EaES455txt.ppt

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Clastic Sedimentology andSequence Stratigraphy(EaES 455)

Instructor: Torbjörn Törnqvist

2450 SES

(312) [email protected]



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Components of EaES 455

• Lectures

• Paper (including oral presentation)

• Labs• Reviews of two published papers

• Field trip (Indiana and/or Minneapolis?)

• More detailed information on the EaES 455homepage:http://www.uic.edu/classes/eaes/eaes455/

http://www.uic.edu/classes/eaes/eaes455/




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Grading

• Written tests (50%)

• Midterm (20%)

• Final (30%)

• Paper (30%)

• Writing (20%)

• Seminar (10%)

• Labs (10%)

• Reviews (10%)



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Literature

• Reading, H.G. (Editor), 1996. Sedimentary Environments: Processes, Facies and Stratigraphy .Blackwell, Oxford, 688 pp. ISBN 0-632-03627-3.

• Emery, D. and Myers, K.J. (Editors), 1996. Sequence Stratigraphy . Blackwell, Oxford, 297 pp. ISBN 0-632-03706-7.

• Lecture notes on EaES 455 homepage





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Introduction

Definitions

• Sedimentology = the study of the processes of formation,

transport and deposition of material which accumulates assediment in continental and marine environments andeventually forms sedimentary rocks

• Stratigraphy = the study of rocks to determine the order andtiming of events in Earth history

• Sedimentary geology sedimentology + stratigraphy

• Sequence stratigraphy = the analysis of genetically relateddepositional units bounded by unconformities and theircorrelative conformities



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Introduction

Historical development of sedimentary geologyand key concepts

• Principle of superposition (Nicolas Steno, 1669)• Uniformitarianism (“the present is the key to the past”)

(James Hutton and Charles Lyell, late 18th to early 19th century)

• Stratigraphy developed already around 1800

• Sedimentology is a relatively new discipline (1960s and 1970s)• Late 1980s and 1990s: revival of stratigraphy (sequence

stratigraphy)



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Introduction

Temporal and spatial scales

• Sedimentology focuses primarily on facies and depositional

environments (how were sediments/sedimentary rocks formed?)• Smaller temporal and spatial scales

• Stratigraphy focuses on the larger scale strata and Earth history(when and where were sediments/sedimentary rocks formed?)

• Larger temporal and spatial scales



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Contents

• Introduction

• Sedimentology - concepts

• Fluvial environments

• Deltaic environments

• Coastal environments

• Offshore marine environments

• Sea-level change

• Sequence stratigraphy – concepts

• Marine sequence stratigraphy

• Nonmarine sequence stratigraphy

• Basin and reservoir modeling

• Reflection



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Sedimentology – concepts

Fluid flow and bedforms

• Unidirectional flow leads predominantly to asymmetric

bedforms (two- or three-dimensional) or plane beds• Current ripples

• Dunes

• Plane beds

• Antidunes

• Oscillatory flow due to waves causes predominantlysymmetric bedforms (wave ripples)

• Combined flow involves both modes of sediment transportand causes low-relief mounds and swales



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Sedimentary structures

• Planar stratification is primarily the product of aggrading

plane beds• Cross stratification is formed by aggrading bedforms

• Planar and trough cross stratification are the result of straight-crested (2D) and linguoid (3D) bedforms, respectively• Small-scale cross stratification (current ripples)

• Large-scale cross stratification (dunes)• Wave cross stratification (wave ripples)

• Hummocky cross stratification (mounds and swales)

• A single unit of cross-stratified material is known as a set;multiple stacked sets of similar nature form co-sets



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• The facies concept refers to the sum of characteristics of asedimentary unit, commonly at a fairly small (cm-m) scale

• Lithology

• Grain size• Sedimentary structures

• Color

• Composition

• Biogenic content

• Lithofacies (physical and chemical characteristics)• Biofacies (macrofossil content)

• Ichnofacies (trace fossils)



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• Facies analysis is the interpretation of strata in terms of depositional environments (or depositional systems), commonlybased on a wide variety of observations

• Facies associations constitute several facies that occur incombination, and typically represent one depositionalenvironment (note that very few individual facies are diagnosticfor one specific setting!)

• Facies successions (or facies sequences) are faciesassociations with a characteristic vertical order

• Walther’s Law (1894) states that two different facies foundsuperimposed on one another and not separated by anunconformity, must have been deposited adjacent to each otherat a given point in time



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• Standardized facies codes have been proposed (e.g., by AndrewMiall), but they are frequently critized

• Sedimentary logs are one-dimensional representations of vertical sedimentary successions

• Architectural elements are the two- or three-dimensional „building blocks‟ of a sediment or a sedimentary rock

• The three-dimensional arrangement of architectural elements isknown as sedimentary architecture

• Since the 1970s, facies analysis has evolved from a focus on

one-dimensional data to three-dimensional data (architectural-element analysis, 3D seismic), recognizing that individualsedimentary logs can rarely provide detailed environmentalinterpretations



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• Sedimentary structures occur at very different scales, from lessthan a mm (thin section) to 100s –1000s of meters (largeoutcrops); most attention is traditionally focused on the

bedform-scale• Microforms (e.g., ripples)

• Mesoforms (e.g., dunes)

• Macroforms (e.g., bars)

