Uppermost Cambrian-lower Ordovician faunas and Laurentian platform sequence stratigraphy, Eastern New York, and Vermont

Journal of Paleontology, Jan 2003 by Landing, Ed, Westrop, Stephen R, Van Aller Hernick, Linda

ABSTRACT-The Cambrian-Ordovician boundary is a type I depositional sequence boundary with dramatic local erosional incision in restricted marine facies on the easternmost New York Promontory. The systemic boundary is bracketed below by Late Cambrian, upper Cordylodus proavus Zone (s.s). conodonts from carbonates of the upper Little Falls Formation (=Whitehall Formation, abandoned). Presumed Lower Ordovician ellesmeraceratoid cephalopods from the upper Little Falls are uppermost Cambrian and among the oldest known in North America. The overlying deepening-shoaling cycle of the Tribes Hill Formation (=Cutting and Great Meadows Formations, abandoned) is the local expression of a lowermost Ordovician (Rossodus manitouensis Zone) depositional sequence recognizable across Laurentia. Complete replacement of conodonts takes place in the late Tremadocian or Tremadocian-Arenigian boundary interval with onlap of the "Fort Ann Formation" across the paleokarst cap of the Tribes Hill. The trilobites Hystricurus sp. and Symphysurina myopia Westrop new species occur in less restricted, thrombolitic facies of the middle Tribes Hill that have the highest conodont diversity. Ulrichodina Furnish, 1938, emend. is regarded as the senior synonym of the conodont Colaptoconus Kennedy, 1994 (=Glyptoconus Kennedy, 1980).

DRAMATIC EUSTATIC EVENTS and fauna replacements occur through the Cambrian-Ordovician boundary interval. Alternative interpretations that include abrupt, multiple eustatic changes; a single eustatic fall-rise event; or progressive eustatic rise have been related to turnovers in latest Cambrian-earliest Ordovician trilobites and conodonts (see early reviews by Miller, 1984; Fortey, 1984; and Ludvigsen et al., 1987). Higher in the Lower Ordovician, nearly complete replacements of Laurentian conodonts and trilobites are locally associated with a stratigraphic break in the Tremadocian-Arenigian boundary interval (Ji and Barnes, 1993). The Laurentian platform sequence in the southern Lake Champlain lowlands, easternmost New York and adjacent Vermont (Fig. 1), is a succession that offers the opportunity to examine relationships between earliest Ordovician sea-level changes and biotic events. This region has a thin Cambrian-Lower Ordovician succession (e.g., Palmer, 1971) due to its location on the slowly subsiding, New York Promontory rift-margin of Laurentia (Thomas, 1977; Williams, 1978), and at the onset of our study we considered it likely that this passive margin recorded Early Ordovician eustatic changes. Recent work had shown that the Cambrian-Ordovician boundary is an unconformity at such western (i.e., inboard) locations on the New York Promontory as the Mohawk River valley. There, sub-Middle Ordovician (Black River and Trenton groups) unconformities on the lowest Ordovician Tribes Hill Formation have erased all record of later Early Ordovician sea-level and biotic changes (Westrop et al., 1993; Landing et al., 1996). This report deals with outboard locations in the southern Lake Champlain lowlands with more complete Lower Ordovician sequences in which sea level history may be reconstructed with the aid of conodont and macrofaunal biostratigraphy.

GEOLOGIC SETTING

Tectonic setting and stratigraphic nomenclature.-An apparently uniform Upper Cambrian-Lower Ordovician lithostratigraphy occurs in authochthonous sequences on the west side and south end of the Lake Champlain lowlands and in the parautochthonous Champlain slice in west-central Vermont (e.g., Fisher, 1984; Fig. 1). However, a confusing stratigraphic nomenclature obscures the regional extent of lithic units and the simple Early Paleozoic evolution of this stretch of the New York Promontory. This complexity reflects the proposal of synonymous units in New York and Vermont, usually without designation or description of type sections, detailed lithic characteristics, or upper or lower contacts. These contacts were often changed arbitrarily, and lateral correlations were commonly established by assertion rather than by biostratigraphic or lithostratigraphic analyses (Fig. 2).

The nomenclature for the autochthonous Upper CambrianLower Ordovician derives from Brainerd and Seely's (1890) spartanly-described subdivisions of the now largely covered East Shoreham section in the Champlain slice (Figs. 1, 2). Upper Cambrian-Lower Ordovician rocks at East Shoreham include the basal, quartz areni te-dominated Potsdam Formation and the more-or-less calcareous and dolomitic rocks of the "Calciferous Formation." The "Calciferous" is now subdivided into formations and members that are set off below by quotation marks when they are regarded as synonyms of Mohawk valley units or have not been adequately proposed or described.

Cambrian-Ordovician boundary and lowest Ordovician.-The Potsdam Formation is overlain by a mixed dolomitic sandstone and quartz arenite facies, typically called "Ticonderoga Formation" (J. Rodgers in Welby, 1961). The "Ticonderoga" is overlain by Rodger's (1937) carbonate-dominated "Whitehall Formation" in the autochthon (e.g., Fisher, 1984). The Cambrian-Ordovician boundary has been placed within the "Whitehall Formation" in recent publications. This was suggested by Flower's (1964, p. 157, 158) unillustrated report of purported Lower Ordovician fossils from two white limestone units that led him to propose a lowest Ordovician "Baldwin Corner Formation" and limit "Whitehall" to the Upper Cambrian, an undesirable restriction not followed in later work (Fig. 2). These "earliest Ordovician" fossils include Sinuopea, a gastropod first described from the Upper Cambrian (Ulrich, 1911), in the "Steves Farm Limestone" and closely septate ellesmeroceratoids, cephalopods now known from the Upper Cambrian (Xu and Lai, 1983), in the "Rathbunville School Limestone" (Fig. 2, see Steves Farm locality, below). Taylor and Halley (1974) later demonstrated that the traditional base of the Ordovician in Laurentia, the base of the Missisquoia Zone, occurred in the middle "Whitehall."