Abstract
Three outcrop sections for the Eocene- Oligocene boundary have been studied in Sinjar anticline northwestern Iraq. This study includes the lithostratigraphy, biostratigraphy and the identification of trace fossils and related ichnofacies in order to report the geological history of this boundary in the frame of sequence stratigraphy. Jaddala Formation, which is composed of marl and marly limestone, represents deep (outer shelf-upper bathyal) Eocene facies, Palani and Tarjil formations represent deposition in deep basinal environments during Early and Late Oligocene sequences. The Eocene- Oligocene boundary in Sinjar area represents a disconformity surface, where the deposition ceased in the Middle Eocene (Lutetian) at the end of Jaddala Formation, forming the firmground Glossifungites Ichnofacies which embrace Thalassinoides ichnotaxa in Jaddala section, and the hardground Trypanites Ichnofacies in Sharafiddin section, while at Sinjar section, a chalky limestone bed deposited at the Early Oligocene (Rupelian) Palani Formation representing Skolithos Ichnofacies. A rapid wide transgression took place at the end of Early Oligocene (Rupelian) leading to the deposition of Palani Formation, and continued through the Late Oligocene (Chattian) leading to the deposition of Tarjil Formation.
Highlights
A series of geological events has been occurred at the Eocene / Oligocene boundary on a worldwide scale, there is an important regression explaining that continuous sections are rare during this boundary, the Eocene/Oligocene boundary in Europe, which is marked by major discontinuities in all environments (Van Couvering et al, 1981)
The Eocene /Oligocene boundary at Sinjar area is a discontinuity surface located between Jaddala Formation of Early – Middle Eocene age and the overlying Palani Formation of Early Oligocene age and Tarjil Formation of Late Oligocene age (Al-Rubai, 2017 and Al-Rubai and Al- Mutwali, 2020)
The disconformity Eocene-Oligocene sequence boundary surface in Jaddala section is overlain by a low thickness conglomerate lag deposits bed (30 cm thick), which represents the erosion products that accumulate during Early Oligocene age at the lower part of Palani Formation, representing the lowstand system tract (LST) of the subsequent new sequence that started at the Early Oligocene (Al-Banna and AlMutwali, 2002)
Summary
A series of geological events has been occurred at the Eocene / Oligocene boundary on a worldwide scale, there is an important regression explaining that continuous sections are rare during this boundary, the Eocene/Oligocene boundary in Europe, which is marked by major discontinuities in all environments (Van Couvering et al, 1981). The Eocene /Oligocene boundary at Sinjar area is a discontinuity surface located between Jaddala Formation of Early – Middle Eocene age and the overlying Palani Formation of Early Oligocene age and Tarjil Formation of Late Oligocene age (Al-Rubai, 2017 and Al-Rubai and Al- Mutwali, 2020). The purpose of this paper is: (1) to give a detailed lithological description to the exposed studied sections of Eocene-Oligocene boundary, (2) to discuss the mechanisms responsible for trace fossils formation and types of ichnofacies that appear during this boundary with emphasis on erosional surfaces and associated Thalassinoides dominated firm ground; and (3) applying of these trace fossils and substrate-depending ichnofacies in sequence stratigraphic interpretation especially in the delineation of this sequence boundary
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