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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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