CLIMATIC CYCLICITY AND TERRIGENOUS SEDIMENT INFLUX TO THE EARLY TURONIAN GREENHORN SEA, SOUTHERN UTAH

Rhythmically alternating beds of limestone and shale in pelagic deposi ts of the Cenomanian-Turonian North American Western Interior Seaway h ave been widely attributed to climatic cyclicity in the Milankovitch b and. Whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial. We present results of a stu dy of early Turonian strata of the Tropic Shale and Tununk Member of t he Mancos Shale in southern Utah, aimed at testing the dilution model in a prodeltaic environment within 130 km of the paleoshoreline. The s tudy transect consisted of four localities spaced along a 110 km onsho re-offshore transect. In each of the four sections, we studied a strat igraphic interval characterized by beds of limestone or marlstone that alternate with beds of marly shale or calcareous shale. Along the tra nsect, content of quartz sand and coarse silt in each of the beds gene rally decreases and content of calcium carbonate increases. In each se ction, quartz sand and coarse silt content varies antithetically with calcium-carbonate content. These observations suggest that the limesto ne/marlstone beds accumulated during times when rates of terrigenous s ediment transport to offshore sites were relatively slow. Conversely, the shaly beds were deposited during times of enhanced terrigenous sed iment flux. These observations support the hypothesis that limestone-s hale bedding rhythms record wet and dry phases of climate cycles. In U tah, the limestone/marlstone beds contain smaller proportions of mixed -layer illite/smectite clays than do the shaly beds. Climatic cyclicit y apparently led to cyclic variations in the proportions of clay miner als supplied to the prodeltaic environments, or to variation in the lo ci of accumulation of different clay minerals. Analysis of intensity o f bioturbation and preservation of fecal pellets also indicates that t he limestone/marlstone beds accumulated under more oxic benthic condit ions 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 t hree gradational lithologies, than as bedding rhythms, composed of two alternating lithologies. Data on grain size and calcium-carbonate con tent suggest that the bedding cycles correspond to shoaling-upward par asequences that developed in response to small-scale changes in relati ve sea level. Our study suggests that climatically modulated changes i n fresh-water discharge and terrigenous sediment supply were mainly re sponsible for these sea-level changes and for cyclic variations in ben thic oxygenation in prodeltaic environments of the early Turonian West ern Interior Seaway.