Abstract
ABSTRACTThe Mirga Mir Formation (Lower Triassic) was studied for its sedimentology and lithostratigraphy from two outcrops (Nazdur and Beduhe areas) c. 20 km apart, in the Northern Iraq‐Kurdistan Region. The collected samples were investigated by standard polarised microscope, X‐ray diffraction and scanning electron microscopy to infer their mineralogical characteristics, provenance and paleodepositional environments. The Mirga Mir Formation is a mixed carbonate‐siliciclastic succession consisting of thin‐, medium‐, occasionally thick‐bedded, and yellowish‐grey limestones, sandy dolomitic and argillaceous limestones, alternating with thin‐ to thick‐bedded grey shale/marl. Thin‐laminated siltstones and fine‐ to coarse‐grained sandstones are seen in the lower and middle parts of the formation. The petrographic study of carbonates and clastics (sandstones) showed that most limestones are carbonate mud (micrite). The skeletal grains consist principally of thin‐shelled pelagic bivalves (Posidonia), serpulid worm tubes (Spirorbis), microgastropods, ammonoids, calcispheres, brachiopods, dasyclad green algae, miliolid foraminifers and ostracods. Non‐skeletal grains include ooids, peloids, intraclasts and extraclasts. Depending on field observations, five different lithologic units were recognised. These are in ascending order: oolitic limestone unit, sandy limestone‐shale unit, shale‐limestone unit, argillaceous limestone‐shale unit and brecciated‐slumped limestone unit. Sandstones occur as thin beds within the lower and middle parts of the studied formation. In addition, thick coarse‐grained sandstone beds occur in the middle part of the Nazdur section only. Petrographically, thin sandstone beds consist of fine‐grained quartz. The thick sandstone bed is an immature, litharenite and consists of rock fragments (mostly sedimentary), quartz and feldspars. The X‐ray diffraction analysis and scanning electron microscope micrographs of shales revealed that the abundant clay minerals in the Mirga Mir Formation are illite, followed by kaolinite and chlorite. In addition to illite‐chlorite mixed layers, non‐clay minerals include calcite and quartz, with low feldspars. Based on detailed microfacies analysis of the limestones, 12 microfacies types from the carbonates and mixed detrital carbonates are distinguished. These facies were subdivided, according to their environmental interpretation, into five facies associations: Offshoal/basinal, Foreshoal/slope, Shoal, Back Shoal/Lagoon. The study introduces a good example of a gently sloping epeiric carbonate ramp that shows a gradual shallowing of the basin from the offshoal/basinal sediments to the foreshoal/slope and backshoal/lagoonal environments.
Published Version
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