LITHOLOGICAL INDICATORS OF OLIGOCENE GAP UNCONFORMITY AT THE BOUNDARY BETWEEN HIGH AND LOW FOLDED ZONES, NE-IRAQ

The Oligocene gap is considered as one of the major and widespread unconformities and interruption in continuity of sedimentary records in northern Iraq. The duration of this gap, in some parts of the area, is the time span of Upper Oligocene. The associated unconformity separates Pila Spi and Anah Formations, while in other parts it separates Pila Spi Formation from Fatha Formation. In this area of the study the conglomerate located between Pila Spi and Anah Formations, called Anah basal conglomerate. Lithologically the contact between Pila Spi and the overlying formations represented by either conglomerate (ortho and polymitic), limestone conglomerate, sedimentary breccia ,paleosol well developed or partially developed. These all indicate different depositional systems and different source areas. From sequence stratigraphy point of view, the studied unconformity indicates a type (1) sequence boundary (SBI).