Sedimentology and stratigraphy of Geli Khana Formation (Anisian-Ladinian), a contourite depositional system in the northeastern passive margin of Arabian plate, northern Iraq-Kurdistan region

摘要

The Middle Triassic Geli Khana Formation of the northeastern part of the Arabian plate marks the establishment of the Neo-Tethys passive margin. The indication of bottom-current activities, within the lower and middle parts of the formation, gives the opportunity to study Middle Triassic facies and depositional settings in northern Iraq. Three sections (two outcrops and one subsurface) were selected to study the sedimentology and stratigraphy of Geli Khana succession. Petrographic investigations of the carbonate and siliciclastic beds on 140 thin sections show both skeletal and non-skeletal grains. The skeletal grains reveal deposition in deep open marine and in shallow warm water, within a gently slope ramp setting. Twelve microfacies were recognized. In the northern thrust zone, these facies were subdivided, according to their environmental interpretation, into three basic types of facies associations: outer ramp/basinal, middle ramp/slope, and inner ramp/lagoon (open and restricted). Restricted lagoon and tidal flat facies association is suggested for the Geli Khana Formation in Well Jabal Kand-1. Typical contourite deposits associated with turbidites are recognized for the first time in the Middle Triassic Geli Khana Formation in the northern thrust zone, northern Iraq, Kurdistan region. The contourites are characterized by thin beds and occasional lenses of sandy limestones, siltstones to fine-grained sandstones with current ripples, laminations (planar and cross), and erosional surfaces. These current structures are associated with thinbedded (5-25 cm) limestones and shales. Deformation structures are characteristic feature of the formation inferring syndepositional slumping and turbidite influence too.