Abstract

There are numerous allocyclic and autocyclic factors which influence fluvial sandbody geometry. Unraveling the relative importance of such factors is challenging because fluvial systems can migrate and avulse across significant distances on the floodplain, so that collecting datasets that cover a large enough area with sufficient detail is difficult. This problem has been addressed by acquiring an oblique helicopter-mounted lidar survey over 16 km of exposed sections from Cretaceous coastal-plain deposits cropping out in the Wasatch Plateau of east-central Utah. The exposures of the Blackhawk Formation occur in steep cliffs, which are continuous for several tens of kilometers, and have been previously inaccessible for study.

The recorded lidar data were used to build detailed photorealistic models (virtual outcrops) from which quantitative dimensional data on fluvial architectural elements (mostly sandstone channel bodies) were collected. Those channelized bodies show significant variation in width and thickness. 26% of the sandbodies were identified as being multistory channel bodies, with a thickness of 9.1 m (± 4.67 m; 1 σ) and a width of 142.6 m (± 180.7 m), which are significantly larger than the measured single-story sandbodies (5.9 m and 96.9 m, respectively). 50% of all sandbodies have aspect ratios smaller 13∶1 and the highest values reach 78∶1.

The spatial constraint of the 397 fluvial sandbodies enabled the recognition of a series of vertical trends within the dataset. These trends include a subtle upward increase in width and thickness of both the mean and maximum values. This is coupled with a general upward decrease in the number of sandbodies and results in a broadly constant channel-to-overbank ratio. A clear correlation between wider channels with increased distance to the shoreline could also be documented.

These observations can be explained by considering the entire Blackhawk Formation as the deposits of a large distributive fluvial system. In this model the key control on channel architecture is the distance to the shoreline, contrasting with previous interpretations of the Blackhawk depositional system in which the rate of accommodation creation in the coastal plain was thought to be the key control on channel geometry and stacking patterns.