A diverse ichnofauna from Eocene‐Miocene rocks of the Makran Range (S.E. Iran)

Abstract

A diverse assemblage of trace fossils has been collected from Lower Eocene to Upper Miocene strata during regional geological mapping of 75,000 km2 of the Makran Range of southeastern Iran. Almost all are excellently preserved and this allows detailed descriptions, which in some cases can be used as a basis for taxonomic revisions of much used but inadequately analysed ichnogenera. 27 ichnogenera have been identified and 25 ichnospecies described. These ichnotaxa include forms which are: straight, un‐branched (Planolites), straight, unbranched but twisted around a horizontal axis (Helicorhaphe), straight, branched (Chondrites, Palaeophycus, Thalassinoides), winding unbranched (Scolicia, Subphyllochorda, Taphrhelminthopsis), meandering or winding branched (Acanthorphaphe), spirals (Spirorhaphe, Spirophycus), networks (Desmograpton, Megagrapton, Paleodictyon, Urohelminthoida), rosettes (Glockerichnus, Lorenzinia) and spreite (Phycosiphon). There are also pellet lined burrows (Ophiomorpha) and rare unbranched vertical burrows (Skolithos). Descriptions are given of some 25 ichnospecies but three ichnogenera (Nereites, Paleodictyon, Urohelminthoida), are subjected to more detailed treatment, with taxonomic revision. All examples of Nereites with a sinuous or winding habit are placed in N. jacksoni Emmons 1844, to separate them from those with tight meanders and spirals for which N. cambrensis Murchison 1839 and N. macleayii Murchison 1839 are available. 6 ichnospecies of Paleodictyon are described and it is recognised that there is considerable gradation in mesh and string size, with significant overlap in the dimensions of these parameters between some ichnospecies where they are used as the main or sole diagnostic criteria. Consequently, the diagnosis of P. carpathicum (Matyasovsky, 1878) is emended to include all regular equidimensional to elongate hexagonal meshes with maximum length 3–10 mm and string diameter 0.1–2.5 mm. As a result, some examples of the following are placed in synonomy with it: P. majus Mene‐ghini 1879, P. miocenicum Sacco 1886, P. regulare Sacco 1886, Glenodictyon caucasicum Papp 1910, P. chattoni Pugin 1955 and P. carpaticum Vialov and Golev 1964. A detailed analysis shows no consistent difference between Urohelminthoida appendiculata (Heer 1877) and U. dertonensis Sacco 1888, so they are united under the former, which has priority, and an emended diagnosis is given. The sequences containing the trace fossils occur in a series of tectonic slices bounded by reverse faults. Stratigraphic position and correlation between these slices is based on detailed paleontological and, more particularly, micropaleontological determinations. The oldest sequence, comprising the Zaboli Unit is Eocene in age and shows a transition from trace fossil barren shelf carbonates to deep water turbidite sandstones and shales containing: Acanthorhaphe, Desmograpton, Helminthoida, Lorenzinia, Paleodictyon, Spirophycus, Spirorhaphe, Taphrhelminthopsis and Urohelminthoida. The Darban, Mosri, Shirinzad, Ruk, Angohran and Shahr Pum units vary in age within the range Middle Eocene to Lower Miocene and consist of sandstone‐shale turbidite sequences thousands of metres thick and have together yielded the following ichnogenera: Chondrites, Cosmorhaphe, Desmograpton, Gordia, Helicorhaphe, Helminthoida, Lorenzinia, Megagrapton, Paleodictyon, Phycosiphon, Scolicia, Spirorhaphe, Subphyllochorda and Urohelminthoida. The Mashkid, Roksha, Jarut, Ghasr Ghand and Band‐e Chaker units vary in age within the range Upper Eocene to Upper Miocene and comprise deeper water turbidite sequences passing upwards into clastic shelf deposits including deltaic, fluvialite, swamp and emergent facies. The turbidites contain: Paleodictyon, Spirorhaphe, Subphyllochorda and Taphrhelminthopsis, while the shallow water sandstones have: Chondrites, Glockerichnus, Ophiomorpha, Palaeo‐phycus, Planolites, Skolithos and Thalassinoides. The Jaghin and Sahan Tang units are of Miocene age and consist of shallow water clastic deltaic sequences with Nereites, Palaeophycus, Planolites and Skolithos. The total number of ichnogenera now known from Miocene turbidites is at least 35 and many of these have also been recorded from the present‐day deep oceans. This suggests that, with further research, trace fossils will probably prove to be at least as abundant and diverse in such environments in the Neogene and Quaternary as in the Paleogene. The occurrence of many forms in the present day oceans should allow deductions on the producers of the more complex and distinctive traces which will be applicable to Tertiary and older examples.