• Bounding-surface hierarchies have been developed to

distinguish different ranks of stratal discontinuity, from laminato basin scale; they are much more readily used in outcropsthan in subsurface data



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• Allogenic (allocyclic) controls are external forces that exert astrong influence on depositional processes; they include sea-level (base-level) change, climate change (e.g., sediment

supply), and tectonism (e.g., subsidence, sediment supply)• Autogenic (autocyclic) controls operate within a given

depositional environment and cause changes while allogeniccontrols may remain constant (e.g., delta-lobe switching)

• The last few decades have seen an enormous shift in emphasis

from autogenic to allogenic processes (sequence stratigraphy)



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• Accommodation is the space available, at any given point intime, for sediments to accumulate; in marine environmentsaccommodation is created or destroyed by relative sea-level

changes• The stratigraphic record is nearly always very incomplete due to

a limited preservation potential, that decreases withincreasing time scales

• Only an extremely small proportion of deposits that are initially

formed actually survive and become preserved in thestratigraphic record (typical orders of magnitude 10-4 –10-6)



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• Facies models are schematic, three-dimensionalrepresentations of specific depositional environments that serveas norms for interpretation and prediction• Facies models are static in the sense that they focus heavily on

autogenic processes and deposits, following Walther‟s Law • Modern processes must constitute the basis for interpreting

ancient products (uniformitarianism works in many cases, butnot always)

• Unconsolidated sediments (~Quaternary) can provide the bridge

between present-day processes and ancient sedimentary rocks(~pre-Quaternary); Quaternary deposits are usually easy tointerpret in terms of depositional environment and have greatpotential for studying 3D facies relationships and allogeniccontrols



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Fluvial environments

• Channel patterns (fluvial styles) of alluvial rivers are commonlyclassified as:

• Braided rivers

• Meandering rivers• Straight rivers

• Anastomosing rivers

• Fluvial style is primarily controlled by specific stream power (Wm-2) and bed-load grain size, but also by bank stability and the

amount of bed load (but not the proportion of suspended load!)

=fluid density; Q=discharge; s=slope (gradient); w=channel width

w

ρgQsω



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• Bars are sandy or gravelly macroforms in channels that areemergent, mostly unvegetated features at low flow stage, andundergo submergence and rapid modification during high

discharge• Point bars form on inner banks and typically accrete laterally,commonly resulting in lateral-accretion surfaces; mid-channel orbraid bars accrete both laterally and downstream

• Bars are always associated with channels; a genetically related

bar/bar complex and channel/channel complex is known as astorey



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• Lateral accretion involves higher-order bounding surfacesdipping perpendicular to paleoflow direction and associatedlower-order bounding surfaces; in the case of downstreamaccretion higher-order bounding surfaces dip parallel topaleoflow direction

• Braided rivers are characterized by a dominance of braid barsexhibiting both lateral and downstream accretion; meanderingrivers primarily contain point bars with lateral accretion; instraight (and most anastomosing) rivers bars are commonlyalmost absent



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• Facies successions in sandy to gravelly channel depositstypically fine upward, from a coarse channel lag, through large-scale to small-scale cross stratified sets (commonly withdecreasing set height), and finally overlain by muddy overbank deposits

• Facies successions produced by different fluvial styles can beextremely similar!

• The geometry and three-dimensional arrangement of

architectural elements therefore provides a much better meansof inferring fluvial styles from the sedimentary record



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• Channel belts consist of channel-bar and channel-filldeposits; the proportion of the two generally decreasesmarkedly from braided rivers to anastomosing rivers

• The geometry of a channel belt (width/thickness ratio) is afunction of the channel width and the degree of lateralmigration; values are typically much higher for braided systems(>>100) than for straight or anastomosing systems (<25)

• Sheets have width/thickness ratios of >50

• Ribbons have width/thickness ratios of <15• Residual-channel deposits are predominantly muddy

(occasionally organic) deposits that accumulate in anabandoned channel where flow velocities are extremely small



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• Overbank environments are dominated by fine-grainedfacies (predominantly muds)

• Natural-levee deposits are wedges („wings‟) of sediment thatform adjacent to the channel, dominated by fine sand and siltexhibiting planar stratification or (climbing) ripple crossstratification

• Crevasse-splay deposits are usually cones of sandy to siltyfacies with both coarsening-upward and fining-upward successions,and are formed by small, secondary channels during peak flow

• Flood-basin deposits are the most distal facies, consistingentirely of muddy sediments deposited from suspension, and arevolumetrically very important (mainly in low-energy fluvial settings)



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• Paleosols (well drained conditions) and occasional peats (poorly drained conditions) occur frequently in overbank environments and are important indicators of variations of clastic aggradation rates and the position relative to active

channels (proximal vs. distal)• The pedofacies concept refers to the maturity of a paleosol,

irrespective of the specific set of pedogenic processes operating,in the case of floodplains mainly controlled by distance to theactive channel

• Lacustrine deposits can be important in overbank environments characterized by high water tables, and are alsofound in distal settings; they are more likely to contain primarysedimentary structures (horizontal lamination) than theirfrequently bioturbated subaerial counterparts



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• Facies models highlight conspicuous differences betweendifferent fluvial styles:

• Channel-belt width/thickness ratio (braided: high; meandering:intermediate; straight/anastomosing: low)

