Monday, March 7, 2011

Interpreting Stratigraphic Columns

Here is an outline of how to approach interpreting the depositional environment represented by a stratigraphic column.

Step 1: Look for sedimentary structures that are characteristic of a specific environment or process
Examples:


  • HCS

  • wave ripples (vs current ripples)

  • herringbone cross stratification

  • reactivation surfaces

  • mud drapes in sandstone

  • bouma sequence

  • mud cracks

  • root casts

  • ripple cross lamination with reverse grading

  • meter-high dunes in fine sand

  • diamictites with facetted clasts and striations

  • lone (or drop) stones in laminated shale


    Step 2: Evaluate how these distinctive structures relate to each other in the stratigraphic column to develop a tentative environmental interpretation


  • 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?


    Step 3: Compare the tentative interpretation to flow 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

    Step 4: 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 upward into a river system? (Then it might be a delta building out into standing water.)


    Step 5: Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable.
    Try to sketch neighboring environments and interpret how they shifted through time. Are your interpreted vertical changes in environments consistent with neighboring environments horizontally? Does you interpretation require any jumps in environments or imply an unconformity? Revise your interpretation until it is consistent with your data.

    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.
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