introduction to sedimentology

Principles of Sedimentology and

Stratigraphy (EaES 350)

Instructor: Torbjörn TörnqvistSES 2450

(312) [email protected]

Teaching assistant: Zenon MateoSES 2474

(312) [email protected]

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Components of EaES 350

• Lectures• Labs• Poster presentation• Field trips (Indiana Dunes; SW Wisconsin)

• More detailed information on EaES 350 homepage: http://www.uic.edu/classes/eaes/eaes350/

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Grading

• Written exams (50%)• Midterm (20%)• Final (30%)

• Labs (30%)• Poster (20%)

• Content (15%)• Visual appeal (5%)

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Literature

• Nichols, G., 1999. Sedimentology and Stratigraphy. Blackwell, Oxford, 355 pp. ISBN 0-632-03578-1.

• Lecture notes on EaES 350 homepage (http://www.uic.edu/classes/eaes/eaes350/)

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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

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Introduction

Context of sedimentary geology within the Earth sciences

• Rock cycle: mountain formation and/or uplift; weathering and erosion; sediment transport, deposition, and diagenesis; metamorphism and igneous rock formation; renewed uplift… etc.

• Structural geology/Tectonics (Geophysics); Geomorphology; Sedimentology/Stratigraphy (Sedimentary geology); Metamorphic geology/Petrology

• Other closely related disciplines: Paleontology, Geochemistry, Geochronology

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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”)

(Charles Lyell, 1830)

• 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

• The stratigraphic record is nearly always very incomplete due to a limited preservation potential, that decreases with increasing time scales

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Introduction

Primary data sources for sedimentologic/stratigraphic studies

• Outcrops (consolidated vs. unconsolidated sediments)• Cores (hand-operated vs. power-driven) • Geophysical data (e.g., wireline logs, seismic, ground-

penetrating radar)

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Introduction

• Modern processes constitute the basis for interpreting ancient products (uniformitarianism works in many cases, but not always)

• Unconsolidated sediments (~Quaternary) vs. sedimentary rocks (~pre-Quaternary)

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Introduction

• General structure of the course: from small scale (sediment grains) to large scale (sedimentary basins); i.e., from sedimentology to stratigraphy

• Clastics and carbonates integrated

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Contents

• Introduction• Unconsolidated clastic sediments• Sedimentary rocks• Diagenesis• Sediment transport and deposition• Sedimentary structures• Facies and depositional environments• Glacial/eolian/lacustrine

environments• Fluvial/deltaic/coastal environments• Shallow/deep marine environments

• Stratigraphic principles• Sequence stratigraphy• Sedimentary basins• Models in sedimentary

geology• Applied sedimentary geology• Reflection

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Unconsolidated clastic sediments

• Particles (or ‘clasts’) are the basic elements of any sediment

• Clastic (terrigenous clastic or siliciclastic) sediments (80-85% of the stratigraphic record) consist of particles derived from pre-existing rocks, as opposed to non-clastic sediments

• Texture• Grain (particle) size• Grain shape• Clast/matrix relationships• Fabric

• Lithology is the general characterization of a sediment or a sedimentary rock (e.g., coarse sand, mudstone)

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• The Udden-Wentworth grain-size scale is based on factors of two: = -log2 (mm)

• Mud (<63 m; >4 )• Clay (<4 m; >8 )• Silt (4–63 m; 4–8 )

• Sand (63–2000 m; -1–4 )• Gravel (>2000 m; <-1 )

• Grain-size (particle-size, granulometric) analysis• The old-fashioned way: direct measurement (gravel) and

sieve/pipette analysis (sand and mud) • The modern technology: laser particle sizing (sand and

mud)

2log

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Moment measures• First moment: mean (cf. median, mode)

• Premier measure of the grain size

• Second moment: variance (cf. standard deviation)• Measure of the degree of sorting

( = standard deviation)

• Third moment: skewness• Measure of the symmetry of the grain-size distribution

/nxμ i

n

1i

n

1i

2i

2 /nμ)(x σ

n

1i

3i /nμ)(x sk

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• Grain shape• Roundness (well rounded to very angular)• Sphericity (high or low)

• Clast/matrix proportion• The matrix is the relatively fine-grained material that lies

between the relatively coarse-grained clasts• Clast-supported sediments (clasts are in direct contact)• Matrix-supported sediments (clasts are entirely surrounded by

matrix)

• Fabric• Preferential orientation of particles in a sediment or

tendency of a rock to break in specific directions

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• Sediment composition• Detrital mineral grains (quartz, feldspar, mica, heavy minerals)• Lithic fragments (polymineral grains or rock fragments)• Detrital mineral grains dominate in silts, lithic fragments

dominate in gravels

• Sediment maturity (degree of change compared to original bedrock: provides evidence on the history of a sediment)• Textural (mud content, sorting, grain shape)• Mineralogical (proportion of stable or resistant minerals)• Pitfalls! (depends strongly on the nature of the original

bedrock)

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Clay minerals

• Clay minerals are phyllosilicates with layered crystal structures• Kandite group (two layers): kaolinite• Smectite group (three layers): montmorillonite, illite, chlorite

• Key physical and chemical characteristics of clay minerals• Platy shape (easy to keep in suspension, very slow settling

rates)• Strong cohesion due to electrostatic charge (relatively

difficult to erode, tendency to flocculate)

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Sedimentary rocks

• Clastic (siliciclastic) rocks (80-85% of the stratigraphic record)

• Carbonate sediments and rocks (10-15% of the stratigraphic record)

• Organic (carbonaceous) sediments and rocks• Evaporites• Volcaniclastic sediments and rocks

