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Late Devonian Conodonts from the Chah-Riseh Area, Central Iran

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Late Devonian Conodonts from the Chah-Riseh Area, Central Iran Records of the Western Australlall Museulll Supplement No. 58: 179-189 (2000). Late Devonian conodonts from the Chah-Riseh area, central Iran Mehdi YazdP, Mansooreh Ghobadi pour1 and Ruth Mawson2 1 Department of Geology, University of Isfahan, Isfahan, Iran. 2 Centre for Ecostratigraphy and Palaeobiology, Macquarie University, NSW, 2109, Australia. Abstract - In order to refine the chronologic framework for the Chah-Riseh area, north-east of Isfahan, limestone samples were collected from a section measured through the most continuous outcrop of limestone thought to span the youngest Late Devonian and/or Early Carboniferous time interval, through an apparent erosional interval into a substantial sequence of dolomitised carbonate. A small Late Devonian (Early expansa Zone) conodont fauna was recovered from the lower half of the section. Above the conodont­ bearing horizons a red shale and quartz arenite horizon indicative of an erosional interval proved to be unfossiliferous. Above that again, the sequence of dolomitised carbonates is dated as Early Permian on the basis of their fauna of foraminifers. The hiatus between the lower limestone and the upper dolomitised limestones encompasses latest Famennian, the entire Carboniferous and possibly the earliest Permian horizons. Based on chronology and lithology, the lower limestone is referred to the Shishtu Formation, and the upper dolomitised carbonates to the Jamal Formation. INTRODUCTION beds of the unit are shown herein to be Famennian Controversy regarding the age of the sequence in (Early expansa Zone) in age. According to lithologic the Chah-Riseh area that underlies Permian and palaeontologic evidence, these deposits are horizons in the region (Figure 1) has arisen because equivalent to the Padeha, Bahram and Shishtu a variety of ages have been assigned to the thin­ formations and suggest that a shallow sea existed in bedded limestone, sandy limestone and muddy the area during the late Famennian. limestone horizons in the area. Based on Portion of the Shishtu Formation in the Ozbak­ palynological data, a late Famennian (Strunian) age kuh Mountains area appears to be equivalent to the was suggested by Hamedani (1996: 320). In Geirud Formation (Member A) in Central Alborz. contrast, an Early Carboniferous age was proposed Stocklin et al. (1965) believed the entire Shishtu by a number of researchers, for example, Djafarian Formation in the Shotori Range to range in age from and Brice (1973: 2128), Zahedi (1976: 14-15), the Late Devonian into the Carboniferous. On the Djafarian (1977: 182), Shirani (1995: 20) and basis of conodont studies in the same area, Yazdi Gholamalian (1997: 42, 82). This study was (1996, 1999) has shown the range to be from undertaken in order to obtain further time Frasnian (late Early hassi Zone to Early constraining data. Carboniferous (at least the anchoralis-Iatus Zone). Stocklin and Setudehnian (1977: 223-225) divided the Shishtu Formation into two parts: Shishtu 1 GEOLOGICAL SETTING (Frasnian-Famennian and ?Tournaisian) and The Chah-Riseh area, northeast of Isfahan in Shishtu 2 (late Tournaisian into early Visean). central Iran, is located close to the Qum-Zefreh Wendt et al. (1997: 292-294, figures 8, 9) concluded Fault Zone that consists of many parallel to that Famennian horizons are present at Howz-e­ subparallel minor faults. The trend of the fault Dorah but did not mention the eroded surface system is NW to SE, the common trend of the below the Permian horizons. Zagros Mountain Range and the Zagros Subduction Zone. Minor faults have disturbed the rock sequences of the area acting as horst and graben RESULTS structures resulting in Devonian sediments being From a 13.5 m interval of predominantly found adjacent to Miocene deposits (Figure 1). limestone horizons, 10 samples were acid leached The oldest rock unit exposed in the Chah-Riseh in an attempt to recover age-diagnostic conodonts. area is a series of sandstone, dolomite, limestone The small fauna recovered from the first 5 horizons and marls. Gholamalian (1997) suggested the basal (Figure 1) is assigned a Late Famennian (Early beds of this unit were Frasnian in age. The highest expansa) age on the basis of the co-occurrence in 180 M. Yazdi, M. Ghobadi pour, R. Mawson Quaternary Oligocene - Miocene Triassic Permian Late Devonian (Famennian) Fault t .... - Covered fault N ----._ I Qanat ' ........ (underground channel) .2200 Elevation (m) vv Vegetation r.$m v : Rock 'li.... System Stage Uthological :tie Lithologiesl Description 60° Unit ~;2 Column (:- 40° + + 10 .r-....... -,,_ 13 Grey, platey - thick bedded / • Tehran \.. •.1 fossiliferous limestone -f> ' II 12 9 -1 At .Tabas $ c: II l c: 11 Chah-Riseh c: tIl ~ • tIl 32 Esfahan + 30° <Il E // '§ 0 8 c: u. / 10 Q) 1:: / a. « iii C'. E / Light yellow, thick bedded tIl 9 sandy dolomite and -, 7 '/. ' . dolomitic limestone ./ .. '/. ..'