CHRONOSTRATIGRAPHIC SIGNIFICANCE OF CATHODOLUMINESCENCE ZONING IN SYNTAXIAL CEMENT - MISSISSIPPIAN LAKE VALLEY FORMATION, NEW-MEXICO

Echinoderm-syntaxial cement crystals have been collected at several st ratigraphic levels within the Lake Valley Formation, which is overlain by a major pre-Pennsylvanian subaerial exposure surface. The crystals were microsampled along growth bands, and yielded high-resolution ele mental and isotopic information that record fluid evolution during the ir growth. Although cement crystals show little variation in cathodolu minescence character and bulk chemistry throughout the regional extent of the formation, intracrystalline patterns in minor element and stab le isotope chemistry allow for the identification of several settings of cementation, including the marine phreatic, marine-meteoric mixing, and meteoric phreatic zones. When placed in a regional-stratigraphic context, crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments. Isotopic, petrogr aphic and stratigraphic constraints indicate that cementation was rela ted to two temporally distinct meteoric systems. Most Lake Valley Form ation syntaxial cement formed in the marine phreatic and marine-meteor ic mixing zones during the earliest phase of cementation. Of this ceme nt, in excess of 60% formed in the marine phreatic zone and lowermost marine-meteoric mixing zone. Smaller volumes precipitated from progres sively fresher mixtures of marine and meteoric fluid during the gradua l expansion of a freshwater lens that developed in response to pre-Pen nsylvanian lowering of sea level. Meteoric phreatic fluids were relati vely unimportant during this episode of cementation. In contrast, ceme nt of entirely meteoric phreatic origin is associated with a later met eoric system, but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre-Pennsylva nian unconformity. Our results indicate that cements in the upper and lower parts of the Lake Valley Formation are genetically and temporall y unrelated. Thus, the cement cathodoluminescence stratigraphy, which has been previously applied to the Lake Valley, can not be valid over the full extent of the formation. This has wider implications. Our dem onstration that diverse and temporally distinct diagenetic settings ca n produce identical CL characteristics shows that caution should be em ployed when using cement CL stratigraphy alone to extend diagenetic in terpretations over large geographic regions or through substantial str atigraphic successions.