Giant sediment-wave field and supercritical-flow bedforms in a Mississippian carbonate ramp, USA

Over the past two decades, high-resolution bathymetry surveys have revealed abundant, large-scale sediment waves on modern marine slopes worldwide. However, few ancient analogs have been recognized, possibly owing to the seismic-scale outcrops required for their identification. Sediment waves are large-scale bedforms, with wavelengths ranging from 100s of meters to several kilometers, and amplitudes between a few to 10s of meters. This study documents a giant, carbonate sediment-wave field in outcrops of a Mississippian Fort Payne Formation in Tennessee and south-central Kentucky. Sediment waves are present in clinothem foresets and bottomsets of a distally steepened, carbonate ramp across an area more than 20,000 km2 in water depths estimated to have ranged between 100-150 m. Furthermore, there are large- and small-scale sedimentary structures with characteristics indicating deposition from Froude-supercritical flows.
Basin physiography, upwelling, density flows, and a prolific, heterozoan carbonate-factory, which produced prodigious amounts of crinoid skeletal grains in the clinothem foresets, were key factors in establishing and maintaining a sediment-wave depositional system. Steep slopes and deep water, which characterize many modern examples, were neither present nor required because of near-constant shedding of crinoid grains and basinward transport by sediment gravity flows and cascading density flows. Upper slope sediment-waves are 100-500 m wave length and include two types: (1) transverse-oriented shaley-crinoidal waves with abundant supercritical-flow bedforms and (2) sinuous to transverse-oriented mud waves crossed by furrows filled with crinoid rudstones interpreted as cyclic steps. Bottomset sediment waves include (1) dune-like bedforms with wave lengths and heights of ~50 m and ~10 m, respectively, and (2) compound sediment waves up to 700 m long and 50 m high. They exhibit upslope and downslope asymmetry. Supercritical-flow bedforms are present in troughs and lee sides. Some sediment waves tend to “clean” and coarsen-upward from shaley carbonates near the base to carbonate with thin shale breaks towards the top.
The presence of sediment waves in this ancient and well-known carbonate ramp, although surprising, cannot be unique. Their discovery should stimulate re-examination of other ancient carbonate slope and basin-floor systems.