SILICA SOURCING IN CHERT NODULES FROM THE LOWER PENNSYLVANIAN PORTION OF THE BIRD SPRING FORMATION, MOUNTAIN SPRINGS PASS, SPRING MOUNTAINS, NEVADA

The goal of this project is to assess the origin of the silica in chert nodules from the lower Pennsylvanian portion (basal 300 m) of the Bird Spring Formation from Mountain Springs Pass, Spring Mountains, Nevada. The lower Bird Spring formation consists of dominantly inner- to restricted-platform carbonate facies that were deposited near the equator. Within the study area are local intervals with stratiform chert layers, lenses, and nodules. Several silica sources are possible, including volcanic ash, aeolian, and/or fluvial input, all potentially mediated by biota. In this study, we used thin section petrography, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analyses on 20 chert nodules from the lower Bird Spring Formation to differentiate between these potential silica sources.

Thin sections and XRD data showed an abundance of quartz with varying amounts of calcite. The thin sections contained some fossils (fusulinids, crinoids, one sponge spicule?) and organic matter. XRF analyses for major and trace elements contained few trends. The most distinct is the strong negative linear 1:1 correlation between CaO and SiO₂, consistent with replacement of calcite by amorphous silica. These two oxides comprise >93 wt% of the samples. The other major and minor oxides are generally present at <1%. Most of the oxides have weak to no correlation with each other, with the exception of Fe₂O₃ and SiO₂, and K₂O and Al₂O₃, both of which are positively correlated. There is also a weaker positive correlation between CaO and SO₃.

No apparent trends in the data indicate the exact source of the silica. The preferred model is one where silica is sourced from periodic ash falls, perhaps from the Klamath arc to the north. This ash could have seeded the Bird Spring sea with nutrients, increasing productivity and allowing for proliferation of silica sponges. This silica would then be remobilized during early diagenesis, replacing carbonate. Aeolian input is locally important in the Bird Spring Formation, but no aeolian grains were observed in these samples. Changes in fluvial input is an unlikely source as the Bird Spring basin was dominantly surrounded by small land masses and fluvial deposits have not been reported from the basin.