Facies Analysis and Depositional Environments of the Ubaid Formation, Western Iraq

Noor T. Altaee,Zaid A. Malak

doi:10.24996/ijs.2021.62.8.11

Abstract

The Early Jurassic (Liassic) sequence crops out in numerous anticlines of the high folded zone of north and north-east Iraq and in the Rutba subzone (including Ubaid Formation) in west Iraq. The present study deals with siliciclastic / carbonate rocks of the 58 m-thick Ubaid Formation at Zor Hauran valley in south western Iraq. The formation consists of two parts; the lower part is composed of pebbly coarse sandstone and greenish to yellowish soft marl alternated with marly dolostone, while the upper part is characterized by light brown, well bedded dolostone, with stromatolite structure in some locations. Oval, light to dark brown nodules of chert are also present. A detailed field lithological description and facies analysis of the Ubaid Formation were performed for thin sections. It is composed of five main microfacies; dolomitized mudstone, dolomitized bioclastic wackstone, dolomitized pelloidal packstone, dolomitized oolitic grainstone, and bindstone, in addition to two lithofacies: marl and pebbly sandstone. These facies reflect the deposition from the environment of the lower supratidal to intertidal zone and the encompassing shallow scaffold secured shoal - marine environment with cautious water circulation.

Highlights

The late Permian - Liassic Megasequence (Ap 6) was deposited on the north and east sides of the inactive Arab plate margin
The unconformity at the megasequence's lowest tough Ap6 is followed by a revived split that has occurred within that passive margin during the Mid-Late Triassic period where a highly confined intra-shelf catchment was found in the Mesopotamian
The Megasequence AP6 consists of three significant second order series, as follows [1]: the Late Permian – Middle Triassic, Late Triassic, and Early Jurassic

Summary

Introduction

The late Permian - Liassic Megasequence (Ap 6) was deposited on the north and east sides of the inactive Arab plate margin. The unconformity at the megasequence's lowest tough Ap6 is followed by a revived split that has occurred within that passive margin during the Mid-Late Triassic period where a highly confined intra-shelf catchment was found in the Mesopotamian. This rifting stage was followed by a slow thermal decline in the Norian–Liassic Period (Figure-1, A) [1]. The Early Jurassic (Liassic) sequence outcrops in numerous anticlines of the high folded region of northeastern Iraq and in the Rutba subzone in the west of Iraq. The repeating toward the Mesopotamia is characterized by a sequence of inner shelf carbonate and sabkha facies [1]

Objectives

Findings

Discussion

Conclusion