lobes we find a couple of important differences that were not discussed by. Thornburg and Kulm. First, ancient depositional lobes are recognized by.
Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy: Discussion and reply
Discussion Mobil Research and Development Corporation, Dallas Research Laboratory, P.O. Box 819047, Dallas, Texas 75381
Lithofacies information concerning modern submarine fans is sparse in the literature, and thus the recently published paper dealing with sedimentation in the ( M e Trench (Thornburg and Kulm, 1987) is valuable for comparing modern with ancient submarine fan processes and products. Once again, however, we find that problems with nomenclature and concept arise from the differing perspectives of the land-based and marine geologist. As an example, Thornburg and Kulm loosely use the term "lobe" to refer to the entire submarine fan system of the modern Chile Trench. In contrast, lobes in ancient fans are depositional lobes that are specific to the outer fan setting (Mutti, 1977, 1985; Mutti and Ricci Lucchi, 1981). Because of the somewhat special setting of the Chile Trench, where the outer fan is truncated by an axial channel trending normal to the fan slope, it is difficult to make a direct comparison of this modern fan with the hypothesized ancient fan settings. Further to the question of lobes, Thornburg and Kulm have defined two types of lobes (erosional and depositional) from the Chile Trench, and more specifically, they have defined the inactive and erosional fan segments as "erosional lobes." We find that this is potentially confusing to those geologists working with ancient sequences because, in their terminology (Mutti, 1977), all lobes are depositional features. When a direct comparison of the internal character of the Chile Trench lobes is made with ancient sequences described as submarine fan lobes we find a couple of important differences that were not discussed by Thornburg and Kulm. First, ancient depositional lobes are recognized by their characteristic thickening-upward cycles (Mutti, 1977), and yet we find no evidence for this sort of systematic bed thickness change in the Chile Trench depositional lobes. Secondly, ancient depositional lobes generally exhibit a mounded seismic character (Sarg and Skjold, 1982; Mitchum, 1985), but this feature is not displayed by the Chile Trench depositional lobes.
The article discussed appeared in the Bulletin, v. 98, p. 33-52. Geological Society of America Bulletin, v. 99, p. 598-600, 1 fig., October 1987. 598
There are also some problems in the labeling of lithofacies from the Chile Trench in terms of the turbidite facies nomenclature for ancient fans. Thornburg and Kulm chose to use the early paper of Mutti and Ricci Lucchi (1972) for their designation of turbidite facies rather than the more comprehensive and much more widely accepted later work (Mutti and Ricci Lucchi, 1975; Mutti, 1977). In so doing, they have correlated the levee facies of the Chile Trench with Facies E of Mutti and Ricci Lucchi (1972). The levee facies of the Chile Trench, however, are composed of graded and internally structureless sands and silts, which is in marked contrast to the generally accepted Facies E of Mutti and Ricci Lucchi (1975) that are typified by nongraded, ripple cross-laminated sandstone. The possibility that the Chile Trench levee deposits may correlate with the Mutti's (1977) channel-margin facies is also negated by the structureless nature of the levee beds. Another difference worthy of mention concerns bed thickness changes. A distinct feature of the Mutti and Ricci Lucchi Facies C in ancient sequences is its thickening-upward cycles (see Table 1 in Mutti and Ricci Lucchi, 1972; also see Figs. 9 and 10 in Mutti, 1977), yet these cycles are. not reported from the Basin 1 facies (M-03 and W-22, Fig. 14C) of the Chile Trench that Thornburg and Kulm label as Facies C. The points here referred to may reflect important differences, such as depositional slope and rate and type of sediment supply, between the trench setting of the Chile fans and the less dynamic tectonic settings of conventional fans upon which popular ancient fan models are based. Although Thornburg and Kulm make reference to the uniqueness of the Chile Trench fans, they do not address the specific differences discussed here. We believe that these differences are important to an effective comparison of modern and ancient fans.
