Core-Log Integration Approach for characterizing a Shallow Basement Section of the East Pacific Rise

ODP Leg 206 and IODP Expeditions 309/312 drilled a complete intact in situ section of the upper oceanic crust formed in a super-fast spreading ridge at Site 1256 in the Guatemala Basin, Pacific Ocean. It starts from a sedimentary cover (~250m thick) followed by an extrusive lavas section that transitions into a sheeted dikes complex and the uppermost plutonic rocks. The shallow igneous basement is characterized by a single cooling unit of basalt, which has been interpreted as a ponded lava flow, commonly referred as the Lava Pond. The high core recovery within the Lava Pond (~93%) has allowed for good correlations between core and downhole logging data providing an understanding of the relationships between local and regional tectonic. Electrical resistivity, photoelectric factor, density, gamma ray, microresistivity, and ultrasonic downhole measurements were used to identify 4 main logging units and 15 subunits within the Lava Pond. These logging units were matched to variations in core-based measurements of magnetic susceptibility, chemistry, and grain size that provide a perspective into the magmatic and tectonic history of the shallow basement in this area. In particular, borehole microresistivity and ultrasonic images provided detailed analyses of shallow, intermediate, and steeply dipping structural features that allowed reorientation of several core pieces. Approximately 800 structural features were identified within the Lava Pond from microresistivity and ultrasonic borehole images. More than 200 structures consisting of veins, shear veins, microfaults, late magmatic veins, and joints were matched to the downhole logs allowing for reorientation of specific core pieces. Overall, changes in mineralogical composition and variations in fracture density provide the basis for the downhole characterization of the Lava Pond. A kinematic analysis using slicken fibers measured on shear veins and microfaults as indicators of movement provide a better understanding of the near field stress environment at this site.