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doi:10.2204/iodp.proc.310.104.2007

Petrophysics

The last deglacial sequence (lithologic Unit I) comprises coralgal frameworks heavily encrusted with microbialites that are locally interlayered with coarse skeletal sands and gravels rich in coral and algal fragments. Large primary cavities in the coralgal framework are open or partially filled with skeletal sands and gravels, including coral and coralline algal fragments, and with Halimeda segments. Microbialites are abundant and represent the major and structural and volumetric component of the reef rock. The open framework and centimeter- to decimeter-sized pores result in a highly variable physical system. Physical properties change on a centimeter scale and may range from low porosity, high density and velocity to 100% open pore space. In situ corals, microbialites, and skeletal sands all have a distinct petrophysical signature (acoustic velocity, porosity, grain density, and color reflectance L* values) but cannot be correlated easily from hole to hole.

The last deglacial sequence at Tiarei has a greater volcaniclastic component than the one at Maraa; this difference is observed on digital image scans and quantified by diffuse color reflectance spectrophotometry and magnetic susceptibility core logs. Minor amounts of noncarbonate material, clay, and/or quartz silt/​sand can dramatically change acoustic velocities (Stafleu et al., 1994; Kenter et al., 1997; Anselmetti and Eberli, 2001). Magnetic susceptibility values are lower in the last deglacial sequence (Unit I) at Maraa than at Tiarei.

The contact between lithologic Units I and II is characterized by an irregular unconformity; abundant solution cavities at the top of the underlying limestone are partly filled with unconsolidated skeletal and volcanic sand, including coralline algal branches, coral gravels, and Halimeda segments. The change in physical properties at this contact is sharp and abrupt. Density, velocity, and magnetic susceptibility increase, and porosity decreases.

The older Pleistocene sequence (lithologic Unit II) is composed of (1) well-lithified skeletal packstone/​grainstone to floatstone/​rudstone, (2) well-lithified coralgal frameworks abundant in coated corals and coral clasts (corals are diagenetically altered, indicating subaerial exposure, and solution cavities have several generations of infilling), and (3) rubbles and gravels primarily composed of coral clasts, limestone clasts, basalt pebbles, and reworked coral colonies. All three subunits are well compacted and have undergone several phases of diagenetic alteration. This results in tight, low-porosity, high-velocity limestones with much less variation of physical properties than observed for the younger sequence. Volcanic pebbles and sands are locally interlayered and/or mixed with carbonate and are associated with an increase in magnetic susceptibility values.

The sequence drilled in Tiarei marginal Hole M0008A consists entirely of volcaniclastic sediments and rocks: clays, un- to semilithified sandstones, basalt gravels, pebbles, and cobbles. Sandstones are commonly moderately lithified and have high densities compared to relatively low velocities. Basalts are high in density and velocity. Magnetic susceptibility in this unit is generally high.

In the “Sedimentology and biological assemblages” section in each site chapter, Units I and II and their subdivisions are defined using lithological and biological assemblage criteria. Intervals characterized by particular physical properties were recognized and can usually be correlated with lithologic Units I and II and their subdivisions. Hole-to-hole correlation of physical properties was possible in some cases.

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