Stratigraphy and ammonite fauna of the upper Shemshak Formation (Toarcian–Aalenian) at Tazareh, eastern Alborz, Iran

Stratigraphy and ammonite fauna of the upper Shemshak Formation

(Toarcian–Aalenian) at Tazareh, eastern Alborz, Iran

K. Seyed-Emami a, F.T. Fursich b,*, M. Wilmsen b, F. Cecca c, M.R. Majidifard d,

G. Schairer e, A. Shekarifard a

a Faculty of Engineering, Tehran University, P.O. Box: 11365-4563, Iranb Institut fur Palaontologie der Universitat, Pleicherwall 1, D-97070 Wurzburg, Germany

c CNRS-UMR 5143 ‘Paleobiodiversite et Paleoenvironnements’, Tour 56-46, 5 eme etage, case 104,

Universite Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 05, Franced Geological Survey of Iran, P.O. Box 131851-1494, Tehran, Iran

e Bayerische Staatssammlung fur Palaontologie und Historische Geologie, Richard-Wagner-Str. 10, D-80333 Munchen, Germany

Received 6 April 2005; accepted 17 October 2005

Abstract

With a thickness of 3900 m, the Tazareh section is one of the thickest developments of the Shemshak Formation in the Alborz range. It overlies

with sharp and disconformable contact the limestones and dolomites of the Lower–Middle Triassic Elikah Formation and is topped, again with a

disconformable contact, by the marls and limestones of the Middle Jurassic Dalichai Formation. The nearly exclusively siliciclastic succession

represents a range of environments, from fluvial channels, flood plains, swamps and lake systems to storm-dominated shelf, and a comparatively

deep marine and partly dysoxic basin. The segment of the section between 2300 and 3500 m is exclusively marine and contains a moderately

diverse ammonite fauna, ranging from the Middle Toarcian to the Upper Aalenian. The ammonite fauna comprises 21 taxa, among them the new

genus Shahrudites with two new species, Shahrudites asseretoi and S. stoecklini from the Middle Aalenian Bradfordensis Zone. The other

ammonites from the Shemshak Formation at Tazareh (as elsewhere in North and Central Iran) are exclusively Tethyan in character and closely

related to faunas from western and central Europe. An ammonite-based correlation of Toarcian–Aalenian successions of the eastern Alborz with

time-equivalent strata of the Lut Block, part of the Central-East Iranian Microcontinent (ca. 500 km to the south), suggests a strong influence of

synsedimentary tectonics during the deposition of the upper Shemshak Formation.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Iran; Eastern Alborz; Shemshak Formation; Jurassic; Ammonites; Biostratigraphy; Correlation

1. Introduction

The Tazareh coal mine is the most important productive

coal mine in the eastern Alborz. It is situated north of the main

road from Tehran to Mashhad, about 30 km northeast of the

historic town of Damghan and 45 km west of Shahrud (Fig. 1).

Because of its economic importance, the Shemshak Formation

of the Alborz range, in the Tazareh area, has been intensively

studied by Iranian and Russian geologists of the National

Iranian Steel Company (Bragin et al., 1976; Repin, 1987). For

their internal use, the geologists of the Steel Company employ

their own lithostratigraphic subdivisions and terms, which are

1367-9120/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2005.10.003

* Corresponding author. Tel.: C49 931 31 2596; fax: C49 931 31 2504.

E-mail address: [email protected] (F.T. Fursich).

not in accordance with the international stratigraphic guide

lines and which have not been adopted by the National

Stratigraphic Committee of Iran. Besides, the studies are filed

in internal reports, and there exist practically no modern

detailed reports on the Shemshak Formation of the eastern

Alborz, and thus of the Tazareh area.

The area under discussion was investigated during May

2004 by a joint Iranian–German–French research team and a

detailed cross-section was measured through the Shemshak

Formation near the Tazareh coal mine (base co-ordinates N

36823 073 00, E 54829 054 00, top co-ordinates N 36826 026 00, E

54829 048 00). There the Shemshak Formation overlies, with

sharp and disconformable contact, the limestones and

dolomites of the Elikah Formation (Lower–Middle Triassic)

and is followed disconformably by the marls and limestones of

Journal of Asian Earth Sciences 28 (2006) 259–275

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Fig. 1. Locality map with distribution of the Shemshak formation in the central and eastern Alborz Mountains, northern Iran. The Tazareh section is marked with an

arrow.

Fig. 2. Stratigraphic framework: (A) Upper Triassic to Upper Jurassic lithostratigraphic units in the eastern Alborz Mountains; (B) standard ammonite zones and

subzones of the Toarcian and Aalenian stages (after Elmi et al., 1997; Contini et al., 1997).

K. Seyed-Emami et al. / Journal of Asian Earth Sciences 28 (2006) 259–275260

K. Seyed-Emami et al. / Journal of Asian Earth Sciences 28 (2006) 259–275 261

the Dalichai Formation (Middle Jurassic; cf. Fig. 2). With a

thickness of 3900 m, the Tazareh section is one of the thickest

developments of the Shemshak Formation in the Alborz range.

The Shemshak Formation at Tazareh is a nearly exclusively

siliciclastic succession, representing a range of environments,

from fluvial and lacustrine to coastal and fully marine. The

upper 1600 m of the section are fully marine, containing a low

to moderately diverse benthic macrofauna. Ammonites occur

at several levels and indicate that the marine phase ranged from

the Middle Toarcian to the Upper Aalenian. Of special interest

are some new taxa of Erycitinae (or Hammatoceratinae), which

are described here for the first time.

2. Geological setting and stratigraphy

Sediments of the Upper Triassic to lower Middle Jurassic

Shemshak Group are widespread throughout the Iran Plate, a

major segment of the Cimmerian Microcontinental Collage

(Sengor, 1990). This terrane was originally part of north-

eastern Gondwana which became detached towards the end of

the Permian, to move northwards and finally to collide with

the Turan Plate (part of Eurasia) towards the end of the

Middle Triassic, or in the early Late Triassic (e.g. Sengor,

1990; Stampfli et al., 1991; Saidi et al., 1997; Seyed-Emami,

2003). As a result of this collision, the Palaeotethys Ocean

was closed in this area. This Early Cimmerian Orogeny

caused a distinct facies change in many areas of Iran: Lower–

Middle Triassic platform carbonates (Shotori Formation,

Elikah Formation) are unconformably overlain by fine- to

coarse-grained siliciclastics, often several thousand metres in

thickness. In general, these sediments are interpreted as the

Fig. 3. Field aspects of the Tazareh section: (A) overview of the Tazareh section, vie

of the section up to the ridge-forming Lower–Middle Toarcian sandstones can be s

(Toarcian–Lower Bajocian) is hidden in the strike-parallel valley ((B)) between the T

built of limestones of the Callovian–Upper Jurassic Lar formation; (B) detail of the u

fine-grained siliciclastic sediments of the middle to outer shelf. Note the enormous

fill of molasse-type basins fed by the rising Cimmerian

mountain chain.

