Chapter 8.elastic Property Corrections Applied to Leg 154 Sediment, Ceara Rise

Sediments recovered from Ocean Drilling Program (ODP) holes undergo elastic rebound and physical expansion when recovered from great depths below the seabed (about 75 m). This expansion contributes to the artificial “growth” of the composite depth section (Hagelberg et al., 1992). Sediment elastic properties, derived from one-dimensional consolidation tests, are used to correct the expanded sediment column back to in situ values. The property that controls this effect is the sediment elastic rebound (Cr), which is a simple function of the effective stress or depth below the seabed and sediment type. For Leg 154 sites, 90% to 95% of the composite section “growth” is attributed to elastic rebound of the sediment. The remaining “growth” is likely caused by intervals of sediment flow-in, identified in the visual description of the split cores. The composite depth scale, although artificially expanded, is an excellent tool for building a composite stratigraphy at one site. However, caution should be used when applying this depth scale for the construction of synthetic seismograms, for calculation of mass accumulation rates, and for any analyses that requires true sediment thickness. For these applications, corrections to the composite depth section are recommended. INTRODUCTION Deep-sea sediments have both elastic and plastic deformation properties. The plastic properties are the predominant control on the “permanent” consolidation or the compaction history of sediments. The elastic properties represent the recoverable strain within sediment when stresses are removed. These plastic and elastic components of consolidation are best described using Terzaghi’s one-dimensional consolidation theory (Terzhagi, 1943). The theory defines a log-linear function of porosity reduction with increasing stress and is called the “virgin compression” curve (Fig. 1). The slope of this log-linear curve is the coefficient of compression (Cc). This type of function was empirically determined by petroleum engineers in the early 1930’s for estimation of porosity-depth functions in sedimentary basins (Athy, 1930). However, the Athy functions can only be crudely applied to sediment under gravitational loading because the functions are described for a few ranges of sediment types defined by grain size. These empirical functions also do not separate the porosity reductions into their elastic and plastic components. In addition to virgin compression, the Terzaghi theory also includes a description of sediment elastic expansion when stresses are reduced, for example, during the recovery of samples from below the seafloor. This study describes the application of the elastic rebound as the largest component of the “growth” of the composite depth scale. The elastic rebound is also used to correct laboratory-measured index properties to equivalent in situ values. METHODS Application of the Terzaghi theory is performed by testing samples under conditions of one-dimensional consolidation. In this study, back-pressured consolidometers were used on samples trimmed to a diameter of 5.1 cm and a height of 1.9 cm. Incremental 1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program). 2Geological Survey of Canada, Atlantic, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada. [email protected] 3UHYLRXV &KDSWHU 7DEOH RI & loads were applied at the end of primary consolidation using a load increment ratio of 1. Figure 1 is an example of results from a typical consolidation test. The amount of elastic rebound in a core taken from any depth below the seafloor is calculated using the log-linear slope of the elastic rebound portion of the consolidation curve (Cr), following the derivation of MacKillop et al. (1995): ∆e = Cr log (Po′) (1) where Po′ is the vertical effective overburden stress (i.e., stress relief resulting from sampling) and ∆e is core expansion represented in terms of void ratio change. The change in void ratio resulting from stress relief and subsequent core expansion is converted to lengthening of the core (∆L) by: A B C Recompression to in situ stress