Ocean floor volcanism: constraints from the integration of core and downhole logging measurements

The volcanic architecture of oceanic crust records the diversity in volcanic activity during its development in the neovolcanic zone of individual ridge systems. Potentially there exists a spectrum of lithological architectures which may primarily be related to the spreading rate and the dynamics of individual magma chambers along different ridges. Recent studies have emphasized the observable spatial variations within different neovolcanic zones, although direct extrapolation into the third dimension can only be achieved by the use of drilling results. To study the structure of the volcanic layer it is essential that individual lithologies (sheet flows, pillow lavas and/or breccias) can be discriminated from the core and/or logging results and mapped within the borehole. Unfortunately a problem with the drilling of the volcanic basement during the Ocean Drilling Program has been the generally low (typically c. 25%) and biased core recoveries, which produce an erroneous picture of the lithological diversity of the volcanics. This problem is further compounded by the difficulty in determining the volcanic stratigraphy, particularly when the key information is lost during coring (i.e. boundaries/contacts). Downhole logging provides near continuous records of the physical/chemical properties of the borehole which when integrated with core measurements, yield a detailed picture of the architecture of the volcanic layer. Logging results from ODP Hole 896A are of sufficient quality that sheet flows, pillow lavas and brecciated units can be discriminated and mapped effectively within the borehole. From their distribution it is evident that sheet flows become more abundant in the lower part of the hole, which probably correlates with ridge axis volcanism whereas, the predominance of pillow lava flows (< 340 mbsf (metres below sea floor)) in the upper part of the hole, is probably related to off-axis volcanism within the neovolcanic zone.