Gerhard Schairer

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.

Toarcian and Aalenian (Jurassic) ammonites from the Shemshak Formation of the Jajarm area (eastern Alborz, Iran)

In the Jajarm area (eastern Alborz Mountains, NE Iran) the ?Upper Triassic — Lower Bajocian siliciclastic Shemshak Formation is up to 2000 m thick. Whereas the lower third of the formation is nearly exclusively non-marine, the upper two-thirds are fully marine. The middle part is characterized by several levels containing a relatively diverse and well preserved assemblage of ammonites of the Toarcian to Aalenian stages. Two sections of the ammonite-bearing strata, spaced 20 km, are presented and correlated by means of ammonite biostratigraphy. The ammonite fauna consists of 27 taxa, some of which are recorded for the first time from the Alborz Mountains. The ammonites are briefly described and their palaeobiogeographic context is reviewed. The ammonite fauna of the Shemshak Formation at Jajarm, as elsewhere in North and Central Iran, is exclusively Northwestern Tethyan in character and is closely related to the faunas of Northwestern and Central Europe.KurzfassungIm Gebiet von Jajarm (östliches Alburz-Gebirge, Nordost-Iran) wird die siliziklastische Shemshak-Formation (?Ober-Trias bis Unter-Bajoc) über 2000 m mächtig. Im unterern Drittel besteht die Formation aus nichtmarinen Sedimenten, während die oberen zwei Drittel voll-marinen Charakter haben. Der mittlere Teil der Formation ist durch einige Ammoniten-Horizonte charakterisiert, die eine recht diverse und gut erhaltene Ammoniten-Fauna des Toarc und Aalen führen. Zwei etwa 20 km voneinander entfernt liegende Profile des Ammoniten-führenden Teils der Shemshak-Formation werden vorgestellt und biostratigraphisch miteinander korreliert. Die Ammonitenfauna setzt sich aus 27 Taxa zusammen, von denen einige zum ersten Mal aus dem Alburz-Gebirge berichtet werden. Auf die kurze systematische Beschreibung der Ammoniten folgt eine Bewertung der paläobiogeographischen Zusammenhänge. Dabei zeigt sich, dass die vorliegende Ammoniten-Fauna von Jajarm, ähnlich wie jene von anderen Lokalitäten im Nord- und Zentraliran, ausschließlich nordwest-tethyalen Charakter besitzt und enge Bezüge zu Ammoniten-Faunen Nordwest- und Mittel-Europas aufweist.