Carbonate Sedimentary Rocks from the Western Pacific: Leg 7, Deep Sea Drilling Project

Abstract

Cenozoic calcareous oozes, chalks and limestones drilled during Leg 7 are mainly biomicrites. Foraminifera, and lesser amounts of radiolarians, are in an abundant matrix of nannofossils and broken foraminifers. The faint parallel lamination and diagenetic changes leading to induration are described and illustrated with photomicrographs.Sediments containing more than 70 per cent calcium carbonate dominate the sequences cored at Sites 62, 63 and 64. A few sections of cores from Site 65 contained about 15 to 40 per cent carbonate. At other Leg 7 sites, sediments generally contain less than 1 per cent carbonate.The carbonates are chalk oozes, chalks and limestones of pelagic origin. With the exception of a few rare foraminiferal turbidite beds in radiolarian ooze sections, the carbonates are dominantly microcrystalline calcite (Figures 1 and 2). The micrite is composed of whole and fragmented coccoliths, discoasters, small fragments of foraminiferal tests, and particles too fine to identify but probably the finest particles of broken nannofossils and foraminifers. Less abundant are whole tests and larger fragments of foraminifers which constitute from 0 to about 40 per cent of the samples in the smear slides and thin sections. Clay, glass shards and other volcanic debris, radiolarian tests, pyrite, and other components are present in varying amounts. These sediments can be classified as biomicrite, calcilutite, or nannofossil (nannoplankton) oozes and chalks.Primary sedimentary structures are common but usually indistinct. Where it can be discerned, bedding is parallel. It can be seen only at lithological boundaries, such as between ash-rich and ash-poor beds, and where textural variation within the carbonate reveal fine lamination. For instance, foraminifer-rich layers 0.5 to 3 millimeters apart can be seen in some thin sections (Figures 3 and 4) or on smoothed surfaces. The fabric of such sediments shows a preferred orientation of the long axes of whole and broken foraminifers that is parallel to the lamination ( Figure 5). These sediments are sparsely packed, that is to say, the sand-sized foraminiferal and radiolarian tests are mainly separated by the micrite matrix and only rarely touch one another (Figures 1, 2 and 5).Mottles are most evident in the younger parts of the carbonate sections, where they show by color contrast rather than by local textural changes. In the upper parts of the holes the oozes are greenish grays, pale purple and light grays, whereas at depth the chalks are whitish greens, white and very pale grays. The rocks of little chroma show few mottles, except for purplish ones, due to pyrite. However, the same rocks have portions where textural and fabric layering is evident adjacent to portions where it is not. The latter portions were probably burrowed, but do not show mottles because of the light color of the rocks.Some of the changes in the sediment with increasing depth in the holes apparently are due to changes in sedimentation with time, and some to diagenesis. The decrease in cl...

Stratigraphy of Maruia and Matiri Formations in their type section (Trent stream, Matiri river, Murchison)

“…Pyrite is present, particularly inside microfossil tests, but also scattered through the sediment, and demonstrates the .local existence of reducing conditions, but the general absence of siderite suggests less reducing conditions than those of the Kaiata Formation. Both the sedimentary and diagenetic mineralogy are closely similar to that of recently described ocean floor carbonate oozes (Moberly and Heath 1971) .…”

Introduction and Explanatory Notes, Leg 81, Deep Sea Drilling Project

“…The criteria of Moberly and Heath (1971) are used for calcareous deposits; subjective estimate or behavior in core cutting is used for others.…”

Introduction and Explanatory Notes, Leg 48, IPOD Phase of the Deep Sea Drilling Project

“…The criteria of Moberly and Heath (1971) are used for calcareous deposits; subjective estimate or behavior in core cutting is used for others. a) Calcareous sediments Soft: Oozes have little strength and are readily deformed under the finer or the broad blade of a spatula.…”