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Stable isotopic values were measured on micrite, sparry calcite, dolomite, inarticulated brachiopods, and conodonts from the Lange Ranch section (central Texas) of the Lower Ordovician Tanyard Formation. The section spans the upper Cordylodus angulatus Zone through the lower Rossodus manitouensis Zone. An ϳ2‰ negative ␦ 13 C shift from Ͼ0‰ to ϽϪ1.5‰ VPDB through the section suggests the lower third of the Rossodus manitouensis Zone was sampled. Consistent with previous studies, the ␦ 18 O values of carbonates are low, ranging from Ϫ3.3‰ to Ϫ8.1‰ VPDB . Phosphate ␦ 18 O values range from 15.4‰ to 17.1‰ VSMOW . Paleotemperature estimates calculated from micrite ␦ 18 O values assuming an ice-free seawater ␦ 18 O value of Ϫ1‰ VSMOW indicate Early Ordovician tropical seawater temperatures averaged 42؇C, whereas ␦ 18 O values of co-occurring biogenic phosphate assuming the same seawater value yield paleotemperature estimates averaging 37؇C. The phosphate values are interpreted as less affected by diagenesis than carbonate values and suggest Early Ordovician tropical paleotemperatures were not more than 10؇C warmer or the oxygen isotopic composition of Early Ordovician hydrosphere was not more than 2‰ lower than present.

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.

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The Ibexian Series (mainly Lower Ordovician), first proposed by Hintze (1982), replaces the "Canadian Series" of North American usage. The lower boundary stratotype of the Ibexian Series is defined at a point in rock 39.1 m (128.3 ft) above the base of the Lava Dam Member of the Notch Peak Formation in the Lava Dam Five section of the Steamboat Pass-Lava Dam composite section of Hintze and others (1988). The lower boundary of the Ibexian Series is coincident with the lowest observed occurrence of Cordylodus andresi Viira and Segeeva in Viira and others, 1987 which also defines the base of the Hirsutodontus hirsutus Subzone of the Cordylodus proavus Zone. This horizon is about 0.1 m (4 in) below the lowest trilobite sample assigned to the Eurekia apopsis Zone. The top of the Ibexian Series in its type area coincides with the correlated base of the Whiterockian Series which is drawn at the base of the Tripodus laevis Conodont Zone and coincidentally with the base of the Paralenorthis-Orthidiella Brachiopod Zone (= Zone L of Ross, 1951). The Ibexian Series is characterized by over one-hundred-fifty conodont species and over onehundred-fifty trilobite and articulate brachiopod species from a composite of 11 measured sections in the type area, located in the southern House Range and southern Confusion Range in the Notch Peak and the The Barn 15-minute quadrangles, Millard County, Utah. The composite stratotype section involves the Lava Dam Member of the Notch Peak Formation, House Limestone, Fillmore Formation, and part of the Wah Wah Limestone. The composite section aggregates 801 m (2,628 ft) of abundantly fossiliferous limestone and subordinate calcareous siliciclastic rocks that formed in a miogeoclinal, shallow water carbonate platform environment. The Ibexian Series is here divided, in ascending order, into the Skullrockian, Stairsian, Tulean, and Blackhillsian Stages and into 11 conodont zones and 14 shelly fossil zones that augment and refine the original 10 Ross (1951) and Hintze (1953) shelly fossil zones, which have been widely accepted for correlation within the North American faunal province for 40 years. In addition nautiloid cephalopods, gastropods, sponges, echinoderms, ostracodes, and graptolites occur in the composite section. Recent work shows that the base of the Ibexian Series can be recognized in low paleolatitude sites in both carbonate shelf and slope facies in the western U.S.