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1.11: 12. Interpreting Stratigraphic Columns

  • Page ID
    1421
  • Step 1: Diagnostic Sedimentary Structures

    Look for sedimentary structures that are characteristic of a specific environment or process

    Examples:

    • hummocky cross stratification - waves plus currents (storms)
    • wave ripples (vs current ripples) - waves (standing water)
    • herringbone cross stratification - bidirectional flow over hours or longer (tides)
    • reactivation surfaces - reshaping of bedforms due to changes in flow (tides)
    • mud drapes in sandstone - flow stops (tides)
    • bouma sequence - rapid flow slowing down (turbidity current)
    • mud cracks - mud contracts (exposed to air)
    • root casts - from plants, usually land plants (land)
    • faint ripple cross lamination with reverse grading - (eolian ripples)
    • meter-high dunes in fine sand - (eolian dunes)
    • diamictites - laminar flow - (debris flows, mud flows, melting ice)
    • diamictites with facetted clasts and striations - (glacial)
    • lone (or drop) stones in laminated shale - large grains rafted over quiet environment (icebergs; trees possible)

    Step 2: Tentative Environmental Interpretation

    Evaluate how these distinctive sedimentary structures relate to each other in the stratigraphic column.

    • Are there several indicators of waves or storms?
    • Are there several indicators of tides?
    • Are there several indicators of wind-deposited sediment?
    • Are there several indicators of glacial activity?

    Evaluate the environment(s) in which these processes occur most commonly to develop a tentative environmental interpretation (or an environmental hypothesis).

    Step 3: Consistency with Other Features

    Compare your environmental hypothesis to the characteristics implied by other sedimentary structures in the column and evaluate whether they are consistent with your tentative environmental interpretation.

    • Examples of other sedimentary structures:
      • Trough cross stratification
      • Planar cross stratification
      • Current ripple cross lamination
      • Planar lamination

    Evaluate whether or not your tentative environmental interpretation is consistent with the presence of these other sedimentary structures. If it is not consistent, evaluate how you can revise your environmental hypothesis to take these other features into account.

    Example: Turbidites vs River Channel Deposits

    If you hypothesized that set of a fining upward sandstones were turbidites (which have planar lamination and current ripple cross lamination), but there is also trough cross stratification in a number of the beds, ask yourself if there is a way for trough cross stratification to form in turbidites. The answer is that trough cross stratification is VERY rare to absent, which is why trough cross stratification is not part of the Bouma sequence of facies defined for turbidites.) You would then look for another environment that includes sandstones with planar lamination and current ripples. Fluvial channels often have those features. Turbidites and fluvial channels do not form in neighboring environments (see Step 5 below), so it is unlikely that both are present. You will thus want to go back to steps 1-3 to identify a more consistent tentative environmental interpretation.

    Example: Turbidites vs Storm Deposits

    If you hypothesized that set of a fining upward sandstones were turbidites (which have planar lamination and current ripple cross lamination), but there are also wave ripples in a number of beds, ask yourself if there is a way for wave ripples to form in turbidites. The answer is no because turbidites are unidirectional flows. However, waves and storms can influence the sediment in the same environments that produce turbidity currents. For example, lakes and the ocean can have both in environments above storm wave base. Thus, the presence of wave ripples in some beds that contain mostly turbidite features suggests that waves were ALSO present in that environment. Thus, your environmental interpretation does not need to be significantly revised. Rather, it can be expanded to include both turbidites and waves.

    Step 4: Vertical Evaluation

    Evaluate how the vertical sequence of sedimentary structures changes to refine or correct your environmental interpretations.

    • Do structures occur in a distinctive pattern that suggests a depositional environment?
      • Is there an erosion surface followed by dune stratification followed by ripple lamination followed by a rooted horizon? (Then it might be migrating river channels or tidal channels if there are indicators of tidal currents.)
      • Do the structures suggest an environment that shallows from deep quiet water upward into a river system? (Then it might be a delta building out into standing water.)

    Sometimes the main sedimentary transport characteristics change through time. For example, a storm influenced environment can become more dominated by tides through time as the geometry of the shoreline changes, as climate changes, or as relative sea level changes. All three of these can change at once, too, because climate and sea level are tightly linked to each other, and shorelines respond to the changing sediment transport processes.

    Step 5: Walther's Law

    Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable. There are two ways to approach this: testing your hypothesis by starting with your predictions, or starting with the data from the stratigraphic column and seeing if you get reasonable environments next to each other.

