Abstract As part of a larger intercollegiate effort to reconstruct late Triassic, presalt sediment provenance and routing environments for the Gulf of Mexico sedimentary basin, an integrated geochronologic approach leveraging more traditional biostratigraphic, sedimentologic, and sequence stratigraphic provenance constraints from geologic cores, cuttings, and geophysical well logs was initiated. This paper presents the initial results of this ongoing study and details detrital zircon U-Pb extraction methodologies while Inductively Coupled Plasma Mass Spectrometry analyses are pending. Eagle Mills Formation sandstone samples were collected from well core and cuttings, at five sub-crop locations extending from Texas to South Carolina to the West Florida shelf, in preparation for U-Pb detrital zircon provenance analysis. Prior to separation of detrital zircon grains, a sedimentologic-stratigraphic analysis was conducted including detailed core description, well log evaluation, and thin-section petrography assessment. These findings confirm a hypothesis that late Triassic Eagle Mills siliciclastics were derived from the erosion of an active horst-graben rift block topography with associated igneous intrusives. Specifically, preliminary results reveal pervasive very finegrained mottled gray to red bed sandstone lithology confirming synrift continental alluvium having little or no marine component, and probable deposition in a warm, humid environment but with increasing aridity. Classic fluvial facies features are highlighted including depositional cross strata typifying dynamic braided to meandering channel belts and alluvial floodplain deposits. Less common siltstone and shale lithologies were likely deposited amidst lower energy subfacies including potential shallow lakes, marshes, and/or ephemeral ponds. Bioturbated trace fossils were only rarely preserved, and there was no evidence of marine or eolian facies incursion. Igneous magmatism was prevalent in most subsurface Eagle Mills Formation samples including intrusive diabase, basalt flows, and volcanic ash.

Abstract U-Pb dating of detrital zircons in fluvial sandstones provides a method for reconstruction of drainage basin and sediment routing systems for ancient sedimentary basins. This paper summarizes a detrital-zircon record of Cenomanian paleodrainage and sediment routing for the Gulf of Mexico and U.S. midcontinent. Detrital zircon data from Cenomanian fluvial deposits of the Gulf of Mexico coastal plain (Tuscaloosa and Woodbine formations), the Central Plains (Dakota Group), and the Colorado Front Range (Dakota Formation) show the Appalachian-Ouachita orogen represented a continental divide between south-draining rivers that delivered sediment to the Gulf of Mexico, and west- and north-draining rivers that delivered sediment to the eastern margins of the Western Interior seaway. Moreover, Cenomanian fluvial deposits of the present-day Colorado Front Range were derived from the Western Cordillera, flowed generally west to east, and discharged to the western margin of the seaway. Western Cordillera-derived fluvial systems are distinctive because of the presence of Mesozoic-age zircons from the Cordilleran magmatic arc: the lack of arc zircons in Cenomanian fluvial deposits that dis-charged to the Gulf of Mexico indicates no connection to the Western Cordillera. Detrital zircon data facilitate reconstruction of contributing drainage area and sediment routing. From these data, the dominant system for the Cenomanian Gulf of Mexico was an ancestral Tennessee River (Tuscaloosa Formation), which flowed axially through the Appalachians, had an estimated channel length of 1200-1600 km, and discharged sediment to the east-central Gulf of Mexico. Smaller rivers drained the Ouachita Mountains of Arkansas and Oklahoma (Woodbine Formation), had length scales of <300 km, and entered the Gulf through the East Texas Basin. From empirical scaling relationships between drainage-basin length and the length of basin-floor fans, these results predict significant basin-floor fans related to the paleo-Tennessee River system and very small fans from the east Texas fluvial systems. This predictive model is consistent with mapped deep-water systems, as the largest fan system was derived from rivers that entered the Gulf of Mexico through the southern Mississippi embayment.

The Mississippi River Valley contains many interesting geologic stories, perhaps none more mysterious and potentially dangerous than that of the New Madrid seismic zone earthquakes. Here, deep within the North American plate, the central Mississippi River Valley was lit up by at least three major earthquakes during the winter of 1811–1812. Perhaps we could ignore this if it had been a single sequence, but the paleoseismic record tells us that there have also been four prehistoric earthquake sequences within the past 2,600 yr. In this book we travel through time to see how the central Mississippi Valley region developed. Since the earthquakes are occurring on deep basement faults, it is only fitting to start in deep Precambrian time and work our way forward in our quest to better understand the geologic framework of this intra-plate seismic zone. Are more earthquakes coming? It is my hope that this book will stimulate readers to pursue this and many other geologic problems in the Mississippi River Valley.