Sedimentation and Diagenesis in Deep Forereef, British Honduras Barrier and Atoll Reefs: ABSTRACT

Abstract

The top of a near-vertical escarpment at 70 m marks the change from luxuriant organic growth that typifies the shallow forereef, to physical and chemical sedimentation that characterizes the deep forereef. Off the southern barrier reef and the leeward side of Glovers Atoll, this escarpment extends to 120 m; on the seaward side of Glovers Atoll, it continues to abyssal depths. Where the base of the escarpment is at 120 m, it is buried by the forereef rise, a wedge of talus and sediment whose slope flattens basinward. The escarpment has numerous projecting ledges, caves up to several meters deep, and occasional near-vertical fissures tens of meters high. Sediments from above mantle and infill all irregularities on the escarpment; sticks and plates of coral are stacked on projecting ledges; algal-plate (Halimeda) sand and lime mud cloak ledges and floor caves and small cavities. Rock samples from the outer meter of the escarpment, between 95 and 110 m deep, are well-cemented mixtures of algal-plate (Halimeda) sand, lime mud, whole and fragmented reef corals, and crustose, coralline algae. Specimens show multiple generations of sponge and mollusk borings, sediment infill, and cementation by magnesium calcite and aragonite that almost obliterates many primary depositional fabrics. Two specimens of unaltered reef corals from this rock have radiocarbon ages of 2,340 and 7,835 years B.P. On the barrier reef the upper forereef rise at the base of the escarpment is a talus slope of blocks, surrounded by algal-plate (Halimeda) sand and lime mud. Talus blocks as large as 20 m clearly are derived from the escarpment; all stages of formation, from jointed rock face to large blocks of limestone slightly displaced from the wall, are visible. Within 100 m of the base of the escarpment, the blocks of talus decrease abruptly and algal-plate (Halimeda) sand is increasingly diluted by pelagic muds. End_of_Article - Last_Page 781------------