Pseudo-bedding and burrow silicification in the Kometan Formation of the Dokan area, northeastern Iraq

Abstract

Silicified burrows and stylolite seams are common in the chalky pelagic limestones of the Kometan Formation (late Turonian–early Campanian) of the Dokan area in northeastern Iraq. The shape of individual and packed network branched chert nodules, which are the result of the early diagenetic mobilization of radiolarian silica, are controlled chiefly by pre-existing Thalassinoides burrow system. Frequent appearance of Thalassinoides may reflect periodical formation of discontinuity (omission) surfaces on the canyon margin (or near the listric fault) of the Dokan area resulted from combination of tectonic and eustatic processes. It is also proven that many so-called bedding planes of the Kometan Formation are merely pressure solution stylolites in massive homogeneous pelagic limestones (pseudo-bedding). It is concluded that the association of silicified burrows (chert nodules) with stylolite veins refers to regressive discontinuity surfaces, while the separation of these two features indicates either winnowed permeable surfaces or true bedding planes. The clay constituent of such stylolites is found to be formed mainly by transformation of smectite during burial processes.