sed/strat midterm 2

i will not be made a fool

river

river

• primary means of transporting sediment across continents

• even hyper arid systems have them

• primary agents of erosion and denudation of continents

recognition of rivers

• sinuous, channeling forming element

• commonly leave course grained deposits (even in sandy/muddy systems)

• relatively gentle grades when depositing sediments ranging from 0.5-0.01

• can be 1-1000’s of km long

• commonly 1-2 m deep (can be more than 15 m)

sinuosity

refers to the length of a line divided by the length of a straight line
between it’s endpoints (this is commonly done along the thalweg)

thalweg

deepest part of a river channel

bar

a composite, constructional bedform usually composed of sand or gravel

meandering rivers

• relatively low slopes relative to discharge

• relatively high discharge relative to load

• relatively low bed load relative to suspended load

• high cohesive strength banks

• commonly muddy and vegetated

• channel migrates through time, both laterally and down stream (does this through helical flow)

no

were meandering rivers around before land plants?

point bar

• succession is coarsest at the base and fines upwards

• shows decreasing flow energy, from trough cross-bedding and rip-up clasts at the base to climbing current ripples and silty suspension deposits

• cross strata are very large, commonly the depth of the whole channel

• grows laterally until the channel avulses or spontaneously changes course

oxbow lakes

• created during small avulsions in a meandering stream

• chute develops that shortens the length of the channel (occurs either across the point bar or at the meander neck)

• will fill with fine-grained sediment deposited during floods and overbank flow

• the site of thick and persistent vegetation

flood plains

• rivers flood, water flows out of the main channel and across this area

• water will carry sediment mainly in suspension

• as low decelerates, material will
  accumulate in an overbank position

levee

a ridge of sediment deposited naturally alongside a river by overflowing water

crevasse splay

• river avulses out of its channel and breaks its levees, diverting main flow into the flood plain

• flow will expand and slow, dropping its load

• this builds a small delta-shaped body of anastomosed channels

swamps

• river networks where the ground water table is very high

• lots of vegetation accumulating

• density of biological activity can create anaerobic conditions (leading to the preservation of organic detritus)

• before burial they’re called peat, after they become coal

• accumulate siltstone and shale (some of which can be organic rich)

braided rivers

• relatively high slopes relative to discharge

• relatively low discharge relative to load

• relatively high bed load to suspended load

• channels are broad and shallow (often 1 m deep)

• evidence for upper flow regime (planar beds) as well as trough cross bedding

• channels separated by bars with varying degrees of stability and vegetation

• flow may be either flashy or perennial

• many deposits coarse grained (commonly sandy, often gravelly)

• common near mountain fronts due to steeper gradients and high bedload components

nodal migration

• during bank-full events where many channels are moving sediment

• sediment collects into large bedforms (bars)

• streams diverge around bars and reconverge on downstream side (convergent point is the node site)

• node site - flow acceleration, double helical flow, and downstream erosion

• erosion migrates downstream, inducing deposition just above the node

anastomosing rivers

• relatively high slopes relative to discharge

• relatively high bed load to suspended load

• channels are broad and shallow (~1m deep)

• deposit large volume of sediment rapidly (due to rapid deceleration of flow due to abrupt decreases in gradient, avulsion, evaporation, or loss of discharge into the ground)

• not very common

terminal splay

• typical setting for an anastomosing system

• river that’s rapidly losing flow into substrate (ex. evaporation)

• arid and semi-arid climates are typical

• flow depth and velocity decreases, leading to deposition of sand sheets near shallow channels'

• channels avulse rapidly and repeatedly over short time and length scales
  creating a complex distributary network

straight rivers

• commonly occur in steep reaches (mountains) or areas with small discharge

• commonly occur in very low gradient distributary channels in coastal plains

• may be a function of underlying structure (faulting/jointing of bedrock)

eolian sand dunes

• high angle (20-35) cross bedding

• individual beds up to 35m

• longer asymptotic bottom sets than marine dunes

• beds are tabular-planar or trough/wedge shaped

• wind ripples lower in amplitude and more asymmetrical than water ones

desert

an arid region, which generally lacks vegetation and cannot support a large population

desert occurrence

• where dry air descends along the 30 N and S latitudes in areas of high pressure/behind mountain ranges that have a rain shadow

• also occur in coastal areas, arctic regions, and far inland in continental interiors

deflation lag/desert pavement

wind is less viscous than water to pick up particles, so the coarsest material is
often left behind

characteristics of eolian sand dunes

• well sorted

• well rounded

• surfaces of quartz are pitted and frosted (not always)

10%

how much of the earth is covered in permanent glaciers?

