Oceanography 540--Marine Geological Processes--Autumn Quarter 2002

Overview of Marine Sedimentation

The term sediment refers to material composed of particles that have settled to bottom of a liquid. Sediments are classified by the processes involved in determining their physical and chemical character. Important distinguishing characteristics of sediments include: The source particles can be either preexisting rock (detrital sediments) or prec ipitates formed in the solution from which they settle (chemical sediments). Sources of marine sedimentary material

The material deposited in marine sediments has its origin on the continents:

Within the suspended load, the materials are those that make up the continents, particularly those that are resistant to weathering or dissolve incongruently, f or example:

Eq 13-2: eq 13-2

These include quartz, feldspar, and clay minerals such as kaolinite and montmorillonite. These clay minerals are products of chemical weathering of primary igneous materials and typically very fine-grained .

Transport of Sediments

Physical processes dominate at ocean margins, where they transfer particles eroded from the land to the sea floor.

Active sedimentation processes (where the sediment modifies the properties and behavior of the suspension) include mass wasting and density currents. Such deposition tends to mask other sedimentary processes. The giant landslides surrounding the Hawaiian islands and the abyssal plains of the Atlantic are striking deep-sea examples of active sedimentation. Because active sedimentation depends on gravitational energy, it does not extend seaward of the trenches along convergent plate boundaries (but can affect the entire ocean basin off passive margins).

Passive sedimentationprocesses are those in which the sediment is carried by but does not modify the normal thermohaline circulation. Examples:

Sediments formed by physical processes have distinctive acoustic signatures of military interest. Hence they have been much studied during the past 50 years.

"Passive" sediments record the history of deep currents, volcanism, aridity, wind trajectories, and iceberg abundances and trajectories. Drift deposits, which can accumulate at hundreds of meters per million years, yield some of the highest resolution paleoceanographic records.

Biological processes dominate sediment formation in areas of high productivity that receive little terrigenous material. The equatorial Pacific and Southern Ocean are examples.

Riverine dissolved material is distributed by ocean circulation and mixin g. The principal pathway for particle production is primary production by a variety of phytoplankton and zooplankton. Inorganic debris from dead organisms becomes the source for underlying sediment oozes. Most of this material is either calcium carbonate or amorphous silica:

CaCOsub 3

SiOsub 2 (The images are local copies from the WWW server at NGDC, National Geophysical Data Center)

Benthic organisms also modify the historical record by actively mixing ("bioturbating") the most recently deposited sediments. Bioturbation, which is addressed later in the course, is effectively a low-pass filter that supresses or eliminates records of events that create layers of sediment thinner than the depth of mixing. Rapidly deposited or anoxic sediments provide the only deep-sea records capable of resolving events shorter than about a millennium.

Chemical processes dominate sedimentation only in deep, low productivity areas shielded from terrigenous material. Precipitates from hydrothermal solutions emanating from mid-ocean ridges are prominent along the flanks of the East Pacific Rise in the South Pacific. Authigenic deposits (formed by the very slow precipitation of oxyhydroxides and silicates? from normal seawater) form distinctive sediments in the central South Pacific as well as ferromanganese nodules (Figure MS-2) and crusts on any surfaces where other sediments are absent or accumulating extremely slowly.

Ferromanganese Nodules

Figure MS-2. Deep sea ferromanganese nodules on the floor of the South Pacific Ocean (individual nodules are 5-10 cm diameter).

Distribution of Sediments

The physical processes affecting sediment distribution are reflected in this map of sediment thickness:


Figure 13-3, from NOAA/NGDC, additional information is available concerning the underlying database Distribution of Recent (Surface) Sediments

The global distribution of surface sediment types is shown here:


surface sediment types

Figure 13-4, from (2)


Except for areas near to the continents, which are dominated by hemipelagic contiental margin sediments and muds (and at high latitudes, glacial marine sediments), the predominant sediments in the deep ocean are biogenic oozes and/or fine-grained clays.

