Field test of autogenic control on alluvial stratigraphy (Ferris Formation, Upper Cretaceous-Paleogene, Wyoming)

Abstract

Internally generated (autogenic) sedimentary processes can obscure signals of tectonic movements, climate conditions, or sea-level fl uctuations in alluvial basins. The stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 4 -10 6 yr. However, recent physical and numerical models have shown that over basin-fi lling time scales, self-organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions. Here, we present the fi rst fi eld-based test for autogenic control on stratigraphy in an ancient alluvial basin. The Ferris Formation (Upper Cretaceous-Paleogene, Hanna Basin, Wyoming) is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material-a pattern that has been proposed as an example of a potentially autogenic stratigraphy. In order to evaluate controls on Ferris Formation stratigraphy, spatial patterns of key channel properties were analyzed. These variables (including sand-body dimensions, paleofl ow depth, maximum clast size, paleocurrent direction, and sediment provenance) were chosen because they can change in response to climate, tectonic, or sea-level forcing and are commonly measurable in ancient alluvial successions. No clear statistical trends were detected in the measured variables, which suggests that external forces did not likely control stratigraphic organization in the unit. This study demonstrates that autogenic dynamics in natural, fi eld-scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed. This fi nding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate, tectonic, and eustatic changes from alluvial basins.

Avulsion cycles and their stratigraphic signature on an experimental backwater‐controlled delta

“…The density of channel‐body clustering is high around the avulsion sites compared to the channel‐body clustering downstream of avulsions (Figure ). Although floodplain deposition of fine sediment was absent in our experiment, these results indicate that the channel‐body clustering may give insight into the location of paleo‐river avulsions in deltaic deposits [e.g., Jerolmack and Paola , ; Hajek et al , ]. Our experimental results of avulsion length scaling with the backwater length (Figure c) also bolster the use of backwater length as a paleohydraulic reconstruction tool [ DiBiase et al , ] where the avulsion length, channel bed slope, and characteristic flow depth are interrelated, i.e., L A ~ L b = h c /S .…”

“…Moreover, river avulsions present a serious hazard to communities living on deltas worldwide, and understanding their mechanics is vital for sustainable management of deltas, and forecasting deltaic evolution in the face of sea level rise, river engineering, and land use changes upstream of river deltas. From a stratigraphic viewpoint, river avulsions control the distribution of sediment and the stacking patterns of channel bodies in the subsurface [e.g., Allen , ; Bridge and Mackey , ; Mackey and Bridge , ; Mohrig et al , ; Hajek et al , ], and thus are of significant importance for unraveling the mechanics of ancient deltaic systems [e.g., Paola , ; Kleinhans , ; Bianchi and Allison , ; DiBiase et al , ], and for characterization of aquifers and hydrocarbon reservoirs [ Bohacs and Suter , ; Ainsworth et al , ].…”

“…Following the setup phase, channel bed aggradation results in an increased likelihood of overbank flows, and the avulsion “trigger” is thought to be a short timescale event, which causes the water flow to leave the channel (e.g., overbank flooding), which may occur via a levee breach (Figure b) [ Mohrig et al , ; Slingerland and Smith , ; Edmonds et al , ; Hajek et al , ], and result in a river avulsion. As overbank flow occurs, several incipient channels may develop on the floodplain—referred to as the “finding phase” (Figure b) [ Hoyal and Sheets , ; Martin et al , ; Reitz and Jerolmack , ]—before the selection of the new (daughter) channel path (Figure c).…”

Testing morphodynamic controls on the location and frequency of river avulsions on fans versus deltas: Huanghe (Yellow River), China

“…Avulsion “set up” arises where a spatial decrease in water surface slope reduces sediment transport capacity, thus producing a spatial convergence in sediment flux and concomitantly generating in‐channel sediment deposition [ Mohrig et al ., ]. The avulsion “trigger” is typically an event whereby water flow leaves the channel (e.g., overbank flooding) and/or where erosion induces a levee‐breach, during floods [e.g., Edmonds et al ., ; Hajek et al ., ]. While triggers are typically associated with flood events, the antecedent processes are critical to influencing the location and frequency of avulsions.…”

Applying Information Theory and Bayesian Inference to Paleoenvironmental Interpretation

“…The applicability of the procedure is limited by our understanding of the forward processes, which controls the quality of the probability assigning step (equation ). For instance, only in recent decades has the community started to realize that not all changes in the deposits D B need to incorporate changes in the received signal S A ′ based on field investigations (Beerbower, ; Hajek et al, ; Lewin, ) and flume experiments (Muto & Steel, ; Van Dijk et al, ). For example, the retreat of the Brent Delta was originally interpreted as a result of a reduction in sediment input or increased subsidence (Helland‐Hansen et al, , ); but more recently it has been shown that constant sediment supply and steady relative sea‐level rise could result in the same overall stratal geometry (Muto & Steel, ).…”