AN UPPER ORDOVICIAN GEOLOGICAL ENIGMA: BEACHROCK BOULDERS STRANDED ON THE LOWER SHOREFACE OF AN EPEIRIC-PLATFORM ISLET

An unusual tempestite with bimodal texture (mud-sand:cobble-boulder) occurs in lower shoreface deposits from a carbonate beach in the Carters Ls (Sandbian) south of Nashville, TN (mile marker 38, east-bound lane, I-840). Upper shoreface and foreshore/backshore deposits indicate the tempestite formed off a small, low-energy, fetch-limited beach. However, the outcrop provides ample evidence of high-energy storm impacts on the shoreface. Tempestites with sandy to granular-pebbly lags are common in the lower shoreface deposits, and two medium-bedded, pebbly, storm-generated beds occur ~1/2 m above and below the boulder bed. Four subround-to-round gravel clasts (a cobble and three boulders), with diameters ranging from 10.5 to 50 cm, are the only visible beachrock clasts in the boulder-bearing tempestite. These were likely derived from beachrock deposits in the beach step (i.e. uppermost shoreface), which match the boulders’ facies. For a few meters shoreward of the boulders, the tempestite layer also contains granule and pebble clasts scoured from a mudstone-wackestone hardground in the substrate. No additional gravel is visible farther shoreward or seaward of the 15 m wide, gravel-bearing part of the layer. Additional fine gravel clasts may be present farther shoreward but unrecognizable as the 2-3^o E-dipping bed becomes inaccessible overhead in the hill-face outcrop. The bed’s substrate is a mud-capped tempestite. The two largest boulders are only partially covered by another 5-10⁺ cm thick, sandy, mud-capped tempestite. The beachrock clasts were eroded from the upper shoreface-foreshore and eventually transported to the lower shoreface, most likely during storm surge return flow (i.e. a storm-driven, downwelling current). Absence of an associated coarse-grained tempestite, fining-upward from a beachrock gravel lag, suggests a subsequent storm or storms removed the finer-grained material from the seabed, lagging only the largest clasts. This subsequent storm activity also explains the presence of smaller, non-beachrock gravels, derived from the lower shoreface hardground.