ORIGIN OF CHERT NODULES IN KOMETAN FORMATION  FROM DOKAN AREA, NORTHEAST IRAQ

T.  Al-Barzinjy, Sherzad

	ORIGIN OF CHERT NODULES IN KOMETAN FORMATION FROM DOKAN AREA, NORTHEAST IRAQ
Iraqi Bulletin of Geology and Mining
Article 1, Volume 4, Issue 1, 2008, Pages 95-104
Author
Sherzad T. Al-Barzinjy
Abstract
Kometan Formation, in most localities, contains sporadic and rare chert nodules, but in Dokan area, these nodules become widespread and associated with well-developed and high amplitude stylolites. They are mainly distributed along or around the bedding planes. The long axes of nodules and stylolite surfaces are aligned parallel to the bedding plane. Field observations and statistical analysis (rose diagram of long axes) showed that nodule elongation and stylolite peaks have no any relation with the known tectonic stress directions of the area. Therefore, the growth of the nodules and stylolites are attributed to deep burial diagenesis of the rocks of the formation under vertical lithostatic pressure and not due to tectonic stress. This result contradicts with the previous studies, which supposed that stylolite peaks have directions of North – South and East – West. Microscopic studies showed that nodules are developed by both displacement and replacement during deep burial. The microscopic criteria for emplacement are: 1- The limestone around nodules shows exceptional crowding of planktonic forams due to the dilatational relations with country rock. 2- They are associated with stylolites, this mean that the nodules compressional environment is favored by nodules and stylolites on expense of host limestone. While the criterion for replacement includes presence of partially dissolved and replaced forams directly at the contact between the chert and limestone. The local accretions of silica for replacement and displacement growth are assisted by diffusion and moving of watery solutions. The presence of widespread chert nodules and stylolites in Dokan area and their absence in other areas is attributed to exertion of the vertical lithostatic differential pressure at Dokan area. While the surrounding areas were performed as pressure shadow or the pressure was hydrostatic type (equal in all direction). The load pressure is estimated indirectly by comparing with calcite compensation depth (CCD) pressure under which limestone dissolve and silica increases in deep oceans.

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