THE KONNAROCK SNOWBALL FIGHT: USING IMAGE ANALYSIS, RHYTHMITE THICKNESS, AND FACIES ASSOCIATION TO DETERMINE THE ORIGIN OF THE NEOPROTEROZOIC KONNAROCK FORMATION

The Neoproterozoic Konnarock Formation is a sedimentary formation exposed in the Blue Ridge of SW Virginia interpreted to have been deposited during Snowball Earth. Despite continued debate about the depositional environment, cyclic processes deposited the rhythmic layered sediments at the base of the Konnarock Fm. We test the hypothesis that the rhythmites represent varves that were deposited in a glacial lake. Field mapping indicates that there are at least 2 different rhythmite facies in the lower Konnarock Fm—a lower, fine-grained, less regularly bedded facies, and an upper, coarser grained, thinly laminated facies. Thin section micromorphology of the rhythmites show couplets of quartz silt or fine sand gradually fining up to mud. The contact between couplets in the finer facies is sometimes sharp and well defined, but boundaries between some couplets are less clear. Contacts between couplets in the coarser facies are consistently well defined and isolated coarse clasts are draped by mud. The micromorphology resembles varves in the coarser facies (sharp lower boundaries with dropstones). The finer facies may also be varves and are affected by turbidity currents (irregular bed thicknesses, gradual fining-up, paucity of dropstones). We also use image analysis (in MATLAB and Photoshop) to determine rhythmite thickness in each facies. The finer facies varies in couplet thickness from 1-14mm. The coarser facies contains more uniform couplets with an average couplet thickness of 2.2 mm, which is consistent with modern glacial varves (e.g., Nicolay Lake in Nunavut, Canada). Through petrographic analysis and analysis of rhythmite thicknesses in both facies, we infer that the Konnarock Fm shows a proglacial sequence and that the formation becomes more glacially influenced upsection (as couplet thickness becomes more uniform, dropstones increase, etc.).