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GE0-3112 Sedimentary processes and products Lecture 13. Sequence stratigraphy Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 14. Changing sea level and sedimentary sequences. - Reading & Levell 1996. Ch. 2. Controls on the sedimentary rock record. Contents ► Stratigraphy ► Why sequence stratigraphy? ► Parasequences ► Systems tracts ► Bounding surfaces Stratigraphy – the subdivision of rocks in time and space ► ► ► ► ► ► ► ► ► ► ► Lithostratigraphy Biostratigraphy Chronostratigraphy Magnetostratigraphy Chemostratigraphy Morphostratigraphy Climatostratigraphy Kinetostratigraphy Tectonostratigraphy Allostratigraphy Sequence stratigraphy What is sequence stratigraphy? ► Packages of strata deposited during a cycle of relative sea-level change and/or changing sediment supply. ► Genetic/interpretative approach: packages related to relative sea-level and/or sediment supply. packages bounded by chronostratigraphic surfaces. Walker 1992 Why use sequence stratigraphy? ► To correlate and predict facies and unconformities: division of the sedimentary record into timerelated genetic units. ► To understand the distribution of sedimentary facies and unconformities in time and space. ► To determine the amplitudes and rates of change of past relative sea-level and, in turn, understand the cyclic and non-cyclic nature of tectonics and climate change (durations of 10 ka - >50 Ma). What criteria do we use? ► Stacking patterns - indicate relative sealevel change and or sediment supply. ► Bounding surfaces Components of a sequence ► Bounding surfaces Sequence boundary Transgressive surface Maximum flooding surface ► Systems tracts LST TST HST/RST ► Parasequences Prothero & Schwab 1996 Parasequences ► Parasequences: the small-scale building blocks of systems tracts and sequences. ► A parasequence represents a proximal to distal change in facies accumulated during a minor cycle in the balance between sediment supply and accomodation. ► Each parasequence is bounded above by a flooding surface. Flooding surfaces Prothero & Schwab 1996 Stacking pattern of parasequences ► Progradational ► Retrogradational ► Aggradational Prothero & Schwab 1996 Sequences ►A sequence is composed of a succession of parasequence sets. ► Each sequence represents one major cycle of change in the balance between accomodation space and sediment. ► A sequence is subdivided into 3 or 4 systems tracts, each representing a specific part of the cycle. Prothero & Schwab 1996 Systems tracts ► Exxon LST, TST, HST (incl. RST) ► Alternative LST, TST, HST, RST (forced RST) Walker 1992 Lowstand ST ► Formed immediately following s.l. lowstand. ► Fluvial incision ceases; progradational to aggradational marine parasequences deposited. ► Active submarine fans below the shelf break. Transgressive ST ► Formed during s.l. rise. ► Accomodation space > sediment supply retrogradational parasequences. ► Base of TST is the transgressive surface (=ravinement erosional surface of shoreface). ► Top of TST is the maximum flooding surface. Highstand ST ► Formed during rising and high s.l. ► Accomodation = sediment supply aggradational to progradational parasequences. Falling stage ST ► Formed during a s.l. fall (forced regression). ► (Included in late HST in Exxon system). ► May be associated with erosion. Walker 1992 Bounding discontinuites Sequence boundary (SB) - surface of subaerial erosion and its correlative marine surface formed during sea-level fall. Corresponds to base of incised valley in proximal areas. ► Transgressive (ravinement) surface (TS) - transgressive surface of marine (shoreface) erosion. ► Marine flooding surface - surface across which there is evidence of an abrupt increase in water depth (may be used to separate parasequewnces). ► Maximum flooding surface (MFS) - surface marking regional transition from trangression to regression and most landward extent of the shoreline - commonly marked by a condensed section (horizon). ► Regressive surface of erosion. (NB. may be removed by subaerial erosion or transgressive surface). ► Prothero & Schwab 1996 Wheeler (time-distance) diagrams Prothero & Schwab 1996 Sequence stratigraphy and global sea-level cycles ► Various orders of global sea-level change distinguisged: 1st order (200-400 m.y.), e.g. lowstand during Permian Pangea. Controlled by major tectonic cycles. 2nd order (10-100 m.y.), e.g. MidCretaceous highstand. Controlled by changes in ocean-ridge spreading rate. ► Global correlation of sequences related to eustatic sea-level curves is difficult or impossible due to local variations in tectonics and sediment supply. Sequence development is dependent on: 1) sea level, 2) tectonics, 3) sediment supply. Walker 1992 An example of sequence stratigraphic subdivision applied to fjord-valley fills Corner, in press ► Deglacial-postglacial regressive fill transgressive- Postglacial Fjord glac ier A Fjord glacier Glaciomarine deposition Meltwater Valley glacier B Meltwater Valley glacie r Deglacial Outlet glacier Valley glacier Meltwater C Glaciofluvial delta Fluvial delta River D Marine deposition River E Fluvial terracing Corner, in press 3 ► TST A IVb ► HST ► RST 2 1 mit Marine li I IVd IVc IIIb Regressive accretionary surface Dv IIb P IIIc I Depositional elements I Fjord floor IId - Prodelta (P) IIc - Marine (P) IIa/IIb - Glaciomarine (Df/Dv) Ice-contact (Df) Depositional episodes/sequence tracts P Postglacial forced regressive (PRST) Dv Deglacial (valley glacier) highstand (DHST) Df Deglacial (fjord glacier) transgressive (DTST) Bounding surfaces Fluvial accretion surface Fluvial erosion surface Cross section X - X’ I IIa III Delta foreset IIIc - Delta foreset (P) IIIa/IIIb - Glaciofluvial delta foreset (Df/Dv) II Df IIc IId Deglacial-postglacial surface Highstand systems tract IV Glacial trough B X III Transgressive systems tract Forced-regressive systems tract I II X’ Marine limit DTST (Df) Regressive accretionary surface IVc DHST (Dv) IVd IIIc P IIb IIa I Dv Df IIc IId Sea level IV Terrace top IVd - Fluvial (P) IVc - Delta topset (P) IVa/IVb - Glaciofluvial delta topset (Df/Dv) Maximum flooding surface PRST (P) Time Deltaic erosion surface Corner, in press Corner, in press Further reading ► Coe (ed.) 2003. The Sedimentary Record of Sea-Level Change. Well illustrated, modern treatment of sequence stratigraphy and depositional successions. ► Emery & Myers 1996. Sequence stratigraphy. Similar to above but more dated. Gives background to nomenclature. ► ► ► E-learning journal. Sequence stratigraphy. Walker 1992, in Walker & James (Ch. 1, 'Facies, Facies Models and Modern Stratigraphic Concepts'). Corner, G.D. (in press, 2006). A transgressive-regressive model of fjord-valley fill: stratigraphy, facies and depositional controls. In Dalrymple, R.W., Leckie, D. and Tillman, R.W. (eds.) ‘Incised-Valley Systems in Time and Space', SEPM Special Publication.