Stratigraphy and ammonite fauna of Upper Jurassic basinal sediments at the eastern margin of the Tabas Block (east-central Iran)

Abstract

In this paper, the stratigraphy and depositional setting of Upper Jurassic basinal sediments at the eastern margin of the Tabas Block of east-central Iran are discussed with special emphasis on the rich ammonite fauna. The basinal sediments comprise the upper part of the Qal’eh Dokhtar Limestone Formation and a new, hitherto not recognized lithostratigraphic unit, the low-weathering Korond Formation, which is formally introduced herein. The recognition of the Korond Formation requires significant changes in the geological maps of the area, because invariably, it has been mapped as the Middle Jurassic Baghamshah Formation. As a result, several large faults become superfluous and the tectonic structure of the area is far simpler than hitherto assumed. The upper Qal’eh Dokhtar Limestone and the Korond formations formed in a lower slope-to-basin environment to the east of a huge, N/S trending carbonate platform (represented by the Esfandiar Limestone Formation) fringing the eastern margin of the Tabas Block. The spread of the basinal Korond Formation onto the eastern occurrences of the Esfandiar Platform in the Late Oxfordian to Early Kimmeridgian indicates a partial drowning of this platform. Most of the described ammonites come either from the condensed drowning unconformity at the top of the Esfandiar Limestone Formation or from the upper 100 m of the Qal’eh Dokhtar Limestone Formation. The ammonite fauna is dominated by perisphinctids and phylloceratids and indicates a Middle to early Late Oxfordian age for the top of the Qal’eh Dokhtar Limestone Formation and a Late Oxfordian to late Early Kimmeridgian age for the Korond Formation. A systematic description of the ammonite fauna is presented.