./.': 8 500km ··F:,·'/. ......'f.. ' .. ,- 6 .'": . ~ . ". ··x .... : 7 :-:••:.' >.' L-~-~ 6 Red shale 5 5 4 "2 4 tIl 'c; c: Platey' - thin bedded 0 c: .Q 3 Legend: Stratigraphic Column > tIl 3 fossihferous nodular Q) 'c; iii Cl c: limestone Q) Q) E 2 iii E u.0 Shale tIl -- ::::!. u. 2 rn c: ~ 1 3 tIl .!!l Dolomite 'c; § .c ~ 0 Cl) > 1 2 (/) Q) 0 Dark shales ~- - Marly limestone l!! Cl - 1ii E - '..'>~' ...... Sandy dolomite <> Thin - platey bedded marly limestone and shale including many brachiopods ~ Limestone 0 <> - Figure 1 Diagrams showing location and stratigraphic details of the section sampled at Chah-Riseh, northeast of Isfahan, central Iran. Late Devonian conodonts from Chah-Riseh, central Iran 181 Sample Gh-l at Chah-Riseh of Pelekysgnatlzlls Pe!ekysgnatlzlls guizlzoensis: Over 1992: 304, figs 5.5­ gllizlzoensis Wang and Wang, Scaplzignatlzlls velifer 6,5.9-12 leptus Ziegler and Sandberg, Clydagnathlls orl11ist~ni Beinert et al. and PolygnatflUs delicatlllus Rhodes, Remarks Austin and Druce (see discussion of the age of the These slightly arched, coniform elements with species below). Other conodont elements recovered longitudinal striae on the lateral surface are M or S are consistent with an Early expansa age. elements of the apparatus. The basal cavity is large Samples 6 to 10, collected above the red shales and asymmetrical. The specimens from Chah-Riseh and quartz arenite horizon in thick bedded, highly resemble coniform elements from south-central dolomitised carbonates, failed to produce Oklahoma (Over 1992: figs 5.5-6, 5.9-12). According conodonts. Thin sectioning of the samples revealed to Wang and Wang (1978), Pe!. guizhoensis occurs in an Early Permian foraminiferan fauna of at least 9 horizons dated as Early expansa Zone to Early species (K. Bell pers. comm. 1999). These are the praesulcata Zone. Over (1992) reports they are focus of a paper by Iranian colleagues. Other fossils common in late Famennian horizons of the upper recovered from these horizons, known elsewhere as Woodford Shale throughout the Arbuckle the Jamal Formation, included bryozoans, Mountains (expansa?-Early praesulcata zones). brachiopods, gastropods, algae and corals. It appears that the red shales and quartz arenite Occurrence are indicative of an erosional surface; the hiatus in Two elements in Sample 1, Chah-Riseh. sedimentation it represents at Chah-Riseh is considerable, spanning latest Famennian (from Middle expansa Zone), the entire Carboniferous and Pelekysgnathus sp. the earliest Perrnian. Similarly, a hiatus representing Figure 2.18 a similar time interval has been identified in the Remarks Zagros area (Ghavidel-Syooki 1997) and in central This coniform M element has a large Iran (Yazdi unpub. data). Yazdi (1996, 1999: 178) asymmetrical basal cavity elongated towards the has also reported such an erosional surface from outer margin. In lateral view, longitudinal striae Howz-e-Dorah in the Shotori Range to the east of ornament the outer half of the element and become Chah-Riseh but here the zones represented by the more pronounced above the elongated margin at hiatus are restricted to Middle praesulcata Zone the base of the element. through to sandbergi Zone. It is postulated, therefore, that uplift in central Iran initiated a west to east regression with the result that the effect of Occurrence the event close to the source of the uplift in central One element in Sample 1, Chah-Riseh. Iran is greater than in the Shotori Range. Order Prioniodinida Sweet, 1988 SYSTEMATIC PALAEONTOLOGY Family Spathagnathodontidae Hass, 1959 All specimens documented are housed in the Geological Department Museum of Isfahan Genus Bispathodus Muller, 1962 University, Iran, catalogued with the prefix EUIC. Type species Spatlzodus spinulicostatus E.B. Branson, 1934 Order Conodontophorida Eichenberg, 1930 Family Icriodontidae Muller and Muller, 1957 Bispathodus bispathodus Ziegler, Sandberg and Austin, 1974 Genus Pelekysgnathus Thomas, 1949 Figure 3 Type species Spathognathodus aculeatus: Ziegler 1962: pI. 13, figs Pelekysgnathlls inclinatus Thomas, 1949 28-30 Bispatlzodus bispatlzodus Ziegler et al. 1974: 102, pI. 3, figs 12, 17, 21 PelekysgnatllUs guizllOensis Wang and Wang, 1978 Figures 2.17, 19 Remarks Pelektjsgnatlzlls gllizlzoensis Wang and Wang 1978: 75, This narrow species has a large, asymmetrical pI. 3, figs 7-10 basal cavity expanded from a little anterior of mid­ length of the unit and tapering to the posterior. In Pelekysgnatlllls guizlzoensis: Over and Barrick 1990: lateral view, the 3 anterior-most denticles are pI. 1, figs 10, 11, 14-16 enlarged, fused, increasing in height from the 182 M. Yazdi, M. Ghobadi pour, R. Mawson 7 12 16 18 Late Devonian conodonts from Chah-Riseh, central Iran 183 anterior extremity and curve backwards to the BispatllOdus aculeatlls pIumulus: Barskov et al. 1984: posterior. A series of numerous, smaller denticles pI. 2, figs. 2--6 continue to the posterior extremity. Viewed from above, three lateral denticles are evident on the BispatllOdus aculeatus pIumullls: Mawson and Talent right side of the free blade. According to Ziegler et 1997: 210 al. (1974) this species ranges in age from middle Bispathodus aculeatus plumullls: Molloy et al. 1997: pI.
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