• Channel-deposit proportion (braided: high; meandering:intermediate; straight/anastomosing: low)

• Overbank-deposit proportion (braided: low; meandering:intermediate; straight/anastomosing: high)

• Overbank-deposit geometry (meandering: wedge-shaped;

straight/anastomosing: highly irregular due to numerous crevassechannels)

• Overbank facies (meandering: well-drained paleosols common;straight/anastomosing: peats and lacustrine deposits common)



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• Avulsion is the sudden diversion of a channel to a new locationon the floodplain, leading to the abandonment of a channel beltand the initiation of a new one

• Avulsions are the inevitable consequence of the increase of cross-valley slope (typically through a crevasse channel) relativeto down-valley slope along the channel, associated with thegrowth of an alluvial ridge

• An avulsion belt constitutes an extensive network of rapidly

aggrading, narrow, crevasse-like channels with geneticallyassociated overbank deposits, that may surround the newchannel belt



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• Alluvial architecture refers to the three-dimensionalarrangement of channel-belt deposits and overbank deposits ina fluvial succession

• The nature of alluvial architecture (e.g., the proportion of channel-belt to overbank deposits) is dependent on fluvial style,aggradation rate, and the frequency of avulsion

• When alluvial architecture is dominated by channel-beltdeposits, the separation of channel belts from storeys can be

extremely difficult



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Deltaic environments

• Deltaic environments are gradational to both fluvial and coastalenvironments

• The density relationship between sediment-laden inflowingwater and the receiving, standing water body varies• Hyperpycnal: inflowing water has a higher density than basin

water, leading to inertia-dominated density currents

• Hypopycnal: inflowing water has a lower density than basin water(buoyancy), leading to separation of bed load and suspended load

• Deltas consist of a subaerial delta plain, and a subaqueous

delta front and prodelta• The delta slope is commonly 1-2° and consists of finer (usuallysilty) facies; the most distal prodelta is dominated by even finersediment



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Delta plain

• Delta plains are commonly characterized by distributaries and

interdistributary areas• The upper delta plain is gradational with floodplains, lacksmarine influence and typically has large flood basins, commonlywith freshwater peats and lacustrine deposits

• The lower delta plain is marine influenced (e.g., tides, salt-waterintrusion) and contains brackish to saline interdistributary bays

(e.g., shallow lagoons, salt marshes, mangroves, tidal flats)• Interdistributary areas commonly change from freshwater

through brackish to saline environments in a downdip direction(e.g., transition from swamps to marshes)

• Minor (secondary) deltas commonly form when distributariesenter lakes or lagoons



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Delta plain

• Distributaries are to a large extent comparable to fluvial

channels, but are commonly at the low-energy end of thespectrum (meandering to straight/anastomosing)

• Delta plain distributaries are usually characterized by narrownatural levees and numerous crevasse splays

• Avulsion (i.e., delta-lobe switching) is frequent due to high

subsidence rates, as well as rapid gradient reduction associatedwith channel progradation



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Delta plain

• In humid climates, delta plains may have an important organic

component (peat that ultimately forms coal)• Hydrosere: vertical succession of organic deposits due to thetransition from a limnic, through a telmatic, to a terrestrialenvironment

• Terrestrialization (= hydrosere): gyttja --> fen peat --> woodpeat --> moss peat (commonly a transition from a

minerotrophic to an ombrotrophic environment)• Paludification (= reversed hydrosere) is caused by a rise of

the (ground)water table

• Peats are essentially the downdip cousins of paleosols,representing prolonged periods of limited clastic sediment influx



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Delta front and prodelta

• Mouth bars form at the upper edge of the delta front, at the

mouth of distributaries (particularly in hypopycnal flows); theyare mostly sandy and tend to coarsen upwards

• Wave action can play an important role in winnowing andreworking of mouth-bar deposits; this may lead to merging withprograding beach ridges and if wave action is very important

mouth bars are entirely transformed• The prodelta is the distal end outside wave or tide influence

where muds accumulate, commonly with limited bioturbation



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• Delta morphology reflects the relative importance of fluvial,tidal, and wave processes, as well as gradient and sedimentsupply

• River-dominated deltas occur in microtidal settings with limitedwave energy, where delta-lobe progradation is significant andredistribution of mouth bars is limited

• Wave-dominated deltas are characterized by mouth barsreworked into shore-parallel sand bodies and beaches

• Tide-dominated deltas exhibit tidal mudflats and mouth bars

that are reworked into elongate sand bodies perpendicular to theshoreline



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• The typical progradational delta succession exhibits atransition from prodelta offshore muds through silty to sandy(mouth bar) deposits (coarsening-upward succession), the lattercommonly with small-scale (climbing) cross stratification and

overlain by:• Distributary channel deposits (sometimes tidal channel deposits)

with larger scale sedimentary structures

• Subaqueous levees grading upward into interdistributary sediments

• Transgression occurs upon delta-lobe switching, leading to:

• Intense wave reworking and transformation of mouth bar/beachridge sands into barrier islands

• Drowning of barrier islands leading to offshore sand shoals

• Increasing salinity and eventual drowning of (part of) the deltaplain



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• Shallow-water deltas are thinner but larger in area than theirdeep-water counterparts

• Deformation processes are very common in deltas due to thehigh sediment rates and associated high pore-fluid pressures

