LITHOLOGICAL CONTROL ON THE OCCURRENCE OF CHLORITE IN THE DIAGENETIC WEALDEN COMPLEX OF THE BILBAO ANTICLINORIUM (BASCO-CANTABRIAN BASIN, NORTHERN SPAIN)

Sandstones and shales from the southern flank of the Bilbao anticline in the Gordexola and Orozko valleys, northwestern Spain, have been sub jected to a detailed sedimentological, mineralogical and geochemical s tudy. They are composed of proximal front-deltaic sandbars interbedded with silty-clayey sediments and correspond to a prograding sequence o f a deltaic system. The clay mineral assemblages of doublets of sandst ones and shales are composed mainly of illite, chlorite and illite-sme ctite mixed-layers R3 (ISII). The clay diagenesis consists of the tran sformation of smectite and I-S mixed-layers to illite, the precipitati on of Fe-rich chlorite in the pore spaces, and the alteration of micas to chlorite. According to petrographic, mineralogical and geochemical analyses, chlorite is more abundant in sandstones than in shales in b oth the <2 mu m and coarser fractions. The relative abundance of chlor ite increases in the sandstones located at the top of coarsening upwar d sandbars. Furthermore, chlorite formation occurs preferentially in t he coarser grained sansdstones previously cemented by ankerite. The ge ochemical and petrological investigations suggest that chlorite formed during burial diagenesis in a relatively closed system.