7 control stratigraphy

17. Controls on basin stratigraphyDriving mechanisms for basin stratigraphyTectonic mechanism: (a) flexure under applied loads (rift basin and foreland basin); (b) fault array evolution; (c) in-plane stress

Eustatic mechanism: (a) change in volume of the ocean basin; (b) changes of ice volume on polar regions

Climate change: Influence on sediment discharge to basins

7.1 Tectonic mechanisms: flexure under applied loads

7.1.1 Effects of flexure on stratigraphy in basins due to stretching

Steers-head geometry

Elastic

Viscoelastic

Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

2In rift basins:During the stage of rifting: fault-controlled Airy-type subsidence.During post-rift stage: flexural-controlled subsidence. The increase of Te with increasing plate thermal age leads to stratigraphic onlappattern for post-rift strata.

The mechanisms of subsidence in stretched basins comprise: (i) fault-controlled initial subsidence caused by mechanical stretching of the upper brittle layer of the lithosphere, (ii) a thermal subsidence caused by the cooling and contraction of the upwelled asthenosphere, and (iii) sediment and water loading.


3 Initial strong onlap onto the basement at the transition from fault-controlled Airy-type subsidence to flexural-controlled subsidence. Lateral heatflow causes thermal uplift on the coastal plain, abruptly terminating onlap. By about 16 Myr after rifting, flexural subsidence outstrips thermal uplift and the sediments again progressively onlap basement.


47.1.2 Role of flexure in generating foreland basin stratigraphy

Transition from passive margin to foreland basins

1. Early stage: thermal age of passive continental margin is an important control on foreland basin development.2. Later stage: the thickness of the overthrust load is more important.

Fig. 4.31


5Wedge-shaped basin geometry and progressive stratigraphic onlap of a foreland basin


6Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

77.1.3 The flexural forebulge unconformity


8Inherited deeper bathymetry: no erosion

Forebulge unconformity in eastern Switzerland

Calculated erosion

Bathymetry before orogenic loading

Observed erosion

Hiatus of the unconformity


9A A

A A

7.1.4 Foreland basin isopachs and pinch-outs

Progressive eastward shift of depocentersduring Sevier orogeny


10


11


12

7.2 Tectonic mechanisms: fault array evolution


13


14


15


16


17


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7.3 Changes of in-plane stress

In-plane stresses acting on a deflected plate may enhance or reduce the curvature of the deflection.

In-plane stress may have buckled layered lithosphere and produced long wavelength lithospheric folds.

Compressive in-plane stress causes basin margin to uplift and basin center to subside.Tensile in-plane stress caused basin margin to subside and basin center to uplift.


19

Figure 3.6 Cartoon showing the relationship between relative sea-level, water depth, eustatic sea-level, tectonics (uplift and subsidence), and accumulated sediment. Note that relative sea-level incorporates subsidence and/or uplift by referring to the position of sea-level with respect to the position of a datum at or near the sea-floor (e.g. basement rocks, top of previous sediment package) as well as eustasy. Eustasy (i.e. global sea-level) is the variation of sea-level with reference to a fixed datum, forexample the centre of the Earth.

7.4 Eustatic mechanisms

Relative sea-level: the distance between a local datum (e.g. top of the basement of a basin) and sea-surface. Relative sea-level change is influenced by: (1) eustasy, (2) basin uplift/subsidence.Water depth: the distance between the sea-bed and the sea-surface or water level.Eustatic sea-level (or eustasy): This is global sea-level and is a measure of the distance between a fixed datum, usually taken as the centre of the Earth, and the sea-surface.

Eustasy, relative sea-level, water depth

Coe et al. (2003)


20

Five possible causes that may cause global sea-level changes

1. Continuing differentiation of lithospheric material as a result of plate tectonic processes. (not important)

2. Changes in the volumetric capacity of the ocean basins caused by sediment influx or removal. (not important)

3. Changes in the volumetric capacity of the ocean basins caused by volume changes in the mid-ocean ridge system. (important for first-order eustasy)

4. Thermal expansion and contraction of the oceanic water reservoirs. (not important, 1K increases, 0.45 m rise in sea level)

5. Changes of available water by abstraction in and melting of polar ice caps and glaciers (glacial eustatic).


21

Considering isotasy in eustatic sea level changes

3w

3m g/cm 1.0 ;g/cm 3.3 If ==

=

w

wmSL S

The isostatic subsidence of the ocean floor is approximately 0.4 of the sea level change ( ). Or the seal level change is 0.7 of the increase in the water depth of the ocean (h2-h1).

SL


22

However, sediment in the ocean is removed by tectonic accretion and subduction at active margins and continued spreading creates new ocean floor. It is likely that the balance between influx and removal of sediment, when averaged over long periods of time, is insufficient to cause rates of sea-level change of more than ~ 1 mm per 1000 yr.


