Way up structure

A way up structure, way up criterion, or geopetal indicator is a characteristic relationship observed in a

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]

• angular unconformities
  provide unequivocal evidence of the relative age of two rock sequences.
• volcaniclastic
  rocks. Minor erosional events during the overall deposition give rise to small-scale angular unconformities. There are three ways a cross bed can be used:
  • Troughs of trough cross beds, in which the concave part of the trough points up.
  • The tangential (end) part of the cross bed is always on the bottom of the cross bed.
  • Many cross beds are stacked, and truncation always happens up section.
• depositional environment
  before using this particular criterion.
• flute casts
  , grooves, and tool marks.
• Load casts - These are formed when a higher density layer (e.g. a sand) is deposited on a lower density layer (e.g. a mud). Characteristic structures include flame structures
  that are forced up from the underlying low density layer.
• Mudcracks - Cracks in a desiccated mud layer, such as found on a dried-out lake bed or tidal flat, are often filled by an overlying sand layer. Mudcracks in cross section typically show a profile in which the filled sand layers are narrow at the bottom and wider at the top resembling a V shape.
• Ripple marks - Ripples are typically rounded at the base (trough) and sharp (or truncated) at the top (crest).
• Neptunian dykes
  - These are formed when the top of a bed is temporarily exposed and a fissure develops by processes such as earthquake activity or solution of carbonates. The fissure becomes filled either during the period of exposure by contemporaneous, possibly wind-blown material, or when deposition resumes, by the same material as the overlying bed.
• Rubbly tops and chilled bases of
  lava flows
  - Basaltic lava flows often have bases that show evidence of rapid cooling against the land surface and broken, rubbly tops caused by the continued flow of partially solidified lava.
• Pillow structures in subaqueous lava flows.
• Trace fossils - The trails of bottom feeders such as Cruziana, u-shaped burrows of suspension feeders such as Diplocraterion and traces of root systems can provide clear evidence of way-up.
• Fossils in life position (e.g.
  articulated crinoids
  ) can give a paleo up direction.
• Void-fill structure (known in some texts as simply a geopetal structure) is a void or cavity in a
  sediments or mineral
  deposits, that indicates an initial horizontal that can be used as a way up structure and as a precise measure of post-depositional tilting. There are two main examples of this:
  • Fossils - Any
    brachiopods. The initial fill is often of sediments with the remaining part filled by calcite,^[2]
    giving a clear geopetal structure.
  • Vesicles - Many
    calcedony),^[3][4]
    can be used as geopetal structures.

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]

• Sedimentary structures

• Tucker, M.E.
  2001. Sedimentary Petrology: an introduction to the origin of sedimentary rocks. 3rd edition. Blackwell Scientific Publications.
• Prothero and Schwab, 1996, Sedimentary Geology,
  ISBN 0-7167-2726-9

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

• Cross-bedding http://www.uoregon.edu/~millerm/crossbeds.html