Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Gurpi Formation; A Transgressive or Regressive Unit?

Document Type : Original Research Paper

Authors
Elaheh Zarei
Ebrahim Ghasemi-Nejad
Department of Geology, University College of Sciences, University of Tehran, Tehran, Iran
Abstract
       Palynological and foraminiferal contents of 30 samples taken systematically from Gurpi Formation at its type section were studied in order to determine water-level fluctuations and the general trend of water-level during the depositional course of the formation.  Statistical studies on foraminiferal contents of the samples show that the planktonic to benthonic ratio (P/B) and diversity decrease while benthonic foraminifera with larger size and epicontinental sea fauna (ESF) and shallow water fauna (SWF) increase in general towards the upper parts of the formation. 
On the other hand, dinoflagellate cyst abundance and diversity show an increasing trend which looks like a contradiction at the first glance.  To describe this apparent controversy and determine the water level trend, changes in composition of palynomorphs and the organic matter preservation state (lability factor) was studied.  Statistical studies show that in the lower parts of the section the lability is low and the ratio of structure-less organic matter (SOM) to marine palynomorph is high.  This indicates a low state of preservation of organic matter for the lower portion.  In the upper parts lability is high while the ratio of SOM to marine palynomorph is low which indicate a better state of preservation for the upper parts. 
Statistical studies on the Gonyaulacoid dinocysts which are eurytopic forms and indices for open marine, inner neritic and outer neritic environments confirm the results gained from the foraminiferal studies.  Combining all these data and comparing them with the well-known Tyson palynofacies analysis diagrams confirm a relatively deep basinal condition for the lower part and a relatively shallower and proximal shelf environmental condition for the upper part of the Gurpi Formation.
Keywords
Gurpi
Foraminifera
Dinoflagellate
Organic matters
Transgressive
Regressive

Subjects

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Scientific Quarterly Journal of Geosciences
Volume 16, Issue 61
Autumn 2006, Vol. 16, No.61
Autumn 2006
Pages 188-199