Environments of Deposition
Learning Objectives
The Earth is not stable, it is being uplifted in some areas and eroded downward in others. Rock
weathering and erosion produces a continuous stream of sediments that are laid down in layers.
Contained within these sediments are clues that geologists use to understand the Earth’s past,
both geologic and biologic. The sediments lithify to form rocks which stand as a record of
Earth’s history.
Environment of Deposition: all physical, chemical, biological and geographic conditions under
which sediments are deposited. Geologists look at environments existing today and the
sediments that they contain, from this they extrapolate the types of rocks these sediments will
form. Using the Principle of Uniformatarianism geologists compare rock units from Earth’s past
to sediments of the Earth’s present and can determine past environments that once existed.
Example- a tidal lagoon: sediment deposited in a tidal lagoon is composed of fine silts and muds,
which lithifies into shales. The environment is often anaerobic and the sediment is a dark gray or
black, producing a similarly colored shale. If a geologist found a black shale in an area then one
possible interpretation for that area would be that it once was covered by a tidal lagoon. Fossils
found within the shale will help to further narrow the possible environment of deposition.
The type of sediment indicates the environment of deposition.
There are three major environments of deposition: marine, transitional and continental.
1. Marine: includes continental shelves, continental slopes, continental rises and abyssal plain.
A) Continental Shelf: nearly flat, gently sloping edge of the continent that extends under the
ocean. Sediments wash off of the continent onto the shelf and range in size from coarse sand
closest to the continent fining to clays. In some areas coral reefs or carbonate muds dominate the
shelf.
B) Continental Slope: more steeply sloping edge of shelf that extends down to the ocean floor.
Sediments on the slope consist of fine silts and clays.
C) Continental Rise: fan shaped deposit of sediment at the base of the continental slope, often
contains turbidite deposits from turbidity currents. Sediments consist of sands, silts and clays
from shelf.
D) Deep Marine: referred to as the abyssal plain, the ocean floor is blanketed with a fine
sediment consisting of the microscopic shells of marine plankton. Calcareous and siliceous
deposits are the most common.
2. Transitional Environments: areas found along the edges of a continent.
A) Beaches: usually composed of sands or gravels. Dunes of cross bedded sands often form.
B) Tidal Flats: mud covered flats that are alternately exposed and inundated.
C) Deltas: formed at the mouth of a river, delta consist of cross bedded sands that fine outward
and upward.
D) Barrier Islands: Semi-permanent sand deposits parallel to the coast lines.
E) Lagoons: area between barrier islands and mainland, lagoons consist of fine silts and muds.
F) Swamps: low area on mainland adjacent to the sea, contain silts, muds and organic deposits.
3. Continental Environments: found on the continent, includes a wide variety of environments.
A) Fluvial Environments: includes many sub-environments. Sediments consist of sands, silts
and clays interlayered by meandering streams. Sands are found in point bars and along stream
banks and bottoms, clays and silts are found on the floodplain.
B) Lakes: silts and clays with organic deposits in some areas.
C) Arid Environments: evaporite deposits common as are alluvial fans.
D) Glacial Environments: large volumes of unsorted sediments.
E) Eolian Deposits: wind blown deposits characterized by uniform size of sediments and cross
bedding.
Each of the environments above contain distinct sediments. This enables geologists to determine
the past environment of an area based on the rock types found there. Other sedimentary features
help geologists determine the environment of deposition.
Sedimentary Features:
I) Color:
A) Black or dark gray: indicates the presence of organic material that was not sufficiently
oxidized. Forms in anaerobic areas such as a restricted basin, lake or swamp. Limited black
bands in a beach deposit are formed from ferromagnesium mineral separation not organic
material.
B) Red, brown, yellow, orange: indicates the presence of oxidized iron. Forms in areas with
abundant oxygen, common in nonmarine deposits.
II) Texture: the size, shape and arrangement of mineral grains in a rock.
A) Size: the Wentworth Scale (p. 69) categorizes sedimentary grain sizes. This is important
because the size of a sedimentary grain infers the mode of transport of the original sediment.
B) Sorting: refers to the size range within a sedimentary deposit, all sediment grains are the
same size or sediment grains of many sizes are present. Sorting can infer environment of
deposition.
C) Rounding: refers to the roundness of the individual sediment grain. Rounding infers the
length of time of sediment transport and hence distance from the sediment source. This is
important if the source has completely eroded.
III) Sedimentary Structures: these are the result of specific depositional processes.
A) Mud Cracks: form from drying of fine grained material after deposition. Important because
it implies that an area was intermittently wet then dry.
B) Cross Bedding: layers of sediments are inclined relative to each other. Cross bedding infers
deposition by either streams or wind.
C) Graded Bedding: repeating sediment layers that consist of material that fines upward.
D) Ripple Marks: sedimentary layer has a rippled surface. Symmetric ripple marks form from
oscillating water movement under gentle waves. Asymmetric ripple marks forms from a constant
current or wind direction, the direction of the ripple indicates current direction.
