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Deep-Water Turbidites and Their Equally Important Shallower Water Cousins*
By
Emiliano Mutti1, Roberto Tinterri1, Pierre Muzzi Magalhaes2, and Gustavo Basta2
Search and Discovery Article #50057 (2007)
Posted November 7, 2007
*Adapted from Extended Abstract prepared for presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007.
1Dipartimento di Scienze della Terra, University of Parma, Italy
2Petrobras S.A., Rio de Janeiro, Brazil
Most modern turbidite systems form at depths in excess of a thousand meters and similarly deep-water settings are generally inferred from exposed or buried ancient systems. These systems are characteristically made up of a variety of facies deposited by sediment gravity flows. Sharp-based, graded sandstone beds alternating with generally thinner mudstone units are probably the most typical deposit of such systems.
Growing evidence shows that almost identical deposits are equally common and stratigraphically important in shallower water settings of exposed ancient basin fills. Such sediments, most of which have long been unfortunately mistaken for storm deposits because of the common occurrence of hummocky cross-stratified sandstones (HCS) or have been assigned a deeper water turbidite origin, are the most genuine expression of deltaic systems dominated by rivers in flood and therefore by hyperpycnal flows. These sand-rich depositional elements commonly grade basinward into thick, delta-slope, mudstone-dominated successions. The correct recognition of the origin and environment of these deposits, which is crucial for differentiating flood-dominated deltaic systems from basinal turbidite systems, can only be accomplished through careful geological mapping, stratigraphic correlations. and facies analyses in both the surface and subsurface..
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The term “turbidites”, as originally defined by Kuenen (1957), was intended to denote deep-marine graded sandstone beds deposited by turbidity currents and exemplified by the Tertiary sandy “flysch” of the northern Apennines, such as the Macigno and Marnosoarenacea formations (Kuenen and Migliorini, 1950). By the early seventies, the meaning of the term had broadened to include the sediments of both modern and ancient deep-water systems in which channelized turbidity currents would spread at channel terminus dissipating their energy in depositional lobes and adjacent basin plains (see review in Mutti and Normark, 1991). Most modern deep-sea fans are characterized by this depositional pattern; similar patterns can also be inferred from ancient depositional systems exposed in thrust-and-fold belts, although, in this case, basin configuration and tectonic setting may considerably modify geometry and facies distribution patterns of channel, lobe, and basin-plain deposits. In exposed ancient settings, sheet systems comprised of sandstone lobes and basin-plain deposits are volumetrically the most important depositional elements (for an updated review see Mutti et al. 2003). As a result, the plan-view geometry and the internal architecture of these systems usually show considerable departures from those of modern deep-sea fans, many of which grow in essentially unconfined basins. It should, however, be mentioned that in many divergent continental margins (e.g., the Brazilian offshore) faulting, folding, salt and mud mobility, as well as volcanic activity, may produce substantial topographic relief that will exert an important control on erosion and deposition. The local geologic context, graded sandstone to mudstone beds (typified by the classic Bouma sequence, although with some caution) alternating with hemipelagic layers containing deep-marine fossil assemblages, and the lack of shallow-marine features produced by wave and tidal action have long been considered as the best evidence for a deep-water origin of turbidite strata. Spectacular examples of deep-water turbidites have been described, among others, from the basin-plain deposits of the Miocene Marnosoarenacea Formation, Northern Apennines, Italy, and the Eocene Hecho Group, south-central Pyrenees, Spain. As defined above, deep-water turbidites form huge volumes in the basin-fill successions of both divergent continental margins and exposed collisional belts, and their importance in the geological record is greatly enhanced by their increasing potential as hydrocarbon reservoirs in many continental margins (e.g., West Africa, Gulf of Mexico, Brazilian offshore).