• Growth faults result from downdip increasing sedimentationrates; they develop contemporaneously with sedimentation

• Mud diapirs may form when thick prodelta deposits are coveredby mouth-bar sands

• Slumping can lead to the anomalous occurrence of shallow-waterfacies in prodelta deposits



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Coastal environments

• The classification of deltas can be extended to include thosedepositional coastal environments that are in large part fed bymarine sediments

• Wave-dominated shorelines

• Tide-dominated shorelines

• Depending on the balance between sediment supply andaccommodation, coastal environments can be regressive (progradation) or transgressive (retrogradation)



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• Waves can be subdivided into swell waves that travel longdistances, and sea waves that are generated more locally

• Waves that approach a shoreline consisting of unconsolidated

sediment will produce a series of environments (oscillatory wavezone, shoaling wave zone, breaker/surf/swash zone) withcharacteristic bedforms (symmetric ripples – asymmetric ripplesor dunes – plane beds)

• Long-shore currents and rip currents can lead to sediment

transport along the shoreline and away from the shorelinerespectively, with associated unidirectional bedforms (commonlydunes)



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• Reflective shorelines have steep, coarse-grained foreshoresand lack breaking waves and associated bars away from theshoreline

• Dissipative shorelines are low-gradient, fine-grained, barred

systems where waves may be entirely attenuated• Many coasts can alternate from more reflective to more

dissipative conditions during fairweather and storm conditions,respectively

• The high-energy shoreline tends to trap coarse-grained (sandy

to gravelly) sediment in what is known as the littoral energyfence; escape of sediment to the shelf occurs by means of: • River mouth bypassing (floods)

• Estuary mouth bypassing (ebb currents)

• Shoreface bypassing (storms)



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• Tides are formed by the gravitational attraction of the Moon andSun on the Earth, combined with the centrifugal force caused bymovement of the Earth around the center of mass of the Earth-Moon system

• Semi-diurnal or diurnal tidal cycles are essentially caused bythe Earth‟s rotation relative to the Moon

• Neap-spring tidal cycles are mainly caused by the alignment of the Moon and the Sun relative to the Earth

• Semi-annual tidal cycles are driven by the interplay of variouscyclicities (including the elliptic orbit of the Moon)

• Tidal currents are modulated by the configuration of oceans andseas, and typically lead to a pattern of circulation; even in smalltidal basins flood currents tend to dominate in different areasthan ebb currents



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• Tide-influenced sedimentary structures can take differentshapes:

• Herringbone cross stratification indicates bipolar flowdirections, but it is rare

• Mud-draped cross strata are much more common, and are theresult of alternating bedform migration during high flow velocitiesand mud deposition during high or low tide (slackwater)

• Tidal bundles are characterized by a sand-mud couplet with varyingthickness; tidal bundle sequences consist of a series of bundles

that can be related to neap-spring cycles• Tidal rhytmites can form in fine-grained facies that aggrade

vertically, to a large part from suspension, and consist of commonlyvery thin (mm-scale), but distinct laminae



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• Beach-ridge strandplains and chenier plains result fromcoastal progradation in sand- and mud-dominated settingsrespectively; both are dominantly fed by sediments transportedby long-shore currents

• Tidal flats occur in a wide variety of settings (e.g., directlyfacing the open sea/ocean, in lagoons behind barrier islands,near tidal inlets) and contain a supratidal zone, an intertidalzone, and tidal channels• Tidal channels can be extremely deep and dynamic and are

commonly filled with large-scale cross-stratified tidal-bundle

sequences and/or laterally accreted heterolithic (sandy and muddy)strata

• Intertidal environments include sandy to muddy tidal flats wheretidal rhytmites may form, commonly bordered by salt marshes ormangroves where muddy facies or peats accumulate



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• Barrier islands form in transgressive settings where beachridges get separated from the mainland by a lagoon

• Lagoons commonly accumulate relatively fine-grained (muddy)facies, especially when tidal range is low

• Washovers bring sheets of relatively coarse-grained (sandy) faciesinto the lagoon during storms

• Tidal inlets vary in number, width, and depth dependent on thetidal range; they are associated with flood-tidal deltas and ebb-tidaldeltas

• Barrier island shorelines can exhibit shoreface retreat or in-placedrowning; prolonged shoreface regression ultimately leads tofilling of the back-barrier lagoon



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• Estuaries are transgressed, drowned river valleys where fluvial,tide, and wave processes interact; they are characterized by anet landward movement of sediment in their seaward part

• Tide-dominated estuaries contain tidal sand bars at the seawardend, separated from the fluvial zone by relatively fine-grained tidalflats (e.g., salt marshes); fluvial channel deposits exhibitheterolithic characteristics and sometimes tidal-bundle sequences

• Wave-dominated estuaries have a coastal barrier with a tidal inletand flood-tidal delta, separated from a bayhead delta by a central

basin where fine-grained sediments (muds) accumulate



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Offshore marine environments

• Shallow marine environments include pericontinentalseas that occur along continental margins and have ashoreline-shelf-slope profile; and epicontinental seas thatcover continental interiors and exhibit a ramp morphology

• Under idealized conditions the offshore-transition and offshoreexhibit a systematic decrease in (wave) energy and grain size;however, such an „equilibrium shelf‟ is commonly notencountered

