Illite/smectite transformation in detrital glaucony during burial diagenesis of sandstone: A study from Siri Canyon – Danish North Sea AFSOON M. KAZEROUNI*, METTE L. K. POULSEN   , HENRIK FRIIS*, JOHAN B. SVENDSEN   and JENS P. V. HANSEN   *Department of Geoscience - Aarhus University, Høegh-Guldbergs Gade 2, DK - 8000 Aarhus C, Denmark (E-mail: afsoon.moatari@geo.au.dk, henrik.friis@geo.au.dk)   DONG E&P, Agern Alle ´ 24-26, DK - 2970 Hørsholm, Denmark ABSTRACT Detrital glaucony in the Palaeogene glauconitic sandstones in Siri Canyon, Danish North Sea, has been analysed from 15 exploration wells by X-ray diffraction, electron microprobe and scanning electron microscopy. These sandstones consist of mixed-layer illite/smectite and have a large variability in chemical composition and structure. In the most shallow wells (ca 1700 m), the glaucony is rich in Fe and consists of mixed-layer illite/smectite with random-interstratification (R = 0). In the depth interval from 1700 to 2000 m, the composition changes as Si is incorporated. The structure changes to ordered R = 1. Further increase in burial leads to the loss of Fe. Ordered R =3 mixed-layer illite/smectite is recognized from burial depths of 2200 m. The proportion of illite in illite/smectite mixed layers increases only slightly with depth and temperature. Although the structural changes generally are associated with chemical changes, they can also take place isochemically when the detrital glaucony is tightly embedded in earlier cement, which prevented chemical exchange. The glaucony transformation in the Siri Canyon sandstones partly reflects a supply of Si and partly significant loss of Fe. Thus, the glaucony transformation relates to the general diagenesis of the host sandstone. These sandstones are cemented by microquartz at an early stage, followed by precipitation of Fe-rich grain-coating berthierine or chlorite. Keywords Burial depth, chlorite, glauconitic, illite, mixed layer, smectite. INTRODUCTION A major issue in the study of sandstone diagen- esis is the timing of various diagenetic phases and the supply of ions for cementation and replace- ment. Often, the release and consumption of various elements do not seem to fit in time, and large-scale exchange in chemically open systems is often concluded on a basin scale, either by convective flow or by diffusion between nearby lithologies (Milliken, 1989; Archer et al., 2004; Thyne, 2001; Stokkendal et al., 2009). The trans- formation of smectite to illite through various phases of mixed-layer illite/smectite (I/S) has been the focus of many studies as an important source for burial cementation of sandstones (Burst, 1969; Perry & Hower, 1970; Hower et al., 1976; Freed, 1981, 1982; Lynch et al., 1997; Abid et al., 2004; Peltonen et al., 2009, and others). At the time of deposition, fine-grained sedi- ments are commonly rich in smectite, while their deeply buried counterparts are generally illite- rich. This change is the result of smectite to illite reactions that consist of the progressive increase in illite layers in mixed-layer I/S during burial diagenesis. Generally, the chemical changes associated with this reaction include the consumption of Al and K and the release of Si and hydrated, exchangeable cations, (Na, Mg, Ca and Fe), Sedimentology (2013) 60, 679–692 doi: 10.1111/j.1365-3091.2012.01356.x Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists 679