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 par ially 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 ¹⁸O values (-3.7 ± 0.2) are assumed to represent primary biogenic calcite. These ¹⁸O values are in the heavy end of the range of values from texturally well-preserved whole brachiopods (-6.6 to -2.5), texturally altered whole brachiopods (-6.7 to -3.7), and texturally altered crinoids and cora s (-6.7 to -2.7). This comparison suggests that marine-derived pore waters were a predominant component during the principal phase of diagenesis. The relatively low ^l8O values of well-preserved brachiopods compared with modern representatives implies that Middle Devonian seawater was either warmer (by about 10°C) or depleted in ¹⁸O (by about 2-3) relative to modern seawater.