SEDIMENTATION IN A TROPICAL, MICROTIDAL, WAVE-DOMINATED COASTAL-PLAINESTUARY

The Mono estuary is an infilled, microtidal estuary located on the wav e-dominated Eight of Benin coast which is subject to very strong eastw ard longshore drift. The estuarine fill comprises a thick unit of lago onal mud deposited in a 'central basin' between upland fluvial deposit s and estuary-mouth wave-tide deposits. This lagoonal fill is capped b y organic-rich tidal flat mud. In addition to tidal flat mud, the supe rficial facies overlying the 'central basin' fill include remnants of spits resting on transgressive/washover sand, an estuary-mouth associa tion of beach, shoreface, flood-tidal delta and tidal inlet deposits, and a thin sheet of fluvial sediments deposited over tidal flat mud. A fter an initial phase of spit intrusion over the infilled central basi n east of the present Mono channel, the whole estuary mouth became bou nded by a regressive barrier formed from sand supplied by the Volta De lta during the middle Holocene eustatic highstand. Barrier progradatio n ceased late in the Holocene following the establishment of an equili brium plan-form shoreline alignment that allowed through-drift of Volt a sand to sediment sinks further downdrift. Over the same period, accr etion, from fluvially supplied sediments, of the estuarine plain close to the limit of spring high tides, or, over much of the lower valley, into a fluvial plain no longer subject to tidal flooding, induced mar ked meandering of the Mono and its tidal distributaries in response to confinement of much of the tidal prism to these channels. The process resulted in erosion of spit/washover and regressive barrier sand, and in reworking of the tidal flat and floodbasin deposits. The strong lo ngshore drift, equilibrium shoreline alignment and the year-round pers istence of a tidal inlet maintained by discharge from the Mono and fro m Lake Aheme have resulted in a stationary barrier that is reworked by a mobile inlet. The Mono example shows that advanced estuarine infill may result in considerable facies reworking, obliteration of certain facies and marked spatial imbrication of fluvial, estuarine and wave-t ide-deposited facies, and confirms patterns of sedimentary change desc ribed for microtidal estuaries on wave-influenced coasts. In addition, this study shows that local environmental factors such as sediment su pply relative to limited accommodation space, and strong longshore dri ft, which may preclude accumulation of sediments in the vicinity of th e estuary mouth, may lead to infilled equilibrium or near-equilibrium estuaries that will not necessarily evolve into deltas.