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Sedimentary rocks

Clastic (siliciclastic) rocks

• Sandstones (20-25% of the stratigraphic record) can be subdivided according to the Pettijohn classification, based on texture and composition (relative proportions of quartz, feldspar, and lithic fragments)• Quartz arenite: quartz-dominated• Arkosic arenite: feldspar-dominated• Lithic arenite: dominance of lithic fragments• Wacke: significantly matrix-supported (>15% mud)

• Quartz wacke• Greywacke (feldspathic or lithic wacke)

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Sedimentary rocks

Clastic (siliciclastic) rocks

• Mudstones (60% of the stratigraphic record) are also known as mudrocks or shales and commonly exhibit a distinct fissility• Claystone• Siltstone

• Conglomerates are consolidated gravels; breccias are conglomerates with dominantly angular clasts• Clast-supported conglomerates• Matrix-supported conglomerates

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Sedimentary rocks

Carbonate sediments and rocks

• Principal minerals: calcite, aragonite (unstable), and dolomite (diagenetic)

• Principal rocks: limestone (>50% CaCO3) and dolomite (dolostone) (CaMg(CO3)2)

• Formation of carbonate sediments and rocks occurs by means of two main processes:• Biomineralization of CaCO3 by organisms

• Direct chemical precipitation

32332 COHCaCO2HCOCa

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Sedimentary rocks


• Biogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others)• Most organisms initially form unconsolidated carbonate

sediments• Coral reefs and microbial mats (e.g., stromatolites) are

examples of more solid carbonate structures

• Chemical precipitation produces non-skeletal carbonate grains of various sizes (e.g., ooids, pisoids, micrite)

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Sedimentary rocks


• Carbonate sand usually consists either of (fragmented) skeletal remains or non-skeletal grains

• Carbonate mud (micrite) is commonly the product either of chemical precipitation or algal/bacterial activity

• Dunham classification of carbonate rocks:• Texturally-based subdivision (cf. clastics): mudstone,

wackestone, packstone, grainstone, rudstone• Organically bound framework during formation:

boundstone

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Sedimentary rocks

Organic (carbonaceous) sediments and rocks

• Peat and organic-rich clastic sediments form in relatively anaerobic (reducing) environments (e.g., mires, lakes, oceans)• Minerotrophic peat: mostly nutrient-rich, groundwater-

fed mires (e.g., floodplains, delta plains, coastal plains)• Ombrotrophic peat: mostly nutrient-poor, rainwater-fed

mires (e.g., relatively high, flat terrains)• Gyttja: organic-rich lake sediment• Sapropel: organic-rich marine sediment

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Sedimentary rocks

Organic (carbonaceous) sediments and rocks

• Coal consists primarily of solid organic matter; the remainder is known as ‘ash’

• Carbonaceous shales have a lower proportion of solid organic matter

• Oil shales (may be formed in anaerobic lake and marine environments) contain organic matter that can be driven off as liquid or gas by heating

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Sedimentary rocks

Evaporites

• Dissolved salts precipitate out of sea water due to concentration (brine formation) during evaporation (1 km of sea water --> 12 m of evaporites)

• Evaporites commonly lithify into consolidated rocks upon formation

• Least soluble compounds precipitate first:• CaCO3 (calcium carbonate)• CaSO4 (calcium sulphate: gypsum or anhydrite)• NaCl (halite: rock salt)• Other, less stable (highly soluble) chlorides

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Sedimentary rocks

Volcaniclastic sediments and rocks

• Lava (cooled magma flows) produces volcaniclastic sediment upon weathering

• Pyroclastic material or tephra (ejected particulate material) can be subdivided into different compositional categories:• Mineral grains• Lithic fragments• Vitric material (volcanic glass or pumice)

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Diagenesis

• Diagenesis ( lithification) includes the full range of alterations sediments undergo after deposition, at relatively low temperatures and pressures (gradational to metamorphism)

• Lithification may occur simultaneously with deposition (in several carbonates, evaporites, and volcaniclastics)

• Physical and chemical diagenetic processes constitute compaction and cementation, respectively

• Diagenesis commonly leads to a reduction of porosity and permeability

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Diagenesis

Compaction

• Compaction is the result of overburden pressure during sediment burial, resulting in a decrease of volume and an increase of density• Compaction is extremely important in organics and muds,

but less important in sands, gravels, and reefal carbonates• Compaction is accompanied by the expulsion of

groundwater and a reduction of porosity• Differential compaction is important when sediments

exhibit a high spatial variability

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Diagenesis

Compaction

• Pressure dissolution results in increasingly interlocking grains, and significantly contributes to lithification

• In limestones, pressure dissolution usually occurs at specific horizons, that may or may not correspond to depositional bedding planes

• Stylolites are irregular pressure dissolution surfaces with higher proportions of residual material and represent more extreme forms of this process

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Diagenesis

Cementation

• Dissolution commonly occurs without high pressures, and subsequent precipitation results in the formation of cement (authigenic minerals)• Calcium carbonate (sparry or micritic)• Silica (commonly microquartz)• Clay minerals

• Cementation reduces both the porosity and the permeability

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Diagenesis

Cementation

• Nodules (irregular) and concretions (rounded) are larger cemented bodies (e.g., silica, calcite, siderite, pyrite)• Chert (flint) is the most widely known type of silica

nodules, especially common in limestones

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Diagenesis

Dolomitization

• Dolomites are mostly formed diagenetically, involving the replacement of calcite or aragonite by dolomite (CaMg(CO3)2)

• Four main models of dolomitization can be distinguished:• Evaporite brine residue/seepage reflux model• Meteoric-marine/groundwater mixing model (obsolete)• Burial compaction/formation water model• Sea water/convection model

