Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Geology - Faculty of Science

Abstract

The Kerman-Tabas region of East-Central Iran contains the thickest and most complete sequence of Early Palaeozoic (Cambrian to Silurian) rocks in Iran and the Middle East, but the stratigraphy is complex.

Detailed reassessment of stratigraphic relationships between the Early Palaeozoic strata, together with new sedimentological, petrological and palaeontological data, indicate that the Kerman-Tabas region was tectonically active during this period, not a passive Palaeo-Tethyan shallow marine platform facies as previously suggested. A rift model, involving horst and graben formation, has been invoked to account for the sedimentation patterns in the Late Precambrian to Silurian sequences of the study area. Four phases of extensional (rift) basin development are recognised here, each bounded by a major sequence boundary, with Type I unconformities occurring at the base of: the Rizu Formation; the Dahu Group; the Katkoyeh Formation; and the Shabdjereh and Niur Formations.

Deposition of the Late Precambrian Morad Formation was followed by a period of upwarping and erosion giving an unconformable, but locally faulted, boundary with the overlying Rizu Formation. Bimodal volcanism in the lagoonal to lacustrine Rizu Formation indicates the initiation of rifting. The overlying Ravar and Banestan Formations were deposited in marginal to shallow marine conditions. A scarcity of diagnostic fossils precludes precise location of the Precambrian-Cambrian boundary in the study area but supratidal to intertidal conditions in the Upper Dolomite Member of the Soltanieh Formation and in the Barut Formation indicate a low stand sea-level in the Early Cambrian. Cambrian strata in the Kerman region, assigned to the Dahu Group (redefined), include the Zaigun Formation, Mohammad Abad Sandstone (new name), Lalun Formation (redefined) and Khoram Abad Sandstone (new name). Lake facies in the Zaigun Formation and fluvial sequences in the Mohammad Abad Sandstone followed extensional tectonic activity and inter-continental basin deposition. Different palaeocurrent trends E and W of the Kuhbanan Fault indicate that this fault was active during deposition. A transgression in the late Early Cambrian formed a ravinement surface at the base of the Lalun Formation overlain by aeolian and beach sandstone. This was followed by supratidal and intertidal facies in the Lalun Formation and a barrier island system in the Khoram Abad Sandstone, indicating progradation and a relatively high sediment input. The Kuhbanan Formation (maximum flooding surface) represents a deeper water facies in the Kerman area in southern Iran than the time equivalent lagoonal facies in the Kalshaneh Formation near Tabas and the supratidal-intertidal facies in Member 1 of the Mila Formation in the Alborz Mountains. Late Cambrian regressive, shallowing-upwards carbonate facies accumulated in the Kerman region, while relatively deeper water facies characterised the Derenjal Formation in the Tabas area and the Mila Formation in the Alborz Mountains.

New palaeontological data indicate that the previously poorly defined Cambrian-Ordovician boundary is located in the uppermost carbonate facies of the Derenjal Formation at Dahaneh-eKolut, Tabas area. Intense Early Ordovician subsidence in the Tabas region led to deposition of the thick siliciclastic and carbonate facies of the Tremadoc Shirgesht Formation. compared with the thin carbonate facies of Member 5 of the Mila Formation in the Alborz Mountains. The Katkoyeh Formation is proposed to accommodate the Ordovician of the Kerman area. Where it contains Arenig to Ashgill faunas including the graptolite Dictyonema ghodsiae Rickards et at. 1994; Arenig trilobites are reported for the first time from the Kalmard region from this formation. Pillow basalt of continental origin in the basal Katkoyeh Formation indicates a period of extensional rifting. The Early Arenig transgression of this formation in the Kalmard and Kerman regions, and probably into the Alborz Mountains (Lashkerak formation), may indicate contemporaneous rift development and a high stand sea-level. Transgressive-regressive sequences in the upper Katkoyeh Formation near Kerman indicate sea-level fluctuations.

This Early Palaeozoic extensional phase led to deposition of the relatively thick Shabdjereh Formation near Kerman, and terminated in the Early Silurian. The laterally equivalent Niur Formation at Dahaneh-e-Kolut is faulted against the Shirgesht Formation; elsewhere it has a disconforrnable contact with the underlying Cambrian Derenjal Formation Extrusion of pillow basalt in the lower Niur Formation can be traced into the NE Alborz Mountains. A basal red bed sequence containing hyaloclastite facies is described for the first time. and a barrier island system occurs in the lower part of the Niur Formation. Two non-marine horizons and several polymictic conglomerate lenses highlight absence of deep marine facies in the Shabdjereh Formation in the Kerman region.

Eustatic sea level changes recorded in the studied sequences are: high stands in the late Early Cambrian and the Early Silurian; a global transgression in the Tremadoc; a further rise in the Arenig; and a regression at the end of the Ordovician.

Provenance studies indicate a major cratonic source for the study area in the Early Palaeozoic with periodic detritus from nearby volcanic sources. Throughout the Early Palaeozoic palaeo-relief in the Kerman area was greater than that in the Tabas area.

A warm, low latitude climate during the Early Palaeozoic of Iran is indicated by red beds, evaporates, mudcracks, salt and gypsum pseudomorphs, caliche and stromatolites seen to varying degrees throughout the Cambrian-Silurian succession. This contrasts with the glacial deposits in the Late Ordovician-Silurian strata of the Arabian Peninsula.

The study area and the Arabian Peninsula have a similar Late Precambrian and Cambrian tectonic style. Different Ordovician and Silurian faunas, palaeoclimates and tectonic conditions distinguish East-Central Iran from Saudi Arabia, but there was a close relationship between Iran, Afghanistan and Turkey.

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