LASEMI, YAGHOOB, Teacher's Training University, Department of Geology, Tehran, Iran; and MOSTAFA GHOMASHI, Sistan-Baluchestan University, Department of Geology, Zahedan, Iran

ABSTRACT: Carbonate Ramp Deposits of the Lower-Middle-Triassic Elika Formation in the Bibi Shahrbanou Area, Northern Iran, Facies, Paleoenvironments, and Sequences

The Elika Formation was deposited in an extensive carbonate platform under an arid climate in Central and Northern Iran. It is underlain by the Middle Permian Ruteh Formation, with a distinct paleokarstic horizon in most places. The formation is overlain, by the fluvial/deltaic deposits of the Lower Jurassic Shemshak Formation.

Detailed field and petrographic investigations of over 1300 meter caronate sequence of the Elika Formation in the Central Elborz Range, in Northern Iran, led to the recognition of several shallow marine (tidal flat, subtidal lagoon, and open marine) facies. These facies include, the heavily bioturbated lime mudstone and the bioclast/ooid wackestone-grains tone of the open marine, the ooid, peloid, intraclast, bioclast mudstone-grains tone of the restricted lagoon, and the stromatolite bounds tone to thin bedded, laminated lime mudstone, with gypsum and anhydrite casts and fenestral fabric of the tidal flat subenvironment. Most of these Triassic carbonates are dolomitized with both sabka type and secondary dolomite distinguished.

A relative sea level rise resulted in the deposition of the transgressive Lower Triassic carbonates over a post Permian paleokarstic horizon. The vertical sequence of the Elika Formation indicates a major shallowing upward cycle. Superimposed on this cycle are numerous small scale shallowing upward cycles. These parasequences are of the mudstone-grainstone facies of the open marine sub-environment in the lower part, and the lagoonal-tidal flat facies towards the upper part of the sequence. A sudden drop of relative sea level at the end of the Middle Triassic resulted in the emergence of the platform and the development of a widespread paleosol horizon. Subsequent Liassic rifting resulted in the deposition of the fluvial/deltaic sequence of the Lower Jurassic Shemshak Formation.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.