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Diagenesis

Coal formation

• Coal formation is primarily the result of compaction and geothermal heating

• A very high proportion of compaction occurs during the peat-accumulation stage• Peat --> lignite --> bituminous coal --> anthracite• Relative increase of carbon over hydrogen and oxygen

(gradual expulsion of H2O, CO2, and CH4)

• Methane (CH4) is an important byproduct of coal formation

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Diagenesis

Hydrocarbon formation

• Diagenetic breakdown of planktonic algae (maturation) leads to the formation of kerogen (long-chain hydrocarbons)

• Liquid hydrocarbons (shorter-chain hydrocarbons) are generally formed at temperatures of 70-100° C (‘oil window’ at 2-3 km depth)

• Methane is released at temperatures over 150° C

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Sediment transport and deposition

Transport media

• Water• Overland flow, channel flow• Waves, tides, ocean currents

• Air• Ice• Gravity

• Rock falls (no transport medium involved)• Debris flows, turbidity currents (water involved)

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Reynolds number (laminar vs. turbulent flow)

u=flow velocity; l=characteristic length (flow depth); =kinematic

viscosity (dynamic viscosity/fluid density)

• Turbulence is promoted by high flow velocities and flow depths, and low viscosities (Re>2000); laminar flow occurs when the reverse is the case (Re<500)

• Air and water are nearly always turbulent

ul

Re

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Froude number (subcritical vs. supercritical flow)

u=flow velocity; d=flow depth; gd=celerity (wave velocity)

• Flow velocities exceeding wave propagation velocities (Fr>1) yield supercritical flow, lower velocities (Fr<1) cause subcritical flow

• A spatial transition from subcritical to supercritical flow (or vice versa) is characterized by a ‘hydraulic jump’

gdu

Fr

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Stokes’ Law (settling velocity in a static fluid)

vg=settling velocity; D=grain diameter; g=grain density;

f=fluid density; =dynamic viscosity

• Stokes’ Law only applies to fine (<100 m), quartz-density grains in water

μ 18

)ρ(ρgDv fg

2

g

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• The Bernouilli effect is the reduction of pressure, proportional to the increase of flow velocity as the flow encounters an obstacle (sediment particle), leading to a lift force and entrainment of the particle

• Drag forces and lift forces act together to cause entrainment of sediment grains

• The boundary layer is that part of the flow influenced by frictional effects

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• A widely used parameter in the context of sediment transport is the shear stress, expressed in N m-2, which can be determined anywhere in a flow or at the bed

=fluid density; d=flow depth; S=slope; =dynamic viscosity;u=flow velocity

• Bed shear stress (0) must be higher than the critical shear stress (c) to enable sediment grains to be transported

dddu

μρgdSτ0

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Transport modes in a turbulent fluid

• Traction (rolling over the bed surface)• Saltation (jumping over the bed surface)• Suspension (permanent transport within the fluid)• Solution (chemical transport)

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• Critical velocities are different for sediment entrainment and deposition, especially in the finer fractions

• Fluid density and viscosity play a key role in determining which particle sizes can be transported

• The amount of sediment transport is not only related to flow velocity (or bed shear stress) and grain size, but also to:• Grain density• Grain shape

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Current ripples

• Once movement of sand grains (<0.7 mm) occurs, current ripples are formed as a result of boundary layer separation, commonly accompanied by a separation vortex

• Current ripples have a stoss side (erosion and transport) and lee side (deposition), the latter with a slope of ~30° (angle of repose)

• Current ripples only form under moderate flow velocities, with a grain size <0.7 mm

• Height: 0.5–3 cm; wavelength: 5–40 cm

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Dunes

• Dunes are distinctly larger than current ripples• There is a relationship between boundary-layer thickness

( flow depth in rivers) and the dimension of dunes• Dunes only form in grain sizes >0.2 mm• Low flow velocities (bed shear stresses) yield straight-

crested bedforms (valid for both dunes and current ripples); higher shear stresses result in sinuous to linguoid crest lines

• Sand waves constitute the largest category of subaqueous dunes

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Plane beds and antidunes

• In coarse sands (>0.7 mm) lower-stage plane beds develop instead of current ripples

• At high (but still subcritical) flow velocities upper-stage plane beds are formed in all sand grain sizes

• Supercritical flow conditions (Fr1 or higher) enable the formation of antidunes, characterized by bedform accretion in an upstream direction

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Waves

• Waves are wind-generated oscillatory motions of water• Wave height is dependent on wind strength and fetch• The depth to which the oscillatory motion due to wave

action extends is known as the wave base; shallow water leads to breaking waves

• Wave ripples are distinct from current ripples due to their symmetry, and include low-energy ‘rolling grain ripples’ and high-energy ‘vortex ripples’

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Tides

• 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• Neap-spring tidal cycles• Annual tidal cycles

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Ocean currents

• The circulation of sea water in the world’s oceans is driven by wind and contrasts in density due to variable temperature and salinity (thermohaline circulation), combined with the Coriolis effect

• Ocean currents transport clay and silt in suspension, and sand as bed load, and their effects are especially important in deep waters, where storms and tides are less important

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Gravity flows

• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous• Low Reynolds numbers

• Turbidity currents have a higher proportion of water, are always subaqueous, and move due to density contrasts• Higher Reynolds numbers

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Sedimentary structures

• 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)

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• Laminae and beds are the basic sedimentary units that produce stratification; the transition between the two is arbitrarily set at 10 mm

• Normal grading is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow velocity), as opposed to reverse grading

• Fining-upward successions and coarsening-upward successions are the products of vertically stacked individual beds

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Cross stratification

• Cross lamination (small-scale cross stratification) is produced by ripples

• Cross bedding (large-scale cross stratification) is produced by dunes

• Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output)

• Straight-crested bedforms lead to planar cross stratification; sinuous or linguoid bedforms produce trough cross stratification

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Cross stratification

• The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘climbing ripple cross lamination’

• Antidunes form cross strata that dip upstream, but these are not commonly preserved

• A single unit of cross-stratified material is known as a set; a succession of sets forms a co-set

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Planar stratification

• Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditions

• Planar stratification can easily be confused with planar cross stratification, depending on the orientation of a section (strike sections!)

