ABSTRACT Detrital zircon U-Pb and (U-Th)/He ages from latest Cretaceous–Eocene strata of the Denver Basin provide novel insights into evolving sediment sourcing, recycling, and dispersal patterns during deposition in an intracontinental foreland basin. In total, 2464 U-Pb and 78 (U-Th)/He analyses of detrital zircons from 21 sandstone samples are presented from outcrop and drill core in the proximal and distal portions of the Denver Basin. Upper Cretaceous samples that predate uplift of the southern Front Range during the Laramide orogeny (Pierre Shale, Fox Hills Sandstone, and Laramie Formation) contain prominent Late Cretaceous (84–77 Ma), Jurassic (169–163 Ma), and Proterozoic (1.69–1.68 Ga) U-Pb ages, along with less abundant Paleozoic through Archean zircon grain ages. These grain ages are consistent with sources in the western U.S. Cordillera, including the Mesozoic Cordilleran magmatic arc and Yavapai-Mazatzal basement, with lesser contributions of Grenville and Appalachian zircon recycled from older sedimentary sequences. Mesozoic zircon (U-Th)/He ages confirm Cordilleran sources and/or recycling from the Sevier orogenic hinterland. Five of the 11 samples from syn-Laramide basin fill (latest Cretaceous–Paleocene D1 Sequence) and all five samples from the overlying Eocene D2 Sequence are dominated by 1.1–1.05 Ga zircon ages that are interpreted to reflect local derivation from the ca. 1.1 Ga Pikes Peak batholith. Corresponding late Mesoproterozoic to early Neoproterozoic zircon (U-Th)/He ages are consistent with local sourcing from the southern Front Range that underwent limited Mesozoic–Cenozoic unroofing. The other six samples from the D1 Sequence yielded detrital zircon U-Pb ages similar to pre-Laramide units, with major U-Pb age peaks at ca. 1.7 and 1.4 Ga but lacking the 1.1 Ga age peak found in the other syn-Laramide samples. One of these samples yielded abundant Mesozoic and Paleozoic (U-Th)/He ages, including prominent Early and Late Cretaceous peaks. We propose that fill of the Denver Basin represents the interplay between locally derived sediment delivered by transverse drainages that emanated from the southern Front Range and a previously unrecognized, possibly extraregional, axial-fluvial system. Transverse alluvial-fluvial fans, preserved in proximal basin fill, record progressive unroofing of southern Front Range basement during D1 and D2 Sequence deposition. Deposits of the upper and lower D1 Sequence across the basin were derived from these fans that emanated from the southern Front Range. However, the finer-grained, middle portion of the D1 Sequence that spans the Cretaceous-Paleogene boundary was deposited by both transverse (proximal basin fill) and axial (distal basin fill) fluvial systems that exhibit contrasting provenance signatures. Although both tectonic and climatic controls likely influenced the stratigraphic development of the Denver Basin, the migration of locally derived fans toward and then away from the thrust front suggests that uplift of the southern Front Range may have peaked at approximately the Cretaceous-Paleogene boundary.

ABSTRACT Analysis of detrital zircon U-Pb ages from the Phanerozoic sedimentary record of central Colorado reveals variability in sediment transport pathways across the middle of the North American continent during the last 500 m.y. that reflects the tectonic and paleogeographic evolution of the region. In total, we present 2222 detrital zircon U-Pb ages from 18 samples collected from a vertical transect in the vicinity of Colorado’s southern Front Range. Of these, 1792 analyses from 13 samples are published herein for the first time. Detrital zircon U-Pb age distributions display a considerable degree of variability that we interpret to reflect derivation from (1) local sediment sources along the southern Front Range or other areas within the Yavapai-Mazatzal Provinces, or (2) distant sediment sources (hundreds to thousands of kilometers), including northern, eastern, or southwestern Laurentia. Local sediment sources dominated during the Cambrian marine transgression onto the North American craton and during local mountain building associated with the formation of the Ancestral and modern Rocky Mountains. Distant sediment sources characterize the remaining ~75% of geologic time and reflect transcontinental sediment transport from the Appalachian or western Cordilleran orogenies. Sediment transport mechanisms to central Colorado are variable and include alluvial, fluvial, marine, and eolian processes, the latter including windblown volcanic ash from the distant mid-Cretaceous Cordilleran arc. Our results highlight the importance of active mountain building and developing topography in controlling sediment dispersal patterns. For example, locally derived sediment is predominantly associated with generation of topography during uplift of the Ancestral and modern Rocky Mountains, whereas sediment derived from distant sources reflects the migrating locus of orogenesis from the Appalachian orogen in the east to western Cordilleran orogenic belts in the west. Alternating episodes of local and distant sediment sources are suggestive of local-to-distant provenance cyclicity, with cycle boundaries occurring at fundamental transitions in sediment transport patterns. Thus, identifying provenance cycles in sedimentary successions can provide insight into variability in drainage networks, which in turn reflects tectonic or other exogenic forcing mechanisms in sediment routing systems.

