Continental Environments
Chapter 9
Continental Environments
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Deserts
Alluvial fans
Rivers (fluvial) and floodplains
Lakes (lacustrine)
Glacial
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“Desert Biome”
http://www.blueplanetbiomes.org/climate.htm
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low-latitude deserts approximately between 18° to 28°,
edge of the equatorial subtropical high pressure belt and trade
winds
Descending air masses – therefore rarely hold much moisture
for precipitation
covers 12 % of the Earth's land surface.
Temperature Range: 16° C
Annual Precipitation: 0.25 cm (0.1 in). All months less than
0.25 cm (0.1 in).
World Climate Zones
http://www.blueplanetbiomes.org/climate.htm
Eolian Cross Sets
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Eolian (Desert) Deposits
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Most between 20-30 latitude
Coarsest clasts remain as deflation lag (desert pavement)
Sands are well sorted & rounded; surface pitting and frosting
Large-scale planar-tabular or trough- and wedge-shaped cross
beds
Diagnostic Features
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TECTONIC SETTING
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GEOMETRY
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Dune fields can cover 100's km2; tabular bodies up to 35 m thick
SEQUENCE
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Mountain rain shadows, associated with desert deposits
Large-scale x-beds, foresets dip 25-30; deflation gravel or pebble lags
SEDIMENTOLOGY
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Well-sorted, well rounded quartz-rich sand; Large scale x-beds
comprised of smaller scale low-amplitude wind ripples
Alluvial fans
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common in modern and ancient deposits;
most common in semi-arid region where pronounced
gradient or abrupt relief exists (highland, hills,
mountains, faulting scarps);
preservation potential high when building into lakes,
rivers, playas, flood plains;
May be divided into upper, mid, and lower fans;
commonly associated with intermittent current dunes
and salt lake (playa) or salt flat (sabkha) deposits.
prograding alluvial fans: overall coarsening-upward
succession.
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Boggs 2001
Alluvial Fans, Baffin Island
http://www.physicalgeography.net/fundamentals/10z.html
Different types of
fans
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Debris-Flow Fans
Braided Fluvial Fans
Lowsinuosity/meandering
Fluvial Fans
Boggs 2001
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Sediment Gravity Flows
Boggs 2001
Proximal Fan
Poorly sorted, large, angular clasts
coarse, thick-bedded, poorly
stratified alluvial fan deposits.
Note lack of channels
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Fluvial Environments
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Deposits of rivers and associated
environments
Widespread in sedimentary rock record
Fluvial Environments
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Morphological components:
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Valleys
Channels
Interfluves (floodplains)
Drainage basins
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Headlands – erosional (generally…)
Coastal plain – depositional (generally…)
Channel:
“Trough” through
which water runs.
Press and Siever, 2001
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Valley: Area
between tops of
slopes on both
sides of the river
Press and Siever, 2001
Floodplain: Flat
area, about level
with the top of the
channel.
Inundated during
high discharges.
Press and Siever, 2001
Channel Types
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Four principal types:
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Meandering – winding, single channels
Braided – multiple channels that change
location
Anastamosing – multiple channels that
maintain their locations
Straight (rare)
Continuous gradation from one type to
another
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Fluvial
Systems
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Straight
(rare)
Meandering
Braided
Anastamosed
Boggs 2001
Components
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Bedforms (dunes, ripples, flat bed)
Bars – larger depositional units
(transverse, linguoid, point)
Channels – different types
Levees
Overbank deposits – floodplain,
crevasse splays, paleosols, etc.
Cutbank
Point Bar
Coarse-grained meandering
Sandy bedforms
Overbank
Fines
Walker and Cant 1984
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Point Bar
Cutbank
Press and Siever, 2001
one km
Timeslice – 3-D seismic data
Offshore, S.E. Asia
Courtesy Henry Posamentier
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Press and Siever, 2001
Modern Mississippi Floodplain
Walker and Cant 1984
Google Earth
Thalweg: deepest part of the
channel, strongest currents
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Lateral Accretion Surfaces – Cretaceous (?)
