A Maastrichtian birth of the Ancestral Mississippi River system: Evidence from the U-Pb detrital zircon geochronology of the McNairy Sandstone, Illinois, USA

Abstract Sediment dispersal patterns are integral to the understanding of Cretaceous paleogeography and tectonics. Detrital zircon U-Pb geochronology of fluvial/deltaic sandstones of the Maastrichtian (∼70 Ma) McNairy Sandstone (s = 5, n = 1458) at the northern end of the Mississippi Embayment documents the initiation of south-flowing drainage in the Laurentian midcontinent. The detrital zircon age spectra of the McNairy Sandstone include zircons from the Superior (>2.5 Ga), Penokean (1.8–1.9 Ga), Yavapai-Mazatzal (1.6–1.8 Ga), Midcontinent Granite-Rhyolite (1.3–1.5 Ga), Grenville (980–1300 Ma), and Taconic-Acadian (350–450 Ma) provinces. Grenville zircons account for 74% of the age spectrum. Taconic-Acadian grains also are important, comprising ∼10% of the age spectrum. The McNairy DZ age spectra are statistically indistinguishable from uppermost Pennsylvanian strata (i.e. the later Des Moinesian Copperas Creek Sandstone) in the Illinois Basin. These DZ age spectra are also indistinguishable from sands in the Modern Ohio and Tennessee River valleys. Based on this similarity, local Illinois Basin source areas contributed a previously unrecognized volume of sediment to the Mississippi Embayment as early as Maastrichtian time. Our interpretation includes a Cenomanian onset of subsidence in the southern Mississippi Embayment initiated by the rapid opening of the Gulf of Mexico. This subsidence propagated northward, and by the Maastrichtian, the Mississippi Embayment was the drainage axis for an Ancestral Mississippi that is recorded in both the Maastrichtian strata in the northern Mississippi Embayment and the Paleogene deposits in the southern part. Our interpretation shows the integration of an Ancestral Mississippi River occurred by the Maastrichtian, that the river continued through the Paleocene, and subsequently developed into the modern Mississippi River system.