Paleoslope Reconstruction In Sandy Suspended-Load-Dominant Rivers

Abstract

Constraining the depositional gradient of ancient alluvial river systems can aid in reconstructing landscapes and estimating paleodischarge by establishing boundaries on the climatic and tectonic history of continental sequences. We present three methods for estimating ancient depositional gradients based on the interpreted mode of sediment transport for the range of particle sizes found on the beds of modern rivers or preserved in channel fills. For sandy rivers with suspension as the dominant mode of sediment transport, these methods take advantage of observations that can be directly obtained from preserved strata, including measurements of paleo flow depth and grain size. The first method relates river slope to Shields number at bankfull flow. The second method is similar to the first but allows for variation in Shields number at bankfull flow with grain diameter. The final method relies on criteria required for both suspended-and bed-material load sediment transport in the same system and is the most comprehensive and physically justified method. We present each method and test them with modern river datasets to verify accuracy and help constrain uncertainty-the first step to adapting them to ancient systems. Results indicate that all methods estimate slope within a factor of two. These methods are potentially very powerful for interpreting sandy fluvial deposits because they can provide reasonably accurate quantitative estimates of paleoslope, an elusive yet important environmental variable.

Transport Scaling of Dune Dimensions in Shallow Flows

“…These values allow a transport stage calculation using and τ * c from Shields‐type curves (e.g., Brownlie, ; Garcia, ; Yalin, ).Alternatively, the suspension threshold can used by calculating and w s using Dietrich () or the simplified method of Ferguson and Church (). The indirect approach of reconstructing a flow depth from dune dimensions in the rock record is similar but involves the following steps: (1) measure cross set thickness and estimate dune height using methods that statistically link cross‐set thickness to dune height (e.g., Leclair & Bridge, ; Paola & Borgman, ); (2) measure channel slope in outcrop or reconstruct slope using methods such as that described by Lynds et al (); and (3) measure grain size. Because h exists on both the right‐ and left‐hand side in equation , it must be solved iteratively.…”

Model for the Formation of Single‐Thread Rivers in Barren Landscapes and Implications for Pre‐Silurian and Martian Fluvial Deposits

“…Bank‐strengthening grain sizes can be estimated by inspection of fine intervals and fed into a bank‐cohesion model (equations –). Paleo‐channel width can then be estimated by iteratively solving equation , using the inferred W / h combined with estimates of paleo‐bed slope (e.g., from bed stress estimates, bed grain size, and flow depth; Lynds et al, ; Trampush et al, ; Mahon & McElroy, ; Figure ). For example, consider a putative ancient channel‐bed deposit from an Amargosa‐like river (Ielpi, ), with grain sizes of ~100 μm and that contains ~1‐m‐tall bar forms and current ripples with wavelengths of ~11 cm.…”

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

“…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 . Given estimates of characteristic flow depth and paleo‐slopes from outcrop observations [e.g., Paola and Mohrig , ; Petter , ; Lynds et al , ; Trampush et al , ], one can make a first‐order prediction of the location of paleo‐avulsions using the estimated backwater length. Further, given observations of inferred avulsion sites within the rock record [e.g., Mohrig et al , ; Chamberlin and Hajek , ], the backwater length scale can be used to infer the location of paleo‐shorelines.…”