This study presents results of outcrop characterization and modeling of lithologic heterogeneity within a well-exposed point bar of the Williams Fork Formation in Coal Canyon, Piceance Basin, Colorado. This deposit represents an intermediate-scale depositional element that developed from a single meandering channel within a low net-to-gross ratio fluvial system. Williams Fork outcrops are analogs to petroleum reservoirs in the Piceance Basin and elsewhere. Analysis and modeling of the point bar involved outcrop measurements and ground-based high-resolution light detection and ranging data; thus, the stratigraphic frameworks accurately represent the channel-fill architecture.Two-and three-dimensional (2-D and 3-D) outcrop models and streamline simulations compare scenarios based on different lithologies, shale drapes, observed grain-size trends, petrophysical properties, and modeling methods. For 2-D models, continuous and discontinuous shale drapes on lateral-accretion surfaces result in a 79% increase and 24% decrease in breakthrough time (BTT), respectively, compared to models without shale drapes. The discontinuous shale drapes in the 2-D and 3-D models cause a 30% and 107% decrease, respectively, in sweep efficiency because they focus fluid flow downward to the base of the point bar. For similar reasons, 2-D models based on grain size exhibit 67-267% shorter BTT and 44-57% lower sweep efficiency compared to other model scenarios. Unlike the 2-D models, the continuous shale drapes in the 3-D models cause the fluid front to spread out and contact more of the reservoir, resulting in 42-53% longer BTT and 41-52% higher sweep efficiency compared to the other models. These results provide additional insight into the significance of intermediate-scale heterogeneity of fluvial reservoirs.

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Paleomagnetic studies of Moenkopi samples from several sites on the Colorado Plateau, including a complete 427‐m section at Sinbad Valley, western Colorado, reveal the following: (1) the presence of both normal and reversed Triassic components in many of the same samples at many localities, (2) the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the Sinbad Valley section, (3) a streaked pattern of the remanence directions in any one member in the Sinbad Valley section similar to that expected from polar wander, (4) a lack of progressive change from the base upward, and (5) the common intermediate remanence directions of weak intensity (suggesting the presence of two or more nearly balanced anti‐parallel components) throughout the formation. The results of our investigation, combined with previously reported petrographic and paleomagnetic studies of the Moenkopi Formation, have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism (CRM) acquired diagenetically over a geologically long time interval. From the available data we have formulated the following model to explain how the CRM was acquired through natural processes acting intermittently at geologically reasonable rates. When initially deposited, the Moenkopi sediments consisted primarily of first‐cycle detritus containing a few percent of iron‐bearing accessory minerals such as biotite, hornblende, magnetite, and ilmenite. At some time after deposition, under favorable subsurface conditions, intrastratal alteration began, involving breakdown of iron‐bearing silicate and oxide minerals, partial replacement of some iron‐rich silicate grains by hematite, martitization of magnetite, growth of hematite pods and blebs in ilmenite and biotite, and precipitation of crystalline hematite in both primary and secondary voids. The authigenesis spanned at least two polarity events and probably continued over several tens of millions of years. Stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies. Because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events, any given lithologic unit tended to be dominated by one polarity. The model given above seems to be capable of explaining not only those magnetic and petrographic features found in the Moenkopi Formation but also those of most other red beds.

Sediments of the Alamosa Formation spanning the upper part of the Gauss and most of the Matuyama Chrons were recovered by coring in the high (2300 m) San Luis Valley of south-central Colorado. The study site is located at the northern end of the Rio Grande rift. Lithologic changes in the core sediments provide evidence of events leading to integration of the San Luis drainage basin into the Rio Grande. The section, which includes the Huckleberry Ridge Ash (2.02 Ma) and spans the entire Matuyama Chron, contains pollen, and invertebrate and vertebrate fossils. Stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the San Luis Valley. At the end of the Olduvai Subchron, summer precipitation decreased, summer pan evaporation increased, and temperatures increased slightly compared to the earlier climate represented in the core. By the end of the Jaramillo Subchron, however, cold/wet and warm/dry cycles become evident and continue into the cold/wet regime associated with the deep-sea oxygenisotope Stage 22 glaciation previously determined from outcrops at the same locality. Correspondence between the Hansen Bluff climatic record and the deep-sea oxygen-isotope record (oxygen-isotope stages from about 110-18) is apparent, indicating that climate at Hansen Bluff was responding to global climatic changes.

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