A CONCEPTUAL MODEL EXPLAINING THE OCCURRENCE OF CHERT IN THE LOWER CARBONIFEROUS LIMESTONES OF THE MIDCONTINENT U.S

The presence of chert in Lower Carboniferous limestones of the midcontinent U.S. is widespread, and herein described with a conceptual model based on micro- to macro-scale attributes of the chert. These attributes constrain the model with thickness distributions of the chert layers, areal distribution and continuity of the chert layers, presence and preservation of fossils and microfossils within the chert, contacts of the chert with inter-layered limestone layers, SEM data, and a study of outcrops.

Based on an assessment of the data available, the source of the chert is hypothesized to be airfall ash from volcanoes that occurred south of the present-day Ouachita Mountains. The volcanoes occurred above the subducting plate of the North American continent, and the ash was deposited areally on environments ranging from moderate marine to shallow marine to exposed land surfaces. A volcanic source of the ash is consistent the SEM images and trace-element geochemistry from outcrops of chert in northern Arkansas, and from tuff occurrences in southern Arkansas. Thickness and continuity of the chert decreases from south to north. The Arkansas novaculite close to the Ouachitas has a thickness greater than 275 meters; the Boone Formation in northern Arkansas has an accumulated chert thickness of ~100 m (continuous layers); and the Burlington Formation in Missouri and Iowa has accumulated chert thicknesses of <10 m, typically as discontinuous nodules. Microscopic and hand-sample size contacts of the chert provide evidence that ash fall on sea water formed silica gels of varying thickness (~cm to >m scale), which variably dominated the carbonate factory production of sediment. In cases of deep water, anoxic conditions dominated, and continuous chert layers are dark gray or black, devoid of fossils, and typically have much greater percentage of pyrite. The contacts of some of the chert from these intervals suggest the weight from the overlying formations compressed the gels prior to induration, contributing to an irregular, gnarly surface. Limestone intervals of the couplets dominated when the volcanoes were quiescent.