Sequence-stratigraphic framework of Lower Triassic Beduh Formation, Northern Thrust Zone, Kurdistan, Iraq

摘要

The Beduh Formation (Lower Triassic) is a rhythmic succession of mudstones and fine-grained to very fine grained sandstones, cropping out in the Northern Thrust Zone, Kurdistan, Iraq. This lithostratigraphic unit is traditionally interpreted as offshore to lower-shoreface facies of a homoclinal ramp system. Facies and sequence-stratigraphic analysis identifies two third-order depositional sequences. These two sequences are restricted within type 2 sequence boundary at the bottom of the formation and type 1 sequence boundary at the top. Each sequence comprises transgressive (TST) and highstand (HST) systems tracts. The analysis of high-frequency (meter-scale) cycles, which is based on the high-resolution sequence stratigraphy analysis of third-order sequence, records five shallowing-upward and three deepening-upward cycles within the Beduh succession at Nazdur section and two shallowing-upward and five deepening-upward cycles in Sararu section. The thicknesses of shallowing- and deepening-upward cycles range from 2 to15 m. This work represents a first study in the understanding of the sedimentary and stratigraphic evolution for one of the common Triassic formations exposed in the Northern Thrust Zone. Our sequence-stratigraphic approach contributes to the understanding of the relationship between depositional model and sea-level changes.