DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY FROM THE DAKOTA FORMATION: EVIDENCE FOR MID-CRETACEOUS TRANSCONTINENTAL FLUVIAL SYSTEMS

U-Pb ages (n=402) and Hf isotope geochemistry (n=100) of detrital zircons from the Dakota Formation provide evidence for westward-flowing fluvial systems that stretched from the Appalachian highlands to the Western Interior Seaway (WIS) during Cretaceous (Albian-Cenomanian) time. The Dakota Formation is exposed in a relatively continuous outcrop belt from southwestern Minnesota to central Kansas. It is the oldest sedimentary unit preserved along the eastern margin of the Western Interior Basin (WIB), a north-south-trending foreland basin system that was inundated by the WIS during Cretaceous time. Given the dearth of Triassic, Jurassic, and Early Cretaceous strata east of the WIB province, the Dakota Formation fluvial and marginal marine strata provide the earliest, most complete record of depositional systems east of the WIB during Mesozoic time. Detrital zircon signatures from four Dakota Formation fluvial sandstones exposed in western Iowa and eastern Nebraska indicate small contributions from Precambrian age belts of the eastern subcontinent and all key plutonic assemblages of the Appalachian orogen. Approximately 78% of detrital zircon grains match the ages of Grenvillian (1.3-1.0 Ga), Pan-African (750-500 Ma), and Paleozoic (500-310 Ma) bedrock sources located within the present-day Appalachian Mountains. The presence of only a very few detrital zircon grains of Mesoproterozoic (1.5-1.0 Ga), Paleoproterozoic (2.5-1.5 Ga), or Archean age (>2.5 Ga) indicates that northern source regions in Minnesota, Wisconsin, and Canada did not contribute a significant volume of sediment. Appalachian-aged grains may have been recycled from Appalachian foreland basin Paleozoic strata; however, these strata remain largely undissected today so their relative contribution was likely small. Recycling of Paleozoic strata within the midcontinent was also an unlikely scenario since most of the Paleozoic strata in the midcontinent are carbonate and shale. In addition, abundant Paleoproterozoic and Archean detrital zircon age populations in Cambrian and older sandstone-rich clastic strata from the upper midcontinent preclude these rocks as a dominant source. Overall, our preliminary dataset from the Dakota Formation suggests the presence of transcontinental fluvial systems during middle Cretaceous time.