Paleohydraulics of chute-and-pool structures in a Paleoproterozoic fluvial sandstone

doi:10.1016/S0037-0738(99)00013-5

Sedimentary Geology

Volume 125, Issues 3–4, May 1999, Pages 129-134

https://doi.org/10.1016/S0037-0738(99)00013-5 Get rights and content

Abstract

Chute-and-pool structures consist of an inclined surface leading into a pool where a hydraulic jump causes deposition of backset laminae. Structures fitting this description are present at the base of a very fine-grained sandstone bed in the fluvial Matinenda Formation of northern Ontario. Reconstruction of the paleohydraulic system indicates that a depth of approximately 1 cm and a velocity of approximately 65 cm/s formed these bedforms during the initial stage of a flow with increasing depth and decreasing velocity. This type of deposit should be most common at the base of sandy beds deposited as emergent bars are overtopped during increasing discharge events.

Introduction

Coarse-grained sandstones of the Matinenda Formation outcrop to the north of Lake Huron and form the basal clastic unit of the Huronian Supergroup. Braided, glacial outwash streams, with high width-to-depth ratios, deposited the sands in channels dominated by migration of dunes and transverse bars (Fralick and Miall, 1987, Fralick and Miall, 1989). A fining trend, from coarse- and very coarse-grained sands to fine- to coarse-grained sands occurs down paleoslope, to the southeast (Fralick, 1985; Fralick and Miall, 1989). An outcrop in the source-distal part of the outcrop area, at Denvic Lake (Fig. 1), contained bedforms morphologically similar to chute-and-pool structures described in the literature (Davies, 1890; Power, 1961; Jopling and Richardson, 1966; Schmincke et al., 1973). In the experiments of Jopling and Richardson (1966)the flow passed down a chute, where it constantly accelerated and decreased in depth. This led to the development of a hydraulic jump where the Froude Number (Fr) dropped below 1. Immediately downstream from the jump a series of steeply dipping backset laminae were deposited. The process necessitates the morphology of an inclined ramp (chute) with a series of backset laminae at its base (pool). This configuration typifies the structures in the study outcrop (Fig. 2A–C).

Two beds are present in this glacially polished outcrop: an underlying coarse-grained, white sandstone, which is a minimum of 1 m thick, and an overlying tan, very fine-grained sandstone, which is at least 1.5 m thick. Five chute-and-pool structures are present along the contact between the two units.

Section snippets

Vertical sequence of sedimentary structures

The chute-and-pool structures are formed on the eroded surface of the underlying coarse-grained sandstone (Fig. 2A–C). Laminae filling the pools dip upstream (Fig. 3) and are composed of discrete layers of very fine-grained and coarse-grained sand. Similarities in grain size and mineralogy between the underlying bed and the coarser laminae indicate that the former was probably the source. The sand in the very fine-grained laminae is similar to the overlying sand.

A microdelta overlies the basal,

Paleohydraulics

Bedform stability diagrams can be used to infer the range of velocity and depth which formed the ripples and dune. Both structures were created by the same flow, the grain size defining the bedform which developed. Bedform stability diagrams of depth vs. velocity for the two grain sizes present indicate that average flow depth was between 3 and 27 cm and velocity was between 32 and 60 cm/s. The lower depth represents the minimum average depth needed to cover the dune.

Flow conditions at the

Discussion

Compiling the data on paleovelocity and paleodepth results in a pattern of slightly decreasing velocity combined with a more substantial depth increase as the bed was deposited (Fig. 4). The depositional event which formed the very fine-grained sand bed began with a very shallow flow of water over the coarse-grained sand. A series of hydraulic jumps eroded pools at the downward terminations of short, inclined segments of the bed. The pools were filled with upcurrent-dipping laminae of

Acknowledgements

Grateful appreciation is extended to Drs. A.V. Jopling and J.B. Southard for many helpful comments which improved earlier drafts of the manuscript. Figures were produced by Mr. S. Spivak and wordprocessing was carried out by Ms. W. Bourke and Ms. E. McDonald. This work was supported by Natural Science and Engineering Research Council of Canada Operating Grants to Dr. A.D. Miall and Dr. P.W. Fralick.

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Chute and pool structures are virtually unknown from fluvial strata owing to their presumed low preservation potential, and indeed most published examples of chute and pool structures come from the deposits of explosive volcanic eruptions, such as the spectacular Pleistocene section at Laacher See, Germany (Schmincke et al., 1973). The sole, published ancient example of chute and pool structure known to the author is a series of small-scale structures from Precambrian rocks in Canada (Fralick, 1999). Here, another, larger-scale bedset of interpreted chute and pool origin is described from the Permian of central Queensland, Australia, and a further, possible example from the Shepody Formation of Nova Scotia is provided.
Fluvial strata dominated internally by sedimentary structures of interpreted upper flow regime origin are moderately common in the rock record, yet their abundance is not appreciated and many examples may go unnoticed. A spectrum of sedimentary structures is recognised, all of which occur over a wide range of scale: 1. cross-bedding with humpback, sigmoidal and ultimately low-angle cross-sectional foreset geometries (interpreted as recording the transition from dune to upper plane bed bedform stability field), 2. planar/flat lamination with parting lineation, characteristic of the upper plane bed phase, 3. flat and low-angle lamination with minor convex-upward elements, characteristic of the transition from upper plane bed to antidune stability fields, 4. convex-upward bedforms, down- and up-palaeocurrent-dipping, low-angle cross-bedding and symmetrical drapes, interpreted as the product of antidunes, and 5. backsets terminating updip against an upstream-dipping erosion surface, interpreted as recording chute and pool conditions. In some fluvial successions, the entirety or substantial portions of channel sandstone bodies may be made up of such structures. These Upper Flow Regime Sheets, Lenses and Scour Fills (UFR) are defined herein as an extension of Miall's [Miall, A.D., 1985. Architectural-element analysis: a new method of facies analysis applied to fluvial deposits. Earth Sci. Rev. 22: 261–308.] Laminated Sand Sheets architectural element. Given the conditions that favour preservation of upper flow regime structures (rapid changes in flow strength), it is suggested that the presence of UFR elements in ancient fluvial successions may indicate sediment accumulation under the influence of a strongly seasonal palaeoclimate that involves a pronounced seasonal peak in precipitation and runoff.

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ExpresSedPaleohydraulics of chute-and-pool structures in a Paleoproterozoic fluvial sandstone

Abstract

Introduction

Section snippets

Vertical sequence of sedimentary structures

Paleohydraulics

Discussion

Acknowledgements

Sediment. Geol.

Mar. Geol.

Structure and origin of glacial sand planes

Geol. Soc. Am. Bull.

Antidunes as trochoidal waves

J. Sediment. Petrol.

Antidunes in the Mount Toby conglomerate (Triassic) mass

J. Sediment. Petrol.

Hydraulic factors controlling the shape of laminae in laboratory deltas

J Sediment. Petrol.

Some principles and techniques used in reconstructing the hydraulic parameters of a paleo-flow regime

J. Sediment. Petrol.

ExpresSed
Paleohydraulics of chute-and-pool structures in a Paleoproterozoic fluvial sandstone