In the Alborz Mountains, this sediment pile has been

designated as Shemshak Formation by Assereto (1966), the

type section of which is located at Shemshak, north of Tehran.

Throughout the mountain range (Fig. 1), thickness and facies

patterns vary considerably, from several hundred metres to

more than 3000 m and from alluvial fans, floodplains, lakes

and deltas to shallow marine shelf and, rarely, deeper, basinal

environments (e.g. Corsin and Stampfli, 1977; Stampfli, 1978;

Rad, 1986; Repin, 1987; Vollmer, 1987; Fursich et al., 2005;

Seyed-Emami et al., 2005). Although most of the sedimentary

succession is non-marine, marine intercalations, documented

by ammonites, have been recorded from the Late Triassic

(mainly Norian) and Sinemurian to the Early Bajocian (e.g.

Nabavi and Seyed-Emami, 1977; Seyed-Emami, 1985, 1987;

Seyed-Emami and Nabavi, 1985; Repin, 2000; Seyed-Emami

et al., 2005).

In many areas of the Alborz Mountains faulting and folding,

as well as the partly soft lithology, make it difficult to establish

complete sections of the Shemshak Formation. However, in the

eastern part of the Alborz, near Tazareh, the sedimentary

succession, from the underlying carbonates of the Elikah

Formation (Triassic) to the marls and limestones of the

overlying Dalichai and Lar formations (Bajocian-Tithonian;

Figs. 2 and 3), is completely exposed. The nearly 4 km thick

section of the Shemshak Formation at Tazareh was studied by

Chayasteh (1978) and Mohammed-Hassan (1978), with

emphasis on the coal-bearing fluvial-lacustrine middle part of

the formation. Here, we discuss the marine, ammonite-bearing

segment between 2050–3050 m (Toarcian–lower Upper Aale-

nian), which corresponds to the transgressive part of the

w to the NNE from the top of the Triassic Elikah Formation. Only the lower part

een. The upper, soft-weathering marine upper part of the Shemshak Formation

oarcian sandstones and the light-grey cliffs in the background (O3000 m high),

pper part of the Shemshak Formation showing Upper Toarcian–Upper Aalenian

thicknesses (ca. 500 m).

Fig. 4. Stratigraphic log of the Toarcian–lower Upper Aalenian part of the Shemshak Formation at Tazareh (2050–3000 m). Ammonite levels 1–9 are indicated

(compare with Table 1).



transgressive–regressive megacycle recognized by Fursich

et al. (2005) for the upper (Toarcian–Bajocian) part of the

Shemshak Formation in the eastern Alborz.

The succession starts with a sequence of coastal plain sand-

and siltstones containing root horizons, plant remains and thin,

discontinuous coal seams, as well as first signs of marine

influence (such as the trace fossil Diplocraterion sp. at ca.

2140 m; Fig. 3). From 2210 m onwards, marine inner shelf

sediments prevail, containing marine bivalves, gastropods and

trace fossils (Thalassinoides sp.) along with abundant plant

material. At 2410 m, a facies change to sharp-based, hummocky

cross-stratified sandstone beds alternating with strongly

bioturbated, fine-grained sand-to siltstones (often with deep

infaunal bivalves in life position) suggests a storm-influenced

mid-shelf setting (see Fursich et al., 2005, for details). With this

facies change, the first (?Lower–Middle Toarcian) ammonites

occur (indeterminable hildoceratids and harpoceratids),

accompanied by a moderately diverse association of benthic

macroinvertebrates (dominated by bivalves, with brachiopods,

gastropods, crinoids and serpulids being minor constituents) and

trace fossils (Fig. 3). Up-section, storm influence decreases

whereas the degree of bioturbation increases, suggesting a

further deepening during the Late Toarcian–Early Aalenian. At

ca. 2850 m, in the upper Lower Aalenian, a change towards

monotonous, bioturbated, olive siltstones with an impoverished

benthos suggests deposition in an outer shelf (‘basinal’) setting,

prevailing throughout the Middle and early Late Aalenian.

Ammonites are relatively common at several levels within the

Middle Toarcian to lower Upper Aalenian part of the succession

and are described below.

Table 1

Ammonite occurrences from the Shemshak Formation at Tazareh

Level Sample Metres Taxa

1 – 2410 Indeterminate harpocer

2 040512–4 2465 Grammoceras cf. thoua

Podagrosites pseudogru

3 040512–5 2516 Pseudogrammoceras fa

4 040512–8 2618–2679 Dumortieria radiosa (S

(Pleydellia) subcompta

lia (Walkericeras) cf. lu

Rulleau, 1997, Paradum

and Caloo-Fortier, 1985

5 040512–18 2693 Leioceras comptum (Re

paucicostatum Rieber,

costatum Rieber, 1963

6 040512–10 2725 Bredyia sp.

7 040512–15 2850 Leioceras comptum (Re

aff. comptum (Reinecke

ceras aff. planinsigne (

8 040512–16 2884 Ludwigia sp., Brasilia c

(Buckman, 1881), Plan

signe (Vacek, 1886), A

(Mayer, 1871), Shahrud

Emami sp. nov., Shahru

Emami sp. nov., Shahru

Emami sp. nov., aff. Sh

9 040512–17 2935 Graphoceras (Graphoc

1902

3. Systematic palaeontology

In order to keep the systematic chapter short, only a few,

mainly regional synonymies are given. For specimens kept in

open nomenclature, the recommendations of Bengtson (1988)

are followed. The material is housed in the collections of the

Bayerische Staatssammlung fur Palaontologie und Geologie,

Munich. The stratigraphic position of the samples is shown in

Fig. 4 (see also Table 1). The following abbreviations were

used to specify the dimensions of specimens: D, diameter in

mm; U, umbilical width as % of diameter; H, whorl height as %

of diameter; W, whorl width as % of diameter.

Class Cephalopoda Cuvier, 1798

Order Ammonoidea Zittel, 1884

Superfamily Lytocerataceae Neumayr, 1875

Family Lytoceratidae Neumayr, 1875

Genus Lytoceras Suess, 1865

Lytoceras sp.

Fig. 5(14(a) and (b))

Material: One fragment (040513–10) from the lower

Dalichai Formation, 106 m above the top of the Shemshak

Formation (not displayed in Fig. 4).

Description and discussion: The fragmentary lytoceratid has

a circular whorl cross-section and very fine, dense and

radiate ribbing.

Distribution: Upper Bajocian?

Superfamily Hildocerataceae Hyatt, 1867

Family Hildoceratidae Hyatt, 1867

Age

atids and hildoceratids (?Early–)Middle Toarcian

rsense (d’ Orbigny, 1843)

nowi Guex, 1975

Late Toarcian, Thouarsense Zone

llaciosum (Bayle, 1878) Late Toarcian, Thouarsense Zone

eebach, 1864), Pleydellia

(Branco, 1879), Pleydel-

gdunensis Elmi and

ortieria tectiformis Elmi

Late Toarcian, Aalensis Zone

inecke, 1818), Leioceras

1963, Leioceras crassi-

Early Aalenian, Opalinum Zone, Comptum

Subzone

Early Aalenian, Opalinum Zone

inecke, 1818), Leioceras

, 1818), Planammato-

Vacek, 1886)

Early Aalenian, Opalinum Zone, Comptum

Subzone

f. bradfordensis

ammatoceras aff. planin-

ccardia diadematoides

ites asseretoi Seyed-

dites asseretoi? Seyed-

dites stoecklini Seyed-

ahrudites sp.