• Tides and ocean currents can strongly complicate shelf hydrodynamics

• Rapid sea-level changes (e.g., during the Quaternary) result inrelict shelf sediments that are genetically unrelated to the presentconditions



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• Wave/storm-dominated shelves ideally exhibit a transitionfrom sands in the lower shoreface, to alternating sands andmuds below fairweather wave base, to muddy facies belowstorm wave base

• Storms have a strong imprint (i.e., storm deposits have a highpreservation potential), since they wipe out fairweather deposits

• Tempestites form during storm events and exhibit acharacteristic facies succession from an erosional basal surfacewith sole marks, to a sandy unit with hummocky crossstratification overlain by wave-rippled sand, finally giving way tomuds



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• Tides lead to circulation around amphidromic points, rangingfrom circular to almost rectilinear depending on the shape of thewater body

• Tide-dominated shelves exhibit a distinct suite of bedformsin relation to current velocity and sediment (sand) supply

• Erosional features, sand ribbons, and sand waves go along withdecreasing flow velocities, commonly associated with mud-draped subaqueous dunes; tidal sand ridges (tens of m high,many km across) are characteristic of shelves with a high supplyof sand



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• Ocean current-dominated shelves are relatively rare;geostrophic ocean currents can lead to the formation of bedforms that are somewhat comparable to those of tide-dominated shelves

• Mud-dominated shelves are usually associated with large,tropical rivers with a high suspended load (e.g., Amazon and Yellow Rivers) that can be transported along the shelf if currents are favorable



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• Deep marine environments include the continental slope andthe deep sea

• Subaqueous mass movements (mostly sediment gravity flows)involve a range of transport mechanisms, including plastic flows

and fluidal flows• Debris flows are commonly laminar and typically do not produce

sedimentary structures

• Turbidity currents are primarily turbulent and more diluted; theycommonly evolve from debris flows

• Debris-flow deposits are poorly sorted, related to the „freezing‟ that occurs once shear stresses can not overcome the internalshear strength

• A key mechanism in turbidity currents is „autosuspension‟ (turbulence --> suspended load --> excess density --> flow -->turbulence)



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• Contrary to debris flows, turbidites exhibit a distinct proximal todistal fining

• The idealized Bouma sequence, consisting of divisions A-E, ismost useful for medium-grained, sand-mud turbidites, but it

must be applied with care• A: Rapidly deposited, massive sand

• B: Planar stratified (upper-stage plane bed) sand

• C: Small-scale (climbing ripple) cross-stratified fine sand

• D: Laminated silt

• E: Homogeneous mud• High-density and low-density turbidity currents give rise to

incomplete, coarse-grained (A) and fine-grained (D-E) turbiditesrespectively

• Contourites are formed by ocean currents and commonly representreworked turbidites



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• Submarine canyons at the shelf edge (commonly related todeltas) are connected to submarine fans on the ocean floor

• The size of submarine fans is inversely related to dominantgrain size (i.e., mud-dominated submarine fans are 104 –106

km2, sand or gravel-dominated submarine fans are 101 –102 km2)

• Submarine fans share several characteristics with deltas; theyconsist of a feeder channel that divides into numerousdistributary channels bordered by natural levees („channel-leveesystems‟) and are subject to avulsions • Proximal fan (trunk channel)

• Medial fan (lobes)

• Distal fan



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• Hemipelagic sediments consist for at least 25% of fine-grained (muddy) terrigenous material that is deposited fromsuspension, commmonly after transport by hemipelagicadvection

• Distal, muddy turbidites merge gradationally into hemipelagicdeposits

• Eolian dust is an important component (~50%) of hemipelagic(and pelagic) facies

• Black shales have a 1 –15% organic-matter content and form in

anoxic bottom waters, sometimes in shallow seas (e.g., WesternInterior Seaway)

• Pelagic sediments are widespread in the open ocean andprimarily have a biogenic origin



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Sea-level change

• Relative sea-level change includes a global component(eustasy) that is uniform worldwide and can be measuredrelative to a fixed datum (e.g., the center of the Earth), andregional to local components (isostasy, tectonism) that are

spatially variable• Eustasy involves changes in ocean-basin volume, as well as

changes in ocean-water volume (amplitudes ~101 –102 m)• Tectono-eustasy (time scales of 10 –100 Myr)

• Glacio-eustasy (time scales of 10 –100 kyr)

• Isostasy refers to crustal movements that are a direct result of loading and unloading by ice or water• Glacio-isostasy

• Hydro-isostasy



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Sea-level change

• Tectonism includes a vast array of crustal movements, rangingfrom large-scale uplifts and basins to small-scale faults

• Steric sea-level changes include density changes (temperature,salinity) and dynamic changes (atmospheric pressure, oceancurrents, wind set-up), but these changes are typically on theorder of a few meters at the most

• The geoid exhibits lows and highs relative to the oblatespheroid due to gravity anomalies; geoidal changes do occurover time, but they are most likely slow



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Sea-level change

• Since isostasy and tectonism are spatially variable, everygeographic location has a unique relative sea-level history(RSL=E+I+T)

• Four characteristic RSL-curves associated with the lastdeglaciation:

• Near-field sites (e.g., Hudson Bay)

• Ice-margin sites (e.g., Norwegian coast)