Colorado
Lateral Accretion Surfaces – Cretaceous Utah
Fluvial Environments
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Floods:
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Streams overspill their banks, innundating
floodplain
“Short-term” increase in discharge
Frequency variable, depends on location
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Press and Siever, 2001
Press and Siever, 2001
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Fluvial Environments
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Floodplain deposits
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Mud (shale) deposited during floods
Crevasse splays – breaches in levees
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Sandy, rippled, fan shape
Rooted horizons
Coal, carbonaceous shale
Paleosols
Floodplain (channels and paleosols)
Cretaceous, New Mexico
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Channel sand surrounded by finegrained floodplain deposits
“Classic” finingupward point bar
succession
Thickness will be
proportional to
channel depth
Walker and Cant 1984
Sandy Braided System
Sand flats
Downstream
accretion
Sandy bedforms
Walker and Cant 1984
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Gravel-bed braided system
Gravel bars & bedforms
Walker and Cant 1984
Sand Flat
(Compound bar)
3-D Dunes
(Linguoid Bars)
Walker and Cant 1984
Brahmaputra River
Bangladesh
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braid plain defined by edge of vegetation
Stacked ?braided fluvial sandstones
Cretaceous, New Mexico
Braided stream facies
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broad and shallow
channel bars (longitudinal, transverse)
are main sites of sediment accumulation
gravel sediment dominant vs. sandgrade sediment dominant
2-D dunes most common: planar
(tabular) x-beds dominant
vertical succession: stacked dunes with
planar x-stratification
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Geometry of Braided River Deposits
Walker and Cant 1984
Anastamosing
Overbank
Fines
Swamps
(peat/coal)
and small
floodplain
lakes
Sandy bedforms
Channel
Walker and Cant 1984
Channel Types
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Morphology of fluvial system controls
sandbody geometry
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Braided: “sheet” sandstones, high sand/shale
ratio
Meandering: sandstones “lenticular” in crosssection, moderate/low sand/shale ratio, finingupward successions
Anastomosing: shoestring sandstones in shale,
low sand/shale ratio
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Lakes
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Open lakes have an outflow (i.e. a river)
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Closed lakes have no outflow
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sediments dominated by terriginous particles
and organic matter
solutes are not carried out of the basin
alkalinity can build up such that carbonates
and evaporites can precipitate.
A lake can alternate between open and
closed with tectonic or hydrologic changes
Lakes
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Deposits in open lakes come mainly from
rivers but may also be deposited by wind,
ice-rafting, and other processes.
Sedimentation in closed lake systems
consists of evaporite minerals, carbonate
muds, sands, and silts.
Lacustrine deposits are often rich in
organic matter
Distribution of Open Lakes
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Distribution of Closed Lakes
Transport mechanisms and kinds of
siliciclastic sediments in lakes with annual
thermal layering
Boggs, 2001
Varves-Ellismere Island
coarser lighter spring runoff, then finer darker fall dieoff
(http://www.geo.umass.edu/climate/TILPHTML/Lakesedssummary.html
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Open Lakes
Boggs 2001
Alkaline Lakes
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Closed lakes subject to limited
replenishment from rainfall
Buildup of salinity as there is no outlet
Evaporites such as Sodium sulfate
deposits of Saskatchewan
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The main uses of sodium sulphate are in the pulp
and paper, powder detergent, glass and dyeing
industries.
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Boggs 2001
Diagnostic Features of
LACUSTRINE DEPOSITS
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TECTONIC SETTING
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Fault grabens or downwarped basins; associated with
other nonmarine settings
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GEOMETRY
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SEQUENCE
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Circular or elongate; lenticular in cross section
Coarsening upwards from laminated shale, marl to ripple
& cross- bedded sand. Cyclicity possible.
SEDIMENTOLOGY
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Mudstones, shales, sandstones; carbonate, gypsum,
halite; dessication features. Nonmarine fossils
Glacial Deposits
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Glacier:
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Large masses of ice on land that show
evidence of being in motion or of once
having moved
Two types: valley glaciers, continental
glaciers
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Press and Siever, 2001
Glacial Deposits
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Glacier formation
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Low Temperature (high latitude or high
altitude)
Snow accumulation > ablation
Snow compacts, turns to ice
Movement
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Gravity as driving force
Basal slip and/or plastic flow
Rates: m/yr (km/yr in surges)
Press and Siever, 2001
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Glacial Deposits
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Flowing ice erodes rock & regolith
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Erosional forms: Striations, U-shaped
valleys, Fjords, Cirques, Roche moutonée
Flowing ice transports sediment
Sediment deposited where ice melts
Press and Siever, 2001
Press and Siever, 2001
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Glacial Deposits
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Deposition:
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“Drift” – material of glacial origin
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Till: deposited directly by ice
Outwash: deposited by glacial meltwater
Loess: deposited by wind
Glaciolacustrine: deposited in glacial lakes
Glaciomarine: deposited in the sea by/close to
ice
Erratic: large boulder in till
Press and Siever, 2001
Boggs, 2001
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Glacial Deposits
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Composite that includes eolian, fluvial,
lacustrine, and even marine
environments
Ice-contact deposits characterized by
extremely poor sorting and lack of
stratification
Lakes and marine deposits may have
dropstones
Glacial Episodes
Glacial
sediments
in N.A.
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Summary
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