Clay Mineralogical Study of the Pliocene-Pleistocene Carcar Formation and the Quaternary Alluvium in Consolacion-Liloan, Cebu Province, Philippines

The Pliocene-Pleistocene Carcar Formation and the Quaternary Alluvium are two major lithologies that rim the entire island province of Cebu. The Carcar Formation (CaF) is predominantly limestone rock mass characterized by rolling to hilly topography while the Quaternary Alluvium (QA1) is generally unconsolidated sedimentary deposit on coastal plain marked by low relief and undulating landscape. Mineralogical examinations of the vertisolic soil/clay horizons that mantle these rock formations were conducted to identify and characterize the nature of clay minerals using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetry (TG), scanning electron microscopy (SEM), and energy dispersive x-ray (EDX). Analytical qualitative results show that the clays derived from the Carcar Formation are chiefly smectite-rich assemblage with supplemental association of mixed-layer kaolinite/smectite. Clays from the Quaternary Alluvium are the same as in the CaF, however, mixed-layer kaolinite/smectite in the QA1 appears relatively richer than in the CaF. Quantitative estimates by computer simulation yield slight to moderate variations in the proportions of kaolinite and smectite in the mixed-layer phase, and correlatively, the degree of ordering is determined to be random in both CaF and QA1 clay assemblages. Kaolinite content participating in the interstratified phase is discerned to be diagnostically higher in the alluvium than in the limestone. Calcite impurities occur greater in clays sourced from the Carcar Formation and are found comparatively rarer in the Quaternary Alluvium. Quartz is noted to be frequently present in minor to moderate amounts in both lithologies but is generally more persistent in the QA1. The mixed-layer kaolinite/smectite phase is observed in both CaF and QA1 and revealed that it is not an exotic clay phase exclusively confined in the red-burning clay area. Illite or illitic facies is found only in the QA1 and is envisaged as authigenically or neogenically formed. Smectite with high rehydration capacity is recorded only in the CaF and is believed to be a result of its structure and beidellitic composition.