- Email: [email protected]
The shell pavement below oceanic turbidites
THE SHELL PAVEMENT BELOW OCEANIC TURBIDITES PH. H. KUENEN
Geological Institute, State University, Groningen (The Netherlands) (Received June 1, 1964) (Resubmitted August 27, 1964)
SUMMARY
Nesteroff discovered, among other interesting facts, that many oceanic turbidite are covered by a thin film of pelagic shells, and he attributed this to deposition fror the tail of the turbidity current. Any later pelagic deposit is supposed to have bee washed away by the following current, and is therefore missing. Nesteroff's findin is important in itself and throws new light on ancient turbidites. However, the presen author favours an explanation through winnowing of the shells from a pelagi stratum by the current that deposited the covering turbidite. This would mean tha the shell pavement does not mark the end but the beginning of a turbidite, and tha the upper part of the pelitic interval with pteropods and planktonic Foraminifer~ is a pelagic deposit. Several arguments are presented in favour of the latter view. A Nesteroff and Heezen are led by their interpretation to doubt similarly the presenc of pelagic strata between flysch turbidites, a brief survey is made of this subject. I shows that most of the ancient turbidite sequences also contain many beds with pelagic part towards the top of the pelitic interval. Up to the present, however, n~ shell pavement has been reported, but careful search will probably reveal example of such fossil pavements.
INTRODUCTION
In two papers NESTEROFF (1961, 1963) has recorded significant facts about recen deep-sea turbidites. His results were obtained by the study of large numbers of deep sea cores collected by U.S.A. expeditions, especially those from the Lamont Geologi cal Observatory. Nesteroff shows that turbidites are more numerous in suitabh localities than hitherto realized, and that grading is detectable in practically all thes~ beds when studied carefully. Furthermore, Nesteroff has discovered that in many deep-sea turbidites th, Marine Geol., 2 (1964) 236-24~
THE SHELL PAVEMENT BELOW OCEANIC TURBIDITE$
~.J,
pelitic upper part consists of two zones, (1) the lower one mainly inorganic with more or less calcium carbonate detritus, (2) the upper part with occasional pelagic Foraminifera and pteropods added. Where covered by a fine-grained turbidite the sequence tends to end with a thin band of forams and pteropods one shell thick. But below coarser beds, presumably deposited from a somewhat faster current, this band is missing. Nesteroff concludes that in the latter case the shells have been washed away. The present paper is concerned with the origin of this thin band. The explanation offered by Nesteroff of the upper band is that the tail of the turbidity current was too slow to carry clay but brought along shells rendered extremely buoyant by organic remains or gas bubbles (see NESTEROFFand HEEZEN, 1963). From this he goes on to conclude that there is no pelagic deposit. As pelagic deposition to an amount of a few centimeters must have occurred in the interval between two successive currents, he draws the inference that the pelagic deposit has been washed away by the successive turbidity current. Nesteroff's finding is of much importance, also because it can throw light on the manner in which ancient turbidites were formed. The present author would like to suggest an alternative explanation, which also leads to a different interpretation of the evidence concerning ancient deposits. He believes pelagic sediment containing a few shells was deposited after the turbidity current had laid down a stratum of clay practically devoid of shells. The next current started to wash away the pelite but left the shells which it was unable to pick up. As a result it created a winnowed pavement of shells protecting what remained of the pelagic sediment. Nesteroff points to two items of supporting evidence for his explanation. He mentions cores taken in the Tongue of the Ocean, Bahamas, where several turbidites are characterised by some special material (oolites, corals, spicules of holothurians, etc.) that is missing in the under- and overlying beds. This contrast is attributable to different source areas. The very fine fraction of this special material is found right up to the top of the bed, and is therefore directly overlain by the next turbidite. This is taken as proof of the absence of a pelagic stratum, because pelagic material should not vary in composition from bed to bed. However, the fine material intermixed up to the top of the calcareous turbidites can be accounted for by the burrowing and ploughing activity of animals, a phenomenon emphasized in most of Nesteroff's descriptions. This activity would mix the upper parts of the turbidites with the gradually accumulating pelagic deposit. There is an argument for this interpretation because the radio-carbon dating of the coarse part ( R u S N A K et al., 1963) shows it to be older than the upper fine part beneath or above. This indicates that the coarse part consists of reworked older material, but that the fine part is largely contemporaneous. Nesteroff's first argument is therefore not considered to be convincing. His second argument, based on the occurrence of the shell band in deep water, will be examined later and will also be found weak.
Marine Geol., 2 (1964) 236-246
OBSTACLES TO NESTEROFF'S EXPLANATION
The explanation of the upper film of shells as representing the end of the underlyir turbidite, from which the pelagic cover has later been stripped, meets with obstacle The arguments can be brought under a few headings.
