Lithostratigraphy of the Upper Triassic–Middle Jurassic Shemshak Group of Northern Iran

Abstract

AbstractThe Upper Triassic–lower Middle Jurassic Shemshak Group is a siliciclastic unit, up to 4000 m in thickness, which is widespread across the Iran Plate of northern and central Iran. The group is sandwiched between two major unconformities: the contact with the underlying platform carbonates of the Elikah and Shotori formations is characterized by karstification and bauxite–laterite deposits; the top represents a sharp change from siliciclastic rocks to rocks of a Middle–Upper Jurassic carbonate platform–basin system. In the Alborz Mountains, the group consists of a Triassic and a Jurassic unit, separated by an unconformity, which is in part angular in the northern part of the mountain range and less conspicuous towards the south. Published lithostratigraphic schemes are based on insufficient biostratigraphic and lithological information. Here we present a new lithostratigraphic scheme for the central and eastern Alborz Mountains modified and enlarged from an unpublished report produced in 1976. Two major facies belts, a northern and a southern belt running more or less parallel to the strike of the mountain chain, can be distinguished. In the north, the Triassic part of the group is composed of the comparatively deep-marine Ekrasar Formation with the Galanderud Member (new name) at the base followed by the Laleband Formation, which represents prodelta–delta front environments. Up-section, the latter is replaced by the fluvial–lacustrine, coal-bearing Kalariz Formation. The equivalent Triassic lithostratigraphic unit in the south is the Shahmirzad Formation, redefined here, with the Parvar Member at the base. The formation represents fluvial, coastal plain and shallow- to marginal-marine environments. In the north, the Jurassic part of the group consists exclusively of the Javaherdeh Formation, coarse conglomerates of alluvial fan–braided river origin, which towards the south grades into the Alasht Formation, rocks of fluvial–lacustrine origin with coal. Further south, the Alasht Formation represents intertonguing marginal-marine–flood-plain environments and is followed by the Shirindasht Formation, sandstones and siltstones, indicative of the storm-dominated shelf, and the Fillzamin Formation (new), which is characterized by comparatively deep-marine shales. In the south, the group ends with the Dansirit Formation of deltaic–coastal-plain origin. This lithostratigraphic scheme reflects the tectono-sedimentary evolution of the Shemshak Foreland Basin of the Alborz Mountains where, during the Late Triassic, a relict marine basin in the north became gradually infilled, whereas in the south non-sedimentation and subaerial erosion prevailed and sediments record largely non-marine–marginal-marine conditions. During the early Lias, the basin was filled with erosional debris of the rising Cimmerian Mountain Chain, deposited largely in non-marine environments. During the early Middle Jurassic, in contrast, rapid subsidence in the south resulted in the deepening and subsequent infilling of a marine basin.