ABSTRACT

Bluefish Channel, north of Key West, Florida, is a prominent ebb channel incised into a thick Recent carbonate bank that overlies Pleistocene limestone. The dominant sediments are Halimeda, foraminifera, molluscs and fragments derived from the Pleistocene bedrock. In as much as the source area lies along the entire course of the channel, sediment mixing results in textural inversions (abnormal size-to-roundness relations) and characteristically negatively-skewed channel sands.

A plot of sorting versus grain size of various energy-level sediments studied, reveals a sinusoidal trend similar to that of Folk and Robles (1964) for the beach environment. The lower deflections of the sinusoidal curve coincide with the best sorted sediments of the study area. With increased sorting and abrasion, the constituents tend to be distributed proportionally in the sediment. The best sorted sediment is mature medium calcarenite, a mixture of four constituents in almost equal proportions.

Some calcarenites deposited at the tidal delta, adjacent to the channel, show the initial stages of penecontemporaneous, submarine cementorion by chemically precipitated aragonite crystals. This process of cementation is qualitatively similar to the formation of Bahamian grapestone as described by Illing (1954).

Hydrodynamically, the channel is characterized by the upper-flow regime. The most conspicuous bed forms are straight-crested megaripples at the channel mouth and adjacent parts of the rapidly growing tidal delta. Here beds of cohesive carbonate muds are exposed to submarine erosion, producing armored mud balls and intraformational flat-pebble conglomerates, identical with some of those interpreted as a product of supratidal environment.