Abstract This field trip highlights recent research into the Laramide uplift, erosion, and sedimentation on the western side of the northern Colorado Front Range. The Laramide history of the North Park-Middle Park basin (designated the Colorado Headwaters Basin in this paper) is distinctly different from that of the Denver basin on the eastern flank of the range. The Denver basin stratigraphy records the transition from Late Cretaceous marine shale to recessional shoreline sandstones to continental, fluvial, marsh, and coal mires environments, followed by orogenic sediments that span the K-T boundary. Upper Cretaceous and Paleogene strata in the Denver basin consist of two mega-fan complexes that are separated by a 9 million-year interval of erosion/non-deposition between about 63 and 54 Ma. In contrast, the marine shale unit on the western flank of the Front Range was deeply eroded over most of the area of the Colorado Headwaters Basin (approximately one km removed) prior to any orogenic sediment accumulation. New 40 Ar- 39 Ar ages indicate the oldest sediments on the western flank of the Front Range were as young as about 61 Ma. They comprise the Windy Gap Volcanic Member of the Middle Park Formation, which consists of coarse, immature volcanic conglomerates derived from nearby alkalic-mafic volcanic edifices that were forming at about 6561 Ma. Clasts of Proterozoic granite, pegmatite, and gneiss (eroded from the uplifted at Laramide basin subsidence, sedimentation, and deformation in north-central Colorado, in Morgan, L.A., and Quane, S.L., eds., Through the Generations: core of the Front Range) seem to arrive in the Colorado Headwaters Basin at different times in different places, but they become dominant in arkosic sandstones and conglomerates about one km above the base of the Colorado Headwaters Basin section. Paleocurrent trends suggest the southern end of the Colorado Headwaters Basin was structurally closed because all fluvial deposits show a northward component of transport. Lacustrine depositional environments are indicated by various sedimentological features in several sections within the >3 km of sediment preserved in the Colorado Headwaters Basin, suggesting this basin may have remained closed throughout the Paleocene and early Eocene. The field trip also addresses middle Eocene(?) folding of the late Laramide basin-fill strata, related to steep reverse faults that offset the Proterozoic crystalline basement. Late Oligocene magmatic activity is indicated by dikes, plugs, and eruptive volcanic rocks in the Rabbit Ears Range and the Never Summer Mountains that span and flank the Colorado Headwaters Basin. These intrusions and eruptions were accompanied by extensional faulting along predominantly northwesterly trends. Erosion accompanied the late Oligocene igneous activity and faulting, leading to deposition of boulder conglomerates and sandstones of the North Park Formation and high-level conglomerates across the landscape that preserve evidence of a paleo-drainage network that drained the volcanic landscape.

Abstract The Morrison Formation is a laterally extensive terrestrial deposit representingan ecologically diverse assemblage of paleoenvironments from the Late Jurassic ofwestern North America. Although the Morrison Formation has recently been interpretedas a semiarid lowland savannah on the basis of geological and paleobiologicalindicators, many microenvironments within this system are more consistent with newinterpretations of the Morrison as a ground-water dominated “wetland” deposit. Herewe report new fossils from a little-studied exposure of the Morrison Formation in andaround Temple Canyon Park near Cañon City, Colorado. The Temple Canyon sectionshows a relatively thin sequence of conglomerate, sandstone, siltstone, mudstone, andlimestone beds representing alluvial to fluvial and possibly lacustrine deposition. Thesection rests on Precambrian basement and is unconformably overlain by the LowerCretaceous Lytle Formation. The mudstone and limestone beds preserve an abundantfossil flora and fauna distinct from those previously described from the MorrisonFormation. The floral assemblage includes species of algae, bryophytes, ferns, ginkgophytes,horsetails, cycads, bennettites, and conifers; together these plants indicatea warm climate with abundant local water supply. The faunal assemblage containsostracodes, conchostracans, traces of aquatic insect larvae, a terrestrial insect bodyfossil, prosobranch and pulmonate gastropods, many species of fish, a possible frog,and rare turtle remains. The presence of prosobranch gastropods, fish, and aquaticinsect larvae suggests a perennial water body with high oxygen content, while thepresence of conchostracans and pulmonate gastropods may indicate some fluctuationin water quality. Keywords: Morrison Formation, Jurassic, Cañon City, Colorado, paleoecology.