Way up structure

A way up structure, way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up (i.e. in the attitude in which they were originally deposited, also known as "stratigraphic up") or have been overturned by subsequent deformation. This technique is particularly important in areas affected by thrusting and where there is a lack of other indications of the relative ages of beds within the sequence, such as in the Precambrian where fossils are rare.

The original definition comes from Bruno Sander in 1936, translated from German to English in 1951, which states:

Geopetal Fabrics - All the widely distributed spatial characters of a fabric that enable us to determine what was the relation of "top" to "bottom" at the time when the rock was formed are termed geopetal fabrics. Such fabrics are mechanical and chemical internal deposition; grains on a boundary surface; cross-bedding, etc.[1]

Examples

Cross-bedding in Zion National Park, Utah. Both the tangential bases on the cross beds and the truncations of overlying layers can be used as a way up indicator.
A dinosaur footprint preserved in positive relief is a trace fossil indicating the bottom (or sole) of the bed (Lower Jurassic Moenave Formation, southwestern Utah).
Geopetal structure in bivalve boring in coral; bivalve shell visible; Matmor Formation (Middle Jurassic), southern Israel.
Fossil with geopetal structure, Utah. Calcite fill is white, carbonate sediments are tan. Line between the two represents paleo-horizontal.

Issues with terms

In general, geopetal seems to be used in the U.S., while way up structure is used in the UK. "Geopetal structure", in some circles, is used exclusively for the void fill example, and nothing else. Some geologists use neither term, and might use something else, like topping indicator.[5][6][7]

See also

References

  1. Bruno Sander, 1951, Rhythmically deposited Triassic limestones and dolomites in Contributions to the study of depositional fabrics, Translated by Eleanora Bliss Knopf for AAPG.
  2. Page on Chert textures from the Rhynie chert, Aberdeen University
  3. Holloway, J.L. 2005. Sedimentation in vesicles: interpretation of geopetal fabrics in amygdaloidal agates, Geological Society of America Abstracts with Programs, Vol. 37, No. 2, p. 11
  4. 2000. Structural Geology observations from Hole 1137A, ODP Leg 183, Texas A&M
  5. The Penguin Dictionary of Geology, by Whitten and Brooks, 1972
  6. Bates and Jackson, 1984, Dictionary of Geological Terms, 3rd ed., Prepared by the American Geological Institute
  7. Glossary of Geology, AGI, 5th Ed., 2005

External links

The Wikibook Historical Geology has a page on the topic of: Way-up structures
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