Chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement: Mississippian Lake Valley Formation, New Mexico

Abstract

Echinoderm-syntax ial cement crystals have been collected at several stratigraphic 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 elemental and isotopic information that record fluid evolution during their growth. Although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation, intracrystalline patterns in minor element and stable 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, petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems. Most Lake Valley Formation syntaxial cement formed in the marine phreatic and marine-meteoric mixing zones during the earliest phase of cementation. Of this cement, in excess of 60% formed in the marine phreatic zone and lowermost marine-meteoric mixing zone. Smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre-Pennsylvanian lowering of sea level. Meteoric phreatic fluids were relatively unimportant during this episode of cementation. In contrast, cement of entirely meteoric phreatic origin is associated with a later meteoric system, but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre-Pennsylvanian unconforrnity. Our results indicate that cements in the upper and lower parts of the Lake Valley Formation are genetically and temporally 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 demonstration that diverse and temporally distinct diagenetic settings can produce identical CL characteristics shows that caution should be employed when using cement CL stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions.