DEVELOPMENT OF A CONIACIAN-SANTONIAN ORBITAL TIME SCALE AND COMPARATIVE ESTIMATES OF PRIMARY PRODUCTION VS. DILUTION FOR LATE CRETACEOUS CHALK FACIES

One of the most distinctive geologic features of the Cretaceous Period was the widespread deposition of chalk facies in epieric seas. Chalks are dominantly composed of the calcareous tests of coccolithophorids and the great areal distribution and temporal duration of this lithofacies demands a unique biogeochemical explanation. Did elevated, but not exceptional production favor calcareous over siliceous phytoplankton? Or do Cretaceous chalks simply reflect deposition in settings starved of siliciclastic input? Chalks are commonly rhythmically bedded and the bedding patterns have been attributed to orbital forcing. Variations in either productivity or siliciclastic dilution have been hypothesized as the depositional mechanism, but the problem of co-dilution has prevented a direct test of these hypotheses. This talk will describe the development of a high resolution orbital time scale for Coniacian-Santonian chalks of the U.S. Western Interior basin (Niobrara Formation) and the use of this time scale to make burial flux calculations upon which estimates of primary production and siliciclastic dilution can be made. The orbital time scale is based on application of advanced Fourier techniques to high-resolution pixel and well log (FMI) data sets in multiple cores from the central part of the basin (Colorado). The cores were sampled for analyses of weight percent CaCO₃ and TOC, bulk density, and porosity. Sedimentation rates derived from a moving window spectral method (Evolutive Harmonic Analysis) and density measurements are combined to calculate accumulation rates for CaCO₃, TOC, and insoluble residue. Ichnofabrics are used to constrain a relative preservation factor. The Niobrara flux estimates are compared to similar estimates for other chalks calculated from published data sets by exporting the orbital time scale via ammonite biozonation. The results of this study allow a direct test of the productivity and dilution hypotheses, and have direct implications for the origin of other Late Cretaceous chalks.