Depositional Sequences Sequences

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1 Depositional Sequences Transgressive and Regressive packages can be bound by unconformities Because sediment can only be preserved during net aggradation and progradation All other times there is either erosion or non-deposition Lowering base-level Sequences: successions of parasequence sets Represents one cycle of change in the balance between accommodation space and sediment Major unconformity-bound packages of sediment Composed of up to four systems tracts Systems tracts: represent a cyclic change in the balance between accommodation space and sediment supply These are made up of at least one parasequence set

2 Sequence Stratigraphy Rather than being based either on correlation of rocks using lithology, fossils, or other stratigraphical techniques, or on facies analysis to construct past sedimentary environments and systems >> sequence stratigraphy combines the two approaches and recognizes packages of strata that was deposited during a cycle of relative sea level change and/or sediment supply Abbreviations: e-fr early forced regression; e-t early transgression; l-t late transgression l-fr late forced regression;

3 Sequence Stratigraphy Packages of strata that are bounded by chronostratigraphic surfaces Unconformities formed during relative sea level fall Flooding surfaces formed during relative sea level rise Amorosi 2006

4 Interplay of base-level and sedimentation at the shoreline The sine curve shows the magnitude of base-level changes through time. The thicker portions on this curve indicate early and late stages of base-level rise, when the rates of base-level rise are outpaced by sedimentation rates. Base level represents the surface that uplands want to erode down to and the surface that basins want to fill up to Catuneanu et al. 2009

5 Interplay of base-level and sedimentation at the shoreline The sine curve below shows the rates of base-level changes. The rates of baselevel change are the highest at the inflection points on the top curve. Transgressions occur when the rates of base-level rise outpace the sedimentation rates. For simplicity, the sedimentation rates are kept constant during the cycle of base-level shifts. Catuneanu et al. 2009

6 Multiple sequence stratigraphic models Catuneanu et al. 2009

7 Nomenclature of systems tracts and timing of sequence boundaries Each of these models are justifiable in the context in which they were proposed Each model describes the variation in a basin due to: Tectonic setting Depositional setting Sediment types Dataset Scale of observation Catuneanu et al. 2009

8 System Tracts Taking unconformity bound sequences and subdividing them into units on the basis of distinctive surfaces in a sequence Based on stacking patterns of parasequence sets and parasequences

9 Genetic types of deposits (degradational) Catuneanu et al. 2009

10 Relative sea level undergoing a slow rise from subsidence + eustasy Sediment supply outpaces sea level rise driving seaward migration of the coast = Progradation

11 Outcrop example: Kope Formation HST Gradual lowering (fall) of eustatic sea level pulsed by relative sea rise

12 Rate of eustatic fall increases, exceeding the rate of subsidence relative fall in sea level Forced Regression coast builds seaward = FSST

13 Outcrop example: FSST Basinward shift in facies Grainstone shoal facies, indicated by the prominently crossbedded grainstone, abruptly overlying the deep subtidal facies below, which consists of alternating beds of siliciclastic mudstone and hummocky cross-laminated to planar laminated calcisiltites

14 Rate of eustatic fall slows, eventually equals the rate of subsidence slow relative rise in sea level Rise in sea level outpaced by sedimentation rate progradational stacking Base of LST marks the Sequence Boundary (greatest extent of subaerial exposure

15 Rate of eustatic rise increases, rate of relative sea level increases outpaces sediment supply = Retrogradational Marked by flooding surfaces = Transgressive Surfaces

16 Outcrop example Gradual rising of eustatic sea level marked by flooding surfaces or hard grounds

17 Outcrop example

18 Rate of eustatic rise will slow + outpaced by rate of sedimentation = Progradation The turnaroud from retrogradational to progradational = Maximum flooding surface deepest water depths

19 End of the Depositional Sequence marked by the return of fall in sea level and the formation of the FSST

20 A complete sequence begins at one boundary (subaerial exposure) to the end of the next boundary Note that as you progress landward not all system tracts will be present or preserved

21 SURFACES AND SYSTEMS TRACTS SB mfs TS SB mfs HST Highstand systems tract TST Transgressive systems tract LST Lowstand systems tract HST Highstand systems tract Depending on position along the depositional profile, one of these important surfaces may merge with another.

