Uppermost Cambrian slope deposits at Highgate Gorge, Vermont: a minor miscorrelation with major consequences for conodont- and trilobite-based chronocorrelation

Abstract

Uppermost Cambrian and lowest Ordovician slope deposits in Highgate Gorge, northwestern Vermont, yield a succession of conodont faunas (and a few associated trilobite species) similar to that observed in coeval North American carbonate-platform sequences. Decimeter-scale sampling of a 15-m-interval in two sections comprising thin-bedded limestone–shale rhythmites alternating with thick-bedded debris flow conglomerates yielded 60 trilobite specimens and more than 5,000 conodont elements from 48 productive horizons. The new biostratigraphic control does not support earlier claims that the lowest occurrence of Cordylodus proavus in the Gorge Formation and presumably in other slope sequences is significantly older than the base of the C. proavus Zone in platform deposits; rather, it demonstrates the isochronous persistence of this boundary across the North American (Laurentian) shelf margin into Iapetan slope deposits. The common occurrence of the deep, cool-water conodont Eoconodontus alisonae and the agnostoid trilobite Lotagnostus hedini in the Eoconodontus Zone at Highgate Gorge makes it possible to extend the correlation even farther from the Laurentian platform into uppermost Cambrian strata in Kazakhstan and China. This new information greatly strengthens arguments in favor of using this zonal boundary for defining the international boundary between the Cambrian and Ordovician Systems.In earlier studies of Highgate Gorge strata, composite treatment of biostratigraphic data from similar but non-correlative intervals (Zones 2 and 3) in two sections created an illusion of significant stratigraphic overlap of C. proavus with older faunas and direct association of some trilobite species for which overlap has never been established. Composite treatment of data from Zones 2 and 3 under designations such as “main zone’ or “upper zone’ should be discontinued and species that have been reported as occurring together in the “main zone’ should not be assumed (on the basis of that association alone) to have come from the same stratigraphic level.

Proposed stratotype for the base of the Lawsonian Stage (Cambrian Stage 10) at the First Appearance Datum of Eoconodontus notchpeakensis (Miller) in the House Range, Utah, USA

“…11 are based on strata having one or more of the taxa that are considered to be diagnostic of the Eoconodontus Zone, i.e., from the lower part of the proposed Lawsonian Stage: Cambrooistodus cambricus, C. minutus, Eoconodontus alisonae, Proconodontus serratus, or the occurrence together of P. muelleri and E. notchpeakensis. In Laurentia these faunas are known from slope facies in the USA in western Nevada , Maryland (Taylor et al 1996), New York (Landing 1993), and Vermont (Taylor et al 1991), in Yukon in western Canada (Tipnis et al 1979, Landing et al 1980, and from Newfoundland in eastern Canada (Barnes 1988). Faunas diagnostic of the Eoconodontus Zone are known in miogeoclinal carbonate platform deposits on Devon Island in arctic Canada (Nowlan 1985), North Greenland (Kurtz & Miller 1978), Alberta and Saskatchewan in western Canada (Derby et al 1972, Westrop et al 1981, Hein & Nowlan 1998, including data from drill cores, Godfrey Nowlan, written communication), Quebec in eastern Canada (Godfrey Nowlan, written communication, data from drill cores), in the USA in eastern Nevada , western Utah ; from mixed clastic-carbonate shelf deposits in Montana, Wyoming, and Colorado (Taylor & Miller, unpublished data;Myrow et al 1999Myrow et al , 2003; cratonic carbonates in Texas and New Mexico (Taylor & Repetski 1995); from carbonates deposited in the Oklahoma aulacogen (Miller et al 1982); from nearshore siliciclastic deposits in Iowa, Minnesota, and Wisconsin (Runkel et al 2007, including data from drill cores); from carbonate shelf deposits in Maryland and northern Virginia (Taylor et al 1992); and from outer-shelf deposits in New York (Landing, 1993).…”

Re-evaluation of the conodont Iapetognathus and implications for the base of the Ordovician System GSSP

“…1989), Kazakhstan (Apollonov et al. 1988), Russia (Pegel 2000) and the USA (Taylor et al. 1991) and its FAD is very close to that of C .…”

“…The agnostoid L. hedini has been recorded from China, (e.g. Lu & Lin 1984, Canada (Ludvigsen et al 1989), Kazakhstan (Apollonov et al 1988), Russia (Pegel 2000) and the USA (Taylor et al 1991) and its FAD is very close to that of C. andresi. The name C. andresi was introduced into literature as a new species, three times.…”

The Lawsonian Stage - the Eoconodontus notchpeakensis FAD and HERB carbon isotope excursion define a globally correlatable terminal Cambrian stage

“…Landing (in Landing et al 2007b, p. 51) re-interpreted purported "too old" specimens of C. proavus (i.e., Taylor et al 1991) from continental slope deposits of northwest Vermont as C. andresi Viira & Sergeeva (in Kaljo et al 1986) -a species that was not described when Landing's (1983) study of the Highgate gorge succession appeared -a study in which C. andresi was reported as C. proavus. Finally, new data indicate a second occurrence of C. andresi with a Hungaia magnifica fauna from slope deposits in northwestern Vermont (E. Landing, unpublished data, 2010).…”

“…In slope and marginal successions, the Eoconodontus notchpeakensis FAD lies at approximately comparable levels based on the resolution allowed by marginal trilobite-based correlations (i.e., Shergold & Geyer 2003) and the associated conodont succession. Thus, the E. notchpeakensis FAD occurs with trilobites of the upper Yukonaspis Zone and Keithia schucherti Fauna in western and eastern Laurentia (e.g., Landing et al 1980, Landing 1983, Taylor et al 1991 -trilobites which suggest a less highly resolved correlation with the Laurentian middle Saukia Zone (e.g., Westrop 1995), in strata correlated with the upper Quadratocephalus Zone in northeast China (Chen 1986), and in the Lophosaukia Zone in Kazakhstan (Apollonova et al 1988, Dubinina 2000.…”