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The Mid-Cimmerian tectonic event (Bajocian) in the Alborz Mountains, Northern Iran: Evidence of the break-up unconformity of the South Caspian Basin

By
Franz Theodor Fürsich
Franz Theodor Fürsich
1
Geozentrum Nordbayern der Universität Erlangen-Nürnberg
,
Fachgruppe PaläoUmwelt
,
Loewenichstrasse 28, D-91054 Erlangen
,
Germany
(e-mail: markus.wilmsen@senckenberg.de)
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Markus Wilmsen
Markus Wilmsen
1
Geozentrum Nordbayern der Universität Erlangen-Nürnberg
,
Fachgruppe PaläoUmwelt
,
Loewenichstrasse 28, D-91054 Erlangen
,
Germany
(e-mail: markus.wilmsen@senckenberg.de)
2
Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Sektion Paläozoologie
,
Königsbrücker Landstrasse 159, D-01109 Dresden
,
Germany
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Kazem Seyed-Emami
Kazem Seyed-Emami
3
School of Mining Engineering, University College of Engineering, University of Tehran
,
P.O. Box 11365-4563
,
Iran
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Mahmoud Reza Majidifard
Mahmoud Reza Majidifard
4
Geological Survey of Iran
,
P.O. Box 131851-1494, Tehran
,
Iran
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Published:
January 01, 2009

Abstract

The Mid-Cimmerian tectonic event of Bajocian age can be documented all across the Iran Plate (Alborz Mountains of northern Iran, NE Iran, east-central Iran) and the southern Koppeh Dagh (northeastern Iran). In the Alborz area, the tectonic event consisted of two main pulses. A distinct unconformity (near the Lower–Upper Bajocian boundary) at or near the base of the Dansirit Formation is the sedimentary expression of rapid basin shallowing due to uplift and erosion. Another unconformity is developed in the early Upper Bajocian, close to or at the top of the Dansirit Formation. Locally, it is expressed as an angular unconformity due to block rotation and is overlain by a thin transgressive conglomerate followed by silty marls of the deep-marine Upper Bajocian–Callovian Dalichai Formation. This upper unconformity signals a rapid subsidence pulse. On the Tabas Block of east-central Iran, a single unconformity can be documented that is time-equivalent to those bounding the Dansirit Formation (i.e. ‘mid-Bajocian’). Local folding gives direct evidence of compressional tectonics, and conglomerates indicate subaerial denudation of older Mesozoic or Palaeozoic strata. After a stratigraphic gap, transgressive sediments of ?Late Bajocian–Bathonian age follow, suggesting a fusion of the lower and upper Mid-Cimmerian unconformities in east-central Iran. Along the southern margin of the Koppeh Dagh Mountains (NE Iran), a Late Bajocian subsidence pulse initiated the opening of the strongly subsiding Kashafrud Basin, an eastwards extension of the South Caspian Basin. In all of these areas, one phase of uplift and erosion took place followed by a pronounced pulse of subsidence running counter to trends of the eustatic sea-level curve. Thus, what is generally understood as the Mid-Cimmerian tectonic event is now thought to consist of a tectonic phase, confined to the Bajocian. This phase is explained as the expression of the onset of sea-floor spreading within the South Caspian Basin situated to the north of the present-day Alborz Mountains. This strongly subsiding basin developed close to the Palaeotethys suture during the Toarcian–Aalenian and went through a change from the rifting- to the spreading-stage during the Bajocian. The Mid-Cimmerian event therefore reflects the break-up unconformity of the South Caspian Basin.

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Contents

Geological Society, London, Special Publications

South Caspian to Central Iran Basins

M.-F. Brunet
M.-F. Brunet
CNRS-INSU and Université Pierre et Marie Curie, France
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M. Wilmsen
M. Wilmsen
Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Germany
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J. W. Granath
J. W. Granath
Granath & Associates Consulting Geology, USA
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Geological Society of London
Volume
312
ISBN electronic:
9781862395602
Publication date:
January 01, 2009

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