22 Types of stratigraphic surfaces Sequence stratigraphic surfaces Used as systems tract boundaries or sequence boundaries Within-trend facies contacts Facies contacts developed within systems tracts Lithological discontinuities within systems tracts Suitable for lithostratigraphic or allostratigraphic analyses Catuneanu 2006

23 SURFACES AND STACKING OF SURFACE-BOUNDED SUCCESSIONS Timing of the seven surfaces of sequence stratigraphy relative to the four events of the base-level cycle Catuneanu et al. 2009

24 Surfaces as a function of depositional setting Catuneanu et al. 2009

25 Transgressive Surface = Red Line Tidal Ravinement Surface = Blue Line Wave Ravinement Surface Wittmer et al. 2015

26 SURFACES: SEQUENCE BOUNDARY (SB) Sequence Boundary (SB): Abrupt and significant basinward shift in depositional environment For example, an offshore mudstone overlain by braided-river sandstone (where s the shoreline?) Envelope sequences (i.e., sequence goes from one SB to an overlying SB) Can sometimes be a significant erosional unconformity in other cases, may be more subtle May cut into and truncate other surfaces Can be identified in seismic-reflection data by overlying onlapping reflectors Sequence boundaries mark an abrupt and significant basinward shift in depositional environment

27 Surfaces: Correlative Conformity Coe 2005

28 SURFACES: TRANSGRESSIVE SURFACE (TS) Transgressive Surface (TS): Marks a landward shift in depositional environments (i.e., putting deeper water environments on top of shallower water or nonmarine environments) For example, subtidal carbonates overlying supratidal evaporites and carbonates Is sometimes erosional, but in other cases is first significant flooding surface Transgressive surface marks an abrupt deepening (landward shift in environments); will merge w/ SB landward and MFS basinward

29 SURFACES: MAXIMUM FLOODING SURFACE (MFS) Maximum flooding surface (mfs): The turnaround between landward-stepping and basinward-stepping stacking patterns In marine environment, marks the time of deepest water and typically of very slow sediment accumulation condensed section (a lot of time) In coastal environments, marks the most landward inundation Commonly expressed as downlap surface in seismic-reflection data In outcrop, may not be single surface, but a relatively thin (couple meters) zone Maximum flooding surfaces (mfs) marks the turnaround between landward- and basinwardstepping packages.

30 SURFACES: MAXIMUM FLOODING SURFACE (MFS) MFS Recognition of MFS can be difficult in strata too far from the shoreline facies.

31 Example: Siliciclastic Sequence Stratigraphy Book Cliffs, Utah - Typical marginal marine, deltaic siliciclastic system sepmstrata.org

32 Example: Siliciclastic Sequence Stratigraphy Book Cliffs, Utah - Typical marginal marine, deltaic siliciclastic system sepmstrata.org

33 Carbonate Sequence Stratigraphy sepmstrata.org

34 Example: Humid Carbonate Rimmed Shelf Sequence Strat

35 Example: Humid Carbonate Isolated Rimmed Platform Sequence Strat

36 Example: Mixed Environments

37 Example: Mixed Environments

38 Example: Mixed Environments

39 Cyclicity of Sea Level Catuneanu et al. 2009

40 THEORETICAL CURVE OF SEA LEVEL VARIATION PLUS SUBSIDENCE Cycle controlling sequence set architecture Short-term fluctuations in eustasy (absolute sea level) combined with longer-term increase in subsidence. Nichols (2009)

41 HIGH-FREQUENCY SEA-LEVEL CHANGES ON LONG-TERM CURVE Even higher-frequency sea-level fluctuations added Nichols (2009)

42 Milankovitch Cycles

43 Outcrop example Ellwood et al. 2013