Abstract The Paleogene sedimentary deposits of the Colorado Headwaters Basin provide a detailed proxy record of regional deformation and basin subsidence during the Laramide orogeny in north-central Colorado and southern Wyoming. This field trip presents extensive evidence from sedimentology, stratigraphy, structure, palynology, and isotope geochronology that shows a complex history that is markedly different from other Laramide synorogenic basins in the vicinity. We show that the basin area was deformed by faulting and folding before, during, and after deposition of the Paleogene rocks. Internal unconformities have been identified that further reflect the interaction of deformation, subsidence, and sedimentation. Uplift of Proterozoic basement blocks that make up the surrounding mountain ranges today occurred late in basin history. Evidence is given to reinterpret the Independence Mountain uplift as the result of significant normal faulting (not thrusting), probably in middle Tertiary time. While the Denver and Cheyenne Basins to the east were subsiding and accumulating sediment during Late Cretaceous time, the Colorado Headwaters Basin region was experiencing vertical uplift and erosion. At least 1200 m of the upper part of the marine Upper Cretaceous Pierre Shale was regionally removed, along with Fox Hills Sandstone shoreline deposits of the receding Interior Seaway as well as any Laramie Formation–type continental deposits. Subsidence did not begin in the Colorado Headwaters Basin until after 60.5 Ma, when coarse, chaotic, debris-flow deposits of the Paleocene Windy Gap Volcanic Member of the Middle Park Formation began to accumulate along the southern basin margin. These volcaniclastic conglomerate deposits were derived from local, mafic-alkalic volcanic sources (and transitory deposits in the drainage basin), and were rapidly transported into a deep lake system by sediment gravity currents. The southern part of the basin subsided rapidly (roughly 750–1000 m/m.y.) and the drainage system delivered increasing proportions of arkosic debris from uplifted Proterozoic basement and more intermediate-composition volcanic-porphyry materials from central Colorado sources. Other margins of the Colorado Headwaters Basin subsided at slightly different times. Subsidence was preceded by variable amounts of gentle tilting and localized block-fault uplifts. The north-central part of the basin that was least-eroded in early Paleocene time was structurally inverted and became the locus of greatest subsidence during later Paleocene-Eocene time. Middle Paleocene coal-mires formed in the topographically lowest eastern part of the basin, but the basin center migrated to the western side by Eocene time when coal was deposited in the Coalmont district. In between, persistent lakes of variable depths characterized the central basin area, as evidenced by well-preserved deltaic facies. Fault-fold deformation within the Colorado Headwaters Basin strongly affected the Paleocene fluvial-lacustrine deposits, as reflected in the steep limbs of anticline-syncline pairs within the McCallum fold belt and the steep margins of the Breccia Spoon syncline. Slivers of Proterozoic basement rock were also elevated on steep reverse faults in late Paleocene time along the Delaney Butte–Sheep Mountain–Boettcher Ridge structure. Eocene deposits, by and large, are only gently folded within the Colorado Headwaters Basin and thus reflect a change in deformation history. The Paleogene deposits of the Colorado Headwaters Basin today represent only a fragment of the original extent of the depositional basin. Basal, coarse conglomerate deposits that suggest proximity to an active basin margin are relatively rare and are limited to the southern and northwestern margins of the relict basin. The northeastern margin of the preserved Paleogene section is conspicuously fine-grained, which indicates that any contemporaneous marginal uplift was far removed from the current extent of preserved fluvial-lacustrine sediments. The conspicuous basement uplifts of Proterozoic rock that flank the current relict Paleogene basin deposits are largely post-middle Eocene in age and are not associated with any Laramide synuplift fluvial deposits. The east-west–trending Independence Mountain fault system that truncates the Colorado Headwaters Basin on the north with an uplifted Proterozoic basement block is reinterpreted in this report. Numerous prior analyses had concluded that the fault was a low-angle, south-directed Laramide thrust that overlapped the northern margin of the basin. We conclude instead that the fault is more likely a Neogene normal fault that truncates all prior structure and belongs to a family of sub-parallel west-northwest–trending normal faults that offset upper Oligocene-Miocene fluvial deposits of the Browns Park–North Park Formations.

Lower Cretaceous strata of Kansas are divisible into two unconformity-bound stratigraphic sequences that are overlain by a third sequence of Upper Cretaceous age. The sequences were deposited on the eastern shelf of the Western Interior foreland basin and are equivalent to sequences defined by Weimer (1984) in the center of the basin in Colorado. The bounding unconformities are present throughout the outcrop belt of central Kansas, and are contiguous with the ones at the bases of the Plainview Formation, J Sandstone, and D Sandstone, respectively, of the Denver basin. A basal Cretaceous sequence consisting of the Lytle Formation of Colorado (informally “Cheyenne Sandstone” of southern Colorado) is restricted to extreme northwest Kansas and is not detailed in this study. The lower sequence of Kansas onlaps this basal Cretaceous sequence and progressively oversteps it to the east. This sequences also onlaps and oversteps Jurassic and Permian strata from west to east. Internally it consists of landward-stepping progradational events of fluvial and nearshore siliciclastics succeeded by open-marine shale. Included in the sequence are the Cheyenne Sandstone (formal) of Kansas (fluvial), the Longford Member of the Kiowa Formation (nearshore), and the Kiowa Formation (marine). Strata deposited during a relative sea-level fall at the top of the sequence are present in Colorado but were mostly removed by erosion in Kansas prior to deposition of the second sequence. The second sequence comprises strata between the unconformity at the top of the Kiowa Formation to the unconformity within the middle of the Dakota Formation of Kansas. It also is composed of landward-stepping progradational events of fluvial and nearshore siliciclastics succeeded by marine shale. Fluvial deposits dominate the sequence in central Kansas, and merge through facies transitions with nearshore and marine strata in western Kansas and eastern Colorado. Included in the sequence are the lower half of the Dakota Formation (fluvial), J Sandstone (fluvial and nearshore), and Huntsman Shale (marine). Strata deposited during a relative sea-level fall at the top of the sequence are present in the subsurface of western Kansas but were removed by erosion prior to deposition of the upper half of the Dakota Formation. The unconformity at the top of the second sequence is the onlap surface and basal bounding unconformity of a third sequence that extends upsection to an unconformity at the Codell Sandstone. As in the two underlying sequences, basal strata of this sequence (D Sandstone) occur in landward-stepping progradational events. This unconformity is distinguished in central Kansas outcrops at the base of a laterally continuous conglomeratic sandstone that represents a seaward shift in facies from meandering to braided stream deposits. The lower unconformity of this sequence is reported as Upper Cretaceous age in eastern Colorado. Therefore the upper one-half of the Dakota Formation in Kansas, including all of the Janssen Clay Member and the upper part of the Terra Cotta Clay Member, is Upper Cretaceous.