DEFINITION OF FACIES

Refers to the appearance and characteristics of a rock,

with implications for its mode of origin. The term usually refers

to sedimentary rocks with descriptions of the rock’s lithology

and inferred environment of deposition. Other less common

usages of the term include metamorphic facies, identified by the

mineral assemblages that form in specific rock types under a set

range in pressure and temperature conditions, and biofacies,

referring to a local assemblage of living or fossil organisms. The

term has also been applied, somewhat unsuccessfully, to differentiate

igneous and even strain variations in different terrains.

If we travel around the planet we will see that different

types of sedimentary rocks are deposited in different places.

Within individual systems, such as the Mississippi River

Delta, or the Gulf of Mexico, there are also lateral changes,

such as differences in grain size, or the type of sedimentary

structures. Sedimentary facies refers to a distinctive group of

characteristics within a body of sediment that differs from

those elsewhere in the same body.

Most sediments change laterally as a result of changes in

original depositional environment. An interesting and useful

concept, known as Walther’s Law, states that the lateral

changes in sedimentary facies can also be found in the vertical

succession of strata in the system, because of the lateral

migration of sedimentary systems. Thus, changes upward in

the stratigraphy also reflect changes laterally.

Sedimentary facies in nonmarine environments include

stream facies, lake facies, glacial facies, and eolian (or winddominated)

facies. Within each of these environments, different

sub-facies can be described and differentiated from each

other. Streams are the principal transporting agent for moving

sediments over land. Stream sediments are known as alluvium,

and the sedimentary environment is known as fluvial. There

are many different sedimentary facies in fluvial systems, and

these include conglomerates and sand deposited in the stream

channel and fine-grained silt and mud deposited on the floodplain

or alluvial plain. Lake sediments are known as lacustrine,

with different facies including the lakeshore and the lake bottom.

Lakeshore deposits include gravel and sand beaches (for

big lakes) and deltas, whereas lake bottom environments

include finely laminated clays and silts and vary under the

right conditions. Glaciers are great movers of sediments. Sediments

deposited directly by glaciers are typically poorly sorted

conglomerates mixed with clay, and many fragments may be

angular. Many sediments from glaciers are reworked by meltwater,

however, and deposited in streams and lakes in front of

the glacier. Sediments deposited by wind are referred to as

eolian deposits. Since air is less dense than water and can hold

less material in suspension, deposits from wind systems tend to

be fine-grained. Sand and sand dunes are typical eolian

deposits, with large-scale cross-laminations in the sand.

Some of the world’s thickest sedimentary deposits are

located on the continental shelves, and these are of considerable

economic importance because they also host the world’s

largest petroleum reserves. The continental shelves are divided

into many different sedimentary environments and facies.

Nearshore environments include estuaries, deltas, beaches, and

shallow marine continental shelves. Many of the sediments

transported by rivers are deposited in estuaries, which are

semi-enclosed bodies of water in which freshwater and seawater

mix. In many cases, estuaries are slowly subsiding, and they

get filled with thick sedimentary deposits such as muds,

dolomitic or limestone muds, silts, and storm deposits. Deltas

are formed where streams and rivers meet the ocean and drop

their loads because of the reduced flow velocity. Deltas are

complex sedimentary systems, with coarse stream channels,

fine-grained interchannel and overbank sediments, and a gradation

into deepwater deposits. Beaches contain the coarse

fraction of material deposited at the oceanfront by rivers and

sea cliff erosion. Quartz is typically very abundant, because of

its resistance to weathering and its abundance in the crust.

Beach sands tend to be well-rounded, as does anything else

such as beach glass, because of the continuous abrasion caused

by the waves dragging the particles back and forth.

Deep marine facies include pelagic muds, siliceous oozes,

and windblown dust that accumulates on the seafloor. The

character of deep marine deposits has changed dramatically

with time since many of the present deep-sea sediments are

produced by the accumulation of the skeletal remains of

organisms that did not exist in the Precambrian.

Facies analysis is a very important tool for reconstructing

ancient paleoenvironments and depositional settings. When

the facies of a rock are understood, facies analysis becomes an

important exploration tool and may help find new locations

for economically significant hydrocarbon reserves or help

understand the tectonic and depositional history of a region.

See also SEDIMENTARY ROCKS.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar