Association of rudists and red clastic facies in the upper part of Aqra Formation, Mawat area, Kurdistan Region, NE Iraq

Abstract

In Mawat area, the Late Maastrichtian Aqra Formation (or lens) is located in the upper part of the Tanjero Formation (Lower–Middle Maastrichtian) and overlain by Red Bed Series (Paleocene–Eocene). This lens consists of about 10–100 m of massive biogenic and well-bedded detrital limestone with rich fossil content such as rudists, large forams (luftusia, omphalocyclus, and orbitoids) gastropods, pelecypods, and echinoderms. The present study discusses the geology of an association of rudists and red clastic facies in the upper part of the Aqra Formation. The tectonic and paleogeographic importance of the association are clarified and correlated with similar occurrence in Turkey and Iran. Sedimentologic and paleontologic evidence proved that all the rudists were preserved in their life position which shows that their orientations are in right side up and evidence of reworking or transporting was not found. The occurrence of these opportunistic rudists, in the red clastics, has one to three very important implications. The first is that it is possible that the red clastics belong to and occur in the Cretaceous (Maastrichtian). This occurrence is the first to prove paleontologically that red clastics are deposited during the Late Cretaceous in northern Iraq. This finding has tectonic implication as red clastics prove that the source area was active and soil development started on the recently uplifted Zagros orogenic belt. The second is that some opportunistic rudists were crossed the Tertiary/Cretaceous boundary and survived at the Early Paleocene. Since evidence from the over world put the age range of the rudists between the Late Jurassic and Late Cretaceous, this possibility is discounted by the present authors. Between the two possibilities, the authors prefer the first one. The third is that the extinction of rudists was due to prevalence of turbid red clatics in the northern Iraq.