Revisiting the “Gercus Basalt within the Eocene Gercus Formation”: Implications for sedimentary origin of a basaltic body, Kurdistan Region, Northern Iraq

Abstract

• A previously described intrusive basaltic body is re-studied. • The re-study proved that it consists of sedimentary rocks (greywacke sandstone). • The greywacke transported from remote volcanic source to its present area. • More than ten evidences are found to prove its sedimentary origin. • These evidence are such as content of bitumen, conglomerate imbricated pebbles. • Other evidences are presence of graded beds, parallel and planer bedding and ooids. • More evidences are absence of basaltic flows and chilled borders and others. Recently, a basaltic body is described geochemically and mineralogically by previous authors within the Gercus Formation, in the Bekhair Anticline (Duhok Governorate, Northern Iraq). They indicated feldspar, anorthoclase, diopside, forsterite and olivine as main minerals of the body with many accessary ones. They added that the body is anorogenic (non-tectonic), extruded on continental crust of Arabian Plate and affected by pervasive alteration with a thickness of 16 m and a width of 4 km. The present study discussed in detail the sedimentary origin of the claimed basaltic body, contesting its intrusive or extrusive igneous origin. We proved that the body consists of a volcaniclastic succession (greywacke), which was derived from remote volcanic source areas and deposited by running water in the basin of the Gercus Formation. These sediments had been transported from a northeastern source area toward southwestern by streams to the deltaic basin of the Eocene basin. For proving its sedimentary origin, we presented many field and petrographic evidences such as content of bitumen, ooid bed, thick or thin planar layers (with parallel and sharp contacts), graded bedding, conglomerate, imbricated pebbles and hosting limestone beds in addition to absence of contact metamorphism, lack of structures such as pillow lava, basaltic flow, crystals zoning, xenoliths, peperites, digitation into host rocks and dilatations features. The previous authors depended on the geochemical and thin section studies for proving its igneous origin but these methods cannot prove if the constituents (whole or broken minerals) of the body are transported or indigenous. While accurate field survey and boundary conditions studies can indicate its origin. The proof of the sedimentary origin is achieved via conjugating evidences of the body boundary, those from its internal architecture and composition. Therefore, we are sure 100% that the body is sedimentary succession not basaltic one.