Depositional Controls on the Ichnology of Ordovician Wave-dominated Marine Facies: New Evidence from the Shirgesht Formation, Central Iran |
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| Authors: | Aram BAYET-GOLL Paul M MYROW Guillermo F ACE?OLAZA Reza MOUSSAVI-HARAMI and Asadollah MAHBOUBI |
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| Institution: | 1 Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 451951159, Zanjan, Iran;2 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Iran,3 Department of Geology, The Colorado College, 14 E. Cache La Poudre, Colorado Springs, Colorado,80903, U.S.A;,4 INSUGEO (Instituto Superior de Correlación Geológica), CONICET - National University of Tucumán, Argentina,2 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Iran and 2 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Iran |
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| Abstract: | The Lower Ordovician Shirgesht Formation in central Iran is composed of siliciclastic and carbonate rocks deposited in diverse coastal and marine shelfal environments (tidal flat, lagoon, shoreface, offshore-shelf and carbonate ramp). Five facies associations contain diverse ichnofossil assemblages that show distinct proximal to distal trends formed in a wide range of physical-chemical conditions. The ethological groups of trace fossils in the Shirgesht Formation reflect a gradient of depositional stress conditions across a wave-influenced shoreline and shelf. Deposits of wave-influenced environments make up a significant component of the geological record of shallow marine settings, and the ability to determine paleoenvironments in detail in such successions is critical for reconstruction of depositional histories and sequence-stratigraphic interpretation.The Cruziana ichnofacies of the study shows highly diverse suites that record the establishment of a benthic community under stable conditions and a long-term colonization window. The Skolithos ichnofacies recognized is a low diversity opportunistic ichnocommunity suite that resulted from colonization after tempestite deposition in a stressed environment. The strata record an onshore to offshore replacement of the Cruziana ichnofacies (with abundant feeding traces of deposit-feeders) by the Skolithos ichnofacies (dominated by suspension-feeders and predators). A transitional zone between the two ichnofacies coincides with the offshore-transition/distal lower-surface deposits. The distribution of ichnofacies, the diversity and range of ethological characteristics reflected by the ichnogenera, and the wide range of wave-dominated coastal facies demonstrate the potential to use individual trace fossils and ichnofacies for significantly refined palaeoenvironmental analysis of wave-dominated coastal settings, particularly in Ordovician successions. |
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| Keywords: | Shirgesht Formation Ordovician facies associations sedimentology iconology ichnofacies |
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