ABSTRACT Rocks of the Upper Cretaceous Tuscaloosa Formation (Cenomanian) and Eutaw Formation (Santonian) in southwestern Georgia and southeastern Alabama record an interval of fluvial and nearshore marine deposition. In the vicinity of Columbus, Georgia, basal units of the Tuscaloosa Formation consist of a residual paleosol built on crystalline rocks of the Appalachian Piedmont covered by conglomeratic sandstones deposited in braided stream systems flowing across the mid-Cenomanian Coastal Plain unconformity. The unconformity, which separates Cretaceous detrital rocks from underlying metamorphic rocks and residual paleosols built on those metamorphic rocks, lies primarily within the Tuscaloosa Formation in this region and is marked at the modern surface by the geomorphic Fall Line. Mapping of the unconformity across the region reveals areas of significant paleorelief associated with a number of distinct paleovalleys incised into the mid-Cenomanian surface. The most distinct of these lie immediately east of the Alabama-Georgia state line, within 15 km of the modern Lower Chattahoochee River Valley. Spatially, these distinct paleovalleys lie immediately north of a Santonian estuarine environment recorded in the Eutaw Formation, disconformably above the Tuscaloosa Formation. Collectively, paleo-valleys in the mid-Cenomanian surface, the fluvial nature of the Tuscaloosa Formation in southwestern Georgia and southeastern Alabama, and the estuarine environment in the younger Eutaw Formation suggest a persistent (~10 m.y.) paleodrainage system that may be a forerunner to the modern Chattahoochee River.

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.

Abstract Fluvial systems possess a range of scaling relationships that reflect drainage-basin controls on water and sediment flux. Quaternary channel-belt thickness (as controlled by bank-full water discharge) has been documented as a reliable first-order proxy for drainage basin size if climatic regimes are independently constrained. In hydrocarbon exploration and production, scaling relationships for fluvial deposits can be utilized to constrain drainage basin size with implications for sequence-stratigraphic interpretations. This study documents the scales of channel belts within Cenomanian fluvial successions from the Gulf of Mexico. Data on single-story channel-belt scales were compiled from well logs and utilized to constrain contributing catchment areas of fluvial systems. The data indicate that the Cenomanian fluvial systems were significantly smaller than the Cenozoic fluvial systems, which can be related to drainage basin reorganization. These scaling relationships can be validated by regional paleogeographic maps and provide additional insight into the variability of sediment routing systems through time.

Abstract: The extreme sensitivity of rivers to subtle changes of slope resulting from tectonic tilting provides a basis for the interpretation of four Cretaceous paleovalley systems mapped on four successive sequence-bounding surfaces in the mid-Cenomanian Dunvegan Formation, deposited in the foredeep of the Western Canada Foreland Basin. The northern part of the study area overlies the Peace River Arch, a long-lived structural entity that is transected by faults trending dominantly NW–SE and NE–SW. The Dunvegan Fm. comprises a series of at least ten deltaic sequences labeled J–A in ascending order, and each spans ~ 100–200 ky. Valleys mapped on the top of sequences F, G, and H have many straight reaches that tend to parallel the rectilinear fault pattern in the underlying Paleozoic rocks, and many reaches are colinear with deep-seated faults. There is no evidence for deflection of rivers along the axis of the foreland basin. Isopach maps show that sequences H, G, and F do not thicken appreciably into the foredeep, suggesting that the contemporaneous rate of plate flexure was low. In-plane stress level is also inferred to have been low. Under these conditions, the forebulge was suppressed and long-lived faults in the Peace River Arch area were “unlocked”, permitting individual blocks to undergo vertical isostatic adjustment on a scale of meters. The low-gradient coastal-plain rivers were strongly influenced by this tectonic topography. In contrast, sequences E–A show marked westward thickening into a newly developed foredeep. Paleovalleys on sequence E do not have a preferred rectilinear pattern, instead being dominantly dendritic; distally, valleys are abruptly deflected to flow parallel to the basin axis. This new paleodrainage pattern is interpreted to reflect subtle uplift of a forebulge in response to renewed plate flexure, causing river deflection to the SE. The level of in-plane stress is inferred to have increased at this time, “locking” faults over the Arch and causing the land surface to deform as a regionally homogeneous plane, favoring dendritic drainage. The inferred position of the forebulge approximates the position of a long-lived Cretaceous paleodrainage divide and also coincides with a terrane suture in the Precambrian basement. The suture is inferred to have been a lithospheric weak zone, repeatedly serving to localize the forebulge. Although vertical displacement on both faults and forebulge cannot conclusively be recognized in cross sections or isopach patterns, it appears that rivers were readily diverted by these extremely subtle topographic features.

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