Architectural-Element Analysis: A New Method of Facies Analysis Applied to Fluvial Deposits

A review of the braided-river depositional environment

This article serves as an introduction to the papers dealing with braided river deposits in this volume. A lithofacies code erected earlier by the writer is expanded to include matrix-supported gravel, low-angle cross stratified sand, erosion surfaces with intraclast conglomerates, and massive mud deposits. The four vertical profile models erected by the writer are expanded to six. A new model, the “Trollheim type” is proposed, to include gravelly deposits characterized by abundant debris flows. The Donjek sequence type is restricted to gravel-dominated cyclic deposits and a new model, the “South Saskatchewan type”, is erected for sand dominated cyclic deposits. The Scott, Platte and Bijou Creek models remain essentially unchanged.

Lithofacies Types and Vertical Profile Models in Braided River Deposits: A Summary

Summary
Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long-term dynamics of fluvial systems and their responses to external or ‘allogenic' controls, can play important roles in research concerning both global change and sequence-stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium.

This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea-level and the concept of accommodation. Late Quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine–Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea-level change on continental margin systems.

The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea-level change. At present, empirical field-based research on fluvial response to climate and sea-level change lags behind: (a) the global change community's understanding of the magnitude and frequency of climate and sea-level change; (b) the sequence-stratigraphic community's desire to interpret climate and, especially, sea-level change as forcing mechanisms; and (c) the modelling community's ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (102−103 years) and longer (104−106 years) time-scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre-Quaternary rock record.

Fluvial responses to climate and sea‐level change: a review and look forward

Brahmaputra River : Channel processes and sedimentation

Facies models : response to sea level change

Five main facies of deep-water clastic rocks can be defined: classic turbidites, massive sandstones, pebbly sandstones, conglomerates, and debris flows (with slumps and slides). The classic turbidites consist of monotonously parallel-interbedded sandstones and shales without channeling; internal sedimentary structures include grading, parallel lamination, and cross-lamination. Massive sandstones are thicker, coarser, and commonly channelized. They lack the sedimentary structures of classic turbidites, but do contain evidence of dewatering during deposition. Pebbly sandstones tend to be well graded, and can contain parallel stratification and large-scale cross-stratification. Conglomerates are characterized by inverse and normal grading, parallel and cross-stratification, nd commonly have a preferred clast fabric (imbrication). Both the pebbly sandstones and conglomerates commonly are channelized. The facies can be fitted into a model of submarine-fan deposition. Modern fans are subdivided into an upper fan (suprafan), characterized by (1) a single deep channel with levees, (2) a middle fan, built up from suprafan lobes that periodically switch in position, and (3) a topographically smooth lower fan. The suprafan lobes have shallow, braided channels on their inner parts, but the outer suprafan lobes are smooth, and grade basinward into the smooth lower fan and basin plain. The smooth suprafan lobes and lower fan are characterized by deposition of the classic turbidite facies, and the braided part of the suprafan lobes by massive and pebbly sandstones. When one lobe is abandoned and another starts to prograde elsewhere, the first lobe is blanketed by mud, forming a potential stratigraphic trap. The upper-fan channel is an area of coarse sediment deposition, or conglomerates where gravel and boulders are supplied to the basin. During fan progradation, thickening- and coarsening-upward facies sequences can be formed in a manner analogous to those of deltas. Fan channels also can be abandoned progressively, forming thinning- and fining-upward sequences similar to those of fluvial or distributary channels. These sequences can be identified on electric logs. Where basin shales act as hydrocarbon-source areas, the classic turbidites can act as conduits, leading the hydrocarbons to the thicker, laterally coalesced massive and pebbly sandstones of the braided suprafan lobes. These bodies can be of the order of 25 km in diameter, and up to 100 m thick. The coarse deposits of the upper-fan channel also might form good reservoirs, being bounded by shales (levee deposits) on either side, and possibly by shales above if the fan-channel system is abandoned. Such channels can be tens of kilometers long, several kilometers wide, and a few hundred meters deep. Reservoirs may be present in all of these environments.

Deep-Water Sandstone Facies and Ancient Submarine Fans: Models for Exploration for Stratigraphic Traps

Wave-and storm-dominated shallow marine systems.

The South Saskatchewan River has a long term average discharge of 275 m3/sec, with flood peaks in the range of 1500 to 3800 m3/sec. South of Saskatoon, the four major types of geomorphological elements recognised are channels, slipface-bounded bars, sand flats and vegetated islands and floodplains. Major channels are 3-5 m deep, up to 200 m wide, and flow around sand flats which are 50-2000 m long, and around vegetated islands up to 1 km long. At areas of flow expansion, long straight-crested cross-channel bars form. During falling stage, a small part of the crest of the cross-channel bar may become emergent, and act as a nucleus for downstream and lateral growth of a new sand flat.



The dominant channel bedforms are dunes, which deposit trough cross bedding. Cross-channel bars deposit large sets of planar tabular cross bedding. Sand flats that grow from a nucleus on a cross-channel bar are mostly composed of smaller planar tabular sets, with some parallel lamination, trough cross-bedding, and ripple cross-lamination. A typical facies sequence related to sand flat growth would consist of in-channel trough cross-bedding, overlain by a large (1-2 m) planar tabular set (cross-channel bar), overlain in turn by a complex association mostly of small planar tabular cross-beds, trough cross-beds and ripple cross-lamination.



By contrast, a second stratigraphic sequence can be proposed, related only to channel aggradation. It would consist dominantly of trough cross-beds, decreasing in scale upward, and possible interrupted by isolated sets of planar tabular cross-bedding if a cross-channel bar formed, but failed to grow into a sand flat. During final filling of the channel, ripple cross-lamination and thin clay layers may be deposited. In the S. Saskatchewan, these sequences are a minimum of 5 m thick, and are overlain by 0.5-1 m of silty and muddy vertical accretion deposits.

Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada

The Kicking Horse River is a gravelly, braided stream characterized by very low winter discharge, a peak spring flood (70 m3s−1 in 1973), and summer diurnal discharge fluctuations (from 18 to 44 m3...

Roger G. Walker

Papers

Facies models : response to sea level change

Deep-Water Sandstone Facies and Ancient Submarine Fans: Models for Exploration for Stratigraphic Traps

Wave-and storm-dominated shallow marine systems.

Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada

Bar evolution and development of stratification in the gravelly, braided, Kicking Horse River, British Columbia