Middle Aalenian, Bradfordensis Zone

eras) decorum Buckman, Late Aalenian, Concavum Zone

Fig. 5. Ammonites from the Shemshak Formation at Tazareh (all figures !0.85). Arrows indicate onset of body chamber: 1 (a)–(c). Paradumortieria tectiformis

Elmi and Caloo-Fortier, specimen 040512-8/1. 2, 7, 10: aff. Shahrudites sp.; 2 specimen 040512-16/30D; 7. specimen 040512-16/31; 10. specimen 040512-16/22. 3

Leioceras paucicostatum Rieber, specimen 040512-18/7 (plasticine cast). 4 Ludwigia sp., specimen 040512-16/25. 5, 12, 18. Graphoceras (Graphoceras) decorum

Buckman; 5 specimen 040512-17/5; 12 (a)–(b). specimen 040512-17/4; 18. specimen 040512-17/12. 6 Grammoceras cf. thouarsense (d’Orbigny), specimen

040512-4/1. 8, 16. Accardia diadematoides (Mayer). 8 specimen 040512-16/24; 16. specimen 040512-16/26. 9 Brasilia cf. bradfordensis (Buckman); a. upper

specimen 040512-16/1a, lower specimen 040512-16/1b; b. ventral view of specimen 16/1a. 11. Leioceras crassicostatum Rieber, specimen 040512-18/1. 13.

Pleydellia subcompta (Branco), specimen 040512-8/6 (plasticine cast). 14. Lytoceras sp., specimen 040513-10. 15 a–b. Leioceras aff. comptum (Reinecke),

specimen 040512-15/7. 17. Podagrosites pseudogunowi Guex, specimen 040512-5/12. 19. Leioceras comptum (Reinecke), specimen 040512-15/2. 20.

Pseudogrammoceras fallaciosum (Bayle), specimen 040512-5/12. 21. Dumortieria radiosa (Seebach), specimen 040512-8/2.



Subfamily Grammoceratinae Buckman, 1904

Genus Grammoceras Hyatt, 1867

Grammoceras cf. thouarsense (d’Orbigny, 1842)

Fig. 5(6)

cf. *1843 Ammonites thouarsense nov. sp.—d’Orbigny:

222, pl. 57, Figs. 1–3.

Material: A fragmentary and juvenile internal mould

(040512-4/1).

Discussion: the evolute form with strongly falcoid ribbing

resembles the inner whorls of G. thouarsense.

Distribution: Upper Toarcian, Thouarsense Zone.

Genus Podagrosites Guex, 1975

Podagrosites pseudogrunowi Guex, 1975

Fig. 5(17(a) and (b))

*1975 Podagrosites latescens pseudogrunowi subsp. n.—

Guex: 105, pl. 3, figs. 4–8, 11; pl. 4, figs. 1 and 3.

2005 Podagrosites pseudogrunowi Guex, 1975 — Seyed–

Emami et al.: 361, fig. 5(F) and (J).

Material: A fragment of a large specimen (040512-4/2).

Discussion: See Seyed-Emami et al. (2005).


Genus Pseudogrammoceras Buckman, 1901

Pseudogrammoceras fallaciosum (Bayle, 1878)

Fig. 5(20)

*1878 Grammoceras fallaciosum nov. sp.—Bayle: pl. 78,

figs. 1–2.

2005 Pseudogrammoceras fallaciosum (Bayle, 1878)—

Seyed-Emami et al.: 361, fig. 5(B) and (C).

Material: 12 mostly fragmentary specimens (040512-5/1–12).



Family Graphoceratidae Buckman, 1905

Subfamily Dumortieriinae Maubeuge, 1950

Genus Dumortieria Haug, 1885

Dumortieria radiosa (Seebach, 1864)

Fig. 5(21)

*1864 Ammonites radiosus Seebach—Seebach: 142, pl. 9,

fig. 2.

1985 Dumortieria radiosa (Seebach)—Seyed-Emami and

Nabavi: 254, fig. 4(a) and (b).

Material: A compressed internal mould (040512-8/2).

Discussion: See Seyed-Emami and Nabavi (1985).

Distribution: Upper Toarcian, Aalensis Zone.

Genus Pleydellia Buckman, 1899

Pleydellia (Pleydellia) subcompta (Branco, 1879)

Fig. 5(13)

*1879 Harpoceras subcomptum n. sp.—Branco: 90, pl. 5, fig.

3, 3(a).

1985 Pleydellia subcompta (Branco)—Seyed-Emami and

Nabavi: 264, figs. 30(a) and (b), 31(a) and (b).

2005 Pleydellia (Pleydellia) subcompta (Branco, 1879)—

Seyed-Emami et al.: 364, fig. 6(E) and (J).

Material: Two external casts (040512-8/5–6).

Discussion: See Seyed-Emami and Nabavi (1985): 264).


Pleydellia (Walkericeras) cf. lugdunensis Elmi and

Rulleau, 1991

cf.*1997 Pleydellia (Walkericeras) lugdunensis nom. nov.—

Elmi et al.: 35, fig. 5; pl. 11, fig. 19.

cf. 2001 Pleydellia (Walkericeras) lugdunensis Elmi et

Rulleau, morphotype delicata Buckman—Rulleau and Elmi:

pl. 18, fig. 6.

2004 Pleydellia (Walkericeras) cf. lugdunensis Elmi et al.,

1997—Seyed-Emami et al.: 84, pl. 1, fig. 8(a) and (b).

Material: Three compressed fragments (040512-8/3, /4, /7).

Description: Relatively evolute Pleydellia with high-ovate

whorl cross-section and a sharp keel. The ribs are falcoid,

commonly fasciculating towards the umbilicus and curving

strongly forward towards the venter.

Discussion: The fragmentary specimens can be compared with

Pleydellia (W.) lugdunensis (see also Seyed-Emami et al.,

2004).


Genus Paradumortieria Elmi and Caloo-Fortier, 1985

Paradumortieria tectiformis Elmi and Caloo-Fortier,

1985 [m]

Fig. 5(1(a)–(c))

*1985 Paradumortieria tectiforme nov. sp.—Elmi and Caloo-

Fortier: 46, pl. 1, figs. 1(a) and (b), 2–7.

2001 Paradumortieria tectiforme Elmi and Caloo-Fortier—

Rulleau and Elmi: pl. 16, fig. 10(b).

Material: One relatively well-preserved internal mould

(040512-8/1).

Dimensions (in mm):

D U H W

27 37 37 29

Description: a relatively evolute form with compressed-

rectangular, slightly tectiform whorl cross-section and a

sharp keel. The ornamentation consists of single, relatively

dense, sharp and falcoid ribs, curving strongly forward on

the ventral side and reaching the keel. A few of the ribs are

paired at the umbilical margin.

