About This Item
Share This Item
The term "geopetal fabric" was introduced by Sander for fabrics in sedimentary rocks "which record the direction of the earth's surface at the time they were being formed." Geopetal fabrics thus can be used to determine the orientation of these rocks (i.e., top, bottom, horizontal, and vertical) when the fabrics developed.
The importance of geopetal fabrics in the interpretation of reef complexes is illustrated by examples from the Devonian of the Canning basin in Western Australia. The most useful geopetal fabrics in these limestones are formed by the carbonate mud and spar fillings of fossils, especially closed brachiopod shells. These shells commonly were partly filled by horizontal layers of carbonate mud, the remaining upper part of each cavity being filled subsequently by sparry calcite. The spar and carbonate-mud layers thus demonstrate top and bottom, and the contact between them usually marks the horizontal at the time of deposition. Similar geopetal fillings occur in gastropods, nautiloids, ammonoids, and (on a microscopic scale) in the cellular structure of stromatoporoids, corals, and bryozo ns. Other geopetal features of these limestones include birdseyes and other voids that were partly filled by carbonate mud, stromatactis structures, umbrella structures (below laminar stromatoporoids or algae), heliotropic algae and stromatolites, certain coral and stromatoporoid growth forms, and bedded fillings of neptunian dikes.
Steep depositional dips commonly are developed in facies associated with reefs, especially in forereef facies, and geopetal fabrics can be used to demonstrate the amount of depositional dip and to distinguish it from subsequent tectonic dip. Reefs themselves are commonly unbedded, and geopetal fabrics may be the only means of determining attitude in these rocks. In addition, allochthonous blocks of reef that have been incorporated in forereef and basin deposits can be distinguished from in-place bioherms through the use of geopetal fabrics. These can show that bioherms are in their original growth positions, whereas allochthonous blocks have haphazard orientations.
End_of_Article - Last_Page 645------------