This curious name was given to structures that help to determine the stratigraphic polarity of beds (way-up criterion). In some cases, they are visible to the naked eye, but are more commonly microscopic. The picture shows exactly a microscopic example in a thin section of a carbonate rock (black and white parts are reversed because of the negative print).
A geopetal structure consists in the double filling of a cavity inside the sediment. The first phase of filling is represented by deposition of fine clastic particles that are filtered by the coarser framework of the sediment. This internal sediment is horizontally layered under the control of gravity. Later on, the remaining void is occupied by chemical cement, whose crystals seal the internal sediment; its upper surface then becomes a fossil level, i.e., an indicator of paleohorizontality. Geopetal structures are very useful in poorly bedded rocks like carbonate reefs and banks, where internal cavities are frequent.
The rock of this example is a calcarenite, i.e., a lithified carbonate sand from the Calcari Grigi Formation of southern Alps.1 The cavity is a leaching vug, caused by dissolution of the already lithified carbonate (see irregular, corroded contour). It is, in practice, a microkarst feature, a kind of miniaturized cave.
Note 1: A. Castellarin and R. Sartori. 1973. Desiccation shrinkage and leaching vugs in the Calcari Grigi Infraliassic tidal flat. Eclogae Geol. Helvetiae 66: 339-343. Back.