Accommodation. Tectonics (local to regional) Subsidence Uplift

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1 Accommodation Accommodation varies over time; it is the distance between a reference horizon and the sea surface (i.e. relative sea level). The space to be filled in with sediments or water What are the primary controls? Eustasy change in global sea level due to: Global tectonics (e.g., mid-ocean rift crustal volume) Climate (e.g., seawater locked up in polar ice caps during glacial times and released during interglacial times) Climate (e.g., thermal expansion of ocean due to warming) Tectonics (local to regional) Subsidence Uplift

2 Variations in Accommodation and Sediment Supply Sediment supply Sedimentary basin (accommodation) the two most important controls in the filling of a basin are accommodation and supply. The patterns of basin fills can be understood by considering how accommodation and supply change over space and time.

3 Variations in Accommodation: Tectonic subsidence and Uplift Sediment supply Sedimentary basin (accommodation) uplift Wheeler (1964) As we zoom in, base level can vary by location and is dynamic w/r/t tectonic and depositional history. subsidence

4 Variations in Accommodation: Tectonic subsidence and Uplift Patzkowsky and Holland 2012

5 What is base-level? Base level represents the surface that uplands want to erode down to and the surface that basins want to fill up to Sea level is the best example of base level, but it is not synonymous with base level

6 How do we measure Accommodation Space? At all times sedimentary basins (systems) are trying to achieve and preserve an equilibrium profile (depositional profile) Where the available accommodation space is balanced by the amount of sediment supplied

7 Topographical profile of the height of the source area and the increase in the rate of discharge downstream

8 Uplift will cause the rivers to cut down (incise) and sediment removed

9 Subsidence of the source area will change the shape of the profile and the river system will re-adjust by eroding and depositing sediment to regain stable profile

10 Change in sea level will alter the equilibrium profile A rise in sea level the position of the coastline will change, river channels infill and gradient flattens out

11 Stratigraphy: Record of changes in Accommodation We know that undisturbed sediments form stacks with the oldest material at the base. These stacks, or stratigraphic columns, can be correlated over distances, because sediment deposition is laterally continuous.

12 Laterally continuous strata

13 How to extrapolate a stratigraphic section for facies analyses and depositional environment interpretation 20 m Let s say you go out to an outcrop and measure a section in which you document, thickness, lithology, grain size, sedimentary structures, biogenic structures, etc. simplified grain-size profile (in detail, there would be much more information depicted) You then interpret several different depositional environments based on these objective facies.

14 How to extrapolate a stratigraphic section for facies analyses and depositional environment interpretation 20 m 1 km You then measure another section laterally some distance away, within the same lithostratigraphic package, and also document the facies and interpret depositional environments.

15 How to extrapolate a stratigraphic section for facies analyses and depositional environment interpretation 20 m 1 km Over the next few weeks of field work, you map out the facies and measure several more sections in some locations, you can physically correlate surfaces that separate distinct facies (and, thus, environments).

16 Surfaces that separate distinct facies 20 m 1 km The combination of surfaces and depositional environment provides a framework to interpret the depositional history this is sequence stratigraphy

17 How would you define a depositional profile?

18 How would you define a depositional profile? A = floodplain of meandering river B = braided river C = estuarine mouth (e.g., barrier system) D = estuarine E = delta plain F = delta mouth bar and delta front G = prodelta For each surface-bounded package, always ask yourself: Which way is the shoreline?

19 SCENARIOS LEADING TO RELATIVE RISE IN SEA LEVEL

20 SCENARIOS LEADING TO RELATIVE FALL IN SEA LEVEL Because we commonly only observe the resulting stratigraphic response, we refer to these changes as relative rise/fall of sea level. (With more information, you can then interpret if it was caused by eustasy, tectonics, or combination of both.)

21 THEORETICAL CURVE OF SEA LEVEL VARIATION PLUS SUBSIDENCE Short-term fluctuations in eustasy (absolute sea level) combined with longer-term increase in subsidence. Nichols (2009)

22 HIGH-FREQUENCY SEA-LEVEL CHANGES ON LONG-TERM CURVE Even higher-frequency sea-level fluctuations added how many orders of accommodation increase/decrease here? Nichols (2009)

23 THINKING ABOUT CORRELATION OF TIME-EQUIVALENT UNITS Three sections (subsurface wells, in this case) show the same upward-coarsening pattern interpreted as shoreface succession. 1 km Basinward 3 m HOW WOULD YOU CORRELATE THESE? from Steve Hubbard

24 THINKING ABOUT CORRELATION OF TIME-EQUIVALENT UNITS from Steve Hubbard One correlation option is to connect up the units of similar lithology. This would be a lithostratigraphic correlation. 1 km Basinward 3 m But, what does the vertical pattern of upward-coarsening, even in just one of the sections, suggest about the depositional stacking?

25 THINKING ABOUT CORRELATION OF TIME-EQUIVALENT UNITS from Steve Hubbard An alternative correlation would be to link up different facies that are timeequivalent. 1 km Basinward 3 m Although this is still a hypothesis to be tested (with more data) it helps us reconstruct paleo-environments with greater accuracy and predict in areas of limited data.

26 THINKING ABOUT CORRELATION OF TIME-EQUIVALENT UNITS An alternative correlation would be to link up different facies that are time-equivalent. 1 km Basinward 3 m Although this is still a hypothesis to be tested (with more data) it helps us reconstruct paleo-environments with greater accuracy and predict in areas of limited data. from Steve Hubbard

27 THINKING ABOUT CORRELATION OF TIME-EQUIVALENT UNITS from Steve Hubbard An alternative correlation would be to link up different facies that are time-equivalent. 1 km Basinward 3 m Although this is still a hypothesis to be tested (with more data, for example) it helps us reconstruct paleo-environments with greater accuracy.

28 ANOTHER EXAMPLE Lithostratigraphic correlation from Steve Hubbard

29 ANOTHER EXAMPLE Chronostratigraphic (sequence stratigraphic) correlation from Steve Hubbard

30 PARASEQUENCES BUILDING BLOCKS OF SEQUENCES These packages of strata representing a consistently evolving depositional environment are called parasequences 1 km Basinward Parasequences come in different styles (depending on environment), but can be thought of as the building blocks of sequences. 3 m Characterizing how these building blocks stack helps us determine larger-scale/longer-term depositional evolution.

31 EXAMPLE OF TWO UPWARD-FINING PARASEQUENCES Parasequences are smaller-scale packages inside sequences. They come in different forms, it depends on the depositional environment. These are from clastic tide-dominated shoreline.

32 EXAMPLE OF A UPWARD-COARSENING SHOREFACE PARASEQUENCE Parasequences are smaller-scale packages inside sequences. They come in different forms, it depends on the depositional environment. This type should look familiar this is the classic coarsening-upward clastic shoreface cycle.

33 EXAMPLE OF A CARBONATE TIDAL FLAT PARASEQUENCE SHALLOWING-UP SUCCESSION figure drawn by J. Fred Read (Virginia Tech emeritus)

34 Outcrop example Progradational Stacking Gradual shallowing upwards Flooding surface Shallow carbonates overlain by offshore shales progressively get more carbonates as there is a net shallowing upward then flooded again and goes back to offshore shales

35 STACKING PATTERNS Arrangement of vertical successions (stacking patterns): Progradational basinward/seaward stepping Retrogradational landward stepping Aggradational builds vertically Degradational abrupt basinward shift in facies Parasequences are the building blocks that can stack in these systematic ways.

36 Set of five retrogradationally stacked parasequences in mixed carbonate and siliciclastic mudstone facies thickly bedded carbonates represent shallow subtidal facies, interbedded carbonates and mudstones were deposited in deep subtidal facies, and mudstone facies indicate offshore facies. Flooding Surfaces Mississippian Bangor Limestone along I-24, just southeast of Monteagle, Tennessee; Courtesy Steve Holland

37 Three parasequences developed along on a tidal flat. parasequences are shallowingupward Ordovician Juniata Formation near Germany Valley, West Virginia; Courtesy Steve Holland

38 Aggradational stacking with a flooding surface (shale)

39 offshore shales overlain by shallow carbonates progressively get more carbonates as there is a net shallowing upward trend Gradual shallowing upwards Flooding surface

40 offshore facies are muddy and characterized by the development of covered slopes Transition zone facies consist of interbedded mudstone and sandstone, whereas shoreface and foreshore facies consist only of sandstone. foreshore facies are commonly overlain by a coal, which bleaches the underlying foreshore to white, in contrast to the buff colors of the shoreface. overall coarsening upward from offshore facies near the bottom to shoreface and foreshore facies near the top. individual flooding surfaces can be recognized by reverals in this trend, they are not as well developed as in a retrogradational set of parasequences classic exposure of the Cretaceous Blackhawk Formation in Gentile Wash near Helper, Utah; Courtesy Steve Holland

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42 MULTIPLE PARASEQUENCES STACK TO FORM LARGER-SCALE PATTERNS individual parasequence individual parasequence

43 ACCOMMODATION/SUPPLY RATIO AND STACKING PATTERNS A/S < 1 Sedimentation rate exceeds Accommodation rate A/S > 1 Accommodation rate exceeds sedimentation rate A/S = 1 Accommodation rate nearly equals sedimentation rate Degradational: Accommodation space is LOST

44 THE SEA COMES IN, THE SEA GOES OUT Remember the concept of onshore-to-offshore facies Recall the concepts of facies and Walther s Law (environments that exist laterally can stack vertically)

45 THE SEA COMES IN, THE SEA GOES OUT Remember the concept of onshore-to-offshore facies As sea level goes up (transgression), linked facies zones move landward creating a retrogradational stacking pattern. TRANSGRESSION: sediment supply is overwhelmed by a relative rise in sea level or when the land subsides tectonically Causes the shoreline to move landward (retrogradational) Brings deep water deposits with fine-grained material landward Defined as fining upward sequences

46 Transgressive Sequences Gradual rising of eustatic sea level marked by flooding surfaces or hard grounds Lexington Limestone (Ordovician), Cincinnati Arch Kentucky; Photo by J. Wittmer

47 THE SEA COMES IN, THE SEA GOES OUT Remember the concept of onshore-to-offshore facies As sea level goes down (regression), the facies belts shift basinward creating a progradational stacking pattern. REGRESSION: Shoreline moves seaward due to an excess sediment supply form the land (progradation) Occurs when the land is tectonically uplifted and the sea retreats OR when there is a relative lowering of sea level Coarse shallow water deposits can build outward over fine sediments Typically coarsening-upwards successions

48 Regressive Sequences Gradual lowering (fall) of eustatic sea level pulsed by relative sea rise TWO TYPES Normal Regression Forced Regression Kope Formation (Ordovician), Cincinnati Arch Kentucky; Courtesy Carl Bret

49 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