The northern Barbados accretionary wedge is located along a convergent margin that is actively accreting oceanic sediments. It develops where Upper Cretaceous Atlantic Ocean crust underthrusts the Caribbean plate in a western direction. The accretionary wedge consists of Quaternary to Miocene calcareous mud, mudstone, and claystone (Moore et al., 1998). Detailed knowledge of the Barbados accretionary wedge has been obtained through several Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) legs (DSDP Leg 78 and ODP Legs 110, 156, and 171A). At the Leg 171A sites, the detachment between the underthrusting oceanic plate and the accretionary prism, which is known as the décollement zone, occurs at ~200-400 m below seafloor (mbsf) (Fig. F1) (for a borehole localization map see Moore, Klaus, et al., 1998). The underthrust sequence consists of lower Miocene to Oligocene mudstone, claystone, and turbidites down to 820 mbsf (Moore, Klaus, et al., 1998).
Deformation and fluid flow in this accretionary prism change the physical properties of the sediments. In some cases, changes in physical properties are localized along discrete faults in response to overpressuring and fluid migration, whereas, in other cases, changes in physical properties reflect variations in the broader stress regime (Shipley et al., 1994). The evolution of these physical properties cannot be comprehensively derived from recovered cores because of elastic rebound and microcracking effects.
One of the main objectives during Leg 171A was to map and understand the evolution of changes in physical properties within the accretionary wedge (Moore, Klaus, et al., 1998). Logging with conventional open-hole wireline logs proved difficult to impossible during previous legs (Legs 110 and 156) because boreholes penetrating the unconsolidated sediments were too unstable, especially near the décollement zone (Jurado et al., 1997). The logging while drilling (LWD) technique was used for the first time by the Ocean Drilling Program during Leg 156. During Leg 171A, this technology was used solely for borehole measurements.
Sensors in the LWD tool are located inside the drill string, 3-13 m above the drill bit. This allows geophysical measurements of the formation to be made shortly after the drill bit has penetrated it and before the borehole is affected by continued drilling or coring operations. Thus, the measurements are not influenced by borehole breakouts or washouts. In addition, because measurement occurs within minutes of the hole being drilled, the effects of borehole wall infiltration are minimized. Geophysical analysis with LWD tools may become a routine procedure in soft, unstable, or overpressured sediments. In a single logging run, data for up to 10 or more physical, chemical, and technical parameters can be obtained (Shipboard Scientific Party, 1998). The interpretation of the resulting data matrices requires a profound geophysical and sedimentological background and can benefit from sophisticated statistical operations.
Multivariate statistical analyses of LWD data have not been common. But the large amount of data from LWD measurements and the demand for a fast, reliable, and objective evaluation and interpretation makes the application of multivariate statistical methods ideal. In this study, the multivariate statistics procedures of factor and cluster analysis are used to obtain quick results from the LWD measurements. Factor analysis is used to rescale and reduce the original data set and to derive a deeper insight into the background processes. Cluster analysis is used to define electrofacies in as objective a manner as possible, which is particularly important for Leg 171A because no cores were collected.