30%

how much of the earth was covered in glaciers during the pleistocene?

snowball earth

• lasted for millions of years

• nearly complete shutdown of earths processes (extreme albedo and cold)

paleoproterozoic, neoproterozoic, ordovician, carboniferous, pleistocene

when did glaciations occur?

glacial deposits

• tills and diamictite (presence of drop stones and striations on clasts or bedrock pavements)

• lodgment till at the base (may be overlain by braided gravel bars or cross bedded sands and gravels

glacial origin

mud flows and debris flows

proterozoic glacial deposits

often overlain by uniquely textured and isotopically anomalous carbonates (cap carbonates) which were formed during post-glacial sea level transgression

deep water marine and pelagic environment

• know the least about

• not presently active

• only active during sea level lows

• below the baselevel of erosion (high preservation potential)

sedimentation on continental shelves

• continuous with coastal plain sequences

• tropical environments- accumulate carbonates

• cold water/high siliciclastic input areas- fine sands/silts/muds

epeiric sea

large seas that accumulate abundant sediments during sea level high stands

depositional processes dictated by

whether the sediments accumulate above/below wave base, or whether tidal currents are strong enough to redistribute particles

tidal ranges large (>2m) and currents fast (50-100cm/s)

asymmetrical sand ribbons or tidal ridges are formed on the continental shelf

tidal currents less than 50cm/s

sheets or waves of sand develop

tidal sand wave

• crest of 3-15m

• wavelength of 150-500m

• composed of low angle surfaces (dipped at 5-6 degrees)

• cross sets no more than a few meters in thickness (differentiates them from eolian sand dunes)

storm dominated coasts

• linear sand ridges with variable cross bedding

• hummocky cross stratification

• formed at depths of 5-15m between fair weather and storm wave base

wave-ripple cross-bedding

• storm and wave influence, no tidal effects

• irregular undulatory lower bounding surface

• less trough like shape

• display effects of rapidly reversing wave flow

• often lenticular and flaser bedding

characteristic stratigraphic profiles

recognized based on whether sea level is regressive, transgressive, or balanced

continental slope

• between shelf and deep ocean

• relatively narrow (10-100km)

• slopes downward (average angle 4-6)

• sediments moved down by gravity/dislodged by storms/earthquakes

continental slope sed. features

• olistoliths

• slumped and deformed shales

• debris flows

• turbidites

turbidity current

• brings sediment to the deep ocean

• it is a gravity current that suspends particles through fluid turbulence

• current velocity is a function of concentration, column height, and gradient

• sediment concentration is highest and grain size coarsest at the base of the flow, decreasing upwards

• has a head, body, and tail

turbidity head

• tall

• has the most energy

• does most of the erosive work

turbidity body

• carries most of the sediment

• can be long-lived and large

• can erode and deposit

turbidity tail

• low concentration part

• always decelerating

• deposits most places (but not very much)de

deposition of a turbidite

these currents mix with the ambient water, which decrease their concentration thereby
slowing the flow and depositing sediment

ignition

• a flow may additionally erode its substrate adding mass to its body

• this increases sediment concentration, accelerating flow and increasing erosion

• due to this continental slopes are generally places of sediment bypass, while basin floors are sites of sediment aggradation

bouma sequence (from material falling from suspension)

• Ta (massive)

• Tb (planar bedded)

• Tc (current rippled)

• Td (planar laminated)

• Te (suspension fallout only)

• these can vary in proportion, not all parts are likely to be present

distributary channel complexes (fans)

• fan shaped bodies of coarse sediment

• accumulate where turbidity current flow is unconfined

• occurs at prominent decreases in gradient

• as confinement and gradient decrease, flows decelerate and drop their load

• proximal fans- Ta beds

• distal fans- Tb and Tc beds

confined channel complexes (slope)