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• Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and by laminae dipping in two directions

• Hummocky cross stratification (HCS) forms during storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swales

• Heterolithic stratification is characterized by alternating sand and mud laminae or beds• Flaser bedding is dominated by sand with isolated, thin

mud drapes• Lenticular bedding is mud-dominated with isolated ripples

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

directions, but are 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

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Gravity-flow deposits

• Debris-flow deposits are typically poorly sorted, matrix-supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal direction

• Turbidites, the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedforms

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• Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream

• Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial• Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)

• Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts)

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• Trace fossils (ichnofossils) are the tracks, trails or burrows left behind in sediments by organisms (e.g., feeding traces, locomotion traces, escape burrows)

• Disturbance of sediments by organisms is known as bioturbation, which can lead to the total destruction of primary sedimentary structures

• Since numerous trace fossils are connected to specific depositional environments, they can be very useful in sedimentologic interpretations

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• Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include:• Slump structures (on slopes)• Dewatering structures (upward escape of water, commonly

due to loading)• Load structures (density contrasts between sand and

underlying wet mud; can in extreme cases cause mud diapirs)

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Facies and depositional environments

• 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

• Facies models are schematic, three-dimensional representations of specific depositional environments that serve as norms for interpretation and prediction

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• Soils are formed by physical, chemical, and biological processes that act at the land surface and lead to the development of A-, B-, and C-horizons

• Paleosols are fossil soils that are increasingly important in the facies analysis of continental strata:• Paleoenvironmental indicators (e.g., climate)• Indicators for sedimentation rates that have been

temporarily halted or strongly reduced

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Glacial/eolian/lacustrine environments

Glacial environments

• Glaciers and ice sheets form where precipitation rates, in the form of snow (accumulation), exceed melting rates (ablation)

• Ice flows as a result of gravity and essentially acts like a high-viscosity fluid exhibiting laminar flow

• Temperate (warm-based) vs. polar (cold-based) glaciers reflect the temperature regime within the ice

• Ice shelves can form when a glacier or ice sheet reaches the coast and extends offshore, and ultimately breaks up into icebergs

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• Abrasion leads to the formation of rock flour (mineralogically diverse silt- and clay-sized sediment grains); plucking results in coarser (up to boulder-sized) material

• Warm-based ice tends to be more erosive (abrasive) than cold-based ice

• Till/tillite (also known as diamict/diamictite) is poorly sorted, angular, and immature• Lodgement till forms by active deposition under the ice

(relatively compact and usually fractured)• Meltout till forms passively during melting

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• Glaciofluvial or fluvioglacial deposits are sediments formed in association with glacial meltwater (e.g., glacial outwash)

• More distal glaciolacustrine and glaciomarine deposits are typically dominated by fine-grained sediment (rock flour), along with ice-rafted debris and dropstones

• The preservation potential of glacial deposits is usually limited, with the exception of tills and glaciomarine deposits associated with big ice sheets

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Eolian environments

• Eolian deposits dominate deserts (mostly at low latitudes, but sometimes arctic), but are also important along shorelines (coastal dunes) and in association with ice sheets (loess)

• Air is a low-density and low-viscosity fluid; therefore high flow velocities are required to enable sediment transport

• Eolian deposits are mostly texturally and mineralogically mature, due to the selective transport of specific grain sizes and the large impact of grain-to-grain collision

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Eolian environments

• Sand dunes are the most common eolian landforms; their geometry and resulting sedimentary structures depend primarily on sediment supply and prevailing wind direction

• Large (>~5 m) sets of cross strata are very commonly eolian in origin

• Eolian sand sheets develop when sediment supply is limited and are characterized by planar stratification; vegetation can contribute to dune formation under such circumstances

• Loess is a homogeneous, very well sorted, silt-dominated sediment that is deposited from suspension; it is commonly associated with ice sheets that produce large quantities of source material (rock flour)

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Lacustrine environments

• Playa (saline) lakes are hydrologically closed, ephemeral water bodies that form in arid environments and are characterized by mud-evaporite couplets

• Freshwater lakes are permanent (commonly hydrologically open) water bodies• Waves and relatively weak wind-driven currents constitute

the main mechanisms of sediment transport• Density stratification develops under seasonal climate

conditions and when currents are limited

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• Coarse sediments mainly occur on lake margins (lacustrine deltas, beaches)

• In the central parts of lakes, deposition occurs from suspension and by means of turbidity currents

• Stratified lakes promote the accumulation of organic matter and the formation of varves; organics are especially important in small lakes

• Carbonates of both chemical and biogenic origin can contribute significantly to lake sediments

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• The final stage of filling of lakes commonly involves an important organic component

• Hydrosere: vertical succession of organic deposits associated with the transition from a limnic, through a telmatic, to a terrestrial environment• Gyttja --> fen peat --> wood peat --> moss peat

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Fluvial/deltaic/coastal environments

Fluvial environments

• Bedrock rivers essentially do not contribute to the stratigraphic record, contrary to alluvial rivers

