GEOCHEMICALLY IDENTIFYING PRIMARY DEPOSITIONAL CHERT FROM EARLY AND LATE DIAGENETIC CHERT IN THE LATE ORDOVICIAN MONTOYA GROUP

The Late Ordovician Aleman Formation is the medial unit in the predominantly subtidal Montoya Group of New Mexico and Texas. Chert comprises up to 70% by volume in this unit and the chert morphologies are highly variable. The deepest water facies in the Aleman Formation is thin-bedded dolosiltite and dolomudstone interbedded with continuous spiculitic chert beds. Midramp facies are skeletal wackestone/packstone and burrowed dolomudstone with discontinuously to nodular beds of chert. Skeletal packstone/grainstone comprise the shallowest water facies. Rare-earth element (REE) plots of the chert can be arranged into five distinct types. The first chert type is comprised primarily of siliceous sponge spicules in continuously bedded chert and dolosiltite that are interpreted as primary accumulations of silica. The second through fourth types of chert have variable nodular chert morphologies and are distinguished by their REE patterns which correlate with the percentage of silica-replaced macrofossils they contain. These cherts formed during early diagenesis and are dominantly midramp facies. The fifth type of chert includes silica-replaced evaporite nodules and fracture fill cherts that crosscuts bedding, which formed during late diagenesis and have a distinct REE pattern. The depositional environment of the chert is supported by the lanthanum/cerium (La/Ce) ratio of ~ 1, but REE plots of the Aleman cherts are enigmatic having overall low abundances and a positive HREE slope. The variability of these REE values likely reflect early silica diagenesis in this unit and the variation of biota replaced by the silica. Upwelling of cool, deeper waters up the ramp supplied large quantities of silica and nutrients to the siliceous sponges whose death and decay produced the primary source of opaline silica. The opaline silica was either (1) lithified, producing primary siliceous deposits, or (2) was dissolved into the pore fluids and precipitated during early diagenesis to form the various chert morphologies in the Aleman Formation.