Abstract

Biogenic components (brachiopods, crinoids, and corals) in Middle Devonian limestones were manually separated from their enclosing matrix and analyzed by textural, elemental, and isotopic methods in order to assess the preservational state of these components. In general, brachiopods in these rocks are better preserved texturally than other constituents, as determined by SEM and cathodoluminescence observations. Elemental variations among the major components are compatible with a differential response to diagenesis governed by the relative stability of the original mineralogy of the component. However, among the most stable, low-Mg calcite brachiopods, the relationship between textural preservation and trace element content is complex. This apparent complexity may be due at least partially to our method of textural characterization of whole brachipod samples. The nonluminescent portions of a set of well-preserved brachipods have textural characteristics and elemental contents which are virtually indistinguishable from modern representatives. Consequently, their delta 18 O values (-3.7 + or - 0.2 per thousand ) are assumed to represent primary biogenic calcite. These delta 18 O values are in the heavy end of the range of values from texturally well-preserved whole brachiopods (-6.6 to -2.5 per thousand ), texturally altered whole brachiopods (-6.7 to -3.7 per thousand ), and texturally altered crinoids and corals (-6.7 to -2.7 per thousand ). This comparison suggests that marine-derived pore waters were a predominant component during the principal phase of diagenesis. The relatively low delta l8 O values of well-preserved brachiopods compared with modern representatives implies that Middle Devonian seawater was either warmer (by about 10 degrees C) or depleted in 18 O (by about 2-3 per thousand ) relative to modern seawater.

First Page Preview

First page PDF preview
You do not currently have access to this article.