• Alluvial fans are relatively steep (>1-2°) cones consisting of coarse-grained facies and constitute the most proximal fluvial depositional environments (usually at the break of slope on the edge of a floodplain)• Debris flows dominate on small and steep alluvial fans• Sheetfloods are common on larger and gentler alluvial fans

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• Ephemeral rivers are dry during a significant part of the year, contrary to perennial rivers

• Floodplains are the areas occupied by river channels, as well as the surrounding, flat (overbank) areas that are subject to flooding

• Discharge is confined to the channel until bankfull discharge is reached; from that point on overbank flow can occur, submerging the entire floodplain

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• Channel patterns (fluvial styles) 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 grain size, but also by bank stability and the amount of bed load

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

ρgQsω

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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

• Braided rivers are characterized by a dominance of braid bars; meandering rivers primarily contain point bars; in straight (and most anastomosing) rivers bars are almost absent

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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 straight or 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)

• 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 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 sediments deposited from suspension, and are volumetrically very important (mainly in low-energy fluvial settings)

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• Paleosols (well drained conditions) and 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

• Lacustrine deposits can be important in overbank environments characterized by high water tables, and are also found in distal settings

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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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• 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

• 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

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Deltaic environments

• Deltas form where a river enters a standing body of water (ocean, sea, lake) and forms a thick deposit that may or may not form protuberances

• The delta plain is the subaerial part of a delta (gradational upstream to a floodplain); the delta front (delta slope and prodelta) is the subaqueous component

• Delta plains are commonly characterized by distributaries and flood basins (upper delta plain) or interdistributary bays (lower delta plain), as well as numerous crevasse splays

• Upper delta plains contain facies assemblages that are very similar to fluvial settings

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• Mouth bars form at the upper edge of the delta front, at the mouth of distributaries; they are mostly sandy and tend to coarsen upwards

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

• Progradation (basinward building) of deltas leads to coarsening-upward successions, and progradation rates depend on sediment supply and basin bathymetry (water depth)

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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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• Coarse-grained deltas are composed of gravelly facies and form where alluvial fans or relatively steep braided rivers enter a water body

• Delta cycles are the result of repetitive switching of delta lobes, comparable to avulsion in fluvial environments; this leads to characteristic vertical successions with progradational facies and transgressive facies

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Coastal environments

• Erosional coasts are commonly characterized by cliffs, whereas constructional coasts can be formed by clastic, carbonate, or evaporite facies

• The morphology of constructional coasts is determined by sediment supply, wave energy, and tidal range, as well as climate and sea-level history

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• Beaches form when sand or gravel is available and wave energy is significant, and result in low-angle cross-stratified deposits and cross strata formed by wave ripples

• Beaches can either be connected directly to the land and form strand plains or chenier plains (the latter consisting of beach ridges separated by muds), or be separated by lagoons or tidal basins (the latter consisting of tidal channels, tidal flats, and salt marshes) and form either spits or barrier islands

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• Barrier islands are especially prolific in environments with a high wave energy and a limited tidal range, that have experienced transgression (relative sea-level rise)

• The tidal inlets between barrier islands are sites of deep erosional scour and are associated with flood-tidal deltas (lagoonal side) and ebb-tidal deltas (seaward side)

• Washovers can form during major storm events, and are found elsewhere across barrier islands

• Coastal dunes are common features associated with sandy beaches

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• Estuaries are semi-enclosed coastal water bodies where fluvial and marine processes interact• Tide-dominated estuaries have tidal channels with bars

and tidal mudflats that contain tidal sedimentary structures (e.g., tidal bundles, heterolithic stratification)

• Wave-dominated estuaries are partly enclosed by a coastal barrier and have well-developed bay-head deltas

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• Carbonate coastal environments can exhibit comparable characteristics as clastic coasts (i.e., barriers and lagoons), consisting of carbonate sands and muds, respectively• Stromatolites (algal or bacterial mats) commonly form on

carbonate-rich tidal flats

• Arid coastal environments are characterized by sabkhas and salinas, coastal plains frequently inundated by saline water and hypersaline lagoons, respectively, where evaporites (notably anhydrite and gypsum) can accumulate

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Shallow/deep marine environments

Shallow marine environments

• Shallow seas can be subdivided into clastic and carbonate-dominated systems, depending mainly on sediment supply and climatic setting

• Idealized models predict a general decrease of grain size with water depth (i.e., away from the shoreline); however, this simple picture is complicated by a large number of factors (e.g., shelf bathymetry)

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• Storm-dominated clastic shelves ideally exhibit a transition from predominantly wave-rippled sands in the upper shoreface, to alternating sands and muds (tempestites with hummocky cross stratification) in the lower shoreface, to muddy facies below storm wave base

• Tide-dominated clastic shelves may exhibit erosional features, sand ribbons, and sand waves 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

• Bioturbation can obliterate many primary sedimentary structures in shelf environments

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• Shallow seas within the photic zone are the premier ‘carbonate factories’

• Carbonate platforms can cover continental shelves or epicontinental seas, when the conditions for carbonate production (temperature, salinity, light conditions) are favorable

• Isolated platforms (atolls) are found in shallow seas surrounded by deep water, like extinct volcanoes

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• Carbonate ramps exhibit processes and characteristics comparable to clastic shelves, with carbonate sands and muds ultimately producing a seaward transition from grainstone to mudstone, commonly with similar sedimentary structures

• Rimmed carbonate shelves consist of a coral reef or carbonate sand barrier at some distance from the mainland; the shelf lagoon can be up to many tens of kilometers wide• Boundstones dominate the reef facies• Shelf lagoon facies are mostly fine-grained and ultimately lead

to the formation of mudstones and wackestones

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Deep marine environments

• The continental slope is a major source of sediment for the deep sea, and is a setting where slumps can occur