• Intermediate-field sites (e.g., mid-Atlantic coast)

• Far-field sites (most of the southern hemisphere)



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Sea-level change

• It is believed that eustatic cycles of different periods haveoperated throughout the Phanerozoic:• First-order (108 yr) and second-order (107 yr) cycles (primarily

tectono-eustatic)

• Third-order (106 yr) cycles (mechanism not well understood)• Fourth-order (105 yr) and fifth-order (104 yr) cycles (primarily

glacio-eustatic)

• Glacio-eustasy has only controlled limited portions of Earthhistory (e.g., the Carboniferous or Late Cenozoic icehouse worldas opposed to the Cretaceous greenhouse world)

• Whereas RSL change has a profound impact on the stratigraphicevolution of numerous sedimentary environments (certainlydeltaic, coastal, and marine), the complex spatial pattern of RSLchange commonly yields responses that are out of phase



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Sequence stratigraphy – concepts

• Sequence stratigraphy highlights the role of allogenic controlson patterns of deposition, as opposed to autogenic controls thatoperate within depositional environments

• Eustasy (sea level)

• Subsidence (basin tectonics)

• Sediment supply (climate and hinterland tectonics)



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• Accommodation is the space available, at any given point intime, for sediments to accumulate; accommodation is created ordestroyed by RSL changes

• Water depth is controlled by changes in accommodation as wellas sedimentation

• Base level is the horizontal surface to which subaerial erosionproceeds; therefore it corresponds to sea level

• Base level is a principal control of accommodation, and, hence,

whether erosion or deposition is likely to occur at any givenlocation; attempts to extend the concept landward arecontroversial



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• Allostratigraphy is a relatively new approach to stratigraphicsubdivision, and is based on the separation of strata based onunconformities or other discontinuities (e.g., paleosols)

• Sequence stratigraphy is the analysis of genetically related

depositional units bounded by unconformities and theircorrelative conformities

• A depositional sequence is a stratigraphic unit bounded at itstop and base by unconformities or their correlative conformities(=allostratigraphic unit), and typically embodies a continuum of depositional environments, from updip (continental) to downdip(deep marine)

• The subtle balance between RSL and sediment supply controlswhether aggradation, regression (progradation), forcedregression, or transgression (retrogradation) will occur



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• A RSL fall on the order of tens of meters or more will lead to abasinward shift of the shoreline and an associated basinwardshift of depositional environments; commonly (but not always)this will be accompanied by subaerial exposure, erosion, and the

formation of a widespread unconformity known as a sequenceboundary

• Sequence boundaries are the key stratigraphic surfaces (high-order bounding surfaces) that separate successive sequencesand are characterized by subaerial exposure/erosion, abasinward shift in facies, a downward shift in coastal onlap, and

onlap of overlying strata• Parasequences are lower order stratal units separated by

(marine) flooding surfaces; they are commonly autogenic andnot necessarily the result of smaller-scale RSL fluctuations



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• Systems tracts are contemporaneous, linked depositionalenvironments (or depositional systems); they are the buildingblocks of sequences and different types of systems tractsrepresent different limbs of a RSL curve

• Falling-stage (forced regressive) systems tract (FSST)

• Lowstand systems tract (LST)

• Transgressive systems tract (TST)

• Highstand systems tract (HST)

• The various systems tracts are characterized by their positionwithin a sequence, by shallowing or deepening upward faciessuccessions, or by parasequence stacking patterns



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• Maximum flooding surfaces form during the culmination of RSL rise, and maximum landward translation of the shoreline,and constitute the stratigraphic surface that separates the TSTand HST

• In the downdip realm (deep sea), where sedimentation ratescan be very low during maximum flooding, condensedsections may develop

• LSTs are separated from overlying TSTs by transgressivesurfaces; transgression is further characterized by coastalonlap

• An alternative approach to sequence analysis uses geneticstratigraphic sequences that are bounded by maximumflooding surfaces



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• In a very general sense, RSL fall leads to reduced depositionand formation of sequence boundaries in updip areas, andincreased deposition in downdip settings (e.g., submarine fans)

• RSL rise leads to trapping of sediment in the updip areas (e.g.,coastal plains with a littoral energy fence) and reduced transferof sediment to the deep sea (hemipelagic deposition; condensedsections)



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• Seismic stratigraphy is based on the principle that seismicreflectors follow stratal patterns and approximate isochrons(time lines)

• Reflection terminations provide the data used to identifysequence-stratigraphic surfaces, systems tracts, and theirinternal stacking patterns

• Technological developments have been prolific:

• Vertical resolution improved to a few tens of meters

• Widespread use of 3D seismic• Seismic data should preferably always be interpreted in

conjunction with well log or core data



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• A better understanding of stratigraphic sequences can beobtained by the construction of chronostratigraphic charts(„Wheeler diagrams‟); these can subsequently be used to infercoastal-onlap curves

• Variations in sediment supply can produce stratal patterns thatare very similar to those formed by RSL change (except forforced regression); in addition, variations in sediment supplycan cause stratigraphic surfaces at different locations to be outof phase

• In principle, sequence-stratigraphic concepts could be appliedwith some modifications to sedimentary successions that areentirely controlled by climate change and/or tectonics (outsidethe realm of RSL control)



