12. Physical Properties and Consolidation of the Calcareous Sediments of Broken and Ninetyeast Ridges

Calcareous sediments cored at Broken and Ninetyeast ridges in the eastern Indian Ocean reveal the transitions from calcareous ooze to chalk and from chalk to limestone. Based on downhole porosity trends and laboratory mechanical consolidation tests, the compaction and lithification of these sedimentary sections occur at stratigraphically shallower depths than comparable marine calcareous sections. An exception to this case is the section at the northern Ninetyeast Ridge site. There, terrigenous silts and clays in the carbonate section appear to be normally consolidated. An analysis of porosity vs. depth trends of sediments from Sites 752 and 757 indicates an estimated 120 to 170 m of sediment were eroded during the middle Eocene uplift of this Broken Ridge. Compositional changes in the calcareous sections alter their compaction trends: inclusion of biogenic opal and/or ash results in higher porosity and lower velocity relative to the carbonate-rich horizons. The changes are observed in laboratory and log measurements and, in places, are significant enough to produce impedance contrasts giving rise to acoustic reflectors. INTRODUCTION The physical properties of marine sediments are sensitive indicators of the changes in lithology, compaction history, and diagenesis. Changes in physical properties are, in turn, responsible for reflectors observed in seismic records. The sediment sections cored along the crest of Broken and Ninetyeast Ridges during Ocean Drilling Program (ODP) Leg 121 are composed primarily of pelagic carbonates. Within these sections, lithologic variations are found in occurrences of ash, differing grain-size distributions, biogenic siliceous intervals, and terrigenous sediments. These changes are reflected in variations of downhole trends in physical properties, in modified compaction behavior of the same sections, and in seismic records. Correlation of these trends and aspects of compaction with lithologic changes can lead to a better understanding of the diagenesis of these sediment columns and their geologic history. ODP Sites 752-755 were cored along a nearly north-south transect across the crest of Broken Ridge (Peirce, Weissel, et al., 1989) (Fig. 1). The sediment section there consists of a pelagic calcareous sediment drape in unconformable contact with an underlying, northward-dipping and truncated sequence. The upper pelagic section is a nearly stark-white nannofossil with foraminifer ooze ranging in age from late Eocene to Holocene (Fig. 2). Coring at these four sites was designed to sample different stratigraphic sections of the dipping sediments. The resultant stratigraphic recovery is a section of ash-rich to calcareous sediments ranging in age from Turonian-Coniacian to early Eocene. Three ODP sites were drilled along the crest of Ninetyeast Ridge (Peirce, Weissel, et al., 1989). Site 756 was drilled at the southerly end of this feature where a 139-m-thick section of Pleistocene to Oligocene nannofossil ooze with foraminifers was recovered. Coring at Site 757 recovered a 212-m-thick section of nannofossil ooze with foraminifers to chalk (Fig. 3). Ash content increases downhole in the basal section of these sediments. Site 758 was drilled at the northern end of Ninetyeast Ridge. The calcareous sediments recovered there consist of about 370 m of 1 Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Program). 2 Woodward-Clyde Consultants, 900 Fourth Avenue, Seattle, WA 98164, U.S.A. Campanian to Holocene sediments with cherts and porcellanites in the Campanian to lower Miocene section. Terrigenous material derived from the Ganges-Brahmaputra drainage system is found in the sediment section, in increasing amounts, from the middle Miocene upward (Fig. 3). Fine-scale changes in terrigenous sediment input and their distinct physical properties are discussed in Farrell and Janecek (this volume). METHODS Sediment index properties (porosity, water content, bulk density) were obtained from shipboard measurements of discrete samples using gravimetric techniques and a pycnometer (Peirce, Weissel, et al., 1989). Bulk densities and porosities of sediments from above the unconformity at Broken Ridge, and from non-lithified sections at the Ninetyeast Ridge sites, were then corrected to in-situ values assuming sediment rebound as defined by Hamilton (1976) for calcareous sediments. In addition to the shipboard measurements, 12 whole-round samples were obtained for shorebased consolidation testing. The whole-rounds were X-radiographed to examine them for any obvious disturbance negating geotechnical testing. Several samples exhibited highly disturbed structures or cracks and were not included for testing. Consolidation tests were performed on 6.35-cm-diameter samples trimmed to 2.54 cm in height. Water contents were obtained from trimmings of these samples and were compared against shipboard measurements to make sure no desiccation had occurred. All samples were backpressured, as described by Lowe et al. (1964), to assure complete saturation. Samples were incrementally loaded until the virgin curve was established following standard geotechnical practice as described in Lambe (1951) and Bowles (1978). RESULTS Two approaches were taken to study the compaction history of the calcareous sections on the two ridges and effects of lithologic controls on the physical properties of sediments. First, downhole trends of these properties, as obtained from laboratory and logging measurements, describe the relative consolidation of these carbonate sediments with a superimposed signal resulting from lithologic variability. The effect of different lithologies and cementation can then be evaluated in terms of its role in modifying the compaction process. The second approach is to compare the mechanical consolidation behavior of samples tested under labo-