Discussion: P. tectiformis is characterized by its small size

(microconch form), sharp ribs, and a rather tectiform whorl

cross-section. From the closely related P. schaireri (Seyed-

Emami) (Seyed-Emami and Nabavi, 1985: 259, figs. 7(a)

and (b), 8(a) and (b) it is distinguished by coarser ribbing

and a comparatively larger adult size.

Distribution: Upper Toarcian, Aalensis Zone, together with

Dumortieria radiosa and Pleydellia subcompta.

Subfamily Leioceratinae Spath, 1936

Genus Leioceras Hyatt, 1867

Leioceras comptum (Reinecke, 1818)

Fig. 5(19(a) and (b))

Fig. 6. Ammonites from the Shemshak formation at Tazareh, except Fig. 6/17 (all figure !0.85, if not indicated otherwise). 1–5, 7. S. asseretoi Seyed-Emami sp.

nov.; 1. specimen 040512-16/35 (holotype), 1c. x1.2; 2 specimen 040512-16/36 (paratype), 2c. x1.2; 3. specimen 040512-16/34, 3c. x1.2; 4. specimen 040512-16/3;

5. specimen 040512-16/4; 7. specimen 040512-16/38, Fig. 7a. x1.5. 6, 8, 9. S. stoecklini Seyed-Emami sp. nov.; 6. specimen 040512-16/6 (paratype); 6b, c: x1.2; 8.

specimen 040512-16/14 (holotype); 9. specimen 040512-16/5. 10, 11. S. asseretoi? Seyed-Emami sp. nov.; 10. specimen 040512-16/23a (an extremely depressed

and coarsely ribbed specimen); 11. specimen 040512-16/23b (left: extremely depressed and coarsely ribbed specimen) and Planammatoceras sp. (right, impression).

12. Bredyia sp., specimen 040512-10. 13–16. Planammatoceras aff. planinsigne (Vacek). 13. specimen 040512-16/7; 14. specimen 040512-16/30; 15. specimen

040512-16/33; 16. specimen 040512-16/29. 17. Ceccaites cf. sieboldi (Oppel), specimen SE-92-1. Badamu formation at Khorand, SW of Ravar, central Iran.


Fig. 7. Whorl sections of Shahrudites Seyed-Emami gen. nov. [m]. 1. S.

asseretoi Seyed-Emami sp. nov. (body chamber of specimen 040512-16/34 at

DZ19 mm, WhZ7 mm). 2. S. stoecklini Seyed-Emami sp. nov. (body

chamber of specimen 040512-16/6 at DZ28 mm, WhZ10 mm).


*1818 Nautilus comptus—Reinecke: 57, pl. 1, figs. 5 and 6.

2005 Leioceras comptum (Reinecke, 1818)—Seyed-Emami

et al.: 368, fig. 7(D) and (J).

Material: Six partly fragmentary specimens (040512-15/2,

/4; 040512-18/4, /8–10).

Dimensions (in mm):

Specimen D U H W

15/2 36 22 47 –

Description: The figured specimen (15/2) is a full-grown

Leioceratinae, apparently with the greater part of the body-

chamber preserved, beginning at DZ20 mm. It is a

moderately involute form with a high-ovate whorl cross-

section and a sharp keel. The umbilicus is narrow, with a

steep wall and a faint peri-umbilical depression on the

outer half of the preserved whorl. The ribbing on the inner

whorl is relatively coarse, becoming finer towards the

aperture and lastly giving way to falcoid and fasciculate

striae.

Discussion: Considering the great intraspecific variability

of L. comptum, our specimen corresponds well to

microconch forms described by Goy and Ureta (1987)

and Henriques and Ureta (in Pavia and Cresta, 2002).

Another very similar specimen is L. (Cypholioceras)

comptum bifidatum Buckman (1909: pl. 8, Fig. 2).

Distribution: Lower Aalenian, Opalinum Zone, Comptum

Subzone.

Leioceras aff. comptum (Reinecke, 1818)

Fig. 5(15(a) and (b))

Material: An apparently full-grown Leioceratinae with the

greater part of the body-whorl and original shell (040512-

15/7).

Dimensions (in mm):

Specimen D U H W

15/7 40 25 – w23

Description: Moderately involute Leioceratinae with

compressed high-ovate to lanceolate whorl cross-section

and a sharp keel. The umbilical border up to DZ30 mm

(beginning of the body-whorl) is rounded; later, however,

it becomes sharp, with a distinct peri-umbilical

depression. The shallow umbilical wall is steep and

slightly concave. The ribbing on the inner whorl (up to

Dw30 mm) is coarse and anguliradiate (primaries

prorsiradiate, secondaries rursiradiate), looking much

like that of Brasilia and irregularly bifurcating on the

inner third of the flank. On the body-whorl the ribbing

becomes much fainter and falciform. Towards the end of

the last whorl the ribbing gives way to fine, falcoid and

fasciculate striae.

Discussion: The described specimen combines some

features of Ludwigia and Leioceras. As already discussed,

the ribbing on the inner-whorl strongly resembles that of

Brasilia, but the lanceolate whorl cross-section and the

fine striae on the last whorl are characteristic of the

Leioceratinae, especially of the Leioceras comptum group.


Subzone.

Leioceras paucicostatum Rieber, 1963

Fig. 5(3)

*1963 Leioceras paucicostatum n. sp.—Rieber: 35, pl. 2,

figs. 3–5, 8 and 9; text-figs. 7(b), (e), (f), (k), 15(a), (b),

(f), (g), (m), (n).

2005 Leioceras paucicostatum Rieber, 1963—Seyed-

Emami et al.: 368, fig. 7(A).

Material: Four fragmentary specimens (040512-18/2–3, /6–7).



Subzone.

Leioceras crassicostatum Rieber, 1963

Fig. 5(11)

Leioceras crassicostatum n. sp.—Rieber (1963): 37, pl. 1, figs.

10–13; fig. 7, diagr. B, K; text-figs. 13(p), 15(r)–(t), 16(o)–(p).

2005 Leioceras crassicostatum Rieber, 1963—Seyed-Emami

et al.: 368, fig. 7(F) and (K).

Material: Three fragmentary specimens (040512-18/1, /5, /11).



Subzone.

Subfamily Graphoceratinae Buckman, 1905

Genus Ludwigia Bayle, 1878

Ludwigia sp.

Fig. 5(4)

Material: One fragmentary specimen (040512-16/25).

Dimensions (in mm):

Specimen D U H W

16/25 36 w25 w44 –

Description: A coarsely ribbed, relatively involute Ludwigia

with an ovate whorl cross-section and a high, distinct keel.

The umbilical border is rounded, the umbilical wall steep.

The ribbing is coarse and consists of short, rather bullate and

prorsiradiate primaries, irregularly branching into distinctly

rursiradiate secondaries. Towards the ventral part, the ribs


become slightly broader, curve forward and disappear

before reaching the keel.