• Debris flows and turbidity currents are the main mechanisms of transport from the continental slope into the deep sea; these processes can be triggered by external forcing (e.g., an earthquake) or by the slope reaching a critical state as a result of ongoing deposition

• Debris-flow deposits and turbidites are often genetically related

• Turbidites can be both clastic (commonly leading to the formation of wackes) or calcareous

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

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

• Submarine fans share several characteristics with deltas; they consist of a feeder channel that divides into numerous distributary channels bordered by natural levees and are subject to avulsions• Proximal fan (trunk channel)• Medial fan (lobes)• Distal fan

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• Basal Bouma-divisions have the highest preservation potential updip; upper Bouma-divisions are more common downdip

• Turbidite lobes characterize the medial fan and may exhibit the most complete Bouma sequences

• The Bouma-model is increasingly challenged, because many turbidites do not conform to it (e.g., ‘high-concentration turbidites’)

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

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• Pelagic sediments primarily have a biogenic origin• Calcareous ooze (e.g., foraminifera) forms above the calcite

compensation depth (CCD) at ~4000 m depth• Siliceous ooze (e.g., radiolarians, diatoms) forms between

the CCD and ~6000 m depth where silica dissolves; it lithifies into cherts

• Hemipelagic sediments consist of fine-grained (muddy) terrigenous material that is deposited from suspension• 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

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Stratigraphic principles

• Lithostratigraphy = subdivision of the stratigraphic record into sediments or rocks by means of lithologic characteristics and stratigraphic position

• Biostratigraphy = subdivision of the stratigraphic record into sediments or rocks by means of fossil content

• Chronostratigraphy = subdivision of the stratigraphic record into bodies of sediment or rock represented by a particular age, separated from underlying and overlying units by isochronous surfaces

• Geochronology = subdivision of Earth history into time intervals

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• Type sections (stratotypes) constitute the standard of reference for definition and recognition of a stratigraphic unit or stratigraphic boundary; they are defined where these are representative and well developed

• Stratigraphic relationships can be inferred from the principle of superposition, unconformities, cross-cutting relationships, ‘included fragments’, and ‘way-up indicators’

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Lithostratigraphy

• The formation is the fundamental unit of lithostratigraphic classification; just as the other lithostratigraphic ranks (groups, members, beds), it should be based on field description (i.e., fossil content and age do not play a role)

• Mode of deposition (genesis) is not a criterion in the distinction of lithostratigraphic units; this requires interpretation and is therefore likely to undergo revision over time

• Lithostratigraphic units should have some degree of overall lithologic homogeneity, although diversity in detail may in itself characterize a lithostratigraphic unit

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Lithostratigraphy

• Lithostratigraphic units are commonly diachronous, as opposed to chronostratigraphic units

• Detailed geologic mapping is usually strongly based on lithostratigraphy, whereas overview geologic maps usually show chronostratigraphic units

• Although objective lithostratigraphic classification should be as simple and straightforward as possible, reality demonstrates that this is not always the case; as a result, in many areas revisions are frequently proposed which can lead to extremely complicated and confusing situations

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Biostratigraphy

• Evolution forms the initial basis for biostratigraphic subdivision, either through the development of an increasing number of new species, or by means of evolution of one particular species

• In general, Earth history shows an increase of the number of taxa, but this process is punctuated by (mass) extinctions

• Depositional environments and geographic contrasts play an important role in determining the nature of fossil assemblages

• The biozone is the fundamental biostratigraphic unit• Biozones are strictly diachronous in most cases; however,

over geological time scales their boundaries can commonly be considered to be isochronous, but their resolving power has limitations!

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Biostratigraphy

• A vast diversity of types of fossils exists; the following criteria are important in determining how useful they are for strictly stratigraphic purposes of correlation:• Abundance and size• Degree of dispersal• Preservation potential• Rate of speciation

• As a result, especially numerous marine microfossils (e.g., forams) are stratigraphically highly useful, whereas others are more valuable for paleoecologic purposes

• Numerous pitfalls exist in the correlation of biozones (e.g., Quaternary pollen zones)

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Chronostratigraphy

• Chronostratigraphic classification of sediments or rocks involves the establishment of time lines (isochrons); this, in turn, forms the basis for paleogeographic reconstruction

• Traditionally, biostratigraphy has formed the most important basis for chronostratigraphic classification

• Numerical dating techniques are becoming increasingly important in defining chronostratigraphic units

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Chronostratigraphy

• Radiometric dating methods are in essence based on the decay of radioactive isotopes

N=number of daughter isotopes; N0=initial number of parent isotopes; =decay constant; t=time

• Radiometric dating involves a large number of isotopes and decay series, with highly variable halflives and applications (age ranges from less than a century to billions of years)

λt0eNN

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Chronostratigraphy

• Mass spectrometry is the most commonly used technique to measure the ratio between different isotopes

• Many sediments and sedimentary rocks are not suitable for radiometric dating; indirect ages can sometimes be obtained through dating of associated igneous rocks (e.g., volcanics)

• Luminescence dating is a relatively new technique that allows quartz and feldspar grains up to several 100 kyr to be dated; it is based on the measurement of a minute light signal that can be released by these grains and that is proportional to time after burial

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Magnetostratigraphy

• The Earth’s magnetic field is constantly subject to change• Secular variations, continuous changes of the position of the

magnetic poles, take place over time scales of 101 to 103 years• Reversals from ‘normal’ polarity to ‘reversed’ polarity occur

over time scales of 104 to 106 years• Fine-grained sediments deposited from suspension can

align themselves according to the ambient geomagnetic field (the same applies to volcanics upon cooling below the ‘Curie point’)

