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The Tully Limestone is an anomalous unit which interrupts the thick sequence of detrital rocks constituting the Catskill Delta. The Tully disconformably overlies gray shale, grades upward into black shale, pinches out to the west, and grades eastward into a thicker sequence of sandstone and siltstone. Intensive stratigraphic work has delineated several key beds which afford detailed correlation within the formation.

Several facies are recognized. Detrital facies include (1) laminated muddy quartz siltstone, with restricted fauna, which records salinity variation and rapid deposition from an active clastic source, and (2) burrowed quartz sandstone, with more diverse fauna, which records slower deposition under more marine conditions with a stabilized clastic source. Carbonate facies indicate protection from clastic influx: (3) abraded calcarenite, containing various abraded and residual grain types, records slow deposition with long-term winnowing and grain battering; (4) chamosite oölite, with evenly coated grains and phosphorite pebbles, records long-term agitation and slow deposition; (5) bedded skeletal calcilutite, forming the greatest part of the Tully, records a quiet marine environment under long-term protection from clastic influx; (6) dark, barren shaly calcilutite records fouling of substrate and increase in fine clastic influx. Three facies are associated with calcilutite mounds: (7a) pure massive mound calcilutite, with smooth-bottomed sparry “stromatactis” and a distinct slender-tabulate-coral assemblage, records rapid local accumulation of carbonate mud; (7b) sparsely fossiliferous, bedded backmound calcilutite, partly surrounded and overridden by the mound, records a quiet, perhaps lagoonal environment on one side of the elongate mound; (7c) encrinite records proliferation of pelmatozoans along the opposite side of the mound.

Deposition of the Tully Limestone was controlled by contemporaneous structures at its east end where an anticline, followed by a down-to-the-east fault (or monocline), formed a clastic trap to the east and acted as barriers to the major westward spread of Catskill Delta clastics. After an initial period of submarine nondeposition, the western protected area became a site of lime-mud accumulation. The pure calcilutite mounds (7) at one of the northernmost New York exposures indicate a northern source of lime mud, probably in a shoal abounding with carbonate-secreting organisms along the west side of the nonorogenic Adirondack Dome, also protected by the clastic trap. Supporting this source direction is the southward trend of decreasing purity of the limestone through Pennsylvania, where the Tully eventually becomes calcareous shale.

An erosional unconformity divides the Tully in New York into two distinct members (Lower and Upper) which underwent slightly different histories. The Lower Member developed west of a rising anticline, which blocked major eastern clastic influx but shed limited sediments west, resulting in alternate deposition of laminated muddy siltstone (1) when active, and burrowed sandstone (2) when stabilized, in the east end of the member. The sandstones grade westward into skeletal calcilutite (5). With continued rise, the anticline was breached, and erosion truncated the east end of the Lower Member. A down-to-the-east fault in the east flank of the anticline maintained the clastic trap but caused the anticline and western platform to act subsequently as a unit. Calcarenites (3) mark the unconformity on the Lower Tully and form the basal bed of the Upper Member where it transgressed in the core of the breached anticline. Upper Tully calcilutite (5) spread evenly over the eroded Lower Tully, thickening over older rocks in the breached anticline core and thinning eastward onto the Chenango Valley shoal, the upthrown side of the fault where chamosite oölite (4) formed. Later deepening and bottom fouling caused Tully Limestone deposition along outcrop to end with barren shaly calcilutite (6), marking the upward transition into black shale. The protecting structures lost effect at greater depths, and clastics ultimately spread westward again, overwhelming any remaining carbonate deposition in this region and resuming normal development of the Catskill Delta.

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