Sed Exam 2: Depositional Environments

glaciers

A large accumulation of ice, snow, and debris that forms on land and moves down slope under its own weight and gravity

from compression of snow → granular ice → firn → glacial ice

dependent on latitude of snowline

alpine glaciers

ice that forms in mountainous areas and flow downhill

valley glaciers

form in valley areas

piedmont glaciers

spread out into lowlands

ice caps

dome-shaped mass of glacier ice that spreads out in all directions

forms in high latitudes/ polar regions

ice sheets

dome-shaped mass of glacier ice that is greater than 50,000 km²

glacier flow

mass balance

balance between accumulation and ablation

accumulation zone: gaining mass

ablation zone: losing mass (melting, calving icebergs, sublimation)

equilibrium line evelation

warmer temperatures have ELA at higher elevations and glacier retreat

colder temperatures have ELA at lower elevations and glaciers expands

last glacial maximum

30 percent of land to 10 percent currently

supraglacial zone

on top of glacier

ice contact zone

ice in contact with bedrock

basal zone

base of glacier where plucking and abrasion occur

proglacial zone

areas beyond glacier influenced by melt processes

till

general term for unconsolidated sediment from glacial melt (very poorly sorted, clays-boulders)

generated from plucking via freeze-thaw at the surface and abrasion at the base and sides

tillite

a type of diamcitite from till becoming rock

glacial erratics

boulders carried far from source

glacial polish

smooth surface from abrasion

giant loofah (glacier) exfoliates face (land)

chatter marks

concentric gouges from boulder in basal ice pressed into bed

indicates flow direction

glacial grooves and striations

grooves scratched into cobbles and boulders transported by ice

rock flour

pulverized silt- to clay sized particles

occurs at mouths of braided streams that drain glaciers

grey, sticky, contribute to bright blue lakes

moraines

ridges, mounds, or sheets of unsorted, unstratified glacial sediment

combination of glacial till and roc flour

deposits classified as unstratified diamicts

types of moraines

terminal, recessional, lateral moraines

marine-terminating glaciers/tidewater glaciers

edge of glacier terminates in the sea instead of on land

sediment dumped and reworked by tides/waves, icebergs can form and carry debris, dropstones in marine cores

eskers

narrow sinuous ridges deposited parallel to glacial advance/retreat

formed by meltwater streams within glaciers

deposits classified as stratified diamicts

streams underneath glaciers deposit sed, occurs on top of topography, so land is not carved

kames

hill or ridges of till formed from meltwater deposits

unstratified heads of eskers

drumlins

ridges, mounds, or sheets of unsorted, unstratified glacial sediment

elongate part at ice edge, indicates direction of glacial flow

unclear how they form (erosional features from subglacial water flow, deposits infilling cavities under the ice, a combo, ice surges)

can contain stratified and/or unstratified till

kettle lakes

lakes that formed from depression created by a heavy block of ice

braided streams

create outwash plain deposits

finger lakes

elongated lake that forms in glacially incised valleys

proglacial lakes

form at edge of glaciers

expand like crazy when ice dams melt

glacial varves

seasonal laminations

bright summer layers

dark, thin winter layers

loess

windblown deposits of glacial flour

important deposits for agriculture and also for paleoclimate and paleoecology

proglacial example sequence

deformed substrate → lodgment till → ice-melt structure → braided-stream bar gravels → cross-bedded sands and gravelly sands → braided stream → glaciofluvial outwash → loess

demonstrates glacial retreat

river

body of flowing water that moves downhill via a channel

alluvial fan

fan-shaped mass of alluvium deposited when the flow of a river decreases in velocity

factors of channel form

channel slope, sediment transport processes, sediment characteristics

channel shape

number of channels and degree of sinuosity

straight, meandering, braided, anastomosing

sinuosity

the deviation of the channel from a straight line

degree (straight, low sinuous) and type (regular, irregular, and toruous)

meandering form

flows in a winding, snake-like pattern with numerous curves and bends, typically forming in flat areas

braided forms

streams with larger bedforms (bars) and accreting islands around which channels diverge and converge

anastomosing form

composed of two or more interconnected channels that enclose flood basin

“islands” but only form from flood flows and doesn’t deposit as often as braided streams

width to depth ratio

straight and meandering are deep and narrow

braided are wide and shallow

gradient

steep versus gentle

straight channels have gentle slopes while braided channels have higher slopes

flow velocity

discharge variability

sediment size, sediment load

stability of channel banks

meandering rivers

single channels, high degree of sinuosity, lower gradient, finer sed load, cohesive banks

meandering river sediments

ripples/dunes and trough cross-bedding

cutbanks (laterally migrating areas of erosion)

point bars (laterally accreting areas of deposition), characterized by cross-bedding, fining upward towards top of bar

oxbow lakes or abandoned meanders

floodplains have finer grained deposition (planar bedding) and deposition from overbank water (vertical accretion)

levees with coarse grained on river side, fine grained on floodplain side

crevasse splay when a stream breaks its natural levee creating a graded deposits and climbing ripples (similar to a bouma sequence)

siderite (FeCO3)

forms through diagenesis in floodplainss under anaerobic, reducing conditions in areas with poor drainage

desiccation cracks and raindrop impressions

can be found in floodplain deposits

braided rivers environmental coditions

steep gradient, high relief areas, large/rapid fluctuations in river discharge, abundance of coarse sediment, noncohesive banks

braided river features

multiple channels that diverge and converge

point bars and mid-channel bars

mid-channel bars

accrete sediment on both sides

often submerged at high flow

longitudinal bars

parallel to flow

teardrop

coarse material on the upstream side and sand on the downstream side

usually form around an obstacle

can get more elongate through time

traverse bars

perpendicular to flow

wide and lobate shaped

sand size material

usually form in sandy streams when flow slows down

can migrate downstream through time

braided rivers stratigraphy

produce vertical stacking of bar deposits from episodes of channel shifting

cross-bedding in sand units (traverse bedding)

