Way up structure

A Way up structure 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) or have been overturned by subsequent deformation. This technique is particularly important in areas affected by thrusting and where there is a lack other indications of the relative ages of beds within the sequence, such as in the Precambrian where fossils are rare.

Examples

* Unconformities - Clear angular unconformities provide unequivocal evidence of the relative age of two rock sequences.
* Cross-bedding (Cross-stratification). - These structures are common in rocks laid down by the action of wind or water currents and in some volcaniclastic rocks. Minor erosional events during the overall deposition give rise to small-scale angular uncomformities.
* Graded bedding - In certain types of clastic sedimentary rock, the grain or clast size varies systematically from the base of the bed to its top. In a normally graded bed the grain or clast size is largest at the base and the bed is said to fine upwards. Beds deposited by density underflows such as turbidites typically show normal grading. Reverse grading or coarsening upwards is a characteristic of some fluvial beds, so it is important to understand the depositional environment before using this particular criterion.
* Sole markings - Many beds deposited by density underflows, such as turbidites, have erosional bases. Although there is no clear angular uncomformity developed in most cases, there are characteristic structures formed at the base of the bed. The include flutes, 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.
* 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.
* 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.
* Geopetal structures.

External links

* Cross-bedding http://www.uoregon.edu/~millerm/crossbeds.html
* Graded bedding http://www.geo.wvu.edu/~donovan/geol101/animations/09.swf

References

* Tucker, M.E. 2001. Sedimentary Petrology: an introduction to the origin of sedimentary rocks. 3rd edition. Blackwell Scientific Publications.