23

Volume of present world ridge system is about 10% of the volume of the ocean water.Change in spreading rate and change in the length of ridge systems influence the ridge volume.


24

Maximum sea level: latest Cretaceous (Maastrichtian)~

25

Total melting of Antarctic land ice would result in an increase in water depth, ranging from 60~75 m. Melting Greenland ice cap > 5 m rise,

Taking into account the isostatic effect, if all the land-locked ice melted, it would cause a 50 m rise in sea level (SL).

In geological term, rate of melting ice caps is a rapid process (~10 mmyr-1)

Melting of Present Ice Cap

Melting of Pleistocene Ice Cap

May cause about 150 m sea level rise.


26

For the last 120,000 yrs, 8 sea-level cycles.Magnitudes: 20~180 m, periods: 100,000 yrs (primary),40,000 yrs, and 20,000 yrs.


27

~


28(c) (, precession) 1923

(a) (eccentricity)41106

(b) (obliquity)41

: (Milankovitch cycles)


29

,

O18/O16

O18/O16Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

30


31

7.5 A primer on process stratigraphy

Process stratigraphy (as defined in Allen and Allen (2005), p.268) is the science of the recognition and interpretation of the genetic structures of stratigraphy. The fundamental aim of process stratigraphy is to understand the driving mechanisms for the range of stratigraphic architectures found in sedimentary basins. The key concept of process stratigraphy is the generation/destroy of accommodation space and the amount of sediments supplied.

The stratigraphy in a sedimentary basin is the result of the interplay of the generation of space or accommodation and the influx of sediments (sediment supply).

Accommodation is the interplay of eustatic sea-level changes, basin subsidence/uplift, local patterns of faulting.

Sediment supply is a function of climate (exerting controls on vegetation, weathering, erosion, and far-field sediment transport to basins) and sediment routing systems for siliciclastic deposits. For carbonate systems, carbonate productivity determines how much carbonate sediments will be generated and carbonate productivity depends on water depth (available of light), water turbidity and temperature, and type of biota.


32

Major Controls on Sedimentary Fill of BasinsThe sedimentary fill of basins is controlled by three major variables (Figure 1.8.1): Subsidence:"The thermal and mechanical properties of the lithosphere exert important controls on the formation of sedimentary basins" (Steckler, 1990). Thermal subsidence rates and the magnitude and distribution of subsidence due to loading vary in basins of different tectonic settings (Steckler and Watts, 1978; Stephenson, 1990). Eustasy (global sea level):Eustasy refers to sea level relative to a fixed datum, such as the center of the earth. Global sea level variations result from changes in either oceanic basin volume or water volume. Eustasycombined with subsidence results in relative sea level variations, which control accommodation for sediment deposition (Posamentier et al., 1988; Posamentier and Vail, 1988). Sediment Supply:"The role of sediment supply in transgressions and regressions is a fundamental one..." (Schlager, 1994). When the rate of sediment supply is greater than the rate of relative sea level rise, accommodation space will be filled.

Vail (1987)Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

33

Process Stratigraphy History 1960s The recognitions of unconformity-bounded sequences of

inter-regional extent (Sloss, 1950, 1963) and the shape of sedimentary bodies is controlled by quantity of sediment supplied (Q), rate of basin subsidence (R), sediment dispersal (D), and composition and texture of the sediment supply (M).

1970s Use of seismic stratigraphy (Payton, 1977, AAPG Memoir 26). The recognitions of seismic reflection horizon representing time linesand sequences bounded by unconformities and their correlative conformities (Exxon group, Payton (eds.), 1977, AAPG Mem. 26)

1987 Global sea-level chart (Haq et al., 1987) 1988 Concept of accommodation was introduced in Wilgus et al.

(1988) Late 1980s and early 1990s Criticism on eustasy as the

overwhelming controls on sequence development and on presumed global synchroneity of key stratigraphic surfaces.

Since 1978 Numerical simulations on stratigraphy to explain & predict stratal geometries within sequences.


34

Stratigraphic cycles can be modeled using expressions for accommodation and sediment supply. A fundamental parameter is the magnitude of the eustaticchange compared to the subsidence rate.

Stratigraphy is packaged into large and small genetic units, from megasequences (or supersequences) to depositional sequences and then parasequences (or higher order genetic units, they may be driven by orbital mechanism - Milankovitch band, or unforced - autocyclic).

7.5.1 Forward modeling of stratigraphic cycles from first principles

Considering the stratigraphic cycles generated under a sinusoidal eustaticvariation in a basin with a background tectonic subsidence rate and with a sediment supply coupled to the relative sea-level variation.

