00. Overview

Clastic Sedimentology and Sequence Stratigraphy

(EaES 455)

Instructor: Torbjörn Törnqvist2450 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 455 homepage: 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 as sediment in continental and marine environments and eventually forms sedimentary rocks

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

• Sedimentary geology sedimentology + stratigraphy• Sequence stratigraphy = the analysis of genetically

related depositional units bounded by unconformities and their correlative conformities

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Introduction

Historical development of sedimentary geology and 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 predominantly symmetric bedforms (wave ripples)

• Combined flow involves both modes of sediment transport and 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 a sedimentary 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), commonly based on a wide variety of observations

• Facies associations constitute several facies that occur in combination, and typically represent one depositional environment (note that very few individual facies are diagnostic for one specific setting!)

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

• Walther’s Law (1894) states that two different facies found superimposed on one another and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time

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• Standardized facies codes have been proposed (e.g., by Andrew Miall), 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 is known 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 individual sedimentary logs can rarely provide detailed environmental interpretations

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• Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters (large outcrops); 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 lamina to basin scale; they are much more readily used in outcrops than in subsurface data

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• Allogenic (allocyclic) controls are external forces that exert a strong 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 allogenic controls 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 in time, for sediments to accumulate; in marine environments accommodation 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 with increasing time scales

• Only an extremely small proportion of deposits that are initially formed actually survive and become preserved in the stratigraphic record (typical orders of magnitude 10-4–10-6)

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• Facies models are schematic, three-dimensional representations of specific depositional environments that serve as 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, but not always)

• Unconsolidated sediments (~Quaternary) can provide the bridge between present-day processes and ancient sedimentary rocks (~pre-Quaternary); Quaternary deposits are usually easy to interpret in terms of depositional environment and have great potential for studying 3D facies relationships and allogenic controls

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Fluvial environments

• Channel patterns (fluvial styles) of alluvial rivers are commonly classified as:• Braided rivers• Meandering rivers• Straight rivers• Anastomosing rivers

• Fluvial style is primarily controlled by specific stream power (W m-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

wρgQs

ω

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• Bars are sandy or gravelly macroforms in channels that are emergent, mostly unvegetated features at low flow stage, and undergo 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 or braid 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 a storey

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• Lateral accretion involves higher-order bounding surfaces dipping perpendicular to paleoflow direction and associated lower-order bounding surfaces; in the case of downstream accretion higher-order bounding surfaces dip parallel to paleoflow direction

• Braided rivers are characterized by a dominance of braid bars exhibiting both lateral and downstream accretion; meandering rivers primarily contain point bars with lateral accretion; in straight (and most anastomosing) rivers bars are commonly almost absent

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

• Facies successions produced by different fluvial styles can be extremely similar!

• The geometry and three-dimensional arrangement of architectural elements therefore provides a much better means of inferring fluvial styles from the sedimentary record

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• Channel belts consist of channel-bar and channel-fill deposits; the proportion of the two generally decreases markedly from braided rivers to anastomosing rivers

• The geometry of a channel belt (width/thickness ratio) is a function of the channel width and the degree of lateral migration; 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 an abandoned channel where flow velocities are extremely small

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• Overbank environments are dominated by fine-grained facies (predominantly muds)• Natural-levee deposits are wedges (‘wings’) of sediment

that form adjacent to the channel, dominated by fine sand and silt exhibiting planar stratification or (climbing) ripple cross stratification

• Crevasse-splay deposits are usually cones of sandy to silty facies 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, consisting entirely of muddy sediments deposited from suspension, and are volumetrically 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 the active channel

• Lacustrine deposits can be important in overbank environments characterized by high water tables, and are also found in distal settings; they are more likely to contain primary sedimentary structures (horizontal lamination) than their frequently bioturbated subaerial counterparts

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• Facies models highlight conspicuous differences between different 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 crevasse channels)

• 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 location on the floodplain, leading to the abandonment of a channel belt and the initiation of a new one

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

• An avulsion belt constitutes an extensive network of rapidly aggrading, narrow, crevasse-like channels with genetically associated overbank deposits, that may surround the new channel belt

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• Alluvial architecture refers to the three-dimensional arrangement of channel-belt deposits and overbank deposits in a 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-belt deposits, 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 coastal environments

• The density relationship between sediment-laden inflowing water 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 (usually silty) facies; the most distal prodelta is dominated by even finer sediment