Discussion: The coarse and slightly anguliradiate ribbing is

reminiscent of some coarsely ribbed Ludwigia of the L.

haugi/crassa group, such as Ludwigia (L.) crassa Horn,

figured by Rulleau and Elmi (2001: pl. 21, Fig. 3(a) and (b)).

Distribution: Probably Middle Aalenian, upper Murchiso-

nae Zone.

Genus Brasilia Buckman, 1898

Brasilia cf. bradfordensis (Buckman, 1881)

Fig. 5(9(a) and (b))

cf.*1881 Harpoceras Murchisonae, var. bradfordiense,

S. Buckman—Buckman: 604.

Material: Four small and partly fragmentary specimens

(040512-16/1a, b, /2, /20).

Dimensions (in mm):

Specimen D U H W

16/1a 23 29 44 28

Description: Moderately involute Graphoceratinae with a

high-ovate to fastigate whorl cross-section and a distinct,

sharp keel. The umbilical border is sharp, with a narrow

and vertical wall. The ribbing is relatively fine, dense and

falcoid. Usually, a slightly stronger and irregularly spaced

rib alternates with one or two finer ribs. The stronger ribs

irregularly bifurcate shortly above the umbilicus. At the

ventro–lateral border the ribs curve forward and disappear

before reaching the keel, so that a narrow, smooth and

slightly tectiform band results. The suture-line of speci-

men 16/1b is partly visible. It is simple and looks much

like that of B. bradfordensis in Rieber (1963: text-fig.

14(g)).

Discussion: the specimens from Iran can be compared to

the inner whorls of some finer ribbed specimens of B.

bradfordensis (Buckman) and B. similis (Buckman). A

very similar specimen is the juvenile specimen of B.

bradfordensis figured by Rieber (1963: pl. 5, fig. 5).

Distribution: Middle Aalenian, Bradfordensis Zone.

Genus Graphoceras Buckman, 1898

Graphoceras (Graphoceras) decorum Buckman,

1902

Fig. 5(5(a) and (b), 12(a) and (b), 18)

1888 Lioceras concavum (Sowerby)—Buckman: 56, pl. 8,

figs. 3–4.

*1902 Graphoceras decorum—Buckman: 3.

1963 Ludwigia concava var. decora (Buckman, 1909)—

Rieber: 60, pl. 7, fig. 9.

1967 Ludwigia (Graphoceras) concava decora (Buckman,

1909—Seyed-Emami: 68, pl. 2, fig. 4; pl. 8, fig. 4(a) and

(b).

1969 Graphoceras (Graphoceras) decorum S. Buckman—

Contini (1969): 67, pl. 5, fig. 10; pl. 12, figs. 4–5; pl. 14,

figs. 46–47; text-fig. 18.

2001 Graphoceras (Graphoceras) decorum Buckman—

Rulleau and Elmi: pl. 23, fig. 4(a) and (b).

Material: Twelve mostly fragmentary juvenile specimens

(040512-17/1–12).

Dimensions:

Specimen D U H W

17/5 25 24 51 –

Description: Slim, moderately involute Graphoceratinae

with high-ovate to lanceolate whorl cross-section and a

sharp keel. The umbilical border is sharp, with a shallow and

vertical umbilical wall. On larger specimens, a slight peri-

umbilical depression is indicated. The ribbing is fine, dense

and strongly falcoid. The ribs bifurcate irregularly on the

inner half of the flank. Many ribs remain undivided. The

suture line is relatively simple and graphoceratid-like.

Discussion: The forms, although exclusively juveniles,

match well with G. decorum Buckman. The ribbing of our

specimens closely resembles that of Ludwigia cornu var.

(Rieber, 1963: pl. 7, fig. 14).

Distribution: Upper Aalenian, Concavum Zone.

Family Hammatoceratidae Buckman, 1887

Subfamily Hammatoceratinae Buckman, 1887

Genus Planammatoceras Buckman, 1922

Planammatoceras aff. planinsigne (Vacek, 1886)

Fig. 6(13–16)

aff.*1886 Hammatoceras planinsigne n. sp.—Vacek: 89, pl.

13, figs. 1 (lectotype), 2–6.

aff. 1982 Planammatoceras (P.) planinsigne Vacek—

Westermann and Riccardi: 19, text-fig. 3 (photographic

reproduction of the lectotype).

aff. 1994 Planammatoceras planinsigne (Vacek, 1886)—

Callomon and Chandler: 25, pl. 4, fig. 1.

aff. 2001 Planammatoceras planinsigne (Vacek), morpho-

type dubari (Maubeuge)—Rulleau and Elmi: 56, text-

fig. 20–7; pl. 27, fig. 1.

Material: Nine mostly fragmentary specimens (040512-

15/5-6; 040512-16/7, /27, /29–30, /32–33, /40).

Dimensions (in mm):

Specimen D U H W

16/7 22 27 45 w32

16/32 31 25 48 29

16/33 35 25 46 –

Description: Moderately involute Hammatoceratinae with

high-ovate whorl cross-section and a high, narrow, hollow-

floored keel. The umbilical border is rounded, the umbilical

wall shallow and slightly overhanging. The inner whorls of

specimens 16/7 and 16/32 are partly visible. They are

evolute, depressed and carry up to a diameter of 12 mm

rather prominent and acute tubercles. Later, the forms

become distinctly involute and bear a narrow and sharp

keel. From a diameter of ca. 12 mm up to about 25 mm,


the ornamentation consists of rectiradiate and relatively

wide-spaced primaries ending in small tubercles, then

bifurcating into two slightly concave secondary ribs at

about the inner third of the flank. Usually, there is an

additional intercalated rib, beginning at the bifurcation

level. From a diameter of about 25 mm (specimen 16/33),

there are no tubercles at the bifurcation point, and the

ribbing becomes denser and consists of stronger primaries,

bifurcating into slightly finer and flexuous secondaries,

which extend up to the keel.

Discussion: Compared with the lectotype of P. planinsigne

reproduced by Westermann and Riccardi (1982: 19, Fig. 3),

the tuberculate stage of our specimens is much shorter and

confined to the inner whorls up to a diameter of about

25 mm. In this respect they resemble P. tenuinsigne

(Vacek, 1886: 88). The differences to P. planiforme

Buckman are also minimal. As already remarked by

Westermann and Riccardi (1982: 19) and Callomon and

Chandler (1994: 25) the three above mentioned species

may belong to a single bio-species, ranging from the

Scissum to Bradfordensis subzones. Two fragmentary

specimens (15/5, 15/6) from a lower level (see Fig. 4)

maintain also a longer tuberculate stage than in P.

planinsigne.

Distribution: P. planinsigne has been recorded from the

Aalenian, upper Opalinum to the Murchisonae and the

lower Bradfordensis Zone. Considering the accompanying

fauna, our specimens probably come from the Middle

Aalenian Bradfordensis Zone.

Genus Bredyia Buckman, 1910

Bredyia sp.

Fig. 6(12(a) and (b))

Material: One fragmentary specimen (040512-10).

Description: Moderately evolute Hammatoceratinae with a

broad, rectangular-ovate whorl cross-section, distinct

shoulders and a strong keel on the last preserved whorl.

