Abstract

The Lower Ordovician middle Beekmantown Group is a very thin carbonate platform succession on the northern New York Promontory that thickens north into the Ottawa aulacogen near Montréal. The Tribes Hill Formation (Rossodus manitouensis Zone) records the earliest Ordovician (late Skullrockian, late early Tremadocian) eustatic high that submerged Laurentia, and produced the lowest Ordovician sequence along the New York Promontory. These dolostones are succeeded in the Beekmantown, New York, area by late Tulean?–Blackhillsian transgressive systems tract quartz arenites of the lower Fort Cassin Formation (Ward Member). The “Fort Ann Formation” (middle Stairsian, upper Tremadocian) of the southern Lake Champlain lowlands (=Theresa Formation sandstones in the Ottawa graben) is absent at Beekmantown, and moderate Stairsian (late Tremadocian) eustatic rise apparently did not inundate the Beekmantown area after Skullrockian–Stairsian boundary interval offlap. Highstand carbonates of the upper Fort Cassin Formation [Sciota Member = “Spellman Formation” and “Ogdensburg Member” of the “Beauharnois Formation” in the Montréal area; designations abandoned] at Beekmantown yield diverse conodonts seemingly characteristic of the Oepikodus communisFahraeusodus marathonensis Zone (new). However, associated trilobites, particularly Carolinites tasmanensis (Etheridge, 1919), show a correlation with the upper Trigonocerca typica (trilobite) Zone of the Utah and the overlying Reutterodus andinus (conodont) Zone. This abrupt early Blackhillsian lithofacies change features the appearance of chitinozoans and conodonts known from marginal successions, and records the Laignet Point highstand (new). This highstand is recognized across Laurentia on the west Newfoundland and southern Midcontinent platforms. It is recorded on the east Laurentian continental slope by lower Oepikodus evae Zone dysoxic black mudstone in the Taconian allochthons. Taxonomic re-evaluations include UlrichodinaBranson and Mehl, 1933, with its genotype species U. abnormalis (Branson and Mehl, 1933) emend., as the senior synonym of ColaptoconusKennedy, 1994; EucharodusKennedy, 1980; and GlyptoconusKennedy, 1980. Paraserratognathus An inAn et al., 1983, emend. is the senior synonym of WandeliaSmith, 1991 and StultodontusJi and Barnes, 1994. TropodusKennedy, 1980 is the senior synonym of ChionoconusSmith, 1991. The trilobite fauna of the Sciota Member includes species of Isoteloides, Benthamaspis, Acidiphorus and Carolinites, of which I. fisheri is new.

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