A unique carbonate bed occurs within the dominantly siliciclastic Catskill Magnafacies (Upper Devonian) of southcentral New York. Associated strata comprise interbedded sandstones and mudstones with a variety of burrows, calcareous concretions, desiccation cracks, and remains of plants, fish, and the bivalve Archanodon . They are interpreted as the deposits of river channels, crevasse splays, levees, and floodbasins. The 0.4-0.5-m-thick carbonate bed is composed dominantly of dolomitic mudstone, containing calcispheres, rare ostracodes, and micron-scale algal filament molds. Laminae are disrupted by vertical and horizontal sheet cracks and burrow tubes which are filled with laminated mudstone, peloids and quartz silt, and spar-mosaic cements. The carbonate bed is interpreted as a freshwater-marsh/shallow-lake deposit, where mud-sized particles originated by precipitation around filaments within cyanobacterial (blue-green algal) mats. Laminae were deposited mainly from suspension and disrupted by burrowing organisms and periodic desiccation. The Everglades and interior of Andros Island provide modern analogues, the paleoclimatic and paleolatitudinal implications of which are consistent with previous reconstructions of Catskill deposition under a tropical climate, 20 degrees -30 degrees from the paleoequator. The carbonate bed probably represents local reduction in siliciclastic deposition rate (associated with river diversion) superimposed upon a regional, increasing deposition rate. There is no evidence of hypersalinity or marine influence.