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Delta plain

• Delta plains are commonly characterized by distributaries and interdistributary areas• The upper delta plain is gradational with floodplains, lacks

marine influence and typically has large flood basins, commonly with freshwater peats and lacustrine deposits

• The lower delta plain is marine influenced (e.g., tides, salt-water intrusion) 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 distributaries enter 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 the spectrum (meandering to straight/anastomosing)

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

• Avulsion (i.e., delta-lobe switching) is frequent due to high subsidence rates, as well as rapid gradient reduction associated with 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 the transition from a limnic, through a telmatic, to a terrestrial environment

• Terrestrialization (= hydrosere): gyttja --> fen peat --> wood peat --> 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); they are mostly sandy and tend to coarsen upwards

• Wave action can play an important role in winnowing and reworking of mouth-bar deposits; this may lead to merging with prograding 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 sediment supply• River-dominated deltas occur in microtidal settings with

limited wave energy, where delta-lobe progradation is significant and redistribution of mouth bars is limited

• Wave-dominated deltas are characterized by mouth bars reworked 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 the shoreline

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• The typical progradational delta succession exhibits a transition from prodelta offshore muds through silty to sandy (mouth bar) deposits (coarsening-upward succession), the latter commonly 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/beach ridge sands into barrier islands

• Drowning of barrier islands leading to offshore sand shoals• Increasing salinity and eventual drowning of (part of) the delta

plain

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

• Deformation processes are very common in deltas due to the high sediment rates and associated high pore-fluid pressures• Growth faults result from downdip increasing

sedimentation rates; they develop contemporaneously with sedimentation

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

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

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Coastal environments

• The classification of deltas can be extended to include those depositional coastal environments that are in large part fed by marine sediments• Wave-dominated shorelines• Tide-dominated shorelines

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

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

• Waves that approach a shoreline consisting of unconsolidated sediment will produce a series of environments (oscillatory wave zone, shoaling wave zone, breaker/surf/swash zone) with characteristic bedforms (symmetric ripples – asymmetric ripples or dunes – plane beds)

• Long-shore currents and rip currents can lead to sediment transport along the shoreline and away from the shoreline respectively, with associated unidirectional bedforms (commonly dunes)

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• Reflective shorelines have steep, coarse-grained foreshores and lack breaking waves and associated bars away from the shoreline

• 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 energy fence; 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 and Sun on the Earth, combined with the centrifugal force caused by movement of the Earth around the center of mass of the Earth-Moon system• Semi-diurnal or diurnal tidal cycles are essentially

caused by the 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

various cyclicities (including the elliptic orbit of the Moon)• Tidal currents are modulated by the configuration of

oceans and seas, and typically lead to a pattern of circulation; even in small tidal basins flood currents tend to dominate in different areas than ebb currents

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• Tide-influenced sedimentary structures can take different shapes:• Herringbone cross stratification indicates bipolar flow

directions, but it is rare• Mud-draped cross strata are much more common, and are

the result of alternating bedform migration during high flow velocities and mud deposition during high or low tide (slackwater)

• Tidal bundles are characterized by a sand-mud couplet with varying thickness; 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 commonly very thin (mm-scale), but distinct laminae

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• Beach-ridge strandplains and chenier plains result from coastal progradation in sand- and mud-dominated settings respectively; both are dominantly fed by sediments transported by long-shore currents

• Tidal flats occur in a wide variety of settings (e.g., directly facing the open sea/ocean, in lagoons behind barrier islands, near tidal inlets) and contain a supratidal zone, an intertidal zone, 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 where tidal rhytmites may form, commonly bordered by salt marshes or mangroves where muddy facies or peats accumulate

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• Barrier islands form in transgressive settings where beach ridges 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) facies into the lagoon during storms• Tidal inlets vary in number, width, and depth dependent on

the tidal range; they are associated with flood-tidal deltas and ebb-tidal deltas

• Barrier island shorelines can exhibit shoreface retreat or in-place drowning; prolonged shoreface regression ultimately leads to filling 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 a net landward movement of sediment in their seaward part• Tide-dominated estuaries contain tidal sand bars at the

seaward end, separated from the fluvial zone by relatively fine-grained tidal flats (e.g., salt marshes); fluvial channel deposits exhibit heterolithic characteristics and sometimes tidal-bundle sequences

