SEQUENCE STRATIGRAPHY AND SYSTEMS TRACT DEVELOPMENT IN EARLY ORDOVICIAN, MIXED CARBONATE-SILICICLASTIC ROCKS OF THE COON VALLEY MEMBER OF THE ONEOTA FORMATION, SOUTHEASTERN MINNESOTA

The Early Ordovician Oneota Formation is divided into the lower Coon Valley Member and the upper Hager City Member. This study focuses on the Coon Valley Member, which is a mixed carbonate-siliciclastic unit exhibiting meter-scale cyclicity. Previous regional sequence stratigraphic studies (Smith et al. 1996; Smith et al. 1993) based on generalized lithofacies distinctions have identified a third-order depositional sequence in the Coon Valley Member, including a basal transgressive systems tract and an upper highstand systems tract. In this study, cycle stacking patterns were used for third-order systems tracts evaluation and identification of the position of a maximum flooding surface. Stratigraphic interpretations were based on one detailed measured section near La Crescent, MN. Facies and subfacies classifications were used to interpret depositional environments. At the La Crescent locality, nine shoaling-upward, meter-scale, subtidal cycles are present in the upper eight meters of the Coon Valley Member. These cycles (tentatively described as fifth-order cycles) are identified by upward-shallowing vertical subtidal subfacies associations. The cycles generally begin with stromatolitic boundstone or oolitic grainstone and shallow to skeletal wackestone subfacies. Fifth-order cycles recognized at the La Crescent locality are bundled into two lower-order cycles (tentatively described as fourth-order cycles) bounded by diastemic surfaces. Each fourth-order cycle contains four or five fifth-order cycles. High-frequency eustatic sea-level change is proposed as the overriding mechanism for cycle formation. Third-order sea-level change is proposed as a control on accommodation space and, therefore, on cycle thickness. Relatively thick cycles are attributed to high rates of third-order sea-level rise producing increased accommodation space. Conversely, relatively thin cycles form at times of low rates of third-order sea-level change. Changes in the thicknesses of fifth-order cycles in the Coon Valley are thought to be dependant on rates of third-order sea-level change.