Geology of the Lower Comoro Fluvio-Deltaic Accumulation: outcrop, resistivity, drilling data, and their implications for groundwater resources

The Lower Comoro Fluvio-deltaic Accumulation (LCFDA) is a major water source west of Dili built mostly by sediment fed by the Comoro River. Electric Sounding (ES) and Electric Resistivity Tomography (ERT) results were validated and combined with borehole and outcrop information to investigate the sedimentary architecture and develop a sedimentary facies model of the LCFDA. The geophysical data allow distinguishing fresh schist from weathered schist and understanding the geometry of the covering deposits. The LCFDA is dominated by gravel and sand, with secondary fine-grained units. Eleven lithofacies were recognized on outcrops. Based on the way these lithofacies articulate with each other, six facies associations are identified. These are channel fill (FAcb1), major channel bar (FAcb2), minor distributary channel (FAcb3), major distributary channel (FAcb4), floodplain (FAfp), and marine-influenced bedload deposits (FAmbd). FAcb1, FAcb3, FAcb4, and FAmbd are all good for groundwater aquifers, while FAfp can act as an aquitard or aquiclude. Depending on the thickness and lateral continuity of fine-grained units intercalated within FAcb2, this facies association can function either as an aquifer or as an aquitard. The sedimentary facies model of the LCFDA provides valuable information about the aquifer geology, aiding groundwater exploration and environmental assessment.