Abstract Continental shelves are the key interfaces between terrestrial sediment source areas and deep-sea depositional systems, promoting the transfer of sediment across continental margins. Work on shelves in the context of entire continental-margin sediment-routing systems has focused on their importance as capacitors of sediment during several to tens of thousands of years of post-glacial shoreline transgression and sea-level highstand. We demonstrate that the tectonically active Oceanside shelf offshore southern California has served as an efficient conveyor of sediment from land to the deep sea during millennia of significant climatic fluctuations. This conveyance is a result of littoral drift of sediment to canyon heads at narrow segments of the shelf. We compare insights from the Oceanside shelf to other shelves across the tectonically active Pacific margin of the United States, and demonstrate the importance of shelf width, climatic forcings and timescale of observation in assessing the role of shelves as sediment capacitors or conveyors.

Late Quaternary turbidite and related gravity-flow deposits have accumulated in basins of the California Borderland under a variety of conditions of sediment supply and sea-level stand. The northern basins (Santa Barbara, Santa Monica, and San Pedro) are closed and thus trap virtually all sediment supplied through submarine canyons and smaller gulley systems along the basin margins. The southern basins (Gulf of Santa Catalina and San Diego Trough) are open, and, under some conditions, turbidity currents flow from one basin to another. Seismic-reflection profiles at a variety of resolutions are used to determine the distribution of late Quaternary turbidites. Patterns of turbidite-dominated deposition during lowstand conditions of oxygen isotope stages 2 and 6 are similar within each of the basins. Chronology is provided by radiocarbon dating of sediment from two Ocean Drilling Program sites, the Mohole test-drill site, and large numbers of piston cores. High-resolution, seismic-stratigraphic frameworks developed for Santa Monica Basin and the open southern basins show rapid lateral shifts in sediment accumulation on scales that range from individual lobe elements to entire fan complexes. More than half of the submarine fans in the Borderland remain active at any given position of relative sea level. Where the continental shelf is narrow, canyons are able to cut headward during sea-level transgression and maintain sediment supply to the basins from rivers and longshore currents during highstands. Rivers with high bedload discharge transfer sediment to submarine fans during both highstand and lowstand conditions.

Abstract Understanding the origin and geometry of largescale erosional surfaces in fluvial and channelized submarine depositional settings is critical for interpreting reservoir architecture and connectivity, as these surfaces strongly influence reservoir heterogeneity. We use simple and fast-running forward stratigraphic models to investigate the geometry and the relative age of complex erosional surfaces that form in both the subaerial and submarine domain. Because low-sinuosity systems tend to have relatively simple incisional and aggradational geometries, we focus on high-sinuosity systems. Fluvial deposits are commonly preserved on terraces that form during incision, and the basal erosional surface is highly time transgressive. Terraces can form without any external influence as a result of cessation of incision at channel cutoff locations. Similar processes and geometries can be observed in systems containing incising submarine channels. However, extensive deposition of fine-grained sediment in the overbank area of submarine channels tends to result in draping and long-term preservation of terrace geometries. This is in contrast with fluvial systems, as the incisional terrace morphology can be quickly buried after valley filling initiates. Once incision ceases and aggradation begins, erosional surfaces become less continuous and form an intricate network inside the larger and longitudinally more continuous valley surface. Depending on the rate of aggradation and local rate of lateral migration, the internal erosional surfaces can be similar in vertical extent to a single channel depth, or to multiple channel depths and one channel bend in plan view. Phases of low aggradation cause these scallop-shaped surfaces to connect in the downslope direction and form an extensive erosional surface, without any significant re-incision. As relatively fine-grained deposits (e.g., shale drapes, slides, and debris-flow deposits) are primarily distributed along geomorphic surfaces, differentiating time-transgressive erosional surfaces from geomorphic ones results in a better prediction of reservoir compartmentalization and fluid flow. Understanding the origin and geometry of valleys and their deposits informs the controls on the sequence stratigraphy of basin margins. That is, most erosional surfaces are time transgressive and some of them reflect the autogenic dynamics of valley formation, rather than external forcing.

Abstract Allogenic and autogenic processes interact to regulate sediment distribution in sedimentary basins. Depositional systems can respond in a complex manner to these processes, complicating interpretation of the controls on the stratigraphic record. Here we used published and constant eustatic curves in a stratigraphic forward model to examine the effects of sea-level variation on deep-water sand delivery on a passive continental margin. We found that: (1) models with constant sea level and those with eustatic fluctuations deliver similar volumes of sand to deep water; (2) both large and small eustatic variations result in similar magnitudes of fluctuations in deep-water sand delivery; and (3) deep-water sand delivery signals show similar periodicities for all models. These results suggest that the characteristics of the imposed eustatic curve may not have a significant impact on the total volume of sand delivered to deep water. We propose that the equilibrium state of the shelf-edge delta, where no net deposition or erosion occurs, could explain the similarity in deep-water sand volumes. We posit that such a state could be induced by the progradation of an initial shelf-edge delta that creates a slope which maximizes the efficiency of sediment delivery across the shelf. Because our models show that autogenic and allogenic processes can result in similar deep-water sand volumes, we conclude that other characteristics of sediment-routing systems, such as sediment supply, must exert strong controls on deep-water sand volume.