Continental Environments

Fluvial environment
: streams and rivers.
Most of the geologic record produced by streams is formed when they flood.
Fluvial deposits record not continuous daily activity, but periodic flooding events.

Two types of stream systems:
1) meandering streams:

characterized by a single meandering channel flanked by sandy point bars, cutbanks, and floodplain deposits.

Dominant deposit types:
point bar deposits
Sand deposited on the channel’s inner, gently sloping banks opposite the cutbanks.
Consist of elongate cross-bedded sand bodies overlying an eroded surface.

floodplain deposits
Silt and muds deposited over and beyond the levee during floods.

Characteristics:
dominated by mud deposits (floodplain deposits)
elongate, crossbedded sand (point bar deposits)
terrestrial/freshwater plants and animals

2) braided streams
: characterized
by vegetated islands, sandflats, and gravel bars around which the stream flows in a constantly branching and rejoining braided pattern.

Tend to occur where there are:
a) rapid fluctuations in stream flow rates
b) higher stream slopes
c) large proportions of coarse sediments
d) easily eroded banks

Characteristics:
dominated by cross-bedded gravel and sand deposits (gravel and sand bars, sandflats, vegetated islands)
little or no mud
terrestrial or freshwater plants and animals

Desert environment
Includes three subenvironments:
1) desert alluvial fans: deposited where intermittent streams confined by narrow valleys emerge onto the margins of desert basins.

Deposition occurs due to the sharp decrease in the stream’s ability to transport sediment as it emerges from the confined valley.

Characteristics:
fan-shaped sediment body that radiates from the source valley
gravel and sand at the mouth, rapidly decreasing down fan to sand, silt, and mud
rounded gravel
terrestrial plant and animal fossils

2) sandy deserts (aka sand seas or ergs): marked by abundant eolian sand dunes.

Sand accumulates in an erg mainly due to the presence of a topographic basin.

Characterized by:
well-sorted sand in large crossbeds
fossils are scarce and are limited to terrestrial animals

3) playa lakes: temporary lakes located on the desert floor that are fed by streams and/or groundwater discharge.

Evaporites accumulate where groundwater discharge occurs.
Minerals include: calcite, dolomite, gypsum, sodium carbonates (trona), and sodium sulfates.

Characterized by:
laminated sand, silt, mud, and evaporites
mudcracks
scarce terrestrial and lacustrine plant and animal fossils


Glacial environment:

characterized by the dominance of ice as a geological agent
(erosion, transportation, and deposition).

Glacial ice accumulates due to three conditions:
1) low temperature
2) high rates of precipitation
3) extremely low rates of evaporation.

Glacial ice flows out from the snow fields where it accumulates under the influence of gravity.

Characterized by:
glacial striations: scratches and grooves carved into a rock surface as the glacier moves over it.

glacial erratics: boulders carried by the glacier into areas where no similar bedrock occurs.

glacial till: unsorted, unstratified sediments deposited directly by glacial ice.

glacial outwash: braided stream deposits of sand and gravel.

glacial varves: alternating light and dark, finely laminated muds, deposited in glacial lakes.

dropstones: gravel dropped from floating ice into finely laminated or unlaminated muds of glacial lakes or the ocean.