coarse material in longitudinal bars

planar and trough cross-bedding in channel deposits

factors controlling river system dynamics

sediment supply vs accommodation, hydrologic conditions (climate), base level, tectonics (uplift/subsidence)

effects from changes in river system dynamics 

channel form adjustments (incision, downcutting, widening, aggradation, progradation)

sediment transfer

incision/downcutting

if a river is above base level, it will down cut and erode it’s bed

base level

lowest level to which water can flow and erode

local base level but also ultimate base level of sea level

widening

a river near base level will start eroding its banks (valley widening and river meandering)

aggradation

vertical accumulation of sediments in the riverbed or floodplain

when the long-term rate of accommodation closely matches the long-term rate of sedimentation

relative balance between sediment supply and accommodation space

progradation

sediment sequence that build forward/outward from a source region (ex. seaward)

when the long-term rate of sedimentation exceeds long-term rate of accommodation

imbalance between sediment supple and accommodation space

impacts of sediment load and transfer

infrastructure (bridges, dams, reservoirs, navigability), river health and ecology, soil erosion from poor agricultural practices and deforestation, uplift and weathering over geologic time (basin infill, continental margin evolution), ocean sediment supply, pollution, hypoxia

suspended sediment discharge

Qs = kQC

Q is discharge

C is suspended sediment

k is conversion factor

Qs is suspended sediment discharge

the total amount of sediment transported by a river given points (as weight per unit volume)

alluvial fans

cone or wedge-shaped, consisting of poorly sorted sediment

form at a break in slope (water velocity = 0)

common in areas of high relief, where there is an abundant sediment supply

sediment transport typically associated with infrequent mass movements or flash flooding

debris-flow dominated fans

characterized by gravity flows of high density, and high viscosity

gullies, levees, lobes (active and abandoned lobes)

stream-flow dominated fans

deposition occurs by fluvial activity (by ephemeral streams)

active depositional lobe/channel occasionally shifts laterally to produce fan shaped cone of sediment

fan stratigraphy

proximal: landslides and gravity flow deposits

mid fan: poorly sorted deposits interbedded with cross-bedded sand

distal: foreset cross strata and trough filled cross strata to just trough fill corss strata

Debris flow vs Stream flow fans

debris flows have lobes of coarse sediment in mud matrix, poorly sorted

stream flow has gravel, sand, and mud and can be well-sorted and cross-bedded

hyperconcentrated flows

mixture of water and sediment in a channel which has properties intermediate between fluvial flow and debris flow

optically stimulated luminescence (OSL) dating

provides measure of time since sediment grains were buried (shielded from light/heat)

OSL methods

quartz and feldspar contain defects in crystal lattices that trap electrons

background radiation in sediment from radioactive decay generate those electrons that get trapped

sunlight/heat releases trapped electrons and zeros/bleaches luminescence signal

like a battery- quartz/feldspar grains are charged by radition (burial) and then depleted by sunlight (exposure)

pros/cons of OSL dating

pros: when radiocarbon is not possible (ex. sediment lack organic matter, sediments are older than 40ka)

cons: requires complete bleaching/zeroing of samples prior to burial, sediment may undergo multiple bleaching/burial cycles

OSL Age calculation

OSL Age (yr) = total dose (Gy) / Dose Rate (Gy/yr)

total dose

the luminescence signal in a sample

or how much radiation was trapped by minerals

dose rate

the radiation the sample was exposed to in the sedimentary environment (measured in the background sediment)

marginal marine environments

boundary between continental and marine depositional

riverine, wave, and tidal processes

clastics, carbonates, and evaporitic material

delta

any deposit (subaerial or subaqueous) formed by fluvial (river) sediments that build out (prograde) into a body of water

reworking of sediments at the river mouth by marine processes

delta formation

occur where rivers slow and drop their sediment load

influenced by sediment carried vs. accommodation and base level

retrogradation

marine transgression

landward migration of delta/shoreline

rate of sediment supply is greater than rate of sea level rise

aggradation

vertical increase in delta

rate of sediment supply - rate of sea level change

progradation

marine regression

seaward migration of a delta/shoreline

rate of sediment supply > rate of sea level rise

delta plain

where river meets ocean (distributary channels, floodplains, interchannel marshes)

delta front

slope down from sea level to ocean floor

(turbidites, slumps, grading)

Prodelta

subtidal to deep shelf

(fine grade, laminated)

topset deposits

gently dipping, delta plain

foreset ndeposits

delta front

steeply dipping

coarse nearshore to fine offshore

bottomset deposits

nearly flat, fine grained clays/muds, farther from shore

factors influencing sediment delivery to deltas

type/amount of sediment delivered AND density contrast between sediment laden outflow and basin water

hypopycnal

density of river water is less than density of basin water

homopycnal

density of river water is equal to the density of basin water

hyperpycnal

density of river water is greater than density of basin water

three types of deltas

fluvial dominated, tide dominated, wave dominated

fluvial dominated deltas

bird’s foot deltas

occurs when the river discharge and sediment transport is strong than reworking by waves/tides

form big delta lobes into the sea

distributaries with marshes, bays, or tidal flats

tide dominated deltas

large tidal ranges or high tidal current speeds

looks like an estuarine bay filled with many stretched island parallel to the main tidal flow

wave dominated deltas

high wave energy is the dominate factor

geometry is more lobate with smooth, arcuate to sharp margins

longshore drift can rework sediment to form barrier islands, bars, spits

delta types and sed supply

river and tide dominated deltas have high sed input

wave dominated deltas have low sed input