Density of the shells If the shells are deposited after the clay, their settling rate in stagnant water must 1: slower than that of the clay floccules. A rough estimate can be made of their densit as follows. If the equivalent diameter of the clay floccules is put at 5 IXand that of tb shells at 2 Ix with a true diameter of 0.5 mm, then the density of the shells includin the buoyant material cannot be more than 0.0001 greater than that of the surrounc ing water in order to settle more slowly than the clay. So close a balance in densit between shells and water is impossible. Moreover, such material would be extremel sensitive to all types of current action on the deep-sea floor and would tend to b moved about by the slightest disturbances and to accumulate in irregular patches.
The gas lift If gas bubbles play a part and if the very slow-settling shells are deposited by th, current in deep water, the bubble must have been in the shells when they were picke~ up. At the point of origin in smaller depth the bubble must have been so much large that it would have rendered the shell lighter than seawater. Conversely, if the shell wa,. picked up from the bottom by the current with a bubble that made it almost float the higher pressure at the point of deposition would compress the gas and increas~ the density of the shell. This would cause the shell to settle more quickly than th~ clay and drop out before, not after, the pelitic material. Evidently buoyancy owin~ to a bubble, as suggested by Nesteroff, is too sensitive to changes in depth to b~ invoked in this problem.
Organic matter The area over which the turbidite is finally spread out is much larger than that of the deposit in shallower water before the take-off. This means that the shells must have formed a bed or part of a bed much thicker than the shell pavement. The accumulation of this mass requires a long time and the organic matter would have disintegrated before the current picked the shells up. Moreover, most of the shells reachin~ the bottom are devoid of decomposable organic matter anyhow. This alternative suggestion for explaining a very low density of the shells is evidently also unsatisfactory.
Marine Geol., 2 (t964) 236-246
THE SHELL PAVEMENT BELOW OCEANIC TURBIDITES
L3~
Motive force o f the current
Nesteroff's hypothesis implies a final stage in which the turbidity current no longer carries clay but only organisms. But a current cannot continue to flow under the influence of gravity unless the density is greater than that of the surrounding clear sea water. Obviously a mass of water without clay and only weighted by some micro-
Stratum
~
'-~ -1 density Great
¢:......;. : . . . . . . . . ~ ' . :.-.:..:
S0.ro.
T
B
Fig.1. Distribution of micro-organisms in typical oceanic turbidite sequences. (After NESTEROFF, 1961.) In the terrigenous deposits (T) micro-organisms occur at three levels: a stratum in the sandy part; scattered through the top part of the lutite; and as a covering stratum. According to the present author the latter is a winnowed concentrate. In the bioclastic deposits (B) they are met with throughout, but there is a concentration at the top. According to the present author the latter is either a winnowed concentrate or the true pelagic part.
organisms with very small excess density forms a liquid virtually equal in density to clear water. Hence it would have no potential energy to sustain the flow. In other words, the mechanism invoked to supply the stratum o f shells, a turbidity current weighted by very light shells only, is unrealistic. No cover to the shell band The thickness o f the supposed pelagic bed, covering the shell band, should vary according to the length o f the interval between the currents. One would expect to encounter cases in which the pelagic cover was too thick to have been entirely removed, thus leaving a stratum above the shell band. Nesteroff does not record having f o u n d this situation. Marine GeoL, 2 (1964) 236-246
Thickness of the shell band There is no logical explanation for the close balance between the available amoul of shells and the extent of the turbidite resulting in a band about one shell thick. An even if the right amount should be present to cover the turbidite, it still remains enil matic why it was spread so evenly over the entire surface. One would expect to fin all transitions in a normal frequency distribution between core samples of beds wit only an occasional shell on top to those with a stratum several shells thick. The fa4 that the band is nearly always a rather complete stratum of just one shell thick forrr good evidence for winnowing, and against deposition from a current.
No pteropod supply Deposits rich in pteropods are rare and occur only near Cuba and on the Mid-AtlantJ Ridge. Hence, the great majority of turbidity currents coming down a submarin canyon or down a continental slope would pass no source with sufficient concentr~ tion of these shells.
Solution of the shells Nesteroff points out that on deep abyssal plains after the shell band has been lai down, it is attacked by solution so that the shells are damaged. They are protecte, in his opinion from complete removal by the cover of pelagic mud gradually accumu lating on top. He claims that true pelagic sediment in such deep localities is free tz lime. This forms his second argument for the turbiditic origin of the band. If this i correct and pelagic sedimentation of clay can save a band of shells from solutior it would also seem possible for planktonic shells, dropped individually, to be thu protected. This would imply that even on the deep abyssal plains separate shells ca: accumulate provided sufficient clay is supplied. The absence of pteropod ooze, deposit poor in protective clay in depths exceeding 2,000 or 3,000 m, is no proof tha the shells cannot survive in greater depth when intermixed with clay. In fact, we kno~ that abyssal water fails to dissolve away all calcium carbonate, for in the Atlanti at 6,000 m the average content of samples is about 10 ~. Whether the shells are claim ed to have been emplaced together, or one by one, does not make much differenc to the problem of preservation.
