EOLIAN DUST AND THE ORIGIN OF PALEOZOIC CHERT IN THE EASTERN UNITED STATES

Paleozoic sedimentary chert in the eastern United States occurs in strata that were deposited under arid conditions. Hypothetically, eolian processes exported copious amounts of chemically reactive siliceous dust from Paleozoic deserts to marine environments where chert subsequently formed. Given the particle size distribution of dust, and the chemical reactivity of fine quartz, it appears that quartzose dust best explains the source of silica for bedded sedimentary chert as well as the empirical correlation of aridity with the occurrence of chert. A dust source also explains the long-held view that chert commonly formed from siliceous gels. If so, then bedded cherts should contain highly etched remnants of detrital quartz. To test the dust hypothesis, chert samples were collected from Ordovician, Devonian, and Mississippian chert-bearing strata in the Appalachian and Illinois basins. Sample collection focused on bedded chert but not specific depositional environments. Analysis of thin sections revealed the presence of detrital quartz dispersed in a chert matrix in all samples. Quartz silt occurred in all samples, and rounded to spherical quartz sand grains in many samples. All detrital grains were intensely corroded in every chert sample. The abundance of silt ranged from sporadic occurrences in very fine-grained chert to siltstone with chert cement.

The stratigraphy and physical character of bedded Paleozoic chert is consistent with deposition of silica by eolian processes. Grain size of detrital particles in chert indicates either the magnitude of dust storms, or the proximity to dust sources, or both, because grain size is sorted downwind. Therefore, silt- and sand-bearing chert beds may have been derived from more proximal sources whereas very fine-grained vitreous chert beds represent deposition in distal settings where the fine size fraction of dust was deposited. Furthermore, the chemical and mineralogical composition of dust logically accounts for the composition of impure chert deposits. In addition, dust would have provided a readily available source of dissolved silica for silica secreting organisms.