The ornamentation consists of coarse ribs with rectiradiate

and rather bullate primaries, branching into two or three

slightly flexuous secondaries. On the broad venter the ribs

curve slightly forward and reach up to the keel. The inner

whorl is strongly depressed with tubercles on the mid-flank

and three or four rectiradiate secondary ribs.

Discussion: The fragmentary specimen with distinct

shoulders and a broad, rather rectangular whorl cross-

section can be compared with Bredyia alleoni (Dumortier)

in Senior (1977: pl. 83, figs. 3 and 4) and Rulleau and Elmi

(2001: pl. 25, fig. 1(a) and (b)).

Distribution: Lower Aalenian, Opalinum Zone.

Genus Accardia Cresta, 1997

Accardia diadematoides (Mayer, 1871)

Fig. 5(8(a) and (b), 16(a) and (b))

*1871 Ammonites diadematoides—Mayer: 243, pl. 8, fig. 9.

1960 Hammatoceras diadematoides Mayer sp.—Lelievre:

26, pl. 5, figs. 5–6.

1963 Hammatoceras diadematoides (Mayer, 1871)—Rie-

ber: 68, pl. 8, figs. 5–8 (figs. 5 and 7: photographic

reproduction of the holotype).

2001 Accardia diadematoides (Mayer)—Rulleau and Elmi:

pl. 31, fig. 1(a) and (b).

Material: One small and one fragment of a larger specimen

(040512-16/24, /26).

Description: Evolute and coarsely ribbed Hammatoceratinae

with broad, ogival whorl cross-section and a high, hollow-

floored keel. The maximum width of the whorl is about mid-

flank, from where it converges towards the umbilicus and

the venter. The umbilical border is rounded and transitional

towards the flank. The ribbing is coarse and consists of blunt,

rectiradiate and widely spaced primaries, ending in promi-

nent and sharp tubercles on the mid-flank. Usually three

slightly prorsiradiate secondaries branch from the tubercles,

ending straight at the keel. One or two additional intercalated

ribs, starting at the same height, are present. On the last half of

the whorls of the smaller specimen, at DZ28 mm, seven

tubercles are counted.

Discussion: The specimens from Iran agree in detail with the

specimens described and figured by Lelievre (1960) and

Rulleau and Elmi (2001). Compared to the original of Mayer

(1871), reproduced by Rieber (1963): pl. 7, Figs. 5 and 7), the

ornamentation of the specimens from Iran is slightly coarser

and wider spaced.

Distribution: According to Cresta (1997: 34; 2002: 186), the

genus Accardia is confined to the Lower and Middle

Aalenian. Rieber (1963) records A. diadematoides from the

Concavum Zone and Rulleau and Elmi (2001) from the upper

Bradfordensis to Concavum zones. The specimens from Iran

were collected together with Brasilia and other ammonites

from the Middle Aalenian Bradfordensis Zone.

Genus Ceccaites Cresta, 1997

Ceccaites cf. sieboldi (Oppel, 1862)

Fig. 6(17(a) and (b))

v. cf *1862 Ammonites Sieboldi—Oppel: 144, pl. 46, fig. 1(a)

and (b).

cf. 1997 Ceccaites sieboldi (Oppel, 1862)—Cresta: 40, fig.

10 (with synonymy).

Material: One fully septate and partly eroded phragmo-

cone from the Badamu Formation at Khorand, south-

west Ravar, central Iran (SE-92-1) (see Seyed-Emami

et al., 1993).

Dimensions (mm):

Specimen D U H W

SE-92-1 73 26 44 –

(Original of Oppel) 76 28 45 w35

Description: Moderately involute Hammatoceratinae with

suboval whorl cross-section and a high, hollow-floored keel.

The umbilical border is rounded with a vertical and slightly

overhanging wall. Maximum thickness is about the mid-

flank, from where it converges towards the umbilicus and

the venter. The ornamentation of the inner whorls consists


of prominent and sharp tubercles. On the last preserved

whorl the tubercles disappear and the rather dense and

slightly flexuous ribbing seems to be confined to the outer

two-thirds of the flank. The inner third of the flank is rather

smooth and exhibits only faint undulations. The suture line

is highly complex.

Discussion: Compared with the original of Oppel (which

was checked by K.S.-E. in the Bayer. Staatssammlung in

Munich), the specimen from Iran is slightly more

compressed and involute. Also, the tuberculate stage is

much shorter. The absence of prominent primaries on the

last whorl of the specimen from Iran may be partly due

to erosion. It must be emphasized that also on the

original specimen of Oppel, the bullate primary ribs fade

at about DZ70 mm, giving way to broad and faint

undulations.

Distribution: In Europe, C. sieboldi has been recorded

from the Middle Aalenian. The specimen from central

Iran comes from condensed Aalenian beds at Khorand

near Ravar.

?Family Erycitidae Spath, 1928

Genus Shahrudites Seyed-Emami gen. nov. [m]

Derivatio nominis: After the town Shahrud, southeast

Alborz, the nearest town to the type locality.

Type species: Shahrudites asseretoi Seyed-Emami sp.

nov. (see below).

Diagnosis: Small and evolute microconch forms with

lappets. Inner whorls depressed and broad-ovate, later

high-ovate with a faint, blunt and rudimentary keel.

Innermost whorls smooth, followed by a stage with

tuberculated ribs. The tubercles disappear shortly after

the onset of the body chamber. Subsequently, slightly

rursiradiate and convex ribs occur, single or irregularly

biplicating at the umbilical margin, or bifurcating higher

up. On the rounded venter the ribs become very faint,

turn forward and cross it in acute chevrons. Septal suture

poorly preserved, slightly complex with a broad and

rather deeply incised lateral saddle.

Discussion: The new genus does not show any

resemblance to any known microconchs of Hammatocer-

atinae or Erycitinae, to which it probably belongs. From

Rhodaniceras (Elmi, 1963), which is now considered as

possible microconch of Pseudammatoceras (Rulleau and

Elmi, 2001), and the microconch forms of Bredyia

(Senior, 1977), the new genus can be readily distin-

guished by its smaller size, spatula-like lappets and the

very week and rudimentary keel. From the microconch

forms of Spinammatoceras (Schindewolf, 1964) and

Malladaites (Linares and Sandoval, 1986) it is distin-

guished by only one, periumbilical row of tubercles and

by the ribbing pattern. Possibly, the new genus occupies

a systematic position between Spinammatoceras and

Malladaites.Many fragments of the new genus occur, together with

several fragments of Planammatoceras aff. planinsigne, in

a single concretion. The inner whorls of both genera up to a

diameter of 12 mm are evolute, depressed and tuberculated

and look much alike. In contrast to Shahrudites,

Planammatoceras aff. planinsigne has a sharp keel,

beginning at a very early stage. Shahrudites may represent

a microconch form of Planammatoceras. However, this

interpretation is questioned by the apparent endemic nature

of the new genus in contrast to the widely distributed

Planammatoceras.

Considering the accompanied fauna, the stratigraphic level

of Shahrudites is probably the Middle Aalenian Bradfor-

densis Zone.

