ABSTRACT Paleocene Lower Wilcox Group sedimentation rates are three times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide–Rocky Mountains region. These increased rates likely represent the erosion of easily weathered Phanerozoic strata that blanketed the Laramide-age basement-cored uplifts. Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital zircons from eight samples from the San Juan Basin and five samples from the Gulf of Mexico Basin. Focusing on the zircons younger than 300 Ma allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: (1) Coast Mountains batholith; (2) North Cascades Range; (3) Idaho batholith; (4) Sierra Nevada batholith; (5) Laramide porphyry copper province; (6) Transverse Ranges; (7) Peninsular Ranges; and (8) Sierra Madre Occidental. The εHf ( t ) results range from +8.9 to –27.0 for the San Juan Basin samples and from +13.0 to –26.6 for the Gulf of Mexico samples. Using the San Juan Basin samples as a proxy for the eroded Mesozoic cover that was shed from the Laramide uplifts, we show that much of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the late Paleocene was derived from reworked cover from the greater Laramide–Rocky Mountains region. However, the Gulf of Mexico samples also include a distinct juvenile suite (εHf [ t ] ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma that we traced to the Coast Mountains batholith in British Columbia, Canada. This transcontinental connection indicates an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58 to 56 Ma. Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith to the Gulf of Mexico. This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km 2 . Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark εHf ( t ) versus U-Pb age plot, which can be used to determine provenance of detrital zircons (85–50 Ma) at the scale of specific region(s) within the Cordillera based on their εHf ( t ) values.

Shelf-margin deltas and linked downslope depositional systems are in most cases fed by alluvial valleys that serve to deliver sediment eroded from the hinterland. Accordingly, alluvial valleys provide the link between processes that control sediment flux to the continental margin and processes that control dispersal into the basin. Current research shows the volume of sediment delivered to the margin will reflect hinterland drainage areas and large-scale relief. Superimposed on this background rate will be an unsteadiness that reflects climate change in hinterland source regions, but the rates and directions of change in sediment supply will vary regionally. Alluvial valleys modulate unsteadiness in sediment supply through changes in sediment storage. However, regional variability in the rates and directions of change in sediment supply insures that responses to climate change are regionally circumscribed, and alluvial valley systems in different regions may respond in opposite ways to the same global climate change. Sea-level change has little effect on the total volume of sediment delivered to the margin, but instead forces channel extension and shortening, which plays a major role in determining the proximal to distal location of the river mouth point source through which sediment is dispersed to the shelf and beyond. Moreover, the widely used concept of incision and complete sediment bypass within incised valley systems during periods of relative sea-level fall should be abandoned. Instead, falling stage to lowstand fluvial deposition is actually common in well-studied Quaternary analog systems, and falling stage sand bodies may comprise a significant proportion of reservoir-quality sands within many incised valley fill depositional sequences. Models for falling stage and lowstand systems tracts should therefore incorporate significant fluvial channel belt deposits that are likely connected to, and feeding, the offlapping shore faces, shelf-margin deltas, and linked downslope systems.

Abstract Deepwater depositional systems are in most cases ultimately fed by rivers that deliver sediment from the hinterland to the continental margin. Fluvial systems therefore provide the link between processes that control sediment supply to the margin and processes that control dispersal to deepwater. This paper reviews controls on fluvial sediment supply to the shelf-margin staging area over time scales of 10 6 yr or less. General controls on fluvial sediment discharge to the coastal oceans are reasonably well known: the newly published global database and empirical model of Syvitski and Milliman (2007) shows that geologic factors of drainage area, relief, and rock types explain more than 60% of between-river variance, whereas climatic factors of temperature and discharge explain 10% and 3%, respectively. Geologic factors vary little over time scales of 10 6 yr or less, whereas climatic factors are highly unsteady over those same time scales. Available literature suggests that unsteadiness due to Milankovitch-scale climate change likely results in variations in sediment yield that are less than ± 50%. The Syvitski and Milliman (2007) empirical model shows a positive correlation between temperature and sediment discharge: it follows that sediment discharge to the coastal oceans is predicted to have been less during glacial climates, relative to interglacials, for most unglaciated river systems of the tropics and lower midlatitudes. Routing of fluvial sediments to the shelf-margin staging area reflects a complicated suite of processes but varies with large-scale controls on drainage-basin size and shelf width. Moreover, in the Quaternary Icehouse world, climate changes are coupled to large-scale changes in global ice volume and resultant high-amplitude glacio-eustasy: river systems respond to sea-level changes of this magnitude by transit back and forth across the shelf, such that fluvial response to sea-level change plays a pivotal role in the modulation of sediment dispersal to the shelf-margin staging area. During highstand conditions, most large rivers discharge to the inner parts of broad shelves: direct dispersal to the shelf margin is favored by sea-level fall and transit of river mouths across the shelf to the staging area. By contrast, short, steep river systems discharge to narrow shelves, and can disperse sediment directly to the shelf margin during highstand, although rates are likely increased during lowstands. In some cases, sea-level fall and the transit of river mouths across the shelf results in the merger of river systems that, in a highstand world, discharge separately to the coastal oceans. Merging of drainage basins can significantly increase the magnitude of individual point-source inputs due to the addition of drainage area, but there will be fewer rivermouth point sources at the shelf margin than what might appear from an examination of present highstand conditions. Application of the Syvitski and Milliman (2007) empirical model to river systems of the Texas Gulf of Mexico coastal plain and shelf illustrates a number of key concepts. An estimated 5°C depression of basin-averaged temperatures during the last glacial maximum, relative to today, results in predicted sediment yields that were 25–30% less than today. The Colorado River extended across the shelf during the last glacial period, increasing drainage area by ˜ 45% because of merging with smaller streams; however, total sediment discharges are predicted to have been similar to today during the last glacial maximum sea-level lowstand. Nevertheless, significantly more sediment is predicted to have been delivered than is necessary to account for observed sediment volumes in the shelf-margin delta. In fact, 30% or more may be unaccounted for, and may represent the fraction of sediment dispersed to the slope and beyond. Relationships between fluvial sediment supply, climate and sea-level change, and dispersal to deepwater systems should be fundamentally different in “greenhouse” periods of Earth history when the amplitude of high-frequency sea-level changes is significantly less. Unsteadiness in fluvial sediment discharge due to climate change should not be modulated by the transit of river mouths and deltas across broad shelves during sea-level fall and rise, river systems should not merge to the same extent, and the number of point sources would not change over short time scales. As a result, sediment dispersal to deepwater should be directly coupled to Milankovitch-scale climate changes.