Multiple Origins for Zoned Cathodoluminescent and Noncathodoluminescent Calcite Cements in Pennsylvanian Limestones
GOLDSTEIN, ROBERT H.,JAMES E. ANDERSON, and ROD A. PHARES, University of Kansas, Lawrence, KS
Noncathodoluminescent calcite containing brightly to moderately luminescent zones is a common early cement in limestones. Three such cements in Upper Pennsylvanian limestones from different areas were studied. AU three units are overlain up-section by Permian evaporites and consist of carbonate-siliciclastic "cyclothems" in which individual cycles were subject to subaerial exposure. With such similar settings, one might predict that petrographically similar calcite cements would have similar origins.
In the Holder Formation (New Mexico), the zoned calcite predates compaction, and cross-cutting relationships with cycle-capping paleosols show that zoned cements precipitated during 15 events of subaerial exposure. Therefore, cements precipitated from freshwater during early and repeated subaerial exposure.
For the Lansing-Kansas City groups in northwestern Kansas, the zoned calcite cements commonly are among the first precipitated but may postdate some compaction. AU-liquid fluid inclusions indicate precipitation below about 50 degrees C, from brines of approximately 23 weight % NaCL equivalent. The brines may have refluxed downward during deposition of Permian evaporites.
A limestone of the Lansing-Kansas City groups of west-central Kansas contains early zoned calcite cement that predates compaction. The cement contains all-liquid fluid inclusions indicating precipitation below about 50 degrees C. Reconnaissance measurements suggest cement precipitation from brackish water between about 0.9 and 3.1 weight % NaCL equivalent. This zoned cement probably precipitated from a low-temperature, marine-freshwater mixing zone.
The presence of nonluminescent calcite containing brighter subzones is not indicative of a single diagenetic environment. Petrographically similar cements from similar settings may originate in markedly different diagenetic environments.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)