Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation.
The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gravity processes resulting in patchy sedimentsize distribution. Active coral growth occurs along the western delta margin.
Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposits are characterized by gravelly, proximal, braidedstream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of tan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.