Miall & sungai

[21-9-14M vbm]
MIALL 1992
Much of the oil in the giant fields at Prudhoe Bay (Alaska),Brent
(North Sea), & Daqing (China) occurs in fluvial reservoirs [1992:119]

Minor chemical sediments are formed in floodplain environments.They
include nodular carbonate deposits formed in fossil soils (paleosols),some coals, & minor evaporites deposited in nonmarine sabkhas [1992:119]

cHANNELS are filled by a wide variety of bars & bedforms. Larger
depositional units are termed BARS (fig.1)

Classifications of the deposits: FACIES

4 styles of fluvial channels: MEANDERING, BRAIDED, ANASTOMOSED & STRAIGHT

The study of alluvial deposits plays a key role in the reconstruction of the plate-tectonic evolution of sedimentary basins, & in deducing the history of relative changes in sea level within sedimentary basins [1992:119-121]

Sediment is transported in rivers by two mechanisms, traction currents & sediment gravity flows~~~~~ Most alluvial sediments are deposited from traction currents, but in certain settings sediment gravity flows may be important [1992:121]

2 terms, COMPETENCE & CAPACITY, are used to describe the ability of a river to transport sediment [1992:121]

Fluvial depositional units can be subdivided into ten natural groups,based on their physical dimensions,their sedimentation rate, & the time scale represented by each type of unit (Table 1, Fig. 3) ~~~~A sedimentological classification of these surfaces (Table 1, Fig. 3) is a useful tool for field studies,facilitating field description & leading to more comprehensive interpretations [1992:121]

Petroleum reservoir geologists recognize at least 5 scales of
heterogeneity for purposes of calculating volumes & rates of
production. 4 of these are shown in Figure 4. Microscopic
heterogeneity is concerned w/ porosity variations at the scale of
individual sand grains (within group 1 of Table 1). Mesoscopic
heterogeneity is that of bedding units & sedimentary structures
(groups 1-4). Macroscopic heterogeneity includes the variability
associated w/ the deposition of channels & bars (groups 5-7).
Megascopic heterogeneity deals w/ the variations across major
sedimentary units & entire basins (groups 8-10) [1992:121]

Depositional rates in modern sediments can be estimated using l4C
techniques, or by studying the evolution of meanders & bars on maps & aerial photographs. These rates are typically far higher than those estimated from ancient geological units. This is because individual depositional units have a limited preservation potential, & any givensedimentary structure, bar or channel may be removed by erosion ratherthan become buried & preserved. [1992:121-122]

The study of sedimentary structures in modern river deposits played an important part in the development of our understanding of their
hydrodynamic formation [1992:122]

A classification erected by Miall (1977, 1978b; Table 2) is now in
widespread use for surface outcrop studies and subsurface core
analysis. Examples of facies types are given in Figures 5 & 6

In gravels, an important distinction is that between matrix-supported (facies Gms; Fig. 5A) and clast-supported types (Figs. 58, C). Matrix support indicates that clasts & matrix were deposited together, a characteristic of debris flows. The other facies are those deposited by traction currents [1992:122]

Sand facies are characterized by a range of sedimentary structures
indicating various conditions of deposition (Fig. 5D-G). Thevertical succession of such facies is commonly repetitive, or cyclic, &
indicates progressive change in depositional conditions. For
example, a vertical decrease in grain size & in scale of cross bedding is a feature of the classic fining-upward point-bar succession (Allen,1963), which is discussed in a later section of this chapter [1992:122]

in fluvialsystems, macroforms include major channels & bars, such as point bars,side bars, sand flats & islands (Fig.7) [1992:124]
MIALL 1992
Allen’s (1983a) study of the Devonian Brownstones of the Welsh Borders represents the first explicit attempt to formalize the concept of a hierarchy of bounding surface in fluvial deposits (fig 2.30) ~~~~~~~ A more elaborate [terperinci] & comprehensive classification was developed as a result (Miall 1988ab), consisting of a sixfold hierarchy of lithosomes & bounding surfaces. This is the system used in this book (w/ additions) & is described in Ch 4. [2006:43-44]

[4-6-14R lab mikropal]


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