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Sedimentology of Gravels and Conglomerates See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/303558500 Sedimentology of gravels and conglomerates Article · January 1984 CITATIONS READS 62 2,031 4 authors, including: Frank Ethridge William A. Wescott Colorado State University 54 PUBLICATIONS 1,207 CITATIONS 41 PUBLICATIONS 619 CITATIONS SEE PROFILE SEE PROFILE Ronald J. Steel University of Texas at Austin; Heriot-Watt University 309 PUBLICATIONS 12,809 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Mixed energy process interaction in a compound-clinoform delta View project River deltas prograding in tidal seaways and straits: towards a conceptual depositional model View project All content following this page was uploaded by Ronald J. Steel on 19 January 2018. The user has requested enhancement of the downloaded file. In: Koster, E. H . and Steel, R. J. (Eds.), Sedimentology of Gravels and Conglomerates. Canadian Society of Petroleum Geologists, Memoir 10 (1984). p. 1-31 . ALLUVIAL AND COASTAL CONGLOMERATES: THEIR SIGNIFICANT FEATURES AND SOME COMMENTS ON GRAVELLY MASS-FLOW DEPOSITS W. NEMEC1 AND R. J. STEEL2 ABSTRACT Conglomerates originating in coastal environments represent mainly beachface, shoreface, fan-deltaic or deltaic mouth bar, and Gilbert-type delta sequences. They show structures, textures and otherfeatures created mainly by the varied influence of waves and fluvial output in the shallow marine setting. Transitional, alluvial/marine systems show a broad range of facies characteristics and sequences, and these are discussed in detail. Conglomerates originating in alluvial environments comprise mainly braided stream and mass flow sequences. The former include regular braided river and fan (distributary) channel deposits, and show textures and structures which vary greatly with source and climatic setting. Braided stream sequences commonly show an upward fining motif, due to falling flood stage or to gradual abandonment of alluvial tracts. Mass flow conglomerates originate from a variety of debris flows in subaerial settings, but fluidal gravelly flows (like many 'sheetfloods' or 'streamfloods') may also be important, and they often become prominent subaqueously (high-density gravelly turbidites). In both instances, the deposits show remarkably varied texture, structure, and fabric. Subaerial flows are often considerably transformed when passing into water. A review of diagnostic features and facies sequences is presented. When interpreting the emplacement mechanics of mass flow conglomerates, particular effort must be made to extract maximum information from the individual bed characteristics. We illustrate with examples that even such basic data as bed thickness and maximum clast size may serve as a valuable source for some genetic inferences. RESUME Les cong!ome>ats formes dans des milieux cotiers represented principalement des sequences de zone infralittorale de grande energie, de zone infratidale, de cdne de dejection ou de fleche barrante deltai'que dans une embouchure, et de delta du type de Gilbert. lis exhibent des structures, des textures et autres particularity crepes principalement par les effets varies des vagues et des regimes fluviatiles dans un contexte marin de faible profondeur. Les conglomeYats deVeloppis dans des milieux alluvionnaires comprennent principalement des sequences de cours d'eau anastomoses et d'ecoulements en masse. Le premier inclut des ddpdts de riviere anastomosed rtguliere et de chenaux de cone de dejection (de distribution); on y observe des textures et des structures qui varient grandement selon la region source et le contexte climatique. Les sequences des cours d'eau anastomosis prlsentent frdquemment un motif de granulomere dlcroissante vers le sommet des couches qui requite d'un retranchement du stade de crue ou de l'abandon graduel des zones alluvionnaires. Les conglomeYats d'icoulement en masse derivent d'un grand nombre de coulees de debris dans des contextes subadriens, mais les coulees graveleuses fluidales commes beaucoup d'innondations en nappe ou de cours d'eau peuvent aussi avoir contribue largement, et souvent elles dominent en milieu sous-aquatique (turbidites graveleuses de densite eievee). Dans les deux cas, les depots exhibent remarquablement des diversites dans la texture, la structure et la fabrique. Les coulees subadriennes sont souvent considerablement transformles par le passage en milieu aqueux. Lors de Interpretation du mlcanisme de mise en place des conglom6rats d'ecoulement en masse, il faut fournir un effort particulier pour acquerir le maximum de renseignements devoiies par les caracteristiques de chaque lit. Nous illustrons au moyen d'exemples qui montrent que rneme des donnles de base comme