Reply L D KULM^^^
}
°f Oceanography, Oregon State University, Corvallis, Oregon 97331
During our recent field work in the Chile Trench, we discovered submarine fans with an unusually asymmetric morphology which developed in the presence of a strong axial gradient. We described the fans as having a "bi-lobate" morphology—specifically an up-gradient "lobe" dominated by depositional processes, and a down-gradient "lobe" dominated by erosional processes, divided along a well-defined escarpment. The term "lobe" is used strictly in a descriptive, morphological sense in that these features developed as radial projections from a point source. In no way did we intend to equate these fan lobes with the depositional sandstone lobes associated with mid-to-outer fan environments in the ancient rock record, as Shanmugam and McPherson suggest. The obvious question then becomes: Where in the Chile Trench can we find an analog for the depositional sandstone lobes which form such prominent and extensive accumulations in the submarine-fan systems of the ancient rock record? Although lenticular sand deposits are documented in the sheeted basins of southern Chile (see our Fig. 6b; also note Facies C analog in core M-07) and broadly lenticular, sheet-like reflectors are characteristic of the depositional lobe of the Callecalle Fan (Fig. 9, profile C-C), neither deposit is fed by a more proximal system of primary fan channels and secondary distributaries. Down-gradient from the canyon mouths and the erosional fan lobes, the axial channel appears to lose relief by becoming wide and shallow, as opposed to bifurcating into smaller distributaries (Schweller and Kulm, 1978). These observations, which deviate from the general architecture of normative fan models, and the fact that characteristic mounded and down-lapping seismic-reflection signatures were not observed, make us reluctant at this time to draw analogies to the depositional sandstone lobes of ancient fans. In the now-classic 1972 paper in which Mutti and Ricci Lucchi introduced their facies classification scheme (translated by Tor Nilsen in 1978), Facies E is described as an arenite-pelite alternation with basal graded and structureless intervals, best defined by Bouma T a / e sequences. Subsequent modifications to the classification incorporate lower flowregime current laminations, notably dune and ripple forms (Walker and Mutti, 1973; Mutti and Ricci Lucchi, 1975; Ricci Lucchi, 1975). In spite of the diversity of sedimentary structures attributed to Facies E, all descriptions invariably emphasize thin-bedding, coarse texture, high sand:shale ratios (near 1:1 or greater), and association with Facies B—all of these traits are shared by our Levee facies. Lenticularity and wedging cannot be observed in piston cores, but we noted scour surfaces beneath some of the coarser beds. In addition to the graded, structureless beds isolated by the factoranalysis technique, we noted in our discussion (p. 46) that the Levee facies is also expressed by thin-bedded, nongraded sands in which the transition from basal arenite to upper pelite is sharp and abrupt. Because a bed thickness parameter was not programmed into our facies model, however, the factor analysis inaccurately relegated such deposits to the Channel facies (see Fig. 15A, cores M-ll and M-15; Fig. 15B, core M-18).
Examples of this type of levee stratigraphy are presented here in Figure 1, all of which were cored from modern levees or channel margins.
7
~
Figure 1. Variations of the Levee facies, Chile Trench. Examples display primarily nongraded beds with sharp contacts between the basal arenite and upper pelite, creating a rhythmic alternation of sand and mud. Sand:mud ratio is near 1:1 and could increase during shoreline regressions or with preferential compaction of the muds during lithification. (A) Core M-ll, 185-280 cm. Bouma sequences T a / e and T b / e are present; T c / e and T dune / C were not encountered. (B) Core M-18,195-310 cm. Primarily T a / e sequences with locally intense bioturbation. (C) Core M-15,100-210 cm. T a / e and Tb/e sequences with better developed distribution grading and local scour beneath some sand beds.
599
600
DISCUSSION AND REPLY
The diversity of internal structure (or lack of it) exhibited by Fades E stratigraphy model must be tested by observations of ancient trench and and the Levee facies of our study may be related to the competing proc- fan systems where vertical sequences can be directly observed. esses of sedimentation under a decelerating current and traction of the sediment bed. Where spillover flow velocities are unusually high—as on the outside banks of channel meanders, or on the seaward wall of the axial COMBINED REFERENCES CITED channel directly across from the erosional fan lobes—tractional processes Mitchum, R. M., Jr., 1985, Seismic stratigraphie expression of submarine fans: American Association of Petroleum Geologists Memoir 39, p. 117-136. may be sufficient 1:0 produce internal laminations or to rework the tops of Mutti, E., 1977, Distinctive thin-bedded turbidite facies and related depositional environments in the Eocene Hecho rapidly congealed sands. The gravity-flow volumes entering the basin, the Group (south-central Pyrenees, Spain): Sedimentology, v. 24, p. 107-131. 1985, Turbidite systems and their relations to depositional sequences, in G. G. Zuffa, ed., Provenance of arenites: grain size distribution of the sediment load, the dimensions and configuraDordrecht, The Netherlands, D. Reidel Publishing Company, p. 65-93. tions of the distributary channel systems, and the proximity to the canyon Mutti, E., and Ricci Luochi, F., 1972, Turbidites of the northern Apennines: Introduction to facies analysis (English translation in 1978): International Geology Review, v. 20, p. 125-166. mouth are factors likely to contribute to the variability of facies character 1975, Turbidite facies and facies associations, m Examples of turbidite facies and facies associations from selected formations of the northern Apennines: International Sedimentological Congress, 9th, Nice, Field trip guidebook observed in levee environments. A - l l , p . 21-36. 1981, Introduction to the excursions on siliciclastic turbidites: IAS European Regional Meeting, 2nd, Bologna, Readers will note that we presented no hard data concerning verticalItaly, Excursion guidebook, p. 1-3. Ricci Lucchi, F., 1975, Depositional cycles in two turbidite formations of northern Apennines (Italy), Journal of sequence analysis (that is, fining-, thinning-, coarsening-, thickeningSedimentary Petrology, v. 45, No. 1, p. 3-43. upward sequences). We refrained from doing so because we do not believe Sarg, J. F., and Skjold, L. J., 1982, Stratigraphie traps in Paleocene sands in the Balder area. North Sea: American Association of Petroleum Geologists Memoir 32, p. 197-206. that facies sequences can be accurately assessed in 2- to S-m piston cores, Schweller, W. J., and Kulm, L. D., 1978, Depositional patterns and channelized sedimentation in active eastern Pacific trenches, in Stanley, D. J., and Kelling, G., eds.. Sedimentation in submarine canyons, fans and trenches: Stroudsespecially given the profound sea-level changes which occurred during burg, Pennsylvania, Dowden, Hutchinson & Ross, p. 311-324. deposition of the late Pleistocene-Holocene sediments. In our discussion Thomburg, T. M., and Kulm, L. D., 1987, Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and Geological Society of America Bulletin, v. 98, p. 33-52. (see our Fig. 16), we present a model of trench stratigraphy which predicts Walker,stratigraphy: R. G., and Mutti, E., 1973, Turbidite facies and facies associations, in Middleton, G. V., and Bouma, A. H., eds., Turbidites and deep water sedimentation: Society of Economic Paleontologists and Mineralogists, Pacific Section, vertical facies successions based on our observations of surficial lithofacies Short Course Notes, p. 119-158. and their spatial distribution, depositional and erosional morphologies, seismic stratigraphie configurations, and the principle that lithofacies superposition occurs primarily by depositional (intrabasinal), tectonic and RECEIVED BY THE SOCIETY (DISCUSSION) FEBRUARY 19,1987; (REPLY) APRIL 2,1987 eustatic (extrabasinal) mechanisms. Naturally, the validity of our trench MANUSCRIPTS MANUSCRIPTS ACCEPTED APRIL 17,1987
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