Deposition of Organic-Rich Sediments

Fig. 3. 23 Important oxygen-depleted environments (shaded areas) associated with deposition of organic-rich sediments (after Brooks et al. 1987).

Deposition of organic-rich sediments further down the shelf and on to the continental slope and rise often occurs as a result of turbidite flows, redistributing organic-rich sediments from delta fronts or from further up the shelf and slope (Summerhayes 1983). While there is a certain amount of pelagic sedimentation, primary production decreases away from the coastline as nutrient levels decline, and detritus is largely recycled before it settles to the sea floor. However, this may not always have been so in the past, when the thermohaline circulation (Box 3.2) did not operate and there may have been widespread anoxia in bottom waters, aiding preservation of sedimentary organic matter (e.g. Cretaceous oceanic anoxic events Section 6.3.4). 

For the most part, sediments are also stratigraphically uniform, showing only a few percentage variation in lithologic composition. Cores from Mountain Lake, which consistently show up-core decreases in carbonate content (to about 60% that at depth), are the only exception. A number of shallow-water cores that contain a thin veneer of organic-rich sediments overlying silt and sand were also excluded from analysis. In most locations the spatial boundary between organic-rich profundal-type sediments and littoral deposits of coarse detritus or massive silt was clearly defined. 

Figure I. Elements of a Petroleum System. All petroleum systems contain 1. at least one formation of organic-rich sediments that has been buried to a sufficient depth by overburden rock such that petroleum is generated and expelled, 2. Pathways (permeable strata and faults) that allow the petroleum to migrate, 3. Reservoir rocks with sufficient porosity and permeability to accumulate economically significant quantities of petroleum, and 4. Sealing rock (low permeability) and structures that retain migrated petroleum within the reservoir rock. The top and bottom of the oil window is approximated as a function of burial depth. In actual basins, these depths are not uniform and vary as a function of organic matter type, regional heat flow from basement, in thermal conductivity of the different lithologies, and burial history (e.g., deposition rates, uplift, erosion, and hiatus events).

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

Because of this trapping effect, estuarine sediments can provide excellent chronological records of contamination histories. An example is provided in Figure 28.24 for sediment collected from two sites located in two watersheds in the Elizabeth River, VA, USA. In general, sedimentary trace metals tend to be closely related to the transport and deposition of fine-grained and organic-rich sediments. The downcore profiles presented... 

A fraction of the eroded mass made of small-size particles is transported by winds over subglobal distances, to be ultimately deposited in the open ocean and on other continents. Rates of sediment accumulation in the open oceans are much lower than for sediments in continental bodies of water, where in lakes the rates are typically of an order of 10° 1 mm yr"1 for terrigenous sediments and are higher for deposition of organic-matter-rich muds in areas of strong primary productivity. 

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