= thh 2sin0Sinusoidal eustatic fluctuation: : wavelength

h0: amplitudea: rate of tectonic subsidence

(linear)t: geological timeat

thhrel +

= 2sin0

Adding the tectonic subsidence rate:


35

0ha=

A number of different relative sea-level scenarios due to the interplay between eustasy and basin subsidence can be expressed in terms of the dimensionless parameter (relative sea-level parameter):

zWhen a

36

Fig. 8.2 The dimensionless parameter varies from 0.2 to 2, corresponding to tectonic subsidence rates from 0.1 to mmyr-1 (grey area). Increasing values of cause the relative sea-level maximum (open circle) to be delayed in the cycle. For the glacial eustatic parameters used, tectonic subsidence must be > mmyr-1 in order for the relative sea-level fall to disappear.

In this diagram, the relative sea-level is equivalent otaccommodation since the curves begin at zero water depth rather than at some point on a graded profile.

Variations in relative sea level through a cycle of eustatic change with wavelength and amplitude h0 in a basin with a linear tectonic subsidence rate a.

(comparison: Taiwan foreland sequences: basin subsidence ranges from 0.95 ~1.9 mmyr-1during the deposition of Liuchungchi to ErchungchiFormations (Chen et al., 2001))

100 kyr is the dominant signal for the Pleistocene glacio-eustatic fluctuations.

(100 m variations in height)

(critical rate of basin subsidence for no relative sea-level fall)


37

Key stratigraphic surfaces (e.g., basal surface of forced regression, sequence boundary, transgressive surface of marine erosion, maximum flooding surfaces) are commonly diachronous, with a phase shift of up to of a eustatic period (). Because (1) the variations of tectonic subsidence rate in co-existing basins, (2) the different response times of sediment routing systems to base level change produces different sediment inputs in coeval basins.

Typical glacio-eustatic cycles have a high enough frequency and amplitude to generate unconformities, even at high tectonic subsidence rates.

Lower frequency/amplitude non-glacial cycles are easily overwhelmed by tectonic subsidence to produce monotonically rising relative sea levels.

Some critical subsidence rates for a monotonic rise in relative sea-level:

3.14 mm yr-1: h0=50 m, = 100 kyr 0.3 mm yr-1: h0=20 m, = 400 kyr 0.06 mm yr-1: h0=10 m, = 1 Myr


38

Considering the effects of sediment supply

+

+

= 12cos2

2212sin)( 00

ttsatthtw

Water depth eustasy

Tectonic subsidence

Sediment supply

Rate of sediment supply (s0): Sediment supply is coupled to the rate of relative sea-level changes, with sediment supply rate peaks at the maximum rate of relative sea-level fall, and is zero at the maximum rate of relative sea-level rise.


39

Fig. 8.3: (a) Variation in water depth as a function of the dimensionless relative sea-level parameter with a constant maximum sedimentation parameter s0=2 mmyr-1. When accommodation is filled, sediment starts to bypass the depositional site (illustrated for the case of =2). The onset of bypass occurs progressively later as tectonic subsidence increases ( increases). At =2. the basin remains water-filled until 40 kyr, after which erosion and sediment bypass take place until the beginning of the next glacio-eustatic cycle.

Water depth and potential sediment accumulation during a cycle of relative sea-level change with variable tectonic subsidence rate and sediment supply, using the glacio-eustatic function.

AB

C

A: onset of bypass for =0.2B: onset of bypass for =1C: onset of bypass for =2


40

Fig. 8.3: (b) Variation in water depth and potential accumulated sediment as a function of the sedimentation velocity s0, with a constant dimensionless relative sea-level parameter of 2, corresponding to a tectonic subsidence rate a of 1 mmyr-1. High sediment supply rates cause the available accommodation to be filled early during the cycle of relative sea-level change, after which sediment is bypassed and eroded (two illustrations at s0=2 and mmyr-1), whereas at low supply rates (s0=0.1 mmyr-1), the accommodation remains unfilled. The evolution of water depth during a cycle of relative sea level, sedimentary facies, and the occurrence of erosional bypass surfaces are all critically dependent on the sediment supply.

s0


41

Fig. 8.4 Stratigraphic cycles generated using the algorithms in the text for a glacio-eustatic cycle (=2) using different values of the maximum sedimentation velocity s0, from 0.1 to 5 mmyr-1. Cycle thickness and water depth trends vary strongly from the thin, sediment starved, deep water case (s0=0.1 mmyr-1) with a nonerosionalflooding surface as an upper boundary, to the thicker, top-truncated shallow-water cycles at higher values of s0.

Cycle thickness

Accommodation-limited: cycles are faciesare determined by accommodation; sediment supply is always adequate.

Sediment supply-limited: depositional space is always great enough to accommodate the sediment supply.


42

Fig. 8.5 Sensitivity of retrogradational versus progradational cycles resulting from variations in tectonic subsidence rate a, with a constant maximum sedimentation velocity so= 2 mmyr-1. (a) Aggradational cycles (parasequences) containing offshore, shoreface (0~25 m) and potentially thin coastal plain deposits are produced at =2 (a=1 mmyr-1), using an intialwater depth of 50 m. Relative sea level and sediment supply are in balance. (b) Progradational cycles produced by a small decrease in to 1.8 (a=0.9 mmyr-1), with and initial water depth of 100 m, showing that younger cycles contain progressively lesser amounts of offshore deposits and more shoreface deposits. (c) Retrogradational cycles produced by a small increase in to 2.2 (a=1.1 mmyr-1), with an initial water depth of zero, showing that coastal plain-shorefacecycles are progressively replaced by shoreface-offshore cycles with time. Small variations in tectonic subsidence rate (relative to the sediment supply) therefore cause major variations in the stacking patterns of cycles.


43

Fig. 8.6 Large scale architecture of depositional units in relation to accommodation and sediment supply (after Galloway, 1989). (a) A rise in relative sea level causes an increase in topset accommodation volume Vta, equal to the product of the relative sea-level rise and the topset area; (b) Stratigraphic patterns change from transgressiveto retrogradational, aggradational, and progradational as the sediment supply increases relative to the topset accommodation. White cicles approximate position of beach (or offlap break).

Shoreline movementsTransgression:()Regression:()

Sediment accumulation resulting from shoreline movement

Transgressive deposits: (transgressive lags)Retrogradation:Aggradation:Progradation:


44

Reading: Paoloa, C. (2000) Quantitative models of sedimentary basin filling. Sedimentology, 47 (Suppl. 1), 121-178.

7.6 Numerical Simulation of Stratigraphy

http://sedpak.geol.sc.edu/doc/help/Chap1.html

Quantitative modeling of the filling of sedimentary basins was begun in 1960s.The goal of modeling is to generate insight.


45

Traditional Use of Sedimentary Simulations

Understand complexities of clastic or carbonate stratigraphy

Identify & model sedimentary systems.

Quantify models that explain & predict stratal geometries within sequences.

Used by specialized experts who design & build the simulations.

Sedimentary process models from outcrops, well log & seismic cross sections used to:


46

Some sedimentary models Short-term local events

SEDSIM (Tetzlaff and Harbaugh, 1989) SEDFLUX (Syvitski et al., 1998a; Syvitski et al., 1998b)

Long-term regional events

PHIL (Bowman et al 1999) SEDPAK (Eberli, et al, 1994) FUZZIM (Nordlund1999a&b) CSM (Syvitski et al., 2002) Robinson and Slingerland, 1998 Steckler et al., 1993.


47

Approaches to modeling Geometric models

Fixed depositional geometries are assumed

Conservation of mass Simple computations through general

nonlinear dynamic models Variations in depositional geometries Variations in surface slope vs

discharge More complex computationally

Paola (2000Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

48

Geometric Model

Eberli, et al, 1994 Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

49

9 For carbonates: rely on a carbonate productivity versus depth function combined with rules for surface transport of sediments.

9 For siliciclastics: require a linkage between catchmentprocesses, fluvial transport, and sediment distribution in the basin.

Numerical models


50

Fig. 8.32 Model for the generation of unforced high resolution cyclicity in peritidal carbonates, after Burgess et al. (2001). (a) Illustration of different processes involved in generating prograding inter- and supratidalislands and autocyclic shallowing-upward cycles; (b) Depth-dependent carbonate productivity relationship and sediment transport rate; (c) Different stages in the evolution of prograding islands. Time 1: landward transport of sediment causes accretion of inter- and supratidal flat. Time 2: Continued accretion drives inter- and supratidal flateprogradation, which causes a sediment starved leeward side to develop. Time 3: Sediment starved lee subsides and prograding island forms, allowing a new subtidalcarbonate factory to develop. Time 4: A second island system develops as the entire process is repeated.


51

Fig. 8.33 Carbonate productivity versus depth from various authors (a), normalized by the maximum production rate in (b), after Paola (2000).


52

Fig. 8.34 Carbonate depositional geometries, modified after Pomar(2001). (a) Range of morphologies from ramps to rimmed platforms; (b) Carbonate productivity in relation to the type of biota. Variations in the dominant biota control the water depth range of maximum productivity. Ramps may be distally steepened where oligophoticorganisms dominate.()

()Basin AnalysisDept. Earth Sci., Nat. Central U.Prepared by Dr. Andrew T. Lin

53

Fig. 8.35 Examples of a geometrical 2-D model for stratigraphy at a passive margin (after Burgess and Allen, 1996), showing how depositional sequences and bounding unconformities can be simulated. (a) Movement of the fluvial and marine profile during relative sea-level rise and fall; (b) Computer-generated stratigraphy with a Type 1 sequence boundary and its distal marine conformity and a transgressive ravinementsurface generated during relative sea-level rise. Small numbers are chrons.


7 control stratigraphy

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