Borehole images of the ocean crust: case histories from the Ocean Drilling Program

Abstract

Abstract Downhole logging is an integral part of the Ocean Drilling Program (ODP), although the choice of tool deployment is primarily a function of the scientific objectives of an individual leg. Following the successful deployment of the Formation Microscanner (FMS) during Leg 126, FMS images have been used to constrain a variety of sedimentary, igneous, metamorphic and tectonic questions in the ocean. However, it is only recently, that the full potential of FMS images and other logging data has been employed in the study of volcanology, which provides important constraints on the evolution of the volcanic pile when fully integrated with core based measurements. Furthermore, many of the holes drilled into the volcanics often have incomplete core recovery and here the logging results can be used to constrain the lithological, chemical and mineralogical variations in such intervals. Using the log based stratigraphy from crust created at slow (Hole 395A) and intermediate (Holes 504B and 896A) spreading centres, it is evident that the proportions of volcanic rock types (pillows/flows/breccias) correlate with the spreading rate. This relationship is suggested from submersible observations, but the ODP data provide additional data to test such hypotheses and to also extended and test it within the third dimension.