• If paleomagnetic changes are independently numerically dated, a resulting magnetostratigraphy can be used to date sedimentary successions

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Sequence stratigraphy

• Sequence stratigraphy constitutes a ‘minor revolution’ in the Earth sciences, and has certainly revitalized stratigraphy

• Sequence stratigraphy highlights the role of ‘allogenic’ (or external) controls on patterns of deposition, as opposed to ‘autogenic’ controls that operate within depositional environments• Eustasy (changes in sea level)• Subsidence (changes in basin tectonics)• Sediment supply (changes in climate and hinterland

tectonics)

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• Accommodation refers to the space available for deposition (closely connected to relative sea level in shallow marine environments); however, application of this concept to subaerial environments is problematic

• An increase of accommodation is necessary to build and preserve a thick stratigraphic succession; this requires eustatic sea-level rise and/or basin subsidence (i.e., relative sea-level rise), as well as sufficient sediment supply

• The subtle balance between relative sea-level change and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur

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

• A relative sea-level 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 formation of a widespread unconformity known as a sequence boundary

• Sequence boundaries are the key stratigraphic surfaces that separate successive sequences

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• Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller-scale relative sea-level fluctuations

• Systems tracts are the building blocks of sequences, and different types of systems tracts represent different limbs of a relative sea-level curve• Falling-stage (forced regressive) systems tract• Lowstand systems tract• Transgressive systems tract• Highstand systems tract

• 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 sea-level rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the transgressive and highstand systems tracts

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

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• In a very general sense, relative sea-level fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased deposition in downdip settings (e.g., submarine fans)

• Relative sea-level rise will lead to trapping of sediment in the updip areas (e.g., coastal plains) and reduced transfer of sediment to the deep sea (pelagic and hemipelagic deposition; condensed sections)

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Clastic environments

• Relative sea-level fall in clastic environments commonly leads to fluvial incision into offshore (shelf) deposits, usually associated with soil formation (paleovalleys with interfluves)

• Relative sea-level rise causes filling of paleovalleys, commonly with estuarine or even shallow marine deposits

• Submarine fans and associated high aggradation rates in the deep sea occur especially during late highstand and lowstand, when sediments are less easily trapped updip of the shelf break

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Carbonate environments

• Relative sea-level fall in carbonate environments can lead to the development of karstic surfaces (dissolution of limestones) or evaporites (e.g., sabkhas), depending on the climate

• Highstands generally expand the area of the carbonate factory (drowning of shelves) and vertical construction of reefs, as well as accumulation of other carbonates is enhanced

• Extreme rates of relative sea-level rise can lead to the drowning of carbonate platforms

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• Sequence-stratigraphic concepts contain numerous pitfalls!

• Variations in sediment supply can produce stratigraphic products that are very similar to those formed by sea-level change

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

• Allogenic incision is easily confused with autogenic scour

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Sea-level change

• Causes of relative sea-level change (amplitudes ~101-102 m)• Tectono-eustasy (time scales of 10-100 Myr)• Glacio-eustasy (time scales of 10-100 kyr)• Local tectonics

• The time scales of these controls have given rise to the distinction of eustatic cycles of different periods• 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)

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Sea-level change

• The global sea-level curve for the Mesozoic and Cenozoic contains first, second, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization• Conceptual problems: the role of differential local tectonics

is extremely difficult to single out• 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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Seismic stratigraphy

• Seismic reflection profiling forms the basis of seismic stratigraphy, which in turn has been the foundation for the development of sequence stratigraphy

• The technique is based on contrasts in acoustic impedance between different materials; reflections of sound or shock waves occur at transitions between different types of sediment or rock

v=sonic velocity; =sediment or rock density

vρAI

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Seismic stratigraphy

• A seismic section consists of a large number of vertical traces; acoustic impedance contrasts that can be correlated between large numbers of traces constitute reflectors

• Seismic reflectors are often believed to approximate isochronous surfaces that may be relevant in a sequence-stratigraphic context

• The vertical resolution of seismic profiling has increased considerably over time, and is now on the order of 101 m, but depths and thicknesses have to be derived from two-way travel times which may occur with the aid of geophysical logs

• 3D seismic imaging is becoming increasingly important

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Cyclostratigraphy

• Subtle changes in the earth’s orbital parameters cause variations in the distribution of solar radiation, known as Milankovitch cycles• Eccentricity (~100 kyr)• Obliquity (~40 kyr)• Precession (~20 kyr)

• When Milankovitch cycles produce sufficiently large climatic changes, they may leave an imprint in the stratigraphic record (e.g., sapropels in deep marine deposits)

• Beware of the ‘magic number’ syndrome!

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Sedimentary basins

• Sedimentary basins are the subsiding areas where sediments accumulate to form stratigraphic successions

• The tectonic setting is the premier criterion to distinguish different types of sedimentary basins• Extensional basins occur within or between plates and

are associated with increased heat flow due to hot mantle plumes

• Collisional basins occur where plates collide, either characterized by subduction of an oceanic plate or continental collision

• Transtensional basins occur where plates move in a strike-slip fashion relative to each other

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Sedimentary basins

Extension

• Rift basins develop in continental crust and constitute the incipient extensional basin type; if the process continues it will ultimately lead to the development of an ocean basin flanked by passive margins, alternatively an intracratonic basin will form

• Rift basins consist of a graben or half-graben separated from surrounding horsts by normal faults; they can be filled with both continental and marine deposits

• Intracratonic basins develop when rifting ceases, which leads to lithospheric cooling due to reduced heat flow; they are commonly large but not very deep

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Sedimentary basins

Extension

• Proto-oceanic troughs form the transitional stage to the development of large ocean basins, and are underlain by incipient oceanic crust

