REGIONAL VARIATION IN LATEST CAMBRIAN TO EARLY ORDOVICIAN MARINE CARBON ISOTOPE RATIOS FROM WESTERN UTAH TO CENTRAL TEXAS

Detailed carbon isotope stratigraphic profiles from more than 20 latest Cambrian to Early Ordovician-aged sections have been obtained along a transect from western Utah to central Texas, permitting the development of temporally equivalent ‘chronoplanes' at high stratigraphic resolution. These chronoplanes reveal small but locally-persistant differences in the absolute value of seawater d¹³C, and suggest that both water depth and regional differences in water mass d¹³C are determinant for local d¹³C values.

Intrabasinal differences in the absolute value and progression of d¹³C values were identified within the El Paso Formation in central New Mexico and western Texas. These differences appear to be strongly linked to depositional environment, and occur in conjunction with the basinwide development of distinctive stromatolite mounds, large algal reefs, and ribbon limestones. A ubiquitous strongly negative carbon isotope event (the ‘Jose Event') in this basin is directly linked to sea-level change. This supports a speculation that the Jose Event may represent the rapid restoration of open marine conditions after a period of basin isolation during relative sea level fall. Locally strong differences in d13C values immediately prior to the Jose Event appear to be related to water depth, suggesting that basinal isolation could have led to the development of a more poorly-oxygenated water mass as open marine circulation became restricted. Temporally concordant carbon isotope profiles from sections in the Great Basin are currently being developed to shed additional light on these questions.