It is our strong conviction that in exposed thrust-and-fold belt basins (e.g., south-central Pyrenees, Spain; Neuquen Basin, Argentina; Tertiary Piedmont Basin, northwestern Italy) similarly huge sedimentary volumes of parallel-sided graded sandstone to mudstone beds develop also as nearshore and shelfal deposits at variable depths along the local shelf depositional profile and, in some cases, in deeper water intrashelf (and intraslope) structural depressions. Vertical and lateral stratigraphic relationships and careful mapping leave no doubt that these sediments are a delta-front element of either fan-delta or river-delta systems – a conclusion strongly supported by the consideration that large volumes of shallow-water sand must obviously be fed by an adjacent river. Essentially, these sediments are sandstone lobes deposited by dense hyperpycnal flows (underflows) emanating from relatively small- and high-gradient flood-dominated fluvial systems. Except for the occurrence of hummocky cross-stratified sandstones (HCS) (see below), abundant fossil debris and locally pervasive bioturbation, these flows produce facies tracts that are very similar to those generated by turbidity currents. Seaward, these sandy sediments grade into prodeltaic mudstone-dominated successions deposited by more dilute and mud-laden hyperpycnal flows, as well as by “normal” river plumes (for a more detailed account on these sediments see Mutti et al. 1996, 2003). Although Goldring and Bridges (1973) a long time ago perceived the importance of these shallow-water and locally highly fossiliferous turbidite-like deposits, which they termed “sublittoral sheet sandstones,” this fundamental type of sedimentation has been unfortunately overlooked by sedimentologists. Due to the occurrence of HCS in many graded sandstone beds, common sedimentological interpretations have long emphasized the storm-related origin of these deposits and associated them to high-energy shorelines (e.g., Walker, 1984; Duke et al., 1991). Clearly, we not deny that storm-generated graded sandstone beds may locally develop seaward of the shoreline, but argue that such processes cannot account for the huge volumes of graded sandstone beds with HCS occurring in exposed collisional basin fills. In thrust-and-fold belt and more generally in tectonically-active basins, shorelines are generally expressed by low-energy facies. The Tertiary of the piggy-back basins of the south-central Pyrenees, Spain, and the Piedmont Basin, Italy, are conspicuous for the absence of high-energy beach deposits. Only tidal currents play a major role here in reworking the proximal and shallower portions of delta systems, particularly in estuarine complexes. However, the action of these currents decreases with depth; as a result the distal and deeper portions of flood-generated delta-front sandstone lobes are virtually unaffected by tides and/or waves, thus resembling the typical alternation of graded sandstone beds and intervening mudstones of deep-water turbidites. Figure 1 illustrates some of the main characteristics of delta-front graded sandstone beds. In particular, we emphasize here the very diagnostic occurrence of ball-and-pillow structures that we interpret as the result of differential loading (density gradient) related to the pinch-and-swell geometry of sandstone beds containing large-scale HCS. Depending upon the gradient and physiography of the shelf, which is, in most cases, related to the local structural control, hyperpycnal flows emanating from river mouths can form attached to river-mouth bars (Mutti et al., 1996; Tinterri, in press) or can move considerable distances away from the shore and thus form important sand accumulations in shelfal regions at water depths which may be well below the depth normally reached by storm-waves and tidal currents. In other cases, these accumulations may form in fault-bounded depressions where gravity flows are forced to decelerate and deposit their sediment load. In such cases, distinguishing between these detached delta-front sandstone bodies and deep-water turbidites may become difficult. The distinction must be based primarily on the geological context, although facies characteristics and trace fossils may provide good criteria for separating the two kinds of deposits. In particular, deltaic graded sandstone beds typically show facies characteristics indicating deposition from poorly efficient flows; the common paucity of fine-grained divisions probably results from the bypass of these sediments toward more distal prodelta regions. The similarities between fluvial-dominated deltas and submarine turbidite systems (particularly deep-sea fans) were recognized since the early canyon-fed submarine fan models (Mutti and Ghibaudo, 1972, their Table II; Mutti and Ricci Lucchi, 1972), which compared the overall depositional patterns of these systems in terms of transfer (channels) and depositional (deltaic channel-mouth bars and turbidite channel-mouth lobes) zones. The comparison has become much easier and more significant with time due to the recognition that fluvial floods generate at river mouths hyperpycnal flows that behave essentially as shallow-water turbidity currents. Therefore, the channel-lobe motif observed in most turbidite systems and the facies tracts produced by energy dissipation at channel terminus can be used, with the necessary caution, also in the interpretation of flood-dominated fluvio-deltaic systems. Most nearshore and shelfal sandstones probably need to be revisited in light of these concepts. In our opinion, part of these deposits have long been mistaken either for beach-related “shoreface deposits” or even “basin-floor turbidites” developed at the base of prograding deltaic clinoforms, which in most cases have a relief varying between a few meters and a few hundred meters – clearly a relief too small to generate turbidity currents of sufficient volume and momentum to reach basinal, deep-water basinal regions.