• Passive margins develop on continental margins along the edges of ocean basins; subsidence is caused by lithospheric cooling and sediment loading, and depending on the environmental setting clastic or carbonate facies may dominate

• Ocean basins are dominated by pelagic deposition (biogenic material and clays) in the central parts and turbidites along the margins

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Sedimentary basins

Collision

• Subduction is a common process at active margins where plates collide and at least one oceanic plate is involved; several types of sedimentary basins can be formed due to subduction, including trench basins, forearc basins, backarc basins, and retroarc foreland basins

• Trench basins can be very deep, and the sedimentary fill depends primarily on whether they are intra-oceanic or proximal to a continent

• Accretionary prisms are ocean sediments that are scraped off the subducting plate; they sometimes form island chains

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Sedimentary basins

Collision

• Forearc basins form between the accretionary prism and the volcanic arc and subside entirely due to sediment loading; like trench basins, their fill depends strongly on whether they are intra-oceanic or proximal to a continent

• Backarc basins are extensional basins that may form on the overriding plate, behind the volcanic arc

• Retroarc foreland basins form as a result of lithospheric loading behind a mountainous arc under a compressional regime; they are commonly filled with continental deposits

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Sedimentary basins

Collision

• Continental collision leads to the creation of orogenic (mountain) belts; lithospheric loading causes the development of peripheral foreland basins, which typically exhibit a fill from deep marine through shallow marine to continental deposits

• Foreland basins can accumulate exceptionally thick (~10 km) stratigraphic successions

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Sedimentary basins

Transtension

• Strike-slip basins form in transtensional regimes and are usually relatively small but also deep; they are commonly filled with coarse facies (e.g., alluvial fans) adjacent to lacustrine or marine deposits

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Models in sedimentary geology

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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• Commonly used models in sedimentary geology:

• Architectural models typically simulate specific depositional environments (e.g., alluvial architecture) and may involve a large set of physical or empirical equations

• Stratigraphic models are widely used to simulate basin-scale stratal patterns (e.g., sequence stratigraphy); many are based on a diffusion equation that relates rates of sediment transport to topographic slopes

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Applied sedimentary geology

• There are two main fields of application of sedimentary geology• Economic issues (e.g., hydrocarbon and groundwater

extraction, sand and gravel resources) constitute the traditional applications of sedimentary geology

• Environmental issues (e.g., coastal management, groundwater pollution studies) are finding rapidly increasing interest

• The two following case studies exemplify applied sedimentary geology:• Reservoir modeling (Rhine-Meuse Delta)• Coastal wetland loss (Mississippi Delta)

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Reservoir modeling

• Fluid flow (e.g., groundwater, including pollutants) through porous media is extremely dependent on sedimentary architecture (3D distribution of porosity and permeability)

• Sands and gravels are mostly highly permeable (aquifers); muds and organics are relatively impermeable (aquitards)

• Prediction of the 3D distribution of sediments with different hydraulic properties (reservoir modeling) is therefore a major challenge for the geoscientist, especially in settings that exhibit a large spatial variability, like many fluvial environments

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Reservoir modeling

• Stochastic modeling of alluvial architecture in the Rhine-Meuse Delta (The Netherlands) was done with a so-called ‘object-based model’ that simulates the distribution of objects defined by specified geometries (in this example channel belts) in 3D space, constrained by well data

• The role of the geoscientist is to determine whether the output of stochastic models makes any sedimentological sense

• Process-based models have a considerable advantage over stochastic models, since these are by definition based on sedimentologically realistic processes

• These techniques are equally applicable in groundwater and hydrocarbon reservoirs!

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Coastal wetland loss

• Wetlands in the coastal zone belong to the most valuable ecosystems (1-2 million $ km-2 yr-1), mainly due to the following services:

• Waste treatment (recovery or breakdown of nutrients)• Biological productivity (food production)• Disturbance regulation (storm protection)• Recreation (tourism)

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• The Mississippi Delta is an area undergoing extremely rapid rates of subsidence due to crustal (tectonic) downwarping and compaction (i.e., high rates of relative sea-level rise)

• Accelerated eustatic sea-level rise (associated with greenhouse warming) and reduced sediment input due to human interference (particularly the construction of artificial levees) strongly amplifies the problem

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• A thorough understanding of deltaic evolution over various time scales is indispensable to restore the sediment budget, and, hence, the ecological equilibrium

• The geoscientist can contribute, for instance, with the following insights:• Sediment dispersal patterns and rates (e.g., progradation

rates of delta lobes and crevasse splays, longshore sediment fluxes)

• Barrier island dynamics

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Reflection

• Uniformitarianism remains a cornerstone of modern sedimentary geology; observations of modern processes of sediment transport and deposition must constitute the basis for the interpretation of ancient products

• Sedimentology and stratigraphy have evolved rapidly over the last decades• The 1960s and 1970s saw a decline of interest in classical

stratigraphy and an emphasis on autogenic processes in depositional environments (process-oriented sedimentology, facies models)

• The 1980s and 1990s saw a revival of stratigraphy and a (sometimes obsessive) focus on allogenic processes (sequence stratigraphy and cyclostratigraphy)

• Integration of the two is a major challenge for the future

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Reflection

• The interpretation of ancient depositional environments is increasingly based on the analysis of three-dimensional sedimentary architecture, rather than the analysis of one-dimensional vertical sections or two-dimensional cross sections or outcrops

• 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

• Apart from traditional interests in economic sedimentary geology (e.g., oil, gas, minerals), environmental sedimentary geology (e.g., groundwater pollution) is becoming more important

introduction to sedimentology

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