Study of sedimentary clastic strata of the Tertiary Cebu Formation, Cebu Province, Philippines was carried out to probe the mineralogy and characteristics of the clays contained in the sediments and to propose the geological implications and the mechanisms of their formation or evolution. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) techniques were employed in the experimentation process. Analytical results indicate kaolinite as the major clay component in almost all the samples collected at selected horizons and locations. Allogenic quartz dominantly comprises the non-phyllosilicate impurities and feldspar virtually nil.
The ubiquitous occurrence of kaolinite with other clays however, is exemplimed in terms of 3 mineral assemblages, namely:(1) interstratimed illite/smectite (I/S)-kaolinite, (2) kaolinite-dominated, and (3) interstratimed kaolinite/smectite (K/S). These characterized mineral assemblages essentially portray distinct heterogeneity of horizons and clay associations that suggest formation of the Cebu Formation under particular processes and environment conditions. The interstratimed illite/smectite-kaolinite assemblage and the kaolinite-dominated assemblage are interpreted to have formed with clastic sediments under favorable conditions related to diagenesis with accompanying coalimcation. The I/S in the interstratimed illite/smectite-kaolinite assemblage is suggestive of an intermediate or transitory phase in the illitization of smectite during diagenesis. Progressive dissolution of smectite and precipitation of illite apparently did not attain completion. The interstratimed kaolinite/smectite assemblage is perceived to have evolved by pedogenesis and is likely to have been catalyzed by intense chemical weathering under tropical climate.