The distribution of biogenic sediments responds to oceanic conditions in three ways:

controls on primary productivity, especially those related to the intensity of upwelling
The particle compositions exhibits increasing silica/carbonate ratio as upwelling rate increases; there are also regional effects (temperature, light) on species distributions
corrosiveness of deep ocean waters to (settling particles and) surficial sediments
Waters at all depths are undersaturated with respect to solid amorphous silica; the driving force for dissolution decreases as deep water ages and develops higher dissolved silica concentration. (Because waters at all depths are undersaturated, organisms must spend energy to precipitate amorphous silica.). The siutation is more involved for calcium carbonate, with surface waters supersaturated and deep waters undersaturated.
Most of the dissolution of biogenic debris occurs at seafloor. Thus the topography influences the temperature and pressure conditions under which dissolution occurs.
We can illustrate the interaction of these three factors in generating a vertical sequence of sediment types lying on basement rock created on the East Pacific Rise somewhat south of the equator.


development of sedimentary sequence on a
moving plate

Figure 13-5, from (20)


Near to the ridge at which the basement rock is created, metalliferous sediment (of hydrothermal origin) dominates the supply of material. Once away from this source of sediment, carbonate material raining from above accumulates. As the plate subsides, the dissolution of carbonate increases such that this material is no longer preserved and only residual clay material accumulates (at a much lower sedimentation rate). As the site passes beneath the equator, the rate of supply of biogenic silica exceeds the rate of dissolution and it accumulates. Further north beyond the equatorial band of productivity, red clay dominates again.


Sediments place events in time. Their study from this perspective is termed stratigraphy: age relations of rock strata (layers). The objectives of stratigraphic studies are to

The markers first employed were the skeleta of organisms preserved in rocks and so the description of geological time is closely linked to the evolution of the plant and animal kingdoms.

Earth's history is divided into ages:


Geologic Time 1 Geologic Time 2

Figures 13-6ab


In ocean sediments, the Cenozoic and Mesozoic ages are represented. These ages can be further divided into periods and epochs and stages:



Figure 13-7



Figure 13-8


(Figures 13-6, 7 and 8 courtesy of the University of California at Berkeley Museum of Paleontology, their copyright notice. Their web server offers an extensive discussion of geological time in relation to evolution of the species.)

In establishing time scales for ocean sediments, stratigraphic correlations can be made in a number of ways:

boundaries between rocks of different type or character (variant: seismic stratigraphy recognizing these boundaries from seismic reflection profiles)
based on fossil content (presence, absence of particular indicator species)
based on ages of materials as established by radiometric methods
based on magnetic reversals recorded in sediments
stable isotope stratigraphy
based on oxygen isotopic composition of carbonate material, a signal dominated by global state of ice storage on the continents during glacial times
For study of ocean sediments the most important tools are biostratigraphy (applied throughout the Mesozoic and Cenozoic), magnetostratigraphy (important for the late Cenozoic sediments (past several My)), and oxygen isotope stratigraphy (for Pleistocene sediments, last 1.8 My)

Indicators of Changes in Ocean Conditions

Our interest in Pleistocene sediments is derived from the record of climate change they contain. There are two important classes of signals:

Deposition Rates
Signals indicating the extent to which material is deposited and preserved. These include absolute rates of net deposition and indices of changing proportions of materials (e.g., Si vs. C, specific species)
Chemical Signatures
Signals involving properties of sedimentary materials which reflect conditions of original deposition, either directly or as a proxy for some other property. Some examples are oxygen isotopic composition reflecting ice storage on the continents (and to a lesser extent temperature of deposition); Sr/Ca reflecting the temperature of deposition (and to a lesser extent upwelling), and Cd/Ca reflecting upwelling.

Background Information on Sediment Classification

Summarizing the classification of sedimentary rocks:


classification of sedimentary rocks

Figure 13-1


sediment size distribution

Figure 13-2, from the Geology 202 Pages at University of British Columbia


Chemical Sediments, both allochems (precipitates formed in basin of deposition) and precipitates (precipitates formed in deposit itself)
 LimestoneDolomiteIron formationEvaporiteChertOrganicsPhosphate
Chemical CompositionCaCO3CaMg(CO3)2Fe silicate oxide carbonateNaCl, CaSO4SiO2CCa3(PO4)2
Mineralscalcite, aragonitedolomitehematite, limonite, sideritehalite, gypsum, anhydrite, other saltsopal, chalcedony, quartzcoal, oil, gasapatite


Marine sediments are further classified:

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