Abstract: Depositional Environments of Yallahs Fan Delta, Southeastern Jamaica

William A. Wescott, Frank G. Ethridge

The Yallahs River drains an area of 163 sq km in the Blue Mountains of southeastern Jamaica. From its headwaters, at an elevation of 1,447.8 m, the river flows southeast for 37 km with an average gradient of 39.12 m/km. Where the river debouches from the mountain front it has built a 10.44-sq km lobate fan delta. The delta is modified significantly by high wave energy and dominant westward longshore drift. Major fluvial environments of the subaerial delta plain include: (1) braided channels characterized by longitudinal bars and consisting of poorly sorted gravel to boulder-size sediments showing pebble imbrication; (2) abandoned channels floored by a minimum of 51 cm of muddy sand; and (3) natural levees composed of silty sands characterized by ripples and ripple-drift c oss-stratification, burrows, roots, and small channel structures. Freshwater ponds ringed by encroaching mangrove swamps are developed locally along the western margins of the delta plain. Erosional beaches common along the eastern margins of the delta consist of coarse-grained gravel to boulder-size sediments seaward of a pronounced wave-cut scarp. These coarse-grained beaches are characterized by thick-bedded sedimentary units, seaward-dipping pebble imbrication, and, locally, by beach rock. Aggradational beaches along the central and western delta margins consist of sand- and pebble-size material and exhibit low-angle seaward-dipping laminations.

Except during flood periods on the Yallahs the river mouth is dammed by a wave-built berm of sand and gravel. The subaqueous delta drops off at an angle of 20 to 30° into the Yallahs basin. Sediment distribution is patchy and ranges from muds to coarse gravels.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma