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The upper Middle Jurassic to lowermost Cretaceous Shal Formation, northwest Iran: ammonite faunas, facies development and geodynamic implications

Middle–Upper Jurassic strata and ammonites from Iran are fairly well-known from the Alborz Mountains of northern Iran, the Koppeh-Dagh in northeast Iran and the thick successions on the Tabas and Lut blocks in Central Iran. However, very few data are available from the Talesh Mountains in northwest Iran where the condensed succession of the Shal Formation respresents the higher part of the Jurassic sequence. Thus, we conducted an integrated study of the Shal Formation in order to provide detailed biostratigraphic data, to characterize its depositional environment by means of (micro-)facies analysis and to place the succession in a geodynamic framework. The Shal Formation rests erosionally along the Mid-Cimmerian unconformity on siliciclastic strata of the Shemshak Group and consists of reddish, nodular bioclastic float- and packstones with abundant ammonites and filaments (rosso ammonitico facies). According to the ammonite faunas, deposition of the Shal Formation started in the upper Bajocian and continued into the early Berriasian. The ammonite association with common Lyto- and Phylloceratidae indicates offshore and deeper marine environments, supported by the rosso ammonitico-type facies that commonly characterizes condensed, current-swept deep-water environments. The deposition of the Shal Formation occurred on the southern rifted shelf of the South Caspian Basin (SCB) that experienced considerable extension and thermo-tectonic subsidence in the aftermath of the Mid-Cimmerian Event. Shallow-water deposition prevailed in the southwest (Lar Formation) while towards the northeast, into the SCB, the Shal Formation characterized condensed deep-water environments on submarine swells, potentially represented by crests of submerged fault blocks.

Details

Author
Markus Wilmsen1, Mahmoud Reza Majidifard2
Institutionen
1Senckenberg Naturhistorische Sammlungen Dresden, Germany; 2Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran
Veranstaltung
GeoSaxonia 2024
Datum
2024
DOI
10.48380/7hze-0q44