Shahrudites asseretoi Seyed-Emami sp. nov.

Fig. 6(1–5, 7); Fig. 7(1)

Derivatio nominis: In honour of Riccardo Assereto, who

contributed much to the geology of Central Alborz and

established the Shemshak Formation.

Holotypus: A fairly complete internal mould with lappets

(040512-16/35).

Paratypus: A fairly complete internal mould with parts of

the original shell (040512-16/36).

Locus typicus: Tazareh Coal Mine, 45 km west of Shahrud,

southeastern Alborz.

Stratum typicum: Shemshak Formation, 2884 m above the

base; Bradfordensis Zone.

Diagnosis: Small, evolute microconch with rudimentary

keel and spatula-like lappets. Inner whorls depressed, outer

whorls high-oval. Inner whorls tuberculated, outer whorls

with single or biplicate ribs, which cross the rounded

venter as very faint chevrons.

Material: Seven well preserved or fragmentary specimens

(040512-16/3, /4, /34–38). Three specimens (040512-

16/23a, b, /30c) are included in S. asseretoi with doubt

(see Fig. 6(10–11)).

Dimensions (mm):

Specimen D U H W

16/34 18 44 39 36

16/36 (paratype) 21 40 36 32

16/23a 22 36 40 –

16/3 24 44 37 33

16/4 24 39 37 –

16/35 (holotype) 24 w44 w37 –

Description: The holotype (specimen 040512-16/35) is a

full-grown specimen with spatula-like apophyses and a

maximum size of about 26 mm. The body chamber begins at

a diameter of about 16 mm and takes up about 3/4 of the last

whorl. It is evolute with a rectangular to high-ovate cross-

section of the last whorl. The venter is slightly arched with a

blunt, rudimentary keel. The umbilical border is rounded,

with a low and steep wall. The innermost whorl is smooth,

later there is a tuberculated stage. On the last whorl, the

ornamentation consists of convex, rather distant and coarse,

single or biplicate ribs. On the ventral shoulder the ribs

curve slightly forward, fade almost and cross the venter as

very faint, acute chevrons.The paratype (specimen 040512-16/36) is a nearly

complete specimen with the greater part of the body


chamber preserved, beginning at a diameter of around

16 mm. The inner whorls are rather depressed, the last

whorl becomes high-ovate. The venter is arched, slightly

tectiform, with a blunt and rudimentary keel. The umbilical

border is rounded with a low, steep umbilical wall. The

ornamentation on the inner whorl consists alternatively of

tuberculated and finer, non-tuberculated ribs. Towards the

aperture there are sharp, single, convex-rursiradiate ribs,

with radial tubercles on the mid-flank, alternating with

finer ribs, which last till the end of the phragmocone. On

the body chamber the ribs become finer, the tubercles fade,

and the ribs bifurcate irregularly and indistinctly on the

middle to outer half of the flank. On the ventral shoulder,

all ribs turn forward and form very faint chevrons on the

venter. The lectotype differs from the holotype by a finer

and denser ornamentation and by the bifurcation point

being higher up on the flank.

Discussion: The new species cannot be placed with any

known coeval ammonite. It may belong somewhere in the

evolutionary lineages of Erycites, Spinammatoceras,

Malladaites, Haplopleuroceras or Zurcheria (Linares and

Sandoval, 1986, 1996). From the closely related S.

stoecklini it is distinguished by more depressed inner

whorls and a distinctly coarser and wider spaced

ornamentation. The specimens 16/23a and 16/23b

(Fig. 6(10, 11)) are extremely depressed and coarsely

ribbed varieties. Here, the rather coarse tuberculate stage of

the inner whorl is followed by short, rectiradiate primaries

ending in rounded tubercles on the inner half of the flank

and then bifurcating into convex-rursiradiate secondaries.

The tubercles vanish on the body chamber, where the

ornamentation consists of coarse and rursiradiate ribs, often

bundled at the umbilicus.


Shahrudites stoecklini Seyed-Emami sp. nov.

Fig. 6(6, 8–9, 7–2)

Derivatio nominis: In honour of Dr. Jovan Stocklin, who

contributed much to our knowledge of the geology of Iran.

Holotypus: A fairly complete specimen with parts of the

original shell (040512-16/14).

Paratypus: A well-preserved fragment with parts of the

original shell (040512-16/6).

Locus typicus: Tazareh Coal Mine, 45 km west of Shahrud,

southeastern Alborz.

Stratum typicum: Shemshak Formation, 2884 m above the

base, Bradfordensis Zone.

Diagnosis: Small, evolute forms with arched venter and

flat, blunt keel. Inner whorls rather depressed-oval, later

rectangular, high-ovate. Inner whorls with tuberculated

ribs, outer whorls subsequently with mostly single, dense,

sharp and slightly flexuous ribs, crossing the venter as very

weak chevrons.

Material: Five specimens (040512-16/5-6, /11, /14, /41).

Dimensions (mm):

Specimen D U H W

16/5 20 42 39 w32 (end of phragmocone

at around 14 mm)

16/6 (paratype) 28 41 37 29

16/14 (holotype 28 41 35 w27

Description: The holotype (040512-16/14) is an evolute and

rather compressed specimen preserved with the greater part

of the body chamber. The cross-section of the last whorl is

high-oval. The venter is arched with a faint and flat keel.

The umbilicus is shallow with a rounded border. The

ornamentation on the inner whorl consists of small tubercles

on the mid-flank. Towards the aperture, there are

rectiradiate tuberculated ribs which irregularly alternate

with finer, non-tuberculated ribs. On the last whorl the

ribbing consists of rather sharp, fine, dense and slightly

flexuous single ribs, crossing the venter as very faint

chevrons. On the shell, there are fine and fasciculating

striae. Peristome and suture line could not be seen.The paratype (040512-16/6) is half of an internal mould,

with parts of the shell on the last preserved whorl, which is

the body chamber. It is evolute with a rectangular-ovate

cross-section of the last whorl. The inner whorls are rather

depressed. The venter is broad and slightly arched, with a

very week, blunt and rudimentary keel. The ornamentation

on the inner whorls predominantly consists of rather

coarsely tuberculated ribs, irregularly alternating with

finer, non-tuberculated ribs. On the last whorl, the ribbing

is composed of dense, sharp, flexuous and slightly

rursiradiate ribs, looking much like Zurcheria or Haplo-

pleuroceras (Linares and Sandoval, 1996). At the transition

to the venter, the ribs become slightly accentuated, turn

forward, almost fade and cross the venter as very faint

chevrons. Peristome and suture line are not seen. Compared

with the holotype, the paratype is slightly more depressed,

with a broad venter and a fairly rectangular whorl cross-

section.

Discussion: S. stoecklini differs from S. asseretoi above all

by its dense, sharp and simple ribbing on the body chamber and

probably by a slightly larger end-size. Also, the inner whorls of

S. stoecklini are less depressed. The ornamentation on the inner

whorls with their distinct tubercles is much the same on both

species, so that the inner whorls up to a diameter of about

12 mm hardly can be distinguished.

