Low-angle crossbedded sandstone

Conditions for bedform formation presented in different hydrodynamic contexts

An addition to the Lithofacies Series

Use this link to read the introduction to the lithofacies series.

General occurrence

One of the subordinate but no less significant sandy lithofacies, low-angle crossbedded sandstone is commonly is interbedded with plane-bed laminated sandstone. Recognition of both lithofacies in the rock record is important because they preserve one of the few lines of evidence for upper flow-regime hydrodynamic conditions. Low-angle crossbedded sandstone is generally found in fluvial and shallow marine depositional environments. Several authors include low-angle antidune bedforms in this category, but in this series antidune bedding will be treated separately.

External structure

Low-angle in this context refers to the inclination of crossbed foresets with respect to their substrate and the discordant contact between crossbed sets. Their low angle confers a low relief surface expression such that recognition of distinct bedform surfaces such as stoss and lee faces is difficult. In many situations the lithofacies appears more wedge-like. The boundaries between crossbed sets tend to be planar.

Internal structure

The lithofacies general occurs in relatively clean, fine- to medium-grained sandstones. Crossbed laminae dips are generally less than 15^o. Likewise, the contacts between crossbed sets also have very shallow dips; in 2D outcrops these contacts are flat. Foresets are usually tangential at their base. If tangential tops are present the foresets have an asymptotic geometry.

Parting lineation is common.

Outcrop view of numerous low-angle crossbed sets in medium-grained sandstone. The discordant contacts between successive crossbed sets are <10o to 12o (a few contacts indicated by arrows). Foreset dips are <15o. The lithofacies is part of a prograding shoreface succession. Pen (left centre) is 15 cm long. Jurassic, Bowser Basin, northern British Columbia. — Outcrop view of numerous low-angle crossbed sets in medium-grained sandstone. The discordant contacts between successive crossbed sets are <*10^o to 12^o* (a few contacts indicated by arrows). Foreset dips are <*15^o*. The lithofacies is part of a prograding shoreface succession. Pen (left centre) is 15 cm long. Jurassic, Bowser Basin, northern British Columbia.

Miall (2006) has noted that the bedforms can accumulate on surfaces that are initially flat or inclined. Distinguishing between the two conditions is possible by comparing the orientation of the lithofacies with underlying lithofacies, such as laterally extensive ripple bedded units that probably originated on relatively flat substrates.

Formation – hydrodynamic conditions

Deposition of low-angle crossbed lithofacies probably takes place under conditions that span the hydrodynamic interval between upper plane bed flow and formation of antidunes; the presence of parting lineation confirms this interpretation. This corresponds to the transition from subcritical (tranquil) to critical flow (standing waves and antidunes) and supercritical flow.

Summaries of the hydraulic conditions for deposition of plane bed lamination, compared with other bedforms, in terms of: 1- Flow Regime (left); after J.C. Harms & R.K. Fahnestock, 1965, 2- Median grain size and mean current velocities (centre); modified from Ashley, G.M. 1990, 3- Reynolds and Froude numbers as a function of flow depth and flow velocity (right); modified slightly from J.R.L. Allen,1992, Fig. 1.21. Upper flow plane bed and low-angle crossbed fields are indicated in bold red type. — *Summaries of the hydraulic conditions for deposition of upper flow regime bedforms, compared with other bedforms. Data for the graphical plots is mostly from flume experiments*:
1- Flow Regime (left); after **J.C. Harms & R.K. Fahnestock**, 1965,
2- Median grain size and mean current velocities (centre); modified from **Ashley, G.M.** 1990,
3- Reynolds and Froude numbers as a function of flow depth and flow velocity (right); modified slightly from **J.R.L. Allen**,1992, Fig. 1.21.
Upper flow plane bed and low-angle crossbed fields are indicated in bold red type.

Common environments

The low-angle crossbedded sandstone lithofacies commonly is associated stratigraphically with upper plane bed laminated sandstone lithofacies. Both lithofacies are important because they preserve upper flow regime hydrodynamic conditions.

In fluvial systems the lithofacies most commonly develops in the proximal regions of crevasse splays that spill from river channels and delta distributary channels. In this setting the lithofacies will be associated stratigraphically and laterally with ripple, interbedded mudstone, and possible organic paleosols that develop across adjacent flood plains. The lithofacies can also fill shallow channel scours.

A section of beach deposits exposed during storm surge erosion and subsequent redistribution of sand to the shoreface. The section contains low-angle crossbeds in well sorted fine- to medium-grained sand – discordant contacts indicated by arrows. The section is parallel to the shoreline (i.e., parallel to depositional strike). Raglan, west coast New Zealand.

Low-angle crossbedded sand and sandstone is common on sandy beaches, particularly in the wave swash and backwash zone. In this case the depositional surface is initially inclined seaward. Like the laminated upper plane bed lithofacies, it will generally occur near the top of coarsening-upward, prograding shoreface successions, as shown in the example below.