Shell concentrations analysis in the Lower Permian carbonate rocks (Khan Formation) in Central Iran (Kalmard area)

Abstract

ABSTRACT Shell beds are sedimentary features that can potentially provide important palaeoenvironmental information. Stratigraphic, taphonomic, and paleontologic features frequency along with their occurrence in the shell beds in carbonate rocks of Khan Formation in Central Iran vary across lithofacies. In Khan Formation, four taphonomic and sedimentologic types of shell beds have been distinguished, which are fair weather wave concentrations (bryozoan dominated shells), storm wave concentrations (articulated brachiopod dominated shells), primary biogenic concentrations (fusulinid dominated shells), and proximal temoestite (including echinoderm dominated, gastropod dominated, bivalve dominated and disarticulated brachiopod dominated shells). Bryozoan shells were formed under strong hydrodynamic energy above fair weather base. The large articulated brachiopod shells are developed below the fair-weather wave-base ; Fusulinid shells formed in lagoon environment of homoclinal carbonate ramp which approve in situ concentration with no transportation. In echinoderm, gastropod and disarticulated bivalve and brachiopod shell beds, the occurrence of disarticulated and convex-up shells in packstone, sharp and erosional base, normal grading, parallel and hummocky lamination in the uppermost part, all indicate proximal tempestites in medium hydrodynamic force between the storm wave-base and fair-weather wave-base. These four types of shell concentrations are found in distinct positions within the depositional sequence of Khan Formation.