[ABSTRACT ONLY. DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] Sandy Bank is 
located at the extreme western edge of the Florida Bay facing the open shelf of the Gulf 
of Mexico. The study focuses on the key features of mudbank development based on the 
nature of sediments, internal framework of sedimentary units, distribution of floral 
and faunal communities and 14 C dates. The sediments were primarily carbonates (90- 
98%) with siliciclastic components constituting 2 - 10% of the sediment. The sediment 
accumulated on a subaerially weathered, fairly irregular Pleistocene bedrock surface 
with a raised seaward margin. Holocene sediments formed a shallow submerged bank 
rising 1 - 2.5 m above the rocky submarine floor. The bank profile was gently 
asymmetric with a flat interior becoming steeper seaward. The entire bank surface 
was carpeted with marine grass. The central part, which is nearly exposed at low tide, 
was colonized by Thalassia only. A mixed grass community of Thalassia and 
Syringodium formed the bank edges with progressive decline in Thalassia at depth. The 
sediments were a mixture of mud (<0.062 mm) and shells (>0.062) with the proportion 
of mud in the sediments ranging from 10-98%. Mollusks were the chief skeletal grains, 
and green algae, sponge spicules, foraminifera, radiolari, etc., made up a maximum of 
5% of the total shell fraction. Mudstones, wackestone and packstone were the principal 
sediment types, representing approximately 45%, 40% and 15% of the bank thickness. 
On the bank surface, mudstone was restricted to the central Thalassia Zone and 
Wackestone was associated with the mixed grass bottom community. Often mudstones 
exhibited weakly defined, very fine lamination. Animal burrows, plant roots rhizomes 
were common organic structures in the muddy bank sediment. Mollusks defined three 
distinct surface zones of brackish (Northern), marine (Gulf), and mixed (Transitional) 
subenvironments. The mosaic of surface environments could be recognized throughout 
the upper half to two thirds of the bank's internal stratigraphy. Sediment cores 
revealed an ascending sequence of dark brown to black peat; pale-orange 
Pseudocyrena-Batillaria mudstone with interlaminated peat; Pseudocyrena- 
Anomalocardia packstone; mangrove and/or Thalassia (rare) mudstone; 
Anomalocardia-Transennella packstone; and a highly bioturbated wackestone and/or 
mudstone with frequent interbedded, thick (2-4 cm) Transennella packstones. The 
internal stratigraphy indicated a progressive vertical change in the depositional 
environment from a semi-restricted coastal swamp through open brackish Bay to the 
recent nearly marine condition. 14 C dates on basal peat; Pseudocyrena-Anomalocardia 
packstone; Anomalocardia-Transennella packstone; and a Transennella packstone in the 
top most unit yielded average ages of 5,213, 4,290, 2,690 and 1,190 yrs 
respectively. Layer cake stacking of 14 C dated units and their lateral persistence 
strongly suggested accumulation of carbonate sediment over the entire mudbank area 
throughout the Holocene. Ubiquitous Thalassia free packstone underlying root and 
rhizome bearing mudstone signifies a temporary killing of grass by the sudden spread of 
coarse shell debris on the bank due to a temporary rise in the sea level during an 
exceptionally intense physical regime. The nature, thickness and site of the mudstones 
appear to have been influenced by the post-agitation rate of settling of suspended fines 
from the water column, the vigor of recolonization and the density of Thalassia on the 
newly established substrate. The overall sediment accumulation rate on the bank varied 
between 27 - 75 cm per 1,000 yrs. However, the central and thickest bank sequence 
reflects a systematic increase in sedimentation rate from the base to the top. The basal 
peat marked the slowest accretion rate of 13 cm per 1,000 yrs and the brackish water 
mangrove- Thalassia mudstone showed a growth rate of 31 cm per 1,000 yrs. This 
increase was attributed to trapping of fine sediment by mangrove roots and sparse 
Thalassia community. The uppermost mudstone, wackestone intercalated units had the 
maximum growth (65-88 cm per 1,000 yrs) due to an intense growth of grass on the 
bank under nearly marine conditions. It is suggested that the Pleistocene bedrock played 
a major role in the initial trapping of fine sediments in protected areas behind the 
weakly raised seaward edge. Later, the spread of marine grass influenced the bank 
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