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Beckenanalyse → 1. Subsidenz

ρwhw + ρshs + ρchc + ρmhm + ρaha = c

Hohe Sediment-Akkumulation und die Bildung Sedimentärer Becken sind nicht möglich ohneentsprechenden Subsidenz. Subsidenz beschreibt die Senkung der Erdoberfläche (Landober-fläche oder Meeresboden) bzw. der Basis des Sedimentbeckens relativ zum Erdmittelpunkt (bzw. eines Bezugniveaus konstanten Drucks in der viskosen Asthenossphäre, s.u.).

Subsidenz ist eng verknüpft mit dem Prinzip der Isostasie: die Hebung (oder Senkung) derErdoberfläche ist eine Funktion der Dichte ρ und Dicke h der unterschiedlichen Lagen (Schalen). Vereinfacht sind dies: Meerwasser (w), Sediment (s), konsolidierte Kruste (c), lithosphärischer Mantel (m) und astenossphärischer Mantel (a). Die Masse über einem Bezugniveau konstanten Drucks in der viskosen Asthenossphäre

Ein Beispiel zur Größenordnung (unter stark vereinfachten Bedingungen, ohne thermische Relaxation): Eine durch Extension bedingte Verdünnung der Kruste auf 15 km (statt normalen ca. 30 km) führt zu einer Subsidenz von Meeresspiegelniveau auf 3,5 km Wassertiefe.

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Initiale (tektonische) vs thermische Subsidenz

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Flexural response of thelithosphere due to crustalor sediment loading

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Leeder1999

Fig. 27.7 Cartoon to show how shortening across a deep thrust fault causes a regional load, which must flexure the whole brittle lithosphere and cause deeper compensatory viscoplastic flow (from Felemings & Jordan 1990).

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HvE / Fazies- und BeckenanalyseEinsele 2000

Backstripping → 1. Dekompaktion

HvE / Fazies- und BeckenanalyseEinsele 2000

2. sediment load – isostasy

(3. paleo-bathymetry & sea-level change)

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Young rift zones

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Pro

fil d

urch

den

Atla

ntik

-Sch

elf v

or B

altim

ore,

US

A

passive margin subsidence models

nach Bouma et al. (1982)

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cm/ka

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Grand Banks area(off Newfoundland)

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Grand Banks area(off Newfoundland)

a, b

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fore

land

bas

ins

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Beckenanalyse → 2. Genetische stratigraphische Konzepte

Ein

sele

200

0

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Ein

sele

200

0

CYCLIC BEDDING

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Das Konzept der Sequenzstratigraphie beruht auf dem Zusammenspiel von Sedimentzufuhr und Akkomodationsraum, letzterer wird gesteu-ert über eustatische Meeresspiegelschwankun-gen und Subsidenz bzw. Hebung (Raten!).

Coe et al. 2003

Sequenzstratigraphie

HvE / Fazies- und Beckenanalyse

sedimentary response to perturbations of the equilibrium profile caused by uplift / subsidence and/or sea level change

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HvE / Fazies- und Beckenanalyse Coe et al. 2003

The parasequence forms the smallest and simplest unit in the framework of sequence stratigraphy. It results from a small-amplitude, short-term oszillation in the balance between sediment supply and accomodation space.

Example: coastal environment:

The simplest way to understand sequence stratigraphy is to consider the interface area between coastal and shallow-marine siliciclastic depositional environments where changes in relative sea level are easiest to interpret.

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Parasequence Sets

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Parasequence Sets

forced regression

HvE / Fazies- und Beckenanalyse

Sequences and System Tracts

A sequence or depositional sequence is composed of a succession of parasequence sets. Each sequence represents one cycle of change in the balance between accomodation space and sediment supply. Sequences generally range in thickness from a few meter to tens or even hundreds of m, and they are the next larger (or longer duration) cycles above parasequences.

Similar to parasequences, sequences are the result of changes in eustatic sea-level and/or subsidence/uplift resulting in a changing relative sea-level ( → accomodation space, see below), and/or changes in sediment supply.

eustatic sea level + subsidence → relative sea-level

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Every sequence is composed of up to four systems tracts, each of which represents a specific part in the cyclic change in the balance between accomodation space and sediment supply. Each systems tract is made up of at least one parasequence set. Different conditions may result in one or more of the systems tracts not being developed or preserved.

The four systems tracts are:- Highstand Systems Tract (HST)- Falling Stage Systems Tract (FSST) - Lowstand Systems Tract (LST) - Transgressive Systems Tract (TST)

The high number of different factors involved in any one geological situation (such as climate, eustatic sea-level change, subsidence rate, sediment supply, lithology, ...) means that the resultant sequences are highly variable. However, every sequence has similar genetic components related to changes in the rate of accomodation space creation and sediment supply.

Fig. 4.8b: blue curve taken from Fig. 4.8a; purple curve combines blue curve with short-term changes in accomodation space associated with the development of parasequences.

relative sea-level curves

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shelf-break margin

ramp margin

Because the geometry of the basin floor is crucial to the evolution of depositional sequences and systems tracts, we will consider in the following to principal types of depositional profiles from the land to the sea:

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The Highstand Systems Tract (HST)

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(c)

shelf-break margin

Coe

et a

l. 20

03

ramp margin(d)

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→ Sequence Boundary (SB)

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Coe

et a

l. 20

03

ramp margin

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HvE / Fazies- und BeckenanalyseCoe et al. 2003

The Falling Stage Systems Tract (FSST)

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(c)

shelf-break marginC

oe e

t al.

2003

submarine fans deposited at base of continental slope

ramp margin(d)

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old young

increasing rate of relative sea-level fallor (and)decreasing rate of sediment supply

Different geometries of Falling Stage Systems Tracts on ramp type margins

HvE / Fazies- und BeckenanalyseCoe et al. 2003

A + B

C + D<<

B

D

Loss of accomodation space caused by relative sea-level fall

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The Lowstand Systems Tract (LST)

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ramp margin

(d)

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shelf-break margin(c)

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→ TransgressiveSurface (TS)

HvE / Fazies- und BeckenanalyseCoe et al. 2003

The Transgressive Systems Tract (TST)

(a)

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shelf-break margin

(c)

ramp margin(d)

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et a

l. 20

03→ Maximum FloodingSurface (MFS)

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