Sedimentologic and stratigraphic properties of Early Cretaceous Neo-Tethys shelf margin of Arabia: The Qamchuqa Formation of the Zagros Folded zone – Kurdistan Region of Iraq

Abstract

The Qamchuqa Formation is well-exposed in the High Folded Zone (HFZ) of northeast Iraq. The formation is highly dolomitized and contains thick reservoir units in the subsurface region south of the HFZ. Deposition of the formation took place at the shelf margin of Early Cretaceous Neo-Tethys Ocean. It merges south- and southeastward to the shallow marine shelf of Arabia and to deep basin accumulations farther north. In the study area, the formation is divided into three members: lower member (250m thick) of interbedded bioclastic calcimudstone/wackestone (MW), bioclastic packstone (BP), bioclastic floatstone (BF) and medium-to coarse-crystalline dolorudstone/dolograinstone (CD). These strata accumulated in low to medium energy, subtidal, fore-shoal setting. The middle member consists of thick succession (325m) of massively bedded CD lithofacies interpreted as high energy shoal facies which represents Barremian-Aptian main shelf-margin succession. The upper member (155m thick) consists of three lithofacies of MW, CD and fine crystalline dolomudstone (FD), in ascending order. It is interpreted as lagoon (MW) through subtidal carbonate sand shoal (CD) to supratidal (penecontemporaneous) dolomudstone (FD). The lower two members represent a long-term Barremian to Aptian shallowing-upward megacycle capped by late Aptian unconformity incurred by relative sea level drop. This temporal gap is coeval with an erosional vacuity recognized throughout the Arabian Peninsula. Sea level rise in early Albian heralded deposition of the upper member which also represents a shallowing-upward cycle surmounted by regionally extensive subaerial unconformity recognized in the Arabian Peninsula. Correlation and stratigraphic position of the formation under regional context has been discussed.