CONTROLS ON MICROFOSSIL-BEARING CHERT: NEW INSIGHTS FROM THE MIDDLE TO UPPER CAMBRIAN BONANZA KING FORMATION

The majority of fossil bacteria are found in chert, yet the mechanisms by which chert either precipitates or replaces limestone are still poorly known. Primary chert facies include hot spring deposits, vein-fill (after chalcedony), and recrystallization of opaline skeletal oozes. Secondary chert implies burial migration of silica-rich fluids and replacement of carbonate minerals. Fossil-bearing chert dates back to the Paleoarchean and examples occur through the Holocene. Most researchers believe that the microfossil-bearing cherts are largely primary. However, the majority of chert deposits are secondary.

We mapped out microfossil-bearing cherts in the Middle to Late Cambrian Bonanza King Formation to resolve controls on chert formation. Some of the Bonanza King cherts are clearly replacing stromatolitic limestone and preserve bacterial filaments. Three stratigraphic sections through the upper ~200 meters of Bonanza King were measured, comprising 133 beds. The following lithologic units were differentiated: limestone laminites, bedded dolowackestone, bedded wackestone, grainstone, packstone, massive chert, mottled dolowackestone, mottled wackestone, domical stromatolitic boundstone, hemispherical stromatolitic boundstone, domical thrombolitic boundstone, maceriate thrombolitic boundstone, dolostone breccia, and limestone breccia. Chert was divided by color and type (bedded, nodular, discontinuous, and knots). The stratigraphic sections were correlated based on a diachronous, unique white chert layer, 20 to 30 cm thick. Facies analysis enabled distinguishing four major facies (intertidal, shallow subtidal, deep subtidal, and very deep subtidal) and six subfacies.

The data suggests that there is no strong correlation between chert distribution and stratigraphic position, lithology, mineralogy, or faulting. There is a connection between facies and chert whereas the majority of replacement chert occurs in the deep subtidal facies (81.2% of beds contain chert). This may be due to the original presence of siliceous sponges, but other hypotheses are plausible. Furthermore, this study provides more evidence that secondary chert can preserve bacterial fossils, bringing into question the principle that only primary chert can fossilize bacteria.