l'epaisseur des lits et la grosseur maximale des fragments peuvent servir d'enseignement pr£cieux pour les deductions g6netiques. INTRODUCTION marine fans (e.g., Walker, 1975; Krause and Oldershaw, 1979;Porebski, 1981;Hein, 1982). In terms offacies analysis, Conglomerates have long attracted attention as possible perhaps the main recent efforts have gone to distinguish indicators of tectonic activity and from the viewpoint of specific sedimentary sequences which may be typical for provenance studies and local palaeotransport directions. particular processes or depositional settings, and to estab­ Only more recently has specific effort gone into the study lish criteria which may serve for the recognition of such of gravelly depositional environments and the interpreta­ settings in other rock successions. tion of conglomerate facies. Although quite a few modern A research direction that has been prominent recently is gravelly environments have been well-investigated, espe­ the analysis of fluvially derived gravels in ancient and cially gravelly alluvium, a great number of ancient deposi­ modern shoreline/near shore environments, notably on fan- tional systems still have no available or well-studied modern deltas. Considerable attention is now being paid to lacus­ analogue. Synthesis of conglomerate facies characteris­ trine fan-delta settings where the deposition of fan-derived tics is therefore approached mainly or entirely from the gravels is influenced by a passive body of water, and also ancient record, and one may find it surprising, indeed, that involves mixing with lake muds, and to marine fan-delta models have been attempted only for certain ancient sub- settings where fan gravels are variously reworked, and 'Geological Institute (A), University of Bergen, Aliegt. 41, 5014 Bergen, Norway. 2Norsk Hydro Research Centre, P.O. Box 4313, 5013 Bergen, Norway. Copyright ® 1984, Canadian Society of Petroleum Geologists 1 Copyright © 2009 by the Canadian Society of Petroleum Geologists. Memoir 10 (1984) 2 W. NEMEC andRJ. STEEL mix with marine sediments under the action of wave and Gravel can also be transported beyond the nearshore tidal processes. In the first part of this contribution we zone, but this requires rather unusual fluvial output or briefly review those aspects of shallow marine and alluvial storm conditions, and happens particularly in regions with conglomerates which we have found to be important to a narrow or non-existent shelf, where shoreline transport their interpretation, and we put some emphasis on transi­ has easy access to slopes beyond. tional subaqueous environments. During the last decade considerable progress has also WAVE-DOMINATED SETTING been made on understanding the sedimentary features of In a wave-dominated setting, the areas of significant conglomerates deposited from sediment gravity flows, par­ gravel accumulation are the beachface and shoref ace zones. ticularly through the 'benchmark' work of Middleton and Within the latter, thick gravel sequences are rare, but Hampton (1973, 1976) and Lowe (1979, 1982). This prog­ significant amounts of thin gravelly beds or pebbly ress pertains mainly to the rheology of flow and the mechan­ sandstones, associated with thicker sandstone sequences, ics of gravel transport, but includes also some suggested are common. depositional models for conglomerate beds. In addition, Beachface conglomerates have only rarely been recognised the classification of sediment gravity flows and their depos­ in ancient successions, perhaps partly because of poor its has been considerably improved. preservation and partly because they tend to be thin and However, there are still many difficulties in the field can be overlooked easily. However, they can be recognised application of this theoretically-based work. In the second by the following features: part of this contribution we attempt to review some impor­ (1) Depositional sequences are well-stratified, usually tant aspects of the classification of sediment gravity flows, because of sandy interbeds, and the individual emphasising application to subaerial deposits and com­ conglomerate beds are laterally more persistent menting on other points of potential practical importance. (Clifton, 1973) and texturally uniform than in equally We suggest, in particular, thatMPS/BTh plots, when based coarse-grained alluvial sequences (Figs. 1A, B, C). on carefully collected data, can serve to make some impor­ Stratification on the beachface generally dips gently tant inferences about the depositing flows. seawards (Fig. 1A) but landward dips may occur in backshore sequences (Maejima, 1982). (2) The vertical variation in shape-sorting between adja­ CONGLOMERATES ORIGINATING cent beds is often
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