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• The global sea-level curve for the Mesozoic and Cenozoic(inferred from coastal-onlap curves) contains first, second, andthird-order eustatic cycles that are supposed to be globallysynchronous, but it is a highly questionable generalization

• Conceptual problems: spatially variable RSL change due todifferential isostatic and tectonic movements undermines thenotion of a globally uniform control

• Dating problems: correlation is primarily based on biostratigraphythat typically has a resolving power comparable to the period of

third-order cycles



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Marine sequence stratigraphy

• The marine realm is considered here to include the shoreline,shelf, continental slope, and deep sea

• The shoreline is perhaps the most sensitive component withrespect to eustatic control; it can migrate along dip over longdistances (sometimes up to 100s of km) as a result of:

• RSL change

• Variations in hinterland sediment supply

• Autogenic processes (e.g., delta-lobe switching)



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• Bruun’s Rule predicts that a shoreface remains more or lessconstant during sea-level change (equilibrium profile), withassociated erosion and deposition; this 2D model is atremendous simplification of reality

• The distinction between forced regression and normalregression is critical to infer the relative roles of RSL changeand sediment supply

• Normal regression constitutes shoreface progradation due toexcess sediment supply

• Forced regression is driven by RSL fall and is associated with aregressive surface of erosion with shoreface strata sharplyoverlying fine-grained, offshore strata



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• Ravinement surfaces form during the landward migration of a shoreline due to transgression

• Wave ravinement surfaces are widespread erosion surfacesformed by the stripping of a relatively thin deposit by wave action

• Tidal ravinement surfaces are more localized, but commonlydeeper erosive features associated with tidal channels

• Shelf-edge deltas form during lowstand when RSL is close to theshelf break; they have a fairly high preservation potential



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• Sedimentation in the deep sea is commonly believed to bestrongly controlled by eustasy:• RSL fall and lowstand brings the shoreline close to, or below the

shelf break, and provides a mechanism for rapid transfer of

sediment to the deep-sea floor; RSL fall is associated with relativelycoarse-grained (sandy) sediment gravity flows, whereas turbiditesforming well-developed submarine fans follow during lowstand

• Accommodation creation on the shelf during RSL rise andhighstand reduces sediment supply to the slope and deep sea, andpredominance of hemipelagic facies

• Many exceptions are possible; for instance, a limited shelf widthand a high sediment supply from the hinterland can combine toallow rapid progradation of shorelines to the shelf edge evenduring highstand



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Nonmarine sequence stratigraphy

• The nonmarine realm is considered here to include allenvironments landward of the shoreline (fluvial, delta plain,coastal plain)

• Updip (nonmarine) sections of stratigraphic sequences not onlyrecord RSL changes (downstream control), but also climaticand tectonic signals from the hinterland (upstream control)



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• The fluvial longitudinal profile (graded profile) is crucial,because changes herein determine whether incision oraggradation occurs, including the formation of sequenceboundaries; it responds to changes in RSL (base level), as well

as climate and tectonics (sediment supply)

• Fluvial scour represents local, autogenic erosion of the channelbed (e.g., in sharp bends or at confluences)

• Fluvial incision constitutes the regional, allogenic degradationof the longitudinal profile, commonly including both lowering of

the channel bed and the genetically associated floodplainsurface

• Distinction of incision vs. scour is crucial!



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• Whenever the longitudinal profile is graded to a more or lessstable RSL for any prolonged time interval, and given sufficientsediment supply, a coastal prism will develop, representing adelta plain (possibly laterally connected to a more extensive

coastal plain), with a very low gradient that increases rapidlyacross the shoreline

• The coastal prism is highly sensitive to erosion during RSL fall;therefore, incision and the formation of sequence boundaries islikely to occur even if RSL does not fall below the shelf edge



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• Fluvial incision leads to valley cutting; paleovalleys (alsoknown as „incised valleys‟) are valleys that have beensubsequently filled with sediment

• Even during incision a fluvial deposit is always left behind

(terraces); rivers act as conveyor belts, not as vacuum cleaners!

• Unequivocal recognition of paleovalleys requires incision that mustsubstantially exceed channel depth, with interfluves topped bymature paleosols

• The distinction between paleovalleys and channel belts is tricky

• RSL fall does not necessarily always lead to the formation of well-developed sequence boundaries (e.g., fluvial systems do notalways respond to RSL fall by means of incision); sequenceboundaries may therefore be very indistinct and difficult to detect



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• Paleovalleys are commonly occupied by estuaries duringtransgression; their stratigraphy is a sensitive recorder of RSLchange

• A typical vertical succession, depending on the position in dipdirection, includes:

• A basal, fluvial FSST/LST overlying a sequence boundary

• An overlying TST that is either fully marine, estuarine, or tide-influenced fluvial

• A capping HST that is again more fluvial-dominated



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• In view of the difficulty to identify parasequence stackingpatterns, identification of systems tracts in upper deltaic tofluvial environments is problematic; however, there is a closerelationship between fluvial style, alluvial architecture, and

systems tracts• FSST/LST: destruction of accommodation; high channel-deposit

proportion

• TST: rapid creation of accommodation; low channel-depositproportion, possibly with tidal influence

• HST: moderate accommodation; intermediate channel-depositproportion



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• Coastal prisms are essentially composed of TSTs and HSTs, andin view of their sensitivity to erosion during RSL fall, theFSST/LST has a relatively high preservation potential; this isparticularly the case when subsidence rates are low

