Abstract

Although delta-building processes and the resultant facies and facies associations have been studied for over a century, there remains a lack of well-documented facies architectural studies of ancient deltas. In this study, architectural-element analysis is applied to determine the basic building blocks of a Cretaceous delta at the top of the upper Turonian Wall Creek Member exposed along the western flanks of the Powder River Basin, Wyoming. We document the bed-scale basic building blocks of this ancient delta, and show them in the context of their facies and stratal variability, and their hierarchical spatial arrangement within a single deltaic parasequence. We also test the idea that external sand-body shape may not reflect the internal facies complexity, as has recently been suggested in studies of several modern deltas, including the Burdekin in Australia, the Brazos in the Gulf of Mexico, and the Baram–Trusan in Borneo.

Five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits. These elements are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally modulated deposit (TM), and bar accretion (BA). Seasonal to decadal river floods are thought to represent the main building phases of the delta, producing channels, bars, and frontal splay elements. During intervening periods, the delta was reworked by waves, storms, and tides, producing tidally modulated and storm sheet elements. Due to the complex interactions of river effluents with waves, storms and tides, the delta is interpreted as mixed-influenced. Regional sandstone isolith, facies, and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front. The channels, bars, frontal splays, and tidally modulated elements are found only near the distributary mouth, whereas the storm sheets occur extensively away from the distributary mouth.

The plan-view morphology of the studied delta shows a smooth-fronted, arcuate to cuspate shape, which should indicate wave dominance. In contrast, our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking. Additional outcrop mapping shows that storm-wave-reworked sands are attached to the flanks of the system. Previously published delta models predict incorrectly that the tidal reworking of a river-flood dominated system should result in a more bird-foot shape versus the lobate geometry that is actually mapped. Our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity.