The Lower Triassic Sorkh Shale Formation of the Tabas block, east central Iran: Succession of a failed-rift basin at the paleotethys margin

Abstract

The Lower Triassic Sorkh Shale Formation is a dominantly red colored marginal marine succession deposited in the north-south trending Tabas Basin of east central Iran. It is correlated with the unconformity-bounded lower limestone member of the Elika Formation of the Alborz Mountains of northern Iran. The Sorkh Shale is bounded by the pre-Triassic and post-Lower Triassic interregional unconformities and consists mainly ofcarbonates, sandstones, and evaporites with shale being a minor constituent. Detailed facies analysis of the Sorkh Shale Formation resulted in recognition of several genetically linked peritidal facies that are grouped into restricted subtidal, carbonate tidal flat, siliciclastic tidal flat, coastal plain and continental evaporite facies associations. These were deposited in a low energy, storm-dominated inner-ramp setting with a very gentle slope that fringed the Tabas Block of east central Iran and passed northward (present-day coordinates) into deeper water facies of the Paleotethys passive margin of northern Cimmerian Continent. Numerous carbonate storm beds containing well-rounded intraclasts, ooids and bioclasts of mixed fauna are present in the Sorkh Shale Formation ofthe northern Tabas Basin. The constituents of the storm beds are absent in the fair weather peritidal facies of the Sorkh Shale Formation, but are present throughout the lower limestone member of the Elika Formation.The Tabas Block, a part of the Cimmerian continent in east central Iran, is a rift basin that developed during Early Ordovician-Silurian Paleotethys rifting. Facies and sequence stratigraphic analyses of the Sorkh Shale Formation has revealed additional evidence supporting the Tabas Block as a failed rift basin related to the Paleotethys passive margin. Absence of constituents of the storm beds in the fair weather peritidal facies of the Sorkh Shale Formation, presence of the constituents of the storm beds in the fair weather facies of the Elika Formation (the Sorkh Shale equivalent in the Alborz Paleotethys margin) and southward paleocurrent directions of carbonate storm beds suggest that the low topographic gradient of the ramp in the Tabas failed rift basin was facing the Paleotethys Ocean, where the storms were generated. In addition, northward paleocurrent directions of the fair weather facies and northward increase in carbonate content of the Sorkh Shale sequence further indicate that the Tabas Basin was tectonically a part of the Paleotethys passive margin. It is apparent that relative sea level, basin geometry and tectonic movements along the bounding faults played significant roles during deposition of the Sorkh Shale Formation by controlling accommodation space and facies variations along the Tabas failed rift basin.

Ancient Carbonate Tidalites

“…21.25 , locality 4), the lower Elika equivalent Sorkh Shale Formation consists almost entirely of tidal fl at facies (Fig. 21.30 ) intercalated with numerous storm beds and comprise the transgressive and highstand systems tracts of a depositional sequence (Ghomashi 2008, Y. Lasemi et al 2008 . Southward, the carbonate-rich peritidal deposits grade into mixed-carbonate and quartz sandstone containing bimodal, spherical, and well-rounded sand grains with polished (frosted) surface, tabular bedding with internal laminations, herringbone cross-laminations/cross bedding, fl aser and lenticular bedding.…”

“…The sand grains are interpreted as coming from aeolian dune sands which were transported over the carbonate tidal fl at (at the time of emergence) and were subsequently reworked in the intertidal environment during the next platform fl ooding (Y. Lasemi et al 2008 ) .…”

Depositional environment of the Bagh-e-Vang Formation – the only occurrence of upper Lower Permian in Iran

“…The Tabas Block is considered to represent a failed rift basin from the beginning of the Devonian to the Late Triassic (Lasemi, 2001). It formed during the Palaeotethys rifting as a result of the Early Ordovician-Silurian continental extension along its bounding Nayband and Kalmard-Kuhbanan Faults (Lasemi, 2001;Lasemi et al, 2008).…”

Depositional environment and sequence stratigraphy of the Khoshyeilagh Formation in Bojnourd and Jajarm (NE Iran)

“…High amounts of micrites demonstrate a low energy environment (Adachi et al, 2004). The dolomitized micrite and fenestral dolomudstone were deposited in upper intertidal environments and according to Flugel (2010), dolomitization in this facies indicates the deposition close to the tidal flat environment (Lasemi et al, 2008). KHMF6 is equivalent of units RMF19 and RMF22 in Flugel classification (Flugel, 2010) that belongs to the tidal flat environment.…”