Depositional Facies of a Mud Shoreface in Suriname, South America--A Mud Analogue to Sandy, Shallow-Marine Deposits

Abstract

ABSTRACT Immense volumes of argillaceous muds are being deposited along the world's longest continuous mud coastline which faces the open equatorial Atlantic. Not only are these muds deposited in an environment normally associated with sand deposits, but, just as in sandstones, sedimentary structures within these muds record the dynamic processes that deposited them. This study investigates a 100-km-long portion of the Suriname coast utilizing cores, bathymetric surveys, and field observations from nearshore and innershelf environments. Using terminology borrowed from sand coastlines, the innershelf/nearshore of Suriname is divided into transition zone, shoreface, foreshore, and backshore. The transition zone consists of a mix of offshore sands and shoreface muds. Shoreface and foreshore are subdivided into bank and interbank zones. These zones are transitory because mud banks, which make up the bank zones, migrate westwardly. Mud banks are up to 5 m high, 50 to 60 km long, 10 to 20 km wide, and oriented at oblique angles to the coastline. These banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped. Banks are attached to the shoreline and commonly produce progradation of the coastline. Substrate within the interbank zone is more consolidat d than within the mud banks. Because of lack of wave dampening, wave energies are relatively high within the interbank zone, producing erosion of the shoreface and shoreline. Sand beaches are generally formed within the erosional interbank shoreline. The vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present. Wave and current energies are recorded by scour-and-fill features, discontinuity features, micro cross-laminations, lenticular laminations and parallel to subparallel laminations. Laminations appear to be created by boundary-layer shearing within fluid muds which are above wave base and made mobile by wave-generated, shore-parallel currents. Large-scale evidence for migration of the mud banks is westward-dipping acoustical reflectors within the banks. These reflectors, which are over a kilometer apart, mark the boundaries of annual migration events. Biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces. Migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds, which were deposited by mud banks, and discontinuity features, which were formed by interbank zones. The coastal deposits of Suriname are a mud analogue to nearshore and innershelf sand deposits. Transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments. Both mud and sand shoreface/foreshore systems reflect sediment input and wave, current, and tidal processes that act on them. Mud banks of Suriname resemble linear sand ridges on the continental shelf of the eastern U.S.A., in shape, oblique orientation to the coastline, and orientation with respect to dominant direction of transport processes. The sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and Recent nearshore and shallow-marine sands.