Ordovician Aleman Ribbon Cherts: An Example of Silicification Prior to Carbonate Lithification

Abstract

ABSTRACT The Ordovician Aleman Formation, southwestern New Mexico, is primarily composed of dolomite with ribbons of chert. The presence of stromatolites, relict evaporite nodules, and other features indicates an intertidal to shallow subtidal depositional environment. The stromatolitic layers are preferentially silicified with dissolution of opaline sponge spicules providing the main, if not sole, source of silica. Silicification occurred in both the vadose and phreatic realms, essentially penecontemporaneous with accumulation of the sediment and concomitant with dolomitization. This is indicated by the preservation in silicified form of original anhydrite lath morphology, central cavities in chert nodules partially filled with geopetal matrix dolomite, and matrix dolomite immediately adjacent to conchoidal fractures of silicified carbonate grainstones. The latter relationship indicates that the chert was rigid and fractured before the carbonate host-sediments were lithified.