Dapingian-lower Darriwilian (Ordovician) stratigraphic gap in the Faraghan Mountains, Zagros Ranges, south-eastern Iran

Abstract

The Lower-Middle Ordovician sediments exposed in the Faraghan Mountains, south-eastern Zagros Ranges, represent a condensed succession of siliciclastic-dominated rocks rich in palynomorph assemblages (acritarchs and subsidiary chitinozoans) and sparse shelly concentrations bearing biostratigraphically significant brachiopods and conodonts. The Lower Ordovician Zard-Kuh Formation comprises coarse-grained siliciclastic deposits rich in Cruziana ichnofossils. The lower 80 m of the overlying Seyahou Formation, late Floian to Katian in age, form a heterolithic succession composed of black and green shales, subarkoses and silty limestones. Its lower part is punctuated by a centimetric phosphoarenite that contains lingulate brachiopods (Atansoria yaseri sp. nov.) and conodonts (Baltoniodus aff. B. triangularis Lindström and Drepanoistodus sp.) that suggest a latest Floian age. The top of the condensed phosphoarenite is marked by a considerable hiatus that ranges the Dapingian and early Darriwilian interval. Overlying the hiatus, the Seyahou Formation comprises two fossiliferous levels, the oldest dated as mid-Darriwilian with chitinozoans characteristic of the Siphonochitina formosa Zone, and the youngest of the Katian Acanthochitina barbata Zone. Mid Ordovician phosphogenesis associated with starvation, reworking, resedimentation, and the onset of distinct stratigraphic gaps was a complex process recorded throughout the Arabian margin of Gondwana. In the Zagros Ranges, maximum flooding and phosphate precipitation are suggested as the counterpart of the Helskjer Drowning Event of Baltoscandia and the third-order maximum flooding surface that punctuates the Siphonochitina formosa Zone in North Africa.•

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

“…It is considered equivalent to the D. artus graptolite Zone, and is associated with a major flooding episode (Paris et al, 2007). This zone and its associated flooding event are also known in Oman (Molyneux et al, 2002) and south eastern Iran, where it occurs above an important hiatus encompassing the Dapingian-earliest Darriwilian interval (Ghavidel-Syooki et al, 2014).…”

“…Moreover, some trace fossils are also present at the Saq/Hanadir boundary (ElKhayal and Romano, 1988;Seilacher, 2000). The details of sedimentary facies and faunas have been described in a previous paper (Le Hérissé et al, 2007), and supplementary information on other stratigraphically equivalent localities can be found in Ghavidel-Syooki et al (2014) and Gutiérrez-Marco et al (2016). The age of the Hanadir Member, originally regarded as Early Ordovician (Helal, 1964), was re-assigned on palynological evidence to the Darriwilian, i.e., late Arenig to Llanvirn of the British terminology (Al-Hajri, 1995).…”

“…The stratigraphy of the Dapingian to early Darriwilian interval is complex all over the world (Gutiérrez-Marco et al, 2013, and particularly on the Arabian margin of Gondwana, where it is characterized by condensed sequences and diastems in Oman (Molyneux et al, 2006), Iran (Ghavidel-Syooki et al, 2014), and Saudi Arabia (Le Hérissé et al, 2007;present study).…”

Discussion on Palaeozoic discontinuities in the Kuh-e Surmeh area (Zagros, Iran)

“…The Seyahou shale interbeds have also yielded well-preserved acritarchs and chitinozoans, which can be used to identify a biostratigraphical succession ranging from the latest Floian Euconochitina brevis Zone to the latest Katian Ancyrochitina merga Zone (Ghavidel-Syooki, 2000;Ghavidel-Syooki et al, 2011). Based on new biostratigraphical data provided by Ghavidel-Syooki et al (2014), new divisions for the Zard-Kuh type section have been proposed: the first lower conglomeratic unit is defined as the Zard-Kuh Fm. and the two latter units are now considered as the Seyahou Fm.…”

“…The three units correspond to (i) a lower conglomeratic unit, 20 m thick, composed of polymictic conglomerate beds alternating with thin shale intervals; this unit is newly defined as the Zard-Kuh Fm. (Ghavidel-Syooki et al, 2014); (ii) a middle heterolithic unit, 460 m thick, composed of shale/sandstone alternations bearing interbedded phosphatic and bioclastic carbonate siltstones, which have yielded a rich and diversified shelly fauna, composed of trilobites, bryozoans, calcitic and linguliformean brachiopods, molluscs, and conodonts; and (iii) an upper unit, 390 m thick, recognizable by thin-bedded, rhythmic claystone/sandstone couplets extremely rich in ichnofossils. These formations range from Floian to Katian in age based on the occurrence of the conodont Baltoniodus triangularis (Lindström, 1955) in the lowermost part of the formation, the trilobite Neseuretinus turcicus Dean, 1967 and Dalmanitina cf.…”