CHAPTER 7: TULLY FORMATION AND PRE-FRASNIAN GENESEE GROUP SUCCESSION

The carbonate-dominated Tully Formation and coeval nearshore siliciclastic Gilboa Formation are the type expression of the North American provincial Taghanic Stage (Polygnathus ansatus-“Ozarkina” semialternans zones), recording the multi-phased Global Taghanic Biocrisis. The Tully-Gilboa interval is overlain by basin-filling, progradational siliciclastic deposits of Acadian Tectophase 4 represented by the Geneseo Formation-Lodi Member succession (Genesee Group). This lower, pre-Frasnian, part of the Genesee Group, corresponds to the sequential hermanni, disparilis, and norrisi conodont zones.

The Tully Limestone in central New York, anomalously composed of clean, micritic limestone, contrasts greatly with underlying Hamilton siliciclastic facies and overlying Geneseo basinal black shale deposits. Taghanic biotas are characterized by appearance of the distinctive Old World Tully fauna and demise of the long-ranging, endemic Hamilton Fauna during an episode of warming and transgression. An apparent cooling event in mid-Tully time permitted a brief, dramatic return of the Hamilton Fauna, followed by stepwise marine deepening, which initiated the classic Taghanic Onlap event during late Tully deposition. This widespread drowning event terminated the Hamilton Biota, allowing for gradual appearance of the lower diversity Genesee Fauna in an evolving foreland basin. Tully shelf deposition in central New York was particularly influenced by carbonate influxes from northward cratonic sources and syndepositional cratonic flexural events, particularly in mid-Tully time. Post-Tully Geneseo, deposition was dominated by widespread transgressive overspread of basinal anoxia associated with flexural foreland basin development as shown by diachronous westward onlap of basal Geneseo black muds as zonally indicated by the pyrite-dominated Leicester lag deposit flooring the Geneseo.

This interval is divided into 3^rd and 4^th-order T-R cycles: third-order Sequence Giv-4, corresponds to the uppermost Moscow Formation and lower Tully Formation, and Giv-5, corresponds to the upper two thirds of the Tully Formation and the entire Geneseo Formation. The diachronous base of Sequence Giv-5 is a widespread erosive surface widely identified as the “Taghanic Unconformity.”