Deposition burial history

Stratigraphy, depositional and burial history of the Epi-Ligurian succession... 

A concretion is a volume of sedimentary rock in which mineral cement fills the spaces between the sediment grains. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. Concretions form within layers of already-deposited sedimentary strata. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. 

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).

Often, the only data available for assessing that history of influences are the physical and chemical properties of the present-day sediment bed itself, which is an integrated product of the entire history of influences. The net sedimentation rate is the rate at which material accumulates in the sediment bed in response to the historical record of settling, deposition, burial, and erosion processes. Numerous forensic and investigative methods are available to provide insight into the historical record contained in bed sediments. These include physical measurements of the bed such as bed elevation measurements and sediment physical properties, and chemical measurements such as radioisotope analyses and vertical contaminant distributions. 

These may contain any of the heavy minerals, the species and proportions present depending on the nature of the rocks from which the sediments were derived, and on the ability of the minerals to withstand the processes of weathering, erosion, transport, deposition, burial, diagenesis, uplift, folding, and other tectonic effects and subsequent history up to the present time. These processes can have a considerable influence on the relative proportions of the heavy minerals, and the most resistant species such as zircon, tourmaline, and rutile may be the only survivors. 

The relevance of the remarks on sulfur content is that, for reasons explained above, it is usually a valid index of the salinity of the environments of deposition. It was remarked earlier that the Eastern and Interior provinces have experienced different temperature/pressure/time histories. It should be added that coals of the Rocky Mountain, Pacific and Alaskan provinces most probably experienced yet further sets of conditions of metamorphism a locally increased geothermal gradient that produced relatively high temperatures at relatively low depths of burial and hence at relatively low pressures of overburden. 

In contrast to the evidence for a late diagenetic timing of arsenic addition, one observation may indicate relatively early introduction of arsenic in the depositional history of the coal beds. As illustrated in Fig. 17 arsenian pyrite locally fills cell lumens (the cavities bounded by the cell wall) of coalified plant debris. These cell lumens, if unfilled, are crushed during burial. Therefore, unless the arsenian (> 0.1 wt. % arsenic) pyrite is a replacement of earlier mineral phase, the presence of arsenian pyrite in uncmshed lumens argues for a relatively early formation. 

Fig. 13. Burial and thermal history diagram of the Namorado turbidites in the Abacora Field (B). Note in the relative sea-level curve of Vail et al. (1977) (A) that at the time of the turbidite deposition there was a s nificant sea-level fall (Cenomanian). The low sea-level stand favoured meteoric water invasion at this time.

Even though this sediment represents marine offshore deep-water turbidites, meteoric flow could have reached the reservoir just after turbidite deposition, during a sea-Ievel lowstand (Fig. 13), when both the platform and part of the slope were partially exposed. The relatively low burial rate during the early history of the Namorado Sandstone favours this hypothesis. Meteoric waters resulted in the formation of blocky calcite that replaces bioclasts or occurs as the inner rims of mouldic pores (see Carvalho et al., 1995). 

The history of the bio eochemical C cycle has been at least partially recorded in the C isotopic composition (S Cpdb) of carbonate (Scarb) and reduced C (5org) in ancient sedimentary and metamorphic rocks. To the extent that sedimentary rocks avoided deep burial and alteration, they have preserved information that indicates the status of the C cycle at the time of their deposition. 

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