62 Earth and Planetary Science Letters, 98 (1990) 62-73 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [DTI Berriasian (Early Cretaceous) radiometric ages from the Grindstone Creek Section, Sacramento Valley, California T.J. Bralower, K.R. Ludwig, J.D. Obradovich and D.L. Jones Department of Geology, Florida International University, University Park, Miami, FL 33199 (U. S.A) Branch of Isotope Geology, U.S. Geological Survey, Denver, CO 80225 (U.S.A.) Department of Geology and Geophysics, University of California, Berkeley, CA 94720 (U.S.A.) Received August 4, 1989; revised version accepted December 13, 1989 The Grindstone Creek Section, Glenn County, Northern California is a sequence of hemipelagic mudstone, siltstone and sandstone interbedded with concretionary limestone and a few thin tufts and bentonites. Two tufts have been collected from a narrow interval of this sequence and subjected to mineralogical and isotopic analyses. U-Pb isotopic analyses of zircon fractions from these volcanic horizons indicate an age of 137.1 + 1.6/-0.6 Ma. A detailed investigation has been conducted on the calcareous nannofossil stratigraphy of this section based on numerous samples with moderately preserved assemblages. The nannoflora is largely of Tethyan affinity, and allows direct correlation with the Berriasian stratotype section, with sections with published magnetostratigraphies and with a DSDP site drilled between known magnetic anomalies. The dated tuffs lie in the lower part of the upper Berriasian Cretarhabdus angustiforatus Zone (Assipetra infracretacea Subzone) and within the narrow range of Rhagodiscus nebulosus. At three different sections, this subzone can be correlated with M-sequence Polarity Zones M16 and M16n. An independent magnetostratigraphic correlation is provided at DSDP Site 387, drilled between anomalies M15 and M16, where basal sediments contain R. nebulosus. Buchia collected within a meter of the lower tuff lie within the B. uncitoides Zone which is Berriasian in age. The upper tuff level, which occurs 65 m above the lower tuff, is situated within the overlying B. pacifica Zone. This zone had previously been correlated with the early Valanginian, but is clearly also partly of Berriasian age based on narmofossilstratigraphy. Our results allow an estimate of the age of the Berriasian-Valanginian and Jurassic-Cretaceous boundaries of 135.1 Ma and 141.1 Ma, respectively, and these fail within the range of, but differ significantiy from, several published time-scales. 1. Introduction The solution of diverse geologic problems re- quires accurate chronostratigraphy. Relative age control, provided by biostratigraphy and magneto- stratigraphy is necessary to interpret the timing of geologic processes. The assignment of isotopic age control to biostratigraphic and magnetostrati- graphic events and intervals, transforms these components into biochronologic and magneto- chronologic units respectively, and this is neces- sary to interpret the rate at which ancient processes occurred. Pertinent examples include seafloor spreading rates [1], sedimentation or accumulation rates of sedimentary components [2], and evolu- tionary and extinction rates of fossil organisms [3]. Aspects of the Mesozoic time-scale have be- nefited from a large amount of recent data. In particular, magnetostratigraphy has become a use- 0012-821X/90/$03.50 © 1990 - Elsevier Science Publishers B.V. ful means of high-resolution stratigraphic correla- tion (e.g. [4,5]). Microfossil biostratigraphies have been widely applied in dating sediments deposited in a spectrum of environments, and the precision, accuracy, and resolution of these techniques is gradually improving (e.g. [6]). Correlation between biostratigraphy and magnetostratigraphy is crucial and has advanced significantly (e.g. [5,7-9]). Bio- stratigraphic control often helps when interpreting magnetostratigraphies bearing little resemblance to the geomagnetic polarity time-scale. By contrast, radiometric control of the Meso- zoic time-scale has improved much less rapidly. The large number of recently published chronolo- gies and the scarcity of new radiometric data is an indication that there is little agreement on either the quality of various available isotopic ages, or on the different techniques used for time-scale construction. The large proportion of isotopic ages