The band not destroyed by burrowing Most of the beds described by NESTEROFF(1961) show plenty of burrowing activit' in the finer grained parts. This has not impaired the sharpness of the thin sheet o shells at the top, although according to his hypothesis the bed was present befor, the burrowing started. Even if some of the above arguments are not forceful, the combination appear
Marine Geol., 2 (1964) 236-24t
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
241
to render the deposition of the shell bed by the tail of a turbidity current most improbable.
ALTERNATIVE EXPLANATIONS
An alternative explanation could be deposition from the current that deposited the covering turbidite. The shells should then be considered the coarsest fraction. But no cases are known of ancient turbidites with all the coarse grains forming a pavement. They are always scattered through a certain thickness. Some of the arguments opposing Nesteroff's view also go against supply by the following current, namely that there is no known source in shallower water from which so much pteropod material could be derived and that there is no obvious reason why the pavement should usually be just one shell thick. If the two possibilities of supply either by the first current (Nesteroff) or by the second current (see above) are discarded, then the only remaining source is autochthonous, and for pelagic organisms this means pelagic sedimentation. The explanation by direct formation of the pavement due to organic sedimentation without clay is likewise unsatisfactory. For if a bottom current prevented clay deposition the pteropods would be dissolved at depths exceeding 5,000 m. Moreover, there should be many instances of greater thickness than one shell. Finally, burrowing would have mixed clay with the shells and destroyed the band. Direct pelagic sedimentation being excluded, there appears to remain but one possibility: that of gradual supply intermixed with clay, followed by removal of some clay by a current leaving a pavement of winnowed shells. If the logic of this reasoning is admitted, it is worth trying whether or not this alternative explanation of the pavement stands up better to the criticism raised against Nesteroff's hypothesis. In the hypothesis of winnowing, the density of the shells is assumed to have remained constant and normal. The turbidity current was nowhere free of clay and its density was distinctly greater than that of clear water. The bed should always lie directly below the covering turbidite. It should generally be one shell thick because from that stage of development onwards the underlying material is protected and the winnowing must come to a natural end when the pavement is created. No external supply of pteropods is required. The bed post-dates the burrowing and was therefore not mixed with the underlying material. The shorter radio-carbon age of the upper part of the sequence is logically explained. This view also accounts for the difference between the lower barren turbiditic part of the pelitic layer as opposed to the upper part with the same shells as in the covering pavement. In opposition to this view it can be claimed that the slowly deposited pelagic clays must show some cohesion and that this should make them resistant to erosion. Hence, if the following turbidity current is fast enough to be able to remove the clay, it should also be competent to wash away the pteropod shells. This argument would equally oppose Nesteroff's hypothesis in which the pelagic clay is removed by the Marine Geol., 2 (1964) 236-246
next current with the band of shells left intact. However, this objection cannot bea much weight. The high resistance of clay to erosion is a geological dogma, base, mainly on Hjulstr/Sm's diagram. (HJULSTROM, 1934-1935, p.298). But SUNDBORq (1956) later pointed out that it only holds for firmly consolidated clays and that n, evidence is available for fresh deposits in quiet water. The present writer is carryin out tests on this problem and these indicate that in sea water unconsolidated clay i eroded by a current of about 30 cm/sec. According to SUNDBORG (1956, p.197 this is the velocity at which a bed of lignite grains of 0.2 mm (density 1.25) is eroded Assuming that forums and pteropods filled with watery clay have a bulk densit somewhat greater than 1.25, then clay can be eroded by a current unable to carr' away the shells. The thickness of the layer that would yield a pavement of shells could b, estimated if the concentration of the shells in the pelagic clay underneath it wet, known. No data are available but probably the winnowing of a few centimeters woult suffice. In abyssal deep-sea cores the upper part is usually almost watery in compo sition and it is reasonable to suppose that erosion requires only a sluggish current. But the turbidites covering the shell pavements are proof in themselves that current able to carry fine sand in suspension is unable to dislodge the shells. We als( know from ancient turbidites that currents depositing the same type of materia have formed flutings by erosion of the underlying mud. These two facts taken togethe provide a strong case for the possibility that the shells can be winnowed out. The most difficult feature to account for on the basis of the view here presente( is the often observed gradual change in colour and carbonate content through th~ pelagic part of the sequence. Perhaps burrowing and ploughing activity durint sedimentation is the answer. It would be interesting to know whether the pavement shells are aligned like thl Foraminifera at the base of a recent turbidite from the Santa Barbara Basin as note~ by HARMAN (1964). But it would not help to choose between winnowing or depositint because in both cases a current is involved. In passing it may be remarked that Nesteroff compares the grain size of calca reous detritus to that of clay and quartz, determined on deep-sea cores. However we do not know the size and composition of the clay floccules in the deep-sea whict may have held quartz dust enclosed. Hence, conclusions on settling rates must b~ viewed with circumspection.
P E L A G I C S T R A T U M 1N A N C I E N T T U R B I