• Vertical stacking of relatively amalgamated channel belts,characteristic of the FSST/LST, leads to sequence boundariesthat are hard to identify („cryptic‟ sequence boundaries)

• Climatic and tectonic controls can operate in an oppositedirection than RSL, rendering nonmarine sequence-stratigraphic

interpretations considerably more difficult than their marinecounterparts



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Basin and reservoir modeling

What is a model?• Models are expressions of our ideas how things work

• Conceptual models (qualitative models)

• Physical models (experimental models)• Flume-operated simulations of sedimentologic or stratigraphic

phenomena at scales ranging from bedforms to basins

• Mathematical models (computer models)• Deterministic models (physically-based or process-based) have

one set of input parameters and therefore yield one unique

outcome• Stochastic models have variable input parameters, commonly

derived from probability-density functions (pdf‟s), and thereforehave multiple outcomes; as a consequence model runs must berepeated many times (realizations) and subsequently „averaged‟



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• Forward models simulate sets of processes and responses in asystem that has specified (assumed) initial boundary conditions(e.g., the evolution of a sedimentary basin given an initialconfiguration)

• Inverse models use observations as a starting point and aimto estimate initial boundary conditions and combinations of processes and responses that have operated to produce theobserved conditions (i.e., flip side of forward models)

• What is the goal of modeling in sedimentary geology?• Understanding processes and responses in sedimentary systems

(experimental and process-based models)

• Prediction of sedimentary architecture and stratigraphy (primarilystochastic models)



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• Architectural models typically simulate specific depositionalenvironments (e.g., alluvial architecture); different approachesare possible, involving different kinds of equations:

• Physical

• Empirical

• Probabilistic

• Stratigraphic models are widely used to simulate basin-scalestratal patterns (e.g., sequence stratigraphy):

• In geometric models the sediment surface is represented by one ormoresurfaces with predetermined geometry

• Many models are based on a diffusion equation that relates rates of sediment transport to topographic slopes



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• A classical approach in sedimentologic/stratigraphic modelinghas been to start from first principles (i.e., basic, small-scaleprocesses of sediment transport) and multiply this to thedesired spatial and temporal scale („upscaling‟)

• The outcomes of this approach have been very disappointing(i.e., upscaling is a very complicated procedure)

• There is no law of nature that says that “complexity +complexity = greater complexity”!



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• Reservoir characterization is the analysis of subsurfacesediments or sedimentary rocks from the perspective of fluidflow through porous media, including issues related to resourcerecovery (e.g., groundwater, hydrocarbons)

• The net-to-gross ratio (proportion of permeable units) is one of the most basic parameters in reservoir studies

• The connectedness between permeable units is anotherimportant parameter



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• Many reservoir models operate on the scale of sedimentaryarchitecture; they are mostly stochastic

• Object-based models simulate the distribution of objects,defined by specified geometries, in 3D space; simulations are

usually constrained by well data• Geostatistical models predict sedimentary facies at unvisited

sites, based on the quantified spatial facies variability derived fromwell data (e.g., sequential indicator simulation)

• Conditioning model output to observations is more easily done

in stochastic models, but process-based models have theadvantage that they tend to provide sedimentologically morerealistic output



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• The challenge for experimental models is to mimic real-worldconditions as well as possible (scaling); this becomesincreasingly difficult with increasing spatial and temporal scales(compare bedforms vs. sedimentary basins)

• Grain size (e.g., how to simulate clays?)• Grain properties (e.g., how to simulate cohesion of sediment

grains?)

• Fluid mechanics (e.g., how to keep the Froude numberreasonable?)

• Experimental models are increasingly used to simulate

sedimentary architecture and basin-scale stratigraphy• One important outcome of experimental modeling is the

recognition of non-linear responses



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Reflection

Why do we care about all this?

• Sedimentary geology is a key element in the understanding of

Earth history in a very broad sense (i.e., this can includeeverything from plate tectonics to global environmental change)

• Apart from traditional interests in economic sedimentarygeology (e.g., oil, gas, minerals), environmentalsedimentary geology (e.g., coastal management,

groundwater pollution) is becoming increasingly important



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Reflection

• The 1960s and 1970s saw a decline of interest in classical stratigraphyand an emphasis on autogenic processes within depositionalenvironments (process-oriented sedimentology, facies models)

• The 1980s and 1990s saw a revival of stratigraphy and a focus on

allogenic processes (sequence stratigraphy)• Quaternary environments play an increasingly important

role, since they allow a relatively straightforward inference of environments of deposition, including their relationships toindependently inferred changes in climate, sea level, and

tectonism by means of numerical dating techniques• Wherever possible, paleoecological evidence should be utilized

in facies analysis or sequence-stratigraphic analysis



Reflection

• Sequence stratigraphy can be considered to encompass twomain components:

• Development of generic and unifying models of sedimentary basinfilling

• Development of global eustasy models

• The first can potentially provide new and basic understanding,including improved capabilities to make subsurface predictions;the latter has proven to be extremely difficult at best

• The fundamental importance of basic sedimentology (i.e., facies

analysis) for sequence stratigraphy is in danger of beingoverlooked („sequence-strat fundamentalism‟ lingerseverywhere!)

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