• long channels of varying sinuosity are cut into the continental slope by ignition turbidity currents

• increase confinement, promote flow acceleration and incision

• dominated by erosion

• full range of turbidite deposits can occur

levee channels

• turbidity currents are tall enough to overflow their channels, they become unconfined and decelerate

• as flows move away from the channel, they get progressively finer grained & lower in concentration

• this effect builds levees that are:

  • fine grained

  • beds thin and fine away from channel

slumps and slides (mass transport complexes)

• depositional process: mass wasting

• distinguishing structures: contorted bedding, inherited bedding

• other: can be extremely large

debris flow

• depositional process: mass wasting

• distinguishing structures: contorted-massive bedding, outsized clasts

• other: can be rather large

abandonment/drape

• depositional process: dilute turbidites, pelagic fall out

• distinguishing structures: lots of Td, Tde beds, organic enrichment

• other: can be very thin and hard to map

pelagic sediment

• depositional process: strictly water column fall out

• distinguishing structures: micro-laminated texture, lack of normal grading

• other: commonly basinal in occurrence

about slope mudstones

silts and clays brought to deep ocean by pelagic sedimentation/settling out of the water column and fine grained turbidity currents

about mass transport complexes

• thick packages of sediment from accumulation of debris flows, slumps, and slides

• usually made of slope mudstones that fail and accumulate locally

• usually fine grained

about abandonment/drape

• these successions are enriched in pelagic sediments and often associated with organic enrichment

• during transgression when sediments are sequestered high on the continental shelf and no sediment reaches the deep ocean

about pelagic sedimentation

• fine grained sediments rain out of the ocean column and slowly accumulate on the sea floor

• may enter as muddy plumes at delta fronts or low concentration turbidity currents that flow into the water column

• biological sediments (diatoms/forams) that are born in the water column also contribute to pelagic ‘rain’

carbonate compensation depth (CCD)

• level at which carbonate dissolves due to the higher pH at great depths (marine snow line)

• deeper than 4km is where it starts to dissolve

base level erosion

• level on Earth’s surface above which sediments must eventually erode, and below which they are deposited

• most of the time the base level is near or at sea level

non marine environments

poorly preserved because they sit above base level of erosion

alluvial fan

• most proximal and coarse grained of sed. environments

• found next to mountain belts

• product of two main depositional processes (debris flow and sheet flow)

• steep upper surfaces, ranging from 16°– 1.5°, with the slope decreasing towards the basin

• slope magnitude depends on the fan’s provenance (muddy, gravelly) and tectonic setting

• always upper flow regime

• river (fluvial) environments have grades of 0.5°– 0.01°

• two main categories (primary and secondary facies)

debris flow

• occurs when all sizes of sediment (boulders to clay) that’re saturated with water moves en mass and is rapidly deposited with little to no stratification (except if multiple debris flow sheets are stacked)

• occasionally preserve reverse grading (especially near their bases)

mud flow

a class of debris flow with mainly fine-grained particles that can move at rapid rates (up to 10 km/hr) also forming narrow lobes

bajada

when alluvial fans coalesce along mountain fronts

midfan sheets

typically well sorted, well stratified, and cross bedded

intersection point

on an alluvial fan, where the main channel shallows to the surface of the existing fan causing sheet floods that form lobes of coarse boulders/cobbles/sands called sieve deposits

carbonate

• most abundant chemical sediment in modern and (most) ancient oceans

• sensitive recorders of the global marine environment (changes in the exogenic carbon cycle)

• subject to diagenetic alteration by a variety of processes

carbonates are sources

• gravel, Ca and Mg for nutrition, and building facing stone

• host rock to Mississippi-type ore deposits, and when fractured can be good oil reservoir rocks

• most likely to contain the globally-distributed marine fossils used in biostratigraphy

carbonates monitor

• changes in the atmosphere

• carbon in the
  early atmosphere reacted with the
  silicate Earth and water to form alkalinity, now stored as carbonate rock in the crust

carbonates form

• most warm, shallow water environments

• main requirements for formation are high concentrations of Ca 2+ and HCO3- (alkalinity), which are
  products of silicate and carbonate weathering on land

alteration of feldspar into kaolinite (weathering process)

