Abstract

The John Henry Member (JHM) of the Straight Cliffs Formation exposed along the Kaiparowits Plateau provides a record of ∼ 6 My of coastal-plain to marginal-marine deposition. To better understand the mechanisms influencing the alluvial architecture of the JHM, a detailed quantitative characterization of the stratigraphic organization and stacking patterns of fluvial channel-belt sandstone bodies was performed. Techniques of point pattern analysis were applied on three datasets consisting of 136, 228, and 55 channel belts collected from two outcrops in Bull Canyon and one outcrop in Rock House Cove; both localities are in the southwestern Kaiparowits Plateau.

The results reveal two major trends in the spatial organization of channel belts through time, starting with a trend of decreasing channel-belt clustering from the lower to middle stratigraphic interval, followed by a reversed trend of increasing channel-belt clustering in the upper interval. Furthermore, up-section changes in channel-belt clustering exhibit strong similarities across the three outcrops as indicated by a correlation analysis based on dynamic time warping. Trends of consistent channel-belt clustering in Bull Canyon and Rock House Cove are interpreted to be mainly the product of changing allogenic conditions influencing autogenic processes. Correlation of clustering trends with reconstructed paleo-hydraulic conditions and previously measured architectural parameters suggests that significant channel-belt clustering likely reflects the combined influence of two major factors: 1) low aggradation rates driven by reduced accommodation and 2) high avulsion frequency where channels are deposited downstream of a backwater-mediated nodal avulsion point.

A scale-dependent clustering analysis using the K-function indicates that channel belts are clustered at small length scales (∼ 10 to 50 m) and random or uniform at larger length scales (> 60 m). Small-scale clustering is interpreted to be the product of autogenic avulsion reoccupation, where abandoned channels that are preserved as topographic lows serve as attractors to new avulsion paths and lead to channel persistence. Conversely, the large-scale pattern of randomness and/or uniformity reflects periodic large-scale shifts in the local depocenter as a result of avulsion-driven compensational stacking of channel belts.