Occurrences of “Turbidite-like” Facies Figure 2 is a diagram of the three main types of occurrence of “turbidite-like” facies in the marine environments of divergent, convergent, and collisional continental margins. The diagram shows (1) a “shallow-water domain” where graded sandstones are essentially attached to their feeder fluvio-deltaic systems, (2) an “intermediate domain” where graded sandstones are mostly trapped in intra-slope basins (in both compressional and extensional regimes), and (3) a “deep-water, basinal domain” where graded sandstones record the final depositional zone of turbidity currents, whatever their origin (hyperpycnal flows, submarine slides or a combination thereof).
A clear perception of the problems above is crucial for a better understanding of depositional processes and facies characteristics of intra-slope basins which appear to be a major target for hydrocarbon exploration worldwide. Probably, graded sandstone beds form a continuum from alluvial to deep-marine environments in response to cyclic periods of fluvial flooding, although in the marine environment submarine slides may play a role of almost equal importance in triggering large-volume turbidity currents.
Duke, W.L., Arnott, R.W.C., and Cheel, R.J., 1991, Shelf sandstones and hummocky cross stratification: New insights on stormy debate: Geology, v. 19, p. 625-628. Goldring, R., and Bridges, P., 1973, Sublittoral sheet sandstones: Journal Sedimentary Petrology, v. 43, p. 736-747. Kuenen, Ph.H., 1957, Sole markings of graded graywacke beds: Journal of Geology, v. 65, p. 231- 258. Kuenen, Ph.H., and Migliorini, C.I., 1950, Turbidity currents as a cause of graded bedding: Journal of Geology, v. 58, p. 91-127. Mutti, E., and Ghibaudo, G., 1972, Un esempio di torbiditi di conoide sottomarina esterna: le Arenarie di San Salvatore (Formazione di Bobbio, Miocene) nell' Appennino di Piacenza: Accademia delle Scienze di Torino, Memorie, Classe di Scienze Fisiche, Matematiche e Naturali, Serie 4, no. 16, 40 p. Mutti, E., Davoli, G., Tinterri R., and Zavala, C., 1996, The importance of fluvio-deltaic systems dominated by catastrophic flooding in tectonically active basins: Memorie di Scienze Geologiche, Universita di Padova, v. 48, p. 233-291. Mutti, E., and Normark, W.R., 1991, An integrated approach to the study of turbidite systems, in Seismic facies and sedimentary processes of submarine fans and turbidite systems, eds. P.Weimer and H. Link: Springer, New York, p. 75-106. Mutti, E., and Ricci Lucchi, F., 1972, Le torbiditi dell'Appennino settentrionale: Introduzione all' analisi di facies: Memori della Societa Geologica Italiana, v. 11, p. 161 199. Mutti, E., Tinterri, R., Benevelli, G., Di Biase, D., and Cavanna, G., 2003, Deltaic, mixed and turbidite sedimentation of ancient foreland basins, in Turbidites: Models and Problems , eds. E. Mutti, G.S. Steffens, C. Pirmez, M. Orlando, and D. Roberts: Marine and Petroleum Geology, v. 20, p. 733-755. Prather, B.E., Booth, J.R., Steffens, G.S., and Craig, P.A., 1998, Classification, lithologic calibration, and stratigraphic succession of seismic facies of intraslope basins, deep-water Gulf of Mexico. AAPG Bull., 82 (5A), 701-728. Tinterri, R., in press, The lower Eocene Roda Sandstone (South-Central Pyrenees): an example of a flood-dominated river-delta system in a tectonically controlled basin: Rivista Italiana di Paleontologia e Stratigrafia. Walker, R.G., 1984, Shelf and shallow marine sands, in Facies Models (second edition), ed. R.G. Walker: Geoscience Canada, p. 141-170.
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