Sedimentology, sequence stratigraphy, and brachiopod biostratigraphy of the Ordovician (Mohawkian) Decorah Formation, Midcontinent, USA

The Ordovician (Mohawkian) Decorah Formation in the upper Mississippi Valley region is identified as a mixed siliciclastic-carbonate unit deposited between stratigraphic units that are dominated by carbonate lithologies. The Decorah was deposited within a vast epeiric sea during one of the largest episodes of marine flooding of the North American craton.; Detailed sedimentology, stratigraphy, and brachiopod biostratigraphy has revealed that the Decorah Formation consists of one depositional sequence bounded by two regionally extensive discontinuities and contains a nested hierarchy of two high-frequency sequences and four cycle sets. The lowest cycle set consists of shale-rich facies that thicken (northwest) toward clastic source areas along the Transcontinental Arch during a time of high clastic influx and abundant freshwater runoff creating at times, a salinity-stratified water column and dysoxic conditions. The remaining three cycle sets consist of carbonate-rich facies that form a reciprocal wedge of sediment that thickens southeast and was deposited during relative flooding of the clastic source areas reducing the clastic sediment and freshwater supply. The carbonate-rich facies was deposited in more oxic conditions with carbonates showing down ramp progradation from south to north.; Brachiopod biostratigraphy revealed that most species had ranges confined to the Decorah Formation and did not have ranges that crossed formation boundaries. Graphic correlation methods using brachiopod species first and last appearance data, as well as the stratigraphic positions of two k-bentonites, outlined changes in sediment accumulation rates from north to south as well as differences in rates between the two major lithofacies (shale vs. carbonate) of the Decorah. Multivariate Q-mode cluster analysis established two major clusters reflecting the shale- versus carbonate-rich lithofacies change. These results were interpreted to indicate that both biofacies and lithofacies patterns reflect a close link between environmental shifts and faunal change.; Correlation of time equivalent rocks in eastern North America reveals large differences suggesting different water masses for the midcontinent and eastern North America. This interpretation suggests that the North America Ordovician epeiric sea was not necessarily a continuous sea but rather subtle topographic changes divided the flooded continent into different depositional areas with distinct physical, biological, and chemical signatures.