Calcareous, quartzose and organic-rich (peaty) sediments accumulating in two modern mangrove estuarine swamps, the Ten Thousand Islands and the Whitewater Bay areas of southwestern Florida, were studied to derive criteria that would aid in recognizing similar ancient coastal swamp deposits. Both areas are at the seaward margin of the Everglades and are associated with an imposing coastal mangrove forest. The sediments forming in these two coastal swamps are modern analogues of ancient limestone, quartzite, and coal beds, and the coastal mangrove forest is the modern counterpart of ancient coal forests of cratonic and miogeosynclinal areas.

Evaluation of results yields three principal criteria which may be useful in distinguishing ancient coastal swamp deposits.

1. Organic matter and carbon/nitrogen ratio:
The content of organic matter and the ratio of organic carbon to nitrogen in surface sediments (upper 0.25-0.50 foot) along the southwestern Florida coast increases toward land on the open marine shelf and attain their highest values (organic matter ranging from 6 to more than 50 per cent and C/N ratio ranging from 17 to 37 per cent) within the coastal swamps. The increase in organic matter and C/N ratio in a shoreward direction is due to a progressive increase in the amount of organic matter contributed to surface sediments by terrestrial plants (mangrove trees) compared with that supplied by plankton and benthos. Fossil coastal swamp deposits, therefore, may be recognized by their exceptionally high organic matter content and C/N ratio with respect to laterally adjacent sediments.

2. Faunal assemblages:
The areal distribution of molluscan faunal assemblages in the vicinity of the coastal swamps of southwestern Florida suggests that biofacies of ancient coastal swamp deposits would trend parallel with the coastline and delineate a direction of decreasing average water salinity toward the land area supplying run-off waters to the coastal swamps.

3. Grain size of carbonate and detrital minerals:
Calcareous particles and detrital quartz along the southwestern Florida coast decrease in grain size from the open marine shelf toward and into the estuarine mangrove swamps. Hence, with support by criteria 1 ind 2, fossil coastal swamp sediments may be differentiated by their tendency to be noticeably finer-grained than adjacent sediments.

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