    Test your environmental hypothesis

    If you have a good mental image of your hypothetical environments, this is a useful process:

    1. Draw a cross section or map view (or both) that includes all of your hypothesized environments. Make sure that it is a reasonable landscape, e.g. something that would be reasonable to see in a satellite image. In other words, do not draw rivers flowing uphill or lakes on mountain peaks, etc.
    2. Indicate things like the relative water depth and the characteristics of sediment transport in each area. Include the sedimentary structures that you predict will form.
    3. Compare these environments to your stratigraphic column. Start with the interval of the column that you have the most confidence in for your environmental interpretation. Mark that environment on your cross section or map. Then go up (or down) in the column to the next facies (rock type) and mark the environment it represents on your cross section or map.
    4. Evaluate whether the two points are consistent with Walther's law. Draw a line from the first environment to the second one. Does that line cross any depositional environment that is absent from your stratigraphic column? If the answer is yes:
      • How can you reconcile the missing environment?
      • Are there features that you missed in the stratigraphic column that could have been deposited in that environment?
      • Is there an erosion surface between the two facies in the stratigraphic column? 
    5. Revise your interpretation of either your depositional environment or the facies in your stratigraphic column until they are consistent with each other. Do not, however, ignore the sedimentary structures in the stratigraphic column because they are the data that will lead to an appropriate interpretation. Revise your cross section or map appropriately.
    6. Repeat steps numbers 3-5 until you have marked every facies in your stratigraphic column on your cross section or map, and all inconsistencies are resolved. Sometimes there are remaining uncertainties because the rock record does not record everything and we miss features when we are collecting the data. If there are still inconsistencies, record them and why you could not resolve them. For example, if there is one set of trough cross stratification in a sequences that otherwise consists of turbidites and storm deposits, you could make a note of where it occurs and provide a tentative interpretation for why, e.g. maybe the flow speed was consistent enough for long enough to produce dunes even though that rarely happens in turbidity currents.

    Recreate environments from your strat column

    If you are having trouble creating a mental image of your hypothesized environments, this is a useful process:

    1. Start with the interval of the strat column that you have the most confidence in for your environmental interpretation. Draw that environment on a map or in a cross section. For example, if there are interbedded sandstones and mudstones, and the sandstones have features consistent with the Bouma sequences, draw a standing body of water with a slope on one side.
    2. Indicate things like the relative water depth and the characteristics of sediment transport in each area. Include the sedimentary structures that you predict will form.
    3. Next, go up (or down) in the column to the next facies (rock type) and draw the environment that it represents on your cross section or map, again indicating the relative water depth, sediment transport processes, and sedimentary structures indicated by the stratigraphic column data.
    4. Evaluate whether the two environments are consistent with Walther's law. Are they typically next to each other in a landscape? If the answer is yes, go the the next facies in the strat column (go back to 3). If the answer is no, evaluate whether any of these are possible:
      • Did you miss an environment represented in your stratigraphic column? Is that environment between the two you drew in your cross section or map?
      • Is there an unconformity in your stratigraphic column that would mean that Walther's Law does not apply? Could erosion have been caused by the flows in the "missing" environment?
      • Is one of your environmental interpretations wrong? What else could it be?
    5. Revise your interpretation of either your depositional environment or the facies in your stratigraphic column until they are consistent with each other. Do not, however, ignore the sedimentary structures in the stratigraphic column because they are the data that will lead to an appropriate interpretation. Revise your cross section or map appropriately.
      • Example: Say you interpreted interbedded mudstones and sandstones with current ripples as turbidites. However, there are also sandstones with an erosional base, thick sections of trough cross stratification fining up into current ripples at the top that you interpret as river point bar deposits. If those two rock types are next to each other vertically, they would violate Walther's Law since turbidites form in lakes and oceans, whereas river channels form on land. You need a shoreline between the two. Possible resolutions to this "illegal" interpretations could include: a) there is an interval between the two with climbing ripples that you missed in the stratigraphic column that could represent a river mouth bar, providing a transition from the standing body of water into the river channel; b) there is an unconformity that is more significant than the erosion at the base of a fluvial channel in the stratigraphic column, and thus there are no rocks preserved that reflect the changes in environments between the standing water and the river channel; or c) the interbedded mudstones and sandstones actually represent floodplain deposits rather than turbidites. You would then go back to the stratigraphic column and evaluate which change in interpretation is best supported by the data in your column.
    6. Repeat steps numbers 3-5 until you have marked every facies in your stratigraphic column on your cross section or map, and all inconsistencies are resolved. Sometimes there are remaining uncertainties because the rock record does not record everything and we miss features when we are collecting the data. If there are still inconsistencies, record them and why you could not resolve them. For example, if there is one set of trough cross stratification in a sequences that otherwise consists of turbidites and storm deposits, you could make a note of where it occurs and provide a tentative interpretation for why, e.g. maybe the flow speed was consistent enough for long enough to produce dunes even though that rarely happens in turbidity currents.

    Summary

    Often, there is some ambiguity about the depositional environment(s) represented in real rocks. By going through this process, you can reach a reasonable interpretation that is well supported by the data. You will also understand where the ambiguities are. This is particularly helpful if it is your own data and you can make more observations by doing more field work. You will be able to focus your efforts on resolving the ambiguities, leading to a more accurate environmental interpretation.