Facies reconstruction from resistivity-at-the-bit images recorded in a submarine felsic volcanic succession

For the first time in history of the Ocean Drilling Program (ODP), Leg 193 drilled into an active hydrothermal vent field hosted in felsic submarine volcanic rocks using Logging-While-Drilling (LWD) techniques and specifically, the Resistivity-at-the-Bit (RAB) tool. In this manuscript, we present the results obtained from penetrating into the uppermost portion of hydrothermally altered dacite volcanic rocks hosting the subsurface of the PACMANUS active black smoker field along the Pual Ridge (Papua New Guinea). ODP Hole 1189C was drilled to a total depth of 166 m below seafloor (mbsf) and continuous electrical images of the borehole wall provide a full 360° coverage of the surrounding formation. For comparison, higher resolution borehole images were also recorded over an interval of approximately 50 m using a wireline Formation Micro Scanner (FMS). These data, together with geological observations obtained from drill cores provide the background for the interpretation of the electrical formation properties. The interpretation of the RAB images shows that coherent dacite facies and two types of breccias with distinctive clast size populations can be distinguished. These have been classified as breccia type I with predominance of coarse clasts containing maximum diameters of 95 cm and breccia type II with predominace of fine clasts containing maximum diameters of 55 cm. Coherent and breccia facies are intercalated in variable proportions with coherent facies becoming gradually dominant in the lower sections of the hole. The RAB data include the upper 10–30 m below surface where coring and traditional wireline logging data were not obtained. The interpretation of the RAB data confirms the principle observation from drill cores that the Roman Ruins site is located in a proximal to medial position relative to the eruptive vent and show that strongly altered fractured rocks extend to the seafloor enabling mixing of hot hydrothermal fluids and ocean bottom water. The continuous coverage of the RAB data is the only means of interpreting sub-bottom variations in the volcanic stratigraphy and the proportions of different volcanic facies.