• Wave-dominated estuaries have a coastal barrier with a tidal inlet and 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 pericontinental seas that occur along continental margins and have a shoreline-shelf-slope profile; and epicontinental seas that cover continental interiors and exhibit a ramp morphology

• Under idealized conditions the offshore-transition and offshore exhibit a systematic decrease in (wave) energy and grain size; however, such an ‘equilibrium shelf’ is commonly not encountered• Tides and ocean currents can strongly complicate shelf

hydrodynamics• Rapid sea-level changes (e.g., during the Quaternary) result

in relict shelf sediments that are genetically unrelated to the present conditions

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• Wave/storm-dominated shelves ideally exhibit a transition from sands in the lower shoreface, to alternating sands and muds below fairweather wave base, to muddy facies below storm wave base

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

• Tempestites form during storm events and exhibit a characteristic facies succession from an erosional basal surface with sole marks, to a sandy unit with hummocky cross stratification overlain by wave-rippled sand, finally giving way to muds

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• Tides lead to circulation around amphidromic points, ranging from circular to almost rectilinear depending on the shape of the water body

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

• Erosional features, sand ribbons, and sand waves go along with decreasing 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 supply of 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 and the 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; they commonly evolve from debris flows• Debris-flow deposits are poorly sorted, related to the

‘freezing’ that occurs once shear stresses can not overcome the internal shear 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 to distal fining

• The idealized Bouma sequence, consisting of divisions A-E, is most 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) turbidites respectively

• Contourites are formed by ocean currents and commonly represent reworked turbidites

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

• The size of submarine fans is inversely related to dominant grain 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; they consist of a feeder channel that divides into numerous distributary channels bordered by natural levees (‘channel-levee systems’) 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 from suspension, commmonly after transport by hemipelagic advection• Distal, muddy turbidites merge gradationally into

hemipelagic deposits• 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., Western Interior Seaway)

• Pelagic sediments are widespread in the open ocean and primarily 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 measured relative to a fixed datum (e.g., the center of the Earth), and regional 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, ranging from large-scale uplifts and basins to small-scale faults

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

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

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Sea-level change

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

• Four characteristic RSL-curves associated with the last deglaciation:• 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 have operated 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 Earth

history (e.g., the Carboniferous or Late Cenozoic icehouse world as opposed to the Cretaceous greenhouse world)

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

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Sequence stratigraphy – concepts

• Sequence stratigraphy highlights the role of allogenic controls on patterns of deposition, as opposed to autogenic controls that operate 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 in time, for sediments to accumulate; accommodation is created or destroyed by RSL changes

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

• Base level is the horizontal surface to which subaerial erosion proceeds; 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 given location; attempts to extend the concept landward are controversial

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

• Sequence stratigraphy is the analysis of genetically related depositional units bounded by unconformities and their correlative conformities

• A depositional sequence is a stratigraphic unit bounded at its top 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 controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur

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• A RSL fall on the order of tens of meters or more will lead to a basinward shift of the shoreline and an associated basinward shift 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 sequence boundary

• Sequence boundaries are the key stratigraphic surfaces (high-order bounding surfaces) that separate successive sequences and are characterized by subaerial exposure/erosion, a basinward 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 and not necessarily the result of smaller-scale RSL fluctuations

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• Systems tracts are contemporaneous, linked depositional environments (or depositional systems); they are the building blocks of sequences and different types of systems tracts represent 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 position within a sequence, by shallowing or deepening upward facies successions, 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 TST and HST

• In the downdip realm (deep sea), where sedimentation rates can be very low during maximum flooding, condensed sections may develop

• LSTs are separated from overlying TSTs by transgressive surfaces; transgression is further characterized by coastal onlap

• An alternative approach to sequence analysis uses genetic stratigraphic sequences that are bounded by maximum flooding surfaces

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• In a very general sense, RSL fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased 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 transfer of sediment to the deep sea (hemipelagic deposition; condensed sections)

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

• Reflection terminations provide the data used to identify sequence-stratigraphic surfaces, systems tracts, and their internal 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 be obtained by the construction of chronostratigraphic charts (‘Wheeler diagrams’); these can subsequently be used to infer coastal-onlap curves

• Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase

• In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the 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, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization• Conceptual problems: spatially variable RSL change due to

differential isostatic and tectonic movements undermines the notion of a globally uniform control

• Dating problems: correlation is primarily based on biostratigraphy that 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 with respect to eustatic control; it can migrate along dip over long distances (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 less constant during sea-level change (equilibrium profile), with associated erosion and deposition; this 2D model is a tremendous simplification of reality

