Architectural analysis of an Early Cambrian braided-river system on the north Gondwana margin: The lower sandstone of the Lalun Formation in the Shirgesht area, central Iran

Abstract

The Lalun Formation (Early Cimmerian), up to 867 m thick, was studied in the Shirgesht area, Central Iran. It was laid down across the northern edge of Gondwana, where fluvial sandstones were covered by marine shales, carbonates and sandstones. The Lower Sandstone unit of the formation is dominated by sand-rich braided river deposits (97%) with a general scarcity of fine-grained deposits (3%), and the unit consists nine lithofacies (Se, Spb, Sm, Sp, St, Sh, Sr, Fl and Fm). Common sedimentary structures in the succession include planar and trough cross-bedded, plane-laminated and ripple cross-laminated sandstone, and mudcracks in laminated and structureless mudstone. Architectural elements are defined as channel forms and fills (CHs and CHm), sheet sandstone bodies (SB, DA, and LS) and mudstone bodies (FF). The Early Cambrian fluvial system was operating in a non-vegetated setting, and it is probable that the major flooding of the system took place on a seasonal basis. Six orders of surfaces are identified in the outcrops. The highest order surface (sixth-order) includes the lower contact of the Lalun Formation (Lower Sandstone), which was laid down on the Zaigun Formation, and the upper contact of the Lower Sandstone with estuarine deposits of the Middle Shale, denoting a marine transgression. The paleoflow direction is consistent with the reconstructed position of the Lalun Formation during the Cambrian and indicates that sediment was transported eastward to northeastward in the northern part of the Gondwana margin. The petrographic analysis suggests craton interior and transitional continental sources under seasonal humid conditions, consistent with the reconstructed position of Iran along the northeast margin of the Arabian Shield.