Abstract

Rhythmically alternating beds of limestone and shale in pelagic deposits of the Cenomanian-Turonian North American Western Interior Seaway have been widely attributed to climatic cyclicity in the Milankovitch band. Whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial. We present results of a study of early Turonian strata of the Tropic Shale and Tununk Member of the Mancos Shale in southern Utah, aimed at testing the dilution model in a prodeltaic environment within 130 km of the paleoshoreline. The study transect consisted of four localities spaced along a 110 km onshore-offshore transect. In each of the four sections, we studied a stratigraphic interval characterized by beds of limestone or marlstone that alternate with beds of marly shale or calcareous shale. Along the transect, content of quartz sand and coarse silt in each of the beds generally decreases and content of calcium carbonate increases. In each section, quartz sand and coarse silt content varies antithetically with calcium-carbonate content. These observations suggest that the limestone/marlstone beds accumulated during times when rates of terrigenous sediment transport to offshore sites were relatively slow. Conversely, the shaly beds were deposited during times of enhanced terrigenous sediment flux. These observations support the hypothesis that limestone-shale bedding rhythms record wet and dry phases of climate cycles. In Utah, the limestone/marlstone beds contain smaller proportions of mixed-layer illite/smectite clays than do the shaly beds. Climatic cyclicity apparently led to cyclic variations in the proportions of clay minerals supplied to the prodeltaic environments, or to variation in the loci of accumulation of different clay minerals. Analysis of intensity of bioturbation and preservation of fecal pellets also indicates that the limestone/marlstone beds accumulated under more oxic benthic conditions than did the shaly beds. Detailed examination of the alternating beds of limestone/marlstone and shale in southern Utah indicates that the deposits are characterized by a distinctive succession of facies. These deposits are better interpreted as bedding cycles, composed of three gradational lithologies, than as bedding rhythms, composed of two alternating lithologies. Data on grain size and calcium-carbonate content suggest that the bedding cycles correspond to shoaling-upward parasequences that developed in response to small-scale changes in relative sea level. Our study suggests that climatically modulated changes in fresh-water discharge and terrigenous sediment supply were mainly responsible for these sea-level changes and for cyclic variations in benthic oxygenation in prodeltaic environments of the early Turonian Western Interior Seaway.

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