Upper Cambrian to Lower Ordovician conodont biostratigraphy and biofacies, Rabbitkettle Formation, District of Mackenzie

Abstract

Conodonts have been recovered from two sections through the Cambrian-Ordovician boundary beds of the upper Rabbitkettle Formation, near the headwaters of the Broken Skull River, western Mackenzie Mountains. The two sections are separated by a thrust fault. The sequence of trilobite faunas from the low-energy outer shelf facies of the Rabbitkettle is equivalent to the Saukiella junia through Symphysuhna brevispicata subzones from the inner carbonate platform in Oklahoma and Texas. Significant differences in trilobite faunas between the Mackenzie Mountains and United States sequences represent contrasting biofacies developments. Similarly, the sparse and low diversity conodont faunas of the Rabbitkettle resemble coeval Appalachian continental slope and outer shelf faunas rather than those reported from the inner carbonate platform in Texas and Utah. The Proconodontus and Cordylodus oklahomensis (new name) zones can be recognized in the Rabbitkettle but cannot be divided into the subzones established in Utah. These data suggest that lithofacies associations and biofacies developments in conodont distribution may prohibit detailed conodontbased correlations of Cambrian-Ordovician boundary beds.

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

“…However, the biostratigraphic data recovered so far from slope facies in that area (Landing et al 1980, Ludvigsen 1982 confirms that the FAD of E. notchpeakensis lies within the range of the Yukonaspis kindlei Fauna, the interval that is equivalent in age to the Saukiella junia Subzone in platform facies. Consequently, the species of such rapidly evolving deep-water trilobite genera such as Elkanaspis, Larifugula, and Kathleenella hold promise for constraining the base of the proposed Lawsonian Stage in northern Laurentian slope facies.…”

“…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).…”

“…Consequently, the position of the base of the proposed Lawsonian Stage is well established within a number of facies-specific successions of trilobite zones and subzones. Such integrated macro-and micro-fossil data are already available from sections deposited across the full environmental spectrum from inner shelf clastics (Runkel et al 2007), across the full breadth of the carbonate platform (Miller et al 1982Westrop et al 1981;Orndorff 1988;Taylor et al 1996Taylor et al , 2009, and into off-platform successions that accumulated in deep water on both the southern (Barnes 1988, Ludvigsen et al 1989 and northern (Landing et al 1980 margins of Laurentia.…”

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

“…2), a GSSP for the Lawsonian Stage (Stage 10) base could be defined on the basis of long-established, readily available, published evidence in a number of countries. For example, the E. notchpeakensis FAD is known in relatively accessible, fossiliferous, trilobite-bearing successions in North and northeast China , Chen 1986, Australia (Nicoll & Shergold 1991), the Maly Karatau in Kazakhstan (Dubinina 2000), and the western Cordillera of Canada and the western and south-central United States (e.g., Landing et al 1980;Taylor & Miller 1981;Westrop et al 1981;Miller et al 1982Miller et al , 2003Miller et al , 2006. Of these potential global reference sections for Stage 10, we propose two well described, fossiliferous, localities on the west Laurentian platform in the Great Basin of western Utah for the Stage 10/Lawsonian Stage GSSP and reference sections.…”

“…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.…”