Physical Controls on Carbonate Intraclasts: Modern Flat Pebbles From Great Salt Lake, Utah

In carbonate-forming environments, authigenic minerals can cement surface sediments into centimeter-sized intraclasts that are later reworked into “flat-pebble” or “edgewise” conglomerates. Flat-pebble conglomerates comprise only a small portion of facies in modern marine environments but are common in ancient strata, implying that seafloor cements were more widespread in the past. Flat-pebble conglomerates nearly disappeared after the Ordovician radiation, yet it is unclear if this decline was due to changing seawater chemistry or if increased infaunalization and bioturbation simply worked to break down nascent clasts. We discovered a process analog that produces flat-pebble conglomerates around the Great Salt Lake, Utah, USA, and studied these facies using field observations, wave models, satellite imagery, petrography, and microanalytic chemical data. Clasts were sourced from wave-rippled grainstone that cemented in situ in offshore environments. Lake floor cements formed under aragonite saturation states that are lower than modern marine settings, suggesting that physical processes are at least as important as chemical ones. Results from our wave models showed that coarse sediments near the field site experience quiescent periods of up to 6 months between suspension events, allowing isopachous cements to form. Using a simple mathematical framework, we show that the main difference between Great Salt Lake and modern, low-energy marine settings is that the latter has enough bioturbating organisms to break up clasts. Observations from Great Salt Lake demonstrate how geologic trends in flat-pebble abundance could largely reflect changes in total infaunal biomass and ecology without requiring regional-to-global changes in seawater chemistry.