2NaAlSi3O8 + 2CO2 + 11H2O → Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO4

calcite

• rhombohedral structure that can substitute up to 5% Mg (high Mg calcite) for Ca

• surface oceans are generally supersaturated with respect to this mineral

aragonite

structure is orthorhombic and its
larger cation sites allow for the incorporation of larger elements, most notably strontium

organisms that calcify

use a range of carbonate minerals to build their shells and homes

equilibrium constants

• monitor the extent of chemical reactions at set temperatures and pressures

• equal to the concentration of products over reactants

reaction moves to the right (at equilibrium conditions)

• for a reaction A+B → C + D (reactants → products), Keq (1) = (C)(D)/(A)(B)

• the reverse of this reaction C + D → A + B, Keq (2) = (A)(B)/(C)(D)

• if K(1) > K(2)

chemical and physical controls on carbonate formation (in oceans and lakes)

• temperature

• pressure

• degree of agitation

• sediment masking

• light availability

• oxidation state

CaCO3 extraction

• immediate effect: promotes skeletal growth

• ultimate effect: forms allochems and mud

photosynthesis

• immediate effect: removes CO2, pH increase

• ultimate effect: promotes precipitation

decay

• immediate effect: adds CO2, pH decrease

• ultimate effect: hinders precipitation

feeding

• immediate effect: bioturbation

• ultimate effect: generates pellets, stirs sediments

bacterial activity

• immediate effect: removes CO2, pH increase

• ultimate effect: calcifies microbial mats

dunham and folk

main classification schemes for limestone

dunham classification

• based on the recognition of depositional textures as well as the abundance of allochthonous and
  autothonous components

allochem

carbonate particle that was formed outside of the depositional area and transported in (hence carbonate sediments can be clastic)

folk classification

• relies on descriptive terms for the allochems linked to the dominant matrix material, either micrite or sparite

• described a classification based on
  textural maturity

common allochems

• coated grains (ooids, grapestones, pisolites, oncolites)

• skeletal fragments

• intraclasts

orthochemical components

form within the depositional area and represent the rock cement, includes:

• micrite

• spar

micrite

very fine grained component that may be an abiotic precipitate, or form due to the photosynthetic actions of nannoplankton

other carbonate minerals (that can form in terrestrial and marine environments)

• under unusual chemical conditions (often diagenetic)

• ankerite (Ca(MgFe)(CO3)2

• siderite (FeCO3)

• common constituents in the Precambrian banded iron-formations, as well as concretions in peat-rich soils and organic-rich sandstones

• dolomite (CaMg(CO3)2)

kinetic barriers (for precipitation of dolomite to be overcome)

• temperature

• salinity (effects of hydration on Mg 2+ ion)

• Mg/Ca ratio (seawater is 5.4, for it to form must be >8)

• sulfate concentration (28 mM in seawater)

• environments where these barriers can be removed are warm, highly evaporitic, and mixed with fresh water sources

coorong lakes (along the southern margin of australia)

modern environment where dolomite appears to be forming as a primary precipitate

coorong lakes

• area of lagoons and alkaline lakes behind modern beach barriers (fed by seawater and groundwater)

• temporary lakes have high pH (8 to 10), and Mg/Ca of up to 20

• dolomite forms in more landward lakes as minute spherical aggregates

carbonate platforms

• factories

• rapid buildup of carbonate in appropriate warm and shallow marine environments

• frequently shallow to the surface where they may be exposed due to sea level fluctuations on various time scales (resulting in erosion and karstification)

to continue carbonate sedimentation

sea level would have to rise (or the platform subside), or the facies would have to migrate out to sea

subdivision of facies into depth restricted zones

• supratidal

• peritidal

• subtidal

• there are extreme variations in water depth, salinity, and organisms

supratidal

• environments that are generally above the tidal range and are thus wet with
  seawater only during storm events

• otherwise dominated by brackish or fresh water sources

• typical environment would be a marsh where it is likely to find high abundances of organic matter and where coal is likely to form