Facies reconstruction of a hydrothermally altered dacite extrusive sequence: Evidence from geophysical downhole logging data (ODP Leg 193)

Abstract

ODP Leg 193 drilled into the PACMANUS hydrothermal field (Papua New Guinea), which is an active hydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting. The PACMANUS hydrothermal field is part of the eastern Manus Basin and is located near the crest of Pual Ridge, a 500 to 700 meters high felsic neo‐volcanic ridge. Two sites, Snowcap (Site 1188) and Roman Ruins (Site 1189), were drilled approximately 800 m apart. Although low core recovery limited the examination of the subsurface geology, geophysical downhole measurements provided continuous records of in‐situ physical properties with depth and were used to reconstruct lithostratigraphic profiles. Downhole logging operations included standard wire line logging and Logging While Drilling (LWD) measurements. Electrical resistivity images of the borehole wall were examined to find characteristic textures and three different volcanic facies were distinguished: coherent dacite, volcaniclastic dacite and brecciated dacite. In addition, intervals with sulfide mineralization were identified based on characteristic responses of the photoelectric factor log. A comparison of the reconstructed profiles from both sites shows considerable differences in the proportions of facies and the thickness of individual layers. Thick units of coherent facies are predominant at Snowcap and indicate a proximal position of the site within the volcanic system of Pual Ridge. At Roman Ruins, thin individual layers with rapid changes in volcanic facies and a higher proportion of volcaniclastic and brecciated dacite suggest a medial position. These differences in volcanic facies show that the volcanic architecture at Pual Ridge is characterized by small‐scale facies changes, emplacement of small‐volume individual lava flows and complex geometries of individual emplacement units. Geophysical logging data suggest that subseafloor hydrothermal activity is focused to the area around one hole at Roman Ruins. Downhole logs from this hole show characteristic cyclic patterns in the electrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization is concentrated at the tops of individual lava flows representing paleo‐seafloor positions.