The ribbing, especially on the last whorl of the paratype,

resembles much “Zurcheria” kermanensis Seyed-Emami

(1967:104, pl. 12, fig. 13(a)–(c)), which was considered as

very close to Malladaites sp. nov. I by Linares and Sandoval

(1986). Actually, the sharp ribbing with chevrons on the

venter and the rudimentary keel of S. stoecklini shows much

similarity to Malladaites sp. nov. I of Linares and Sandoval

(1986:218, pl. 1, figs. 13–15). The main differences to

Malladaites are the much coarser and unituberculated ribs

on the inner whorls of Shahrudites and especially of S.

stoecklini. Linares and Sandoval (1986:219) mention also

some coarser tuberculated specimens of their new species.


Fig. 8. Correlation of Toarcian to Aalenian ammonite-bearing successions located in the eastern Alborz Mountains (Tazareh and Jajarm area) and the northern Lut

Block (Kuh-e-Shisui); for key of symbols see Fig. 4. Map modified from Alavi et al. (1997); CEIM, Central-East Iranian Microcontinent). For more detailed

stratigraphic information on the Kuh-e-Shisui section, see Seyed-Emami et al. (2004).



aff. Shahrudites sp.

Fig. 5(2, 7, 10)

Material: Three small specimens (040412-16/22, /30D, /31).

Dimensions (mm):

Specimen D U H W

16/31 21 36 35 29 (end of phragmocon at

about 14 mm)

Description: Specimen 16/31 is a small, rather evolute and

compressed form, with a rectangular to high-ovate whorl cross-

section and a very weak, rudimentary keel. The ornamentation

consists of widely-spaced, slightly rursiradiate ribs, often

biplicating at the umbilicus. Of these pairs, the adapertural rib

is usually slightly coarser. On the venter, the ribs turn forward,

almost fade and cross the venter as very faint chevrons. The

inner whorls are not tuberculated.The suture line of specimen 16/31 is relatively simple and

much reminiscent of Graphoceratinae. The ventral lobus is as

deep as the lateral lobus. The lateral saddle is rather broad with

a flat denticulation. There appears to be a suture crowding

(“Lobendrangung”), indicating that the specimen is fully

grown.

Discussion: The ribbing pattern and the rudimentary keel of

the described forms resemble the genus Shahrudites, with

which they were found in a single concretion. From

Shahrudites they differ by a narrower umbilicus, more

depressed whorl cross-section and the absence of tubercles.

In addition, the suture line is much less complex and

graphoceratid-like.


4. Stratigraphic correlation and conclusions

In the Middle Toarcian-lower Upper Aalenian part of the

Tazareh section, nine ammonite levels can be recognized

(Fig. 4, Table 1). Apart from the representatives of the new

genus Shahrudites, the ammonite fauna from the Shemshak

Formation at Tazareh is exclusively Tethyan in character and

closely related to faunas from western and central Europe,

allowing a correlation with the NW European standard

ammonite zonation (Elmi et al., 1997; Contini et al., 1997;

cf. Fig. 2). The ammonite levels allow to recognize the

(?Lower-) Middle Toarcian, the Upper Toarcian Thouarsense

and Aalensis zones as well as the Lower Aalenian Opalinum

Zone, the Middle Aalenian Murchisonae and Bradfordensis

zones, and the Upper Aalenian Concavum Zone.

At Tazareh and elsewhere in the Alborz Mountains (e.g.

Seyed-Emami et al., 2005) as well as in Central Iran, the

Middle Toarcian is documented by a rather poor fauna of

Hildoceratinae. In the Upper Toarcian, besides a few speci-

mens of Hammatoceratinae, Grammoceratinae and Dumortier-

iinae are by far the dominant faunal elements. In the lower part

of the Upper Toarcian, the fauna consists almost exclusively of

the genera Grammoceras, Pseudogrammoceras and Podagro-

sites, whereas representatives of the genus Pleydellia

predominate in the Aalensis Zone. Everywhere in north and

central Iran, the Lower Aalenian contains a rather rich fauna of

Leioceratinae and few Hammatoceratinae.

An ammonite-based correlation of the Toarcian-Aalenian of

the Shemshak Group at Tazareh with that of the Jajarm area

(eastern Alborz; Seyed-Emami et al., 2005) and with the Kuh-

e-Shisui section of the Lut Block further south on the Central-

East Iranian Microcontinent (CEIM; Seyed-Emami et al.,

2004) is presented in Fig. 8. This WSW–ENE–SSE correlation

over more than 600 km shows several interesting relationships:

During the Early Jurassic (Tenuicostatum and Serpentinus

zones), fully marine conditions were established on the Lut

Block, whereas in the eastern Alborz, fluvial to at the most

coastal plain sedimentation prevailed. This early Toarcian

transgression is well documented also further west of Tazareh,

at Shahmirzad (central Alborz), by ammonites of the families

Dactylioceratidae and Hildoceratidae (Harpoceratinae; see

Repin, 1987, 2000). In fact, the earliest transgressions within

the Shemshak Formation occurred at Shahmirzad in

the Sinemurian and Upper Pliensbachian (Nabavi and

Seyed-Mami, 1977; Seyed-Emami, 1977). With the Middle

Toarcian (Bifrons Zone at Jajarm), marine environments

were also established in the eastern Alborz, followed by

a continuous deepening associated with high accumulation

rates (amounting to very high subsidence rates; see Fursich

et al., 2005) during the Upper Toarcian to Aalenian (especially

the Tazareh section is very expanded in this interval). In

contrast to that development, the Lut Block succession shows

regressive tendencies (sandstones) and strongly reduced

thicknesses throughout the Middle and Upper Toarcian and

especially during the the Aalenian. In the uppermost Aalenian–

Early Bajocian, a renewed retrogradational trend is recorded on

the Lut Block by increasingly finer and deeper marine

sediments (marly silts), whereas in the Alborz, the prograda-

tional sedimentation pattern indicate a gradual infilling of the

Alborz trough (Fig. 8). These reversed patterns in relative sea-

level changes, thicknesses and sedimentation across the Iran

Plate strongly argue for a significant impact of synsedimentary

tectonics during the deposition of the upper Shemshak

Formation.

Acknowledgements

We thank Prof. Dr M. Gaetani and Prof. Dr S. Cresta for their

insightful reviews of the manuscript. The present study is part of a

joint research programme of Tehran University (Centre of

Intelligence), the Institute of Palaeontology of Wurzburg

University, the UPMC, Paris, and the Geological Survey of

Iran. We acknowledge financial support by the Alexander von

Humboldt-Foundation (within the framework of an institutional

partnership between Wurzburg and Tehran University) and the

Middle East Basin Evolution Programme (MEBE, project 02–

03). We also thank Eng. Sedighifar, the head of the East Alborz

Coal Company, for his hospitality and logistic help during the

field work. K. S.-E thanks Prof. Dr R. Leinfelder, head of the

Palaeontological Institute of Munich University, for providing


research facilities during his stay in Munich and G. Janssen

(Munich) for his excellent photographic work.

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