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Recent investigation of the Tully Limestone has resulted in the recognition of several general facies. Certain key beds appear to be relatively synchronous units that permit environmental reconstruction of several phases of deposition.
In general, Tully deposition occurred in a carbonate mud-depositing sea with an eastern clastic source that exerted decreasing influence through time.
The lower part of the Tully Limestone consists of arenaceous deltaic beds of limited areal extent. On the east, abundant Chonetes aurora and occasional Leptostrophia represent the restricted fauna of the brackish nearshore environment. Westward, the addition of other brachiopods, notably Hypothyridina, Atrypa, and Schizophoria, characterizes the more marine facies.
Above the lower part of the formation, sandy limestone overlies the beveled eastern end of the delta and grades eastward into an iron-rich oolite which contains pebbles of dark siltstone. Deposition was extended also to the west, where purer carbonate mudstone is developed.
The upper part of the Tully Limestone consists of carbonate mudstone characterized by the presence of metriophyllid and auloporid corals, styliolines, trilobites, and pelmatozoans. Toward the middle of this part, and representing normal marine conditions, is a diverse fauna developed to the east in dark calcareous shale and to the west in limestone.
Erosion surfaces with burrows, channels, local carbonate-pebble conglomerates and filled mud cracks are evidence for the intermittent nature of carbonate mud deposition and are observed especially in the central and western regions.
In the upper part of the formation near the town of Borodino, two elongate mounds of pure carbonate mudstone, about 15 feet high, contain structures similar to stromatactis and grade laterally into much thinner pelmatozoan calcarenites. These mounds are overlain by similar calcarenites which locally contain pebbles of carbonate mudstone. Whereas the lower mound contains a network of auloporid corals that could have been responsible for its development, the upper mound lacks auloporids. The position of the upper mound on the western flank, and quite possibly in the lee, of the lower mound suggests a current-controlled origin.
Tully deposition was ended by the reducing environment of the Geneseo Black Shale, which encroached progressively from the west. Local lingering of the carbonate environment is shown by dark argillaceous limestone at the top of the section which contain only scattered remnants of the Tully fauna.
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