Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region

Abstract

High-resolution sedimentologic, biostratigraphic, and stable isotope data from numerous measured sections across Colorado reveal a complex architecture for lower Paleozoic strata in the central Cordilleran region. A lack of precise age control in previous studies had resulted in misidentification and miscorrelation of units between separate ranges. Corrections of these errors made possible by our improved data set indicate the following depositional history. The quartz-rich sandstone of the Sawatch Formation was deposited during onlap of the Precambrian erosion surface in the early Late Cambrian. The overlying Dotsero Formation, a regionally extensive carbonateand shale-rich succession records blanketlike deposition with only minor facies changes across the state. An extremely widespread, meter-scale stromatolite bed, the Clinetop Bed, caps the Dotsero Formation in most areas. However, a latest Cambrian erosional episode removed 9-11 m of the upper Dotsero Formation, including the Clinetop Bed, from just east of the Homestake shear zone in the Sawatch Range eastward to the Mosquito Range.

Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block

“…The facies are identical to those of the Cambrian inner detrital belt of Laurentia, which was situated between a cratonic hinterland and a more distal carbonate belt (Lochman-Balk, 1971;Li and Droser, 1997;Myrow et al, 2003Myrow et al, , 2004Myrow et al, , 2012Runkel et al, 2012). In this setting, there was interplay between in situ carbonate sediment production and temporal changes in fi ne-grained detrital mud input linked to unsteady runoff and sediment transport (Myrow et al, 2012).…”

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

“…In central Texas a glauconite greensand occurs at this level in the San Saba Limestone Member of the Wilberns Formation. A relative rise of sea level at the base of Sequence 5B is recorded by a thin stromatolite biostrome at Lawson Cove, and in western Colorado a widespread stromatolite unit, the Clinetop Bed, occurs at this level in the Manitou Formation (Myrow et al 2003). In the Black Mountain section, Australia, an eolian sandstone, the Lily Creek Member of the Chatsworth Limestone, occurs at this level (Nicoll & Shergold 1991).…”

“…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). Outside Laurentia, faunas of the Eoconodontus Zone are known in Argentina (Heredia 1999), Australia (Nicoll & Shergold 1991), Estonia (Kaljo et al 1986, Viira et al 1987, Iran (Müller 1973, Ghaderi et al 2008, Kazakhstan (Apollonov et al 1988, Popov & Tolmacheva 1995, Dubinina 2001, Korea (Lee 2002, Lee et al 2009), Siberia (Kanygin et al 1989…”

Carbon isotope composition of Upper Cambrian to Lower Ordovician carbonate in Korea, and its bearing on the Cambrian–Ordovician boundary and Lower Ordovician paleoceanography

“…However, at present it is not certain that the small scale bumps reflect global carbon isotope values because small scale bumps in Korea and Argentina can be explained by contributions of allochems to the overall whole rock signature and by local variation in eperic sea (Holmden et al, 1998;Immenhauser et al, 2003). Moreover, the small scale bumps are not observed at other sections of the same age (Myrow et al, 2003). The above comparison indicates that Lower Ordovician d 13 C curve from the Mungok Formation matches with that of Buggisch et al (2003) that was obtained from a sedimentary basin located in a paleogeographically quite different position.…”

“…which commonly include characteristic negative or positive excursions of carbon isotope values (Marshall and Middleton, 1990;Wadleigh and Veizer, 1992;Qing and Veizer, 1994;Carden, 1995;Gao et al, 1996;Ludvigson et al, 1996;Tobin and Walker, 1996;Saltzman et al, 1998;Montaň ez et al, 2000;Shield et al, 2003). Carbon isotopic curves during the Early Ordovician have been investigated by several researchers (Gao and Land, 1991;Myrow et al, 2003;Saltzman, 2005;Bergström et al, 2009), however global significance of the Lower Ordovician carbon isotopic curve is yet certain. Buggisch et al (2003) presented for a biostratigraphically well-constrained high resolution d 13 C curve for the Early and Middle Ordovician time interval based on the investigation of carbonates of the La Silla and San Juan Formations in Argentina, which serves as a good reference to compare with.…”