EARLY SILICIFICATION OF UPPER ORDOVICIAN DEEP-WATER CHERT-LIMESTONE RHYTHMITES, CENTRAL NEVADA

The Upper Ordovician Hanson Creek Formation of central Nevada is composed of 250 m of deep- to shallow-water carbonates, fine siliciclastics, and chert. The uppermost 40 m is characterized by thin, rhythmically interbedded, deep-water limestone and replacement chert (rhythmites). Limestone-chert couplets average 20 cm thick and layers are laterally extensive over 100’s m. Limestone layers (~10 cm) are composed of dark, laminated, pelletal microspar with sparse siliceous sponge spicules; chert layers (~10 cm) are composed of laminated, organic-rich, dolomitic, calcitic chert with abundant sponge spicules and sparse radiolarians. The rhythmites were deposited below storm-wave base during a global sea-level rise related to melting of Late Ordovician glaciers and immediately after the Late Ordovician global mass extinction. The interbedding between limestone and chert layers is interpreted to represent short-term (millennial-scale) fluctuations in upwelling. Increased intensity of upwelling provided the silica for fossil-rich chert layers.

Evidence for very early silicification of limestone includes: limestone compaction around chert beds/nodules, only minor radiolarian compaction in cherts, and chert breccia fragments in submarine debris flow deposits lying 8 m above the rhythmites. Within chert layers ~50% of spicules and radiolarians are preserved as silica; the remaining are partially replaced by calcite with some silica preserved in fossil centers. At the scale of SEM observations, siliceous fossils are poorly preserved and are variably composed of silica, calcite or dolomite.

Combined field and microscope observations suggest that during rhythmite deposition there were short-time intervals when there were more siliceous sponge spicules being deposited (now chert layers). Soon after deposition of sponge spicule-rich layers either: 1) many spicules dissolved and reprecipitated locally within layers as siliceous cement, while the carbonate in spicule-rich layers dissolved and reprecipitated in adjacent spicule-poor layers (now limestone) as calcite cement; or 2) many spicules dissolved and reprecipitated locally within layers as siliceous cement, while the carbonate matrix in the spicule-rich layers recrystallized locally or partially replaced the originally siliceous fossils.