Abstract: Lime Mud Deposition in Enclosed Lagoon, Bight of Abaco, Bahamas
Mark Boardman, A. C. Neumann
Three cores characteristic of the muddy carbonate Holocene sediment blanket in the Bight of Abaco, Bahamas, provide new insights into tropical-lagoon deposition. Factors controlling deposition include preexisting topography, sea-level changes, and physical energy from both normal waves and storms which appear to produce recognizably distinct products of winnowing.
The earliest sediment is a thin eolian soil of lateritic clay minerals and quartz followed by brackish-schizohaline carbonate sediments and finally normal marine carbonate sediments. The transitions from one environment to the next are related directly to the Holocene rise of sea level and to preexisting topography. Soil formation continued until sea level had risen to the bottom of the basin (about 9.4 m below sea level at 6100 years B.P.) when marine waters infiltrated through the limestone basement and created schizohaline conditions controlled by seasonal rains and evaporation. Schizohaline conditions prevailed until sea level had risen above the basin sill (2.8 m below present sea level at 3640 years B.P.) and stable marine conditions began.
Mechanisms of sediment winnowing are storm waves, currents, and low-energy wind waves and are reflected in the size distribution of the mud fraction. Export of fine-grained carbonate material from the basin became possible only after sea level rose above a rock sill and stable marine conditions began. Thus, schizohaline sediments retain the original texture and composition of naturally produced products of skeletal breakdown, as seen by a high percentage of clay-sized particles.
Available sources of mud are skeletal breakdown and organic and inorganic precipitation. Examination of the coarse-silt fraction (62 to 16µ) by SEM reveals that all of the coarse silt is biogenic and that foraminiferal fragments account for most of the low- and high-magnesium calcite. Origin of the aragonitic coarse silt is not apparent because of a lack of distinctive characteristics, but algal breakdown is likely. Inorganic precipitation is evident only at the base of one core and is probably due to the initial interaction of marine and brackish waters.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC