NDLI: The upper Shemshak Formation (Toarcian–Aalenian) of the Eastern Alborz (Iran): Biota and palaeoenvironments during a transgressive

Content Provider	Springer Nature Link
Author	Fürsich, Franz T. Wilmsen, Markus Seyed Emami, Kazem Cecca, Fabrizio Majidifard, Mahmoud R.
Copyright Year	2005
Abstract	The siliciclastic, up to 4,000 m thick Upper Triassic–Bajocian Shemshak Formation is widespread across the Iran Plate, especially in the Alborz Mountains of northern Iran. In contrast to its lower, generally non-marine part, the upper part is marine. Based on the Tazareh section of the eastern Alborz, an integrated analysis of this marine interval is presented. The 1,700 m thick marine sedimentary succession records a gradual deepening from inner to mid and outer shelf environments from the Middle Toarcian to early Late Aalenian. During the Late Aalenian–Early Bajocian, the trend was reversed and infilling of the basin by a large delta system occurred. This general facies development reflects a nearly symmetrical transgressive–regressive (T–R) megacycle, terminated by the inter-regional mid-Cimmerian tectonic event. A renewed transgression in the early Late Bajocian initiated a subsequent sedimentary megacycle. The bioturbated mid and outer shelf sediments contain a low to moderately diverse benthic fauna dominated by deep burrowing bivalves, often preserved in the growth position. A hierarchy of four orders of sedimentary cycles can be recognized (parasequences, parasequence sets, unconformity-bounded third-order depositional sequences, and the 13 ma long second-order T–R megacycle). A regional correlation with the Jajarm area (200 km to the east) shows a very similar temporal facies pattern of the upper Shemshak Formation. The eastern Alborz T–R cycle is completely out-of-phase with other (eustatic) sea-level curves, suggesting regional tectonic control. Rough estimates of subsidence rates give an average value of 126 m/ma. However, much higher values for the Aalenian (230 m/ma), particularly the Late Aalenian (700 m/ma), indicate a distinct increase in subsidence rate towards the Early Bajocian mid-Cimmerian tectonic event. These high subsidence rates suggest that the sediments of the Shemshak Formation of the eastern Alborz formed in a (young) rift basin.
Starting Page	365
Ending Page	384
Page Count	20
File Format	PDF
ISSN	01729179
Journal	Facies
Volume Number	51
Issue Number	1-4
e-ISSN	16124820
Language	English
Publisher	Springer-Verlag
Publisher Date	2005-06-04
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Jurassic Shemshak Formation Alborz Mountains Iran Facies analysis
Content Type	Text
Resource Type	Article
Subject	Geology Stratigraphy Paleontology

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