• The distinction between forced regression and normal regression is critical to infer the relative roles of RSL change and sediment supply• Normal regression constitutes shoreface progradation due

to excess sediment supply• Forced regression is driven by RSL fall and is associated

with a regressive surface of erosion with shoreface strata sharply overlying 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

surfaces formed by the stripping of a relatively thin deposit by wave action

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

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

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• Sedimentation in the deep sea is commonly believed to be strongly 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 relatively coarse-grained (sandy) sediment gravity flows, whereas turbidites forming well-developed submarine fans follow during lowstand

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

• Many exceptions are possible; for instance, a limited shelf width and a high sediment supply from the hinterland can combine to allow rapid progradation of shorelines to the shelf edge even during highstand

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Nonmarine sequence stratigraphy

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

• Updip (nonmarine) sections of stratigraphic sequences not only record RSL changes (downstream control), but also climatic and 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 or aggradation occurs, including the formation of sequence boundaries; it responds to changes in RSL (base level), as well as climate and tectonics (sediment supply)

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

• Fluvial incision constitutes the regional, allogenic degradation of the longitudinal profile, commonly including both lowering of the channel bed and the genetically associated floodplain surface

• Distinction of incision vs. scour is crucial!

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• Whenever the longitudinal profile is graded to a more or less stable RSL for any prolonged time interval, and given sufficient sediment supply, a coastal prism will develop, representing a delta plain (possibly laterally connected to a more extensive coastal plain), with a very low gradient that increases rapidly across the shoreline

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

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• Fluvial incision leads to valley cutting; paleovalleys (also known as ‘incised valleys’) are valleys that have been subsequently 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 must substantially exceed channel depth, with interfluves topped by mature 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 not always respond to RSL fall by means of incision); sequence boundaries may therefore be very indistinct and difficult to detect

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• Paleovalleys are commonly occupied by estuaries during transgression; their stratigraphy is a sensitive recorder of RSL change

• A typical vertical succession, depending on the position in dip direction, 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 stacking patterns, identification of systems tracts in upper deltaic to fluvial environments is problematic; however, there is a close relationship between fluvial style, alluvial architecture, and systems tracts• FSST/LST: destruction of accommodation; high channel-

deposit proportion• TST: rapid creation of accommodation; low channel-deposit

proportion, possibly with tidal influence• HST: moderate accommodation; intermediate channel-

deposit proportion

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

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

• Climatic and tectonic controls can operate in an opposite direction than RSL, rendering nonmarine sequence-stratigraphic interpretations considerably more difficult than their marine counterparts

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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 therefore have multiple outcomes; as a consequence model runs must be repeated many times (realizations) and subsequently ‘averaged’

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

• Inverse models use observations as a starting point and aim to estimate initial boundary conditions and combinations of processes and responses that have operated to produce the observed 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

(primarily stochastic models)

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• Architectural models typically simulate specific depositional environments (e.g., alluvial architecture); different approaches are possible, involving different kinds of equations:• Physical• Empirical• Probabilistic

• Stratigraphic models are widely used to simulate basin-scale stratal patterns (e.g., sequence stratigraphy):• In geometric models the sediment surface is represented by

one or moresurfaces 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 modeling has been to start from first principles (i.e., basic, small-scale processes of sediment transport) and multiply this to the desired 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 subsurface sediments or sedimentary rocks from the perspective of fluid flow through porous media, including issues related to resource recovery (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 another

important parameter

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• Many reservoir models operate on the scale of sedimentary architecture; 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 from well data (e.g., sequential indicator simulation)

• Conditioning model output to observations is more easily done in stochastic models, but process-based models have the advantage that they tend to provide sedimentologically more realistic output

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• The challenge for experimental models is to mimic real-world conditions as well as possible (scaling); this becomes increasingly 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 number

reasonable?)• 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 include everything from plate tectonics to global environmental change)

• Apart from traditional interests in economic sedimentary geology (e.g., oil, gas, minerals), environmental sedimentary 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 stratigraphy and an emphasis on autogenic processes within depositional environments (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 to independently 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

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Reflection

• Sequence stratigraphy can be considered to encompass two main components:• Development of generic and unifying models of

sedimentary basin filling• 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 being overlooked (‘sequence-strat fundamentalism’ lingers everywhere!)

Documents

00. Overview