COASTAL GEOLOGY OF BLUEFIELDS BAY, WESTMORELAND PARISH, JAMAICA: SUCCESSIONS OF NON-MARINE TO MARGINAL-MARINE DEBRIS FLOWS IN AN ACTIVE TECTONIC SETTING

Recognizing connections between surficial processes, geomorphology, structural features, and ancient deposits helps elucidate our understanding of the complex geologic history of southwestern Jamaica. Bluefields Bay, located in eastern Westmoreland Parish between Bluff Point and Belmont Point, is flanked to the northeast by Bluefields Mountain, which rises 800 m above sea level, trending northwest-southeast. Narrow subparallel linear valleys stretch southeastward, separating Bluefields Mountain from the low hills near Belmont to the south. A fault has been mapped in one valley but numerous lineations support the idea that multiple faults and sets of fractures or faults are oriented at acute angles and perpendicular to the northwest-southeast trend. There are only two mapped geologic units near Bluefields Bay; the Eocene–Miocene White Limestone Group crops out in the mountains and hills, and Quaternary alluvium fills valleys. The Coastal Group (mid-Miocene–Pleistocene) has never been described from Bluefields Bay, however, outcrops described here should be assigned to that group.

Cliff exposures along the west-central part of Bluefields Bay consist of at least a ten-meter-thick succession of clast-supported carbonate conglomerate and breccia. Meter-scale beds are inversely graded with erosional bases. Fine-grained limestone clasts dominantly vary from 2–10 cm in maximum dimension, are sub-angular to sub-rounded, and were derived from the White Limestone Group. Fine-grained interstitial matrix partly to completely fills some flows, alternating with layers characterized by open porosity. We interpret this succession as a series of subaerial debris flows, where a few layers exhibit internal, sieve sedimentation.

Near Belmont Point, a series of flank margin caves and undercut erosional remnants have formed in a 5-m-thick, yellow-weathering debris-flow unit that includes marine fauna and burrows. None of the exposures has been deformed, indicating a relatively recent age. A modern analog for debris-flow deposition occurred in 1979, when a debris field and marine fan-delta formed at the mouth of Bluefields River. The occurrences of ancient and modern sediment-gravity flows illustrate the long-standing capacity for generating such deposits in a tropical setting and tectonically active area.