About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
Volume:
Issue:
First Page:
Last Page:
Title:
Author(s):
Article Type:
Abstract:
As carbonate sediments are lithified to limestone, two major porosity changes commonly occur. First, interstitial pore space is partly or, less commonly, totally obliterated as the grains are bound together; second, moldic porosity usually is developed. In sediments devoid of CaCO3 mud, porosity development tends to be more pronounced than porosity elimination; yet preliminary studies indicate that, where CaCO3-mud matrix is found, the moldic pore space is more likely to be retained. Moldic porosity is formed by the dissolution of aragonitic grains which are abundant in shallow-water marine sediments, but is not formed at the expense of grains composed of the two types of calcite (high-magnesian and low-magnesian). Aragonite and both types of calcite are synthesized predominantly by organisms. Because different biologic groups selectively synthesize carbonate material of either aragonite or one of the two types of calcite, a predictable relation exists between biological activity and the tendency to form moldic porosity.
The tendency for aragonitic grains to develop porosity is variable. Shell material is more strongly affected by dissolution and consequent moldic porosity development than are the more finely crystalline ooids, which may resist dissolution. Yet, ooids are more likely to form moldic ("oomoldic") porosity than are pelletal and cryptocrystalline grains. Where the process of diagenesis continues, the newly created pore spaces are occluded by drusy calcite mosaic.
End_of_Article - Last_Page 614------------