Fluvial architecture of the lower Williams Fork Formation (middle Mesaverde Group), Douglas Creek Arch, Colorado

The main coal-bearing and lowermost upper intervals of the upper Cretaceous middle Mesaverde Group in the Douglas Creek Arch, northwestern Colorado are representative of marginal-marine to lower coastal-plain depositional environments. These strata serve as outcrop analogs to reservoir elements that are laterally equivalent to similar deposits in the adjacent Piceance and Uinta basins. The study interval is relatively low net-to-gross (<50% sandstone) and includes approximately 540 ft (164.6 m) of sandstone and mudstone, with two thin coal layers occurring near the base of the study interval. Sandstones within the study area are mainly very fine to fine grained, with a small percentage occurring as medium grained, and commonly exhibit ripple lamination and cross-stratification. Paleocurrent data obtained from outcrops (N=172, vector mean=86.5°) indicates an overa ll easterly direction of sediment transport.;Review of facies and architectural-element geometries indicates a lower coastal-plain depositional environment for the entire interval, with possible tidal influences on the mainly fluvial succession. Architectural elements found in the study area include (1) single-story channel body, (2) multi-story channel body, (3) crevasse splay, and (4) lacustrine. Eleven facies were defined based on lithology and sedimentary structures. Gamma-ray data for each facies suggests no clear correlation based solely on sedimentary structure variability. Architectural-element gamma-ray analysis indicates that sandstones predominantly show lower gamma-ray values than siltstones; however, overlap occurs between architectural elements, which may pose problems for interpretation based on well-log data alone.;Five detailed measured sections were used to correlate sandstone bodies within the 0.5 mi2 (1.3 km2) study area in efforts to understand stratigraphic variability. Apparent widths, thicknesses and geometries aided in identifying architectural elements, which were then reviewed to determine spatial variability of sandstone bodies. Apparent widths range from 7 to 1149 ft (2.3 to 350.3 m), and maximum thicknesses range from 0.3 to 25.5 ft (0.1 to 7.8 m). Single-story channel bodies occur most commonly in the lower and upper thirds of the study interval, with apparent widths ranging from 7 to 69 ft (2.3 to 21.0 m), maximum thicknesses ranging from 0.3 to 11 ft (0.1 to 3.4 m), and width/thickness ratios ranging from 9 to 79. Multi-story channel bodies occur in most commonly in the middle third of the study interval, with apparent widths ranging from 40 to 1149 ft (12.2 to 350.3 m), maximum thicknesses ranging from 0.7 to 25.5 ft (0.2 to 7.8 m), and width/thickness ratios ranging from 10 to 118. Channel bodies tended to be more laterally continuous and thicker near the middle portion of the study interval, with multi-story channel bodies most commonly occurring above and below a potential sequence boundary. This increase in amalgamation may indicate an increase in accommodation rate, marking a change from a highstand systems tract to a lowstand systems tract. Crevasse splays tend to occur in the lower net-to-gross portions of the interval, with apparent widths ranging from 7 to 178 ft (2.3 to 54.3 m), maximum thicknesses ranging from 0.3 to 2.7 ft (0.1 to 0.8 m), and width/thickness ratios ranging from 26 to 103.;Apparent widths and maximum thicknesses of similar architectural elements from the present study (N=43) were compared data to previous studies conducted in Coal Canyon, Colorado (Cole and Cumella, 2005, N=136; Pranter et al., 2009, N=668). Depositional environments and sandstone-body types were very similar, but dimensional data were less consistent. General width/thickness trends for the Coal Canyon data indicate greater apparent widths and thicknesses in channel bodies than crevasse splays. Single-story channel bodies in Coal Canyon have apparent width values ranging from 44 to 1700 ft (13.4 to 518.2 m), maximum thickness values ranging from 3.9 to 29.9 ft (1.2 to 9.1 m) and width/thickness ratios ranging from 6 to 149. Like West Creek, multi-story channel bodies in Coal Canyon are larger than single-story channel bodies, with apparent width values ranging from 53 to 2791 ft (16.2 to 850.7 m), maximum thicknesses ranging from 5 to 47.1 ft (1.5 to 14.4 m) and width/thickness ratios ranging from 4 to 221. Crevasse splays have apparent width values ranging from 40.1 to 843.3 ft (12.2 to 257.0 m), maximum thickness values ranging from 0.5 to 15 ft (0.2 to 4.6 m) and width/thickness ratios ranging from 9.8 to 718.2. Comparison between the two data sets indicates variation in sandstone-body geometry and magnitude, with crevasse-splay architectural elements showing the most variation. Interpretive differences, sandstone-body orientations, exposure quality, measurement methods, and erosion are the likely causes of variation between the two data sets.