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doi:10.2204/iodp.proc.310.203.2009

Introduction

Understanding the controls of petrophysical properties of porous media is a key issue in interpreting data from seismic sections or from acoustic logs of sedimentary sequences.

Unfortunately, the unpredictable character of porosity in carbonates complicates the relationship between physical and geological properties. Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. All of these modifications alter the elastic properties of the rock and, therefore, the acoustic velocity. The result is a dynamic relationship between primary depositional lithology, diagenesis, porosity, rock texture, and acoustic velocity (Anselmetti and Eberli, 1993). Mixed carbonate-clastic settings present a special challenge in this respect. Carbonate platforms attached or proximal to hinterland may suffer from clastic influx, particularly during periods of sea level fall or lowstand (e.g., Playford, 1980; Sonnenfeld and Cross, 1993; Rankey et al., 1999). Also during other moments of sea level position, riverine transport and oceanic longshore currents may carry clastic material into carbonate provinces (e.g., Dunbar and Dickens, 2003; Cunningham et al., 2003). Few studies exist that investigate acoustic velocity distribution in mixed carbonate-clastic sediments. Kenter and Ivanov (1995) investigated parameters controlling acoustic properties of separate carbonate and volcaniclastic sediments from Ocean Drilling Program Sites 866 (Early Cretaceous shallow-water carbonates, Resolution Guyot) and 869 (Late Cretaceous to Miocene sediment apron, Wodejebato Guyot and Pikini [formerly Bikini] Atoll). Kenter et al. (1997b) focused on mineralogical and diagenetic controls on acoustic properties of Permian samples from New Mexico. In a following study they used these measurement values for synthetic seismic modeling of large outcrop sections in the same study area (Kenter et al., 2001). Anselmetti et al. (1997) investigated the physical properties of Neogene subsurface deposits of the Florida Keys to document the inherent reflectivity of different lithologies and compared these with seismic-reflection patterns from offshore seismic lines.

Extensive work over the last few decades has established some relatively simple relations between acoustic velocities and important rock parameters such as porosity and density. The classic velocity transforms, Wyllie’s time-average equation (Wyllie et al., 1956), Raymer-Hunt-Gardner’s modified time-average equation (Raymer et al., 1980), and Gardner’s empirical relation (Gardner et al., 1974), allow prediction of P-wave velocity from porosity or bulk density. Such transforms are simple and convenient but generally fail to account for variations in velocity at a given porosity value. For mixed carbonate-siliciclastics, such empirical relationships have substantial limitations because they do not take into account the type of porosity, diagenetic parameters, or mixed mineralogy. Global trends in most published data, however, do follow the time-average and Gardner’s equation (Gardner et al., 1974). Recently, Kenter et al. (2007) published a new methodology to extract rock texture information from acoustic velocity data alone through cross-plots of Poisson’s ratio versus P-wave velocity. Verwer et al. (2008) amended that specific diagenetic overprint can be postulated from the position of samples in that particular space.

During Integrated Ocean Drilling Program Expedition 310 (Camoin et al., 2005) 37 boreholes across 22 sites were drilled around Tahiti Island (French Polynesia) (Fig. F1A) to accomplish the following objectives:

• To establish the course of postglacial sea level rise at Tahixti,
• To define sea-surface temperature (SST) variations for the region over the 20–10 ka period, and
• To analyze the impact of sea-level changes on reef growth and geometry (Camoin et al., 2005).

To meet these objectives, the late deglacial reef sequence, which consists of successive reef terraces seaward of the living barrier reef, was cored from the DP Hunter (Fig. F1B) during October and November 2005. In addition to the mission’s scientific objectives, the expedition provided the unique opportunity to study the physical characteristics of young carbonate sediments, Holocene and Pleistocene in age, prior to any diagenetic overprint, such as mechanical compaction or cementation and recrystallization.

In this paper we report on the controls on the acoustic properties of Holocene and Pleistocene “young” carbonate reef sediments recovered from Expedition 310 Sites M0005–M0026 (Camoin et al., 2005). The analysis makes use of the combined petrophysical and petrographical approach of Vernik and Nur (1992) and Kenter et al. (2007) to study the acoustic behavior of mixed carbonate-volcaniclastic sedimentary rocks. We investigated and modeled the relation between carbonate fraction, volcaniclastic material (predominantly igneous minerals and minor clay), and porosity at effective stresses to 10 MPa. In addition, we compared and evaluated the variations in velocity, density, and porosity between the discrete sample set, core-log data, and downhole logging data.

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