Slope Sediment-Waves, Gullies, and Cyclic Steps of Great Bahama Bank and Mississippian Ramp Carbonates—Modern-Ancient Analogues for Supercritical-Flow Deposits?

Sediment waves, gullies, cyclic steps, and upstream-migrating sandy bedforms are common on modern slopes worldwide. Studies have proposed that many of them were deposited from Froude-supercritical sediment gravity flows and density cascades. However, few examples of these bedforms have been reported from ancient rocks, especially carbonates. Here we compare modern cyclic steps and gullies from Great Bahama and Mississippian examples from the Fort Payne Formation in Tennessee.

Fort Payne Formation lower-middle slope facies are mixed shale-carbonates. Roadcuts expose muddy (calcareous shale) sediment-waves and channel-filling crinoid rudstones deposited by alternating Fr-subcritical-supercritical flows. Sediment waves are 150 m long and 7–10 m high, asymmetric-upslope with stoss-side dips of 10–11° and lee-side dips of 8°. Rudstone-filled gullies are 35–90 m wide, 3–7 m thick, sharp-based but do not incise underlying shale. Sections parallel to paleoflow indicate that rudstones are upslope-migrating, sigmoidal bodies 80 m long, 1–4 m thick with convex-upward backset beds. Their 3D architecture and scale are similar to upstream-migrating sandy bedforms in submarine channels (Englert et al. 2021).

Along the slope of Great Bahama Bank, a hyperpycnal offbank flow with estimated velocities of 0.5–2.5 m/s entrains inner platform sediment. On the lower slope, cyclic steps 150–200 m wide and up to 3 m high cover the seafloor. The stoss sides and internal stratification dip 5–7° upslope. Downslope dips are 3–4°. Mean grain size of the cyclic step sediment is <250 μm and grain density of the pure carbonates is 2g cm-3.