may have taken place in the shallow water near shore environment within about 50m depth (e.g. Murray, 1991).
Sequence stratigraphy Kashafrud Formation in this section is composed of two depositional sequence (DS1 and DS2). The lower boundary of the first sequence (DS1), based on angular unconformity (above Triassic limestone of Elika Formation) and fluvial conglomerate deposits, is type I (SB1). The upper boundary of DS1 is SB2 and is located in proximal delta front deposits with no subaerial exposure evidences. The thickness of the first sequence is about 400 m and consists of LST, TST and HST. The LST is composed of continental sediments (137 m) that is covered by transgressive surface. This surface reveals by massive shale (50 m) of distal delta fronts and maximum flooding surface is located in the upper part of these deposits. HST (215 m) is composed of 12 shallowing and coarsening parasequences. DS2 with thickness of about 349 m is separated from DS1with sequence boundary type 2. This depositional sequence is only composed of TST with Fm and Fl lithofacies that transitionally changes to deeper water black shale that follows by marl and fine grain carbonate sediments of the Chaman-Bid Formation. Therefore, the maximum flooding surface is probably located within the Chaman-Bid Formation. Interpreted sea level fluctuation curve of the study area can be related to regional geological history and sometimes can be relatively correlated with global sea level curve (Haq et al, 1987)