Beckenanalyse 1. Subsidenz Hohe Sediment-Akkumulation und die Bildung Sedimentärer Becken sind nicht möglich ohne entsprechenden Subsidenz. Subsidenz beschreibt die Senkung der Erdoberfläche (Landoberfläche 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) der Erdoberfläche 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 ρ w h w + ρ s h s + ρ c h c + ρ m h m + ρ a h a = c 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. 1
Initiale (tektonische) vs thermische Subsidenz 2
Flexural response of the lithosphere due to crustal or sediment loading 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). Leeder 1999 3
Backstripping 1. Dekompaktion 2. sediment load isostasy (3. paleo-bathymetry & sea-level change) 4
Young rift zones passive margin subsidence models Profil durch den Atlantik-Schelf vor Baltimore, USA nach Bouma et al. (1982) 5
cm/ka Grand Banks area (off Newfoundland) 6
Grand Banks area (off Newfoundland) a, b foreland basins 7
Beckenanalyse 2. Genetische stratigraphische Konzepte CYCLIC BEDDING 8
Sequenzstratigraphie Das Konzept der Sequenzstratigraphie beruht auf dem Zusammenspiel von Sedimentzufuhr und Akkomodationsraum, letzterer wird gesteuert über eustatische Meeresspiegelschwankungen und Subsidenz bzw. Hebung (Raten!). sedimentary response to perturbations of the equilibrium profile caused by uplift / subsidence and/or sea level change 9
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. 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: 10
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Parasequence Sets 12
Parasequence Sets forced regression 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 13
relative sea-level curves 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. 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. 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: shelf-break margin ramp margin 14
The Highstand Systems Tract (HST) shelf-break margin (c) (d) ramp margin 15
Sequence Boundary (SB) Coe et al. 2003 ramp margin 16
The Falling Stage Systems Tract (FSST) shelf-break margin (c) submarine fans deposited at base of continental slope (d) ramp margin 17
Different geometries of Falling Stage Systems Tracts on ramp type margins increasing rate of relative sea-level fall or (and) decreasing rate of sediment supply old young Loss of accomodation space caused by relative sea-level fall A + B << B C + D D 18
The Lowstand Systems Tract (LST) ramp margin (d) (c) shelf-break margin 19
Transgressive Surface (TS) The Transgressive Systems Tract (TST) (a) 20
shelf-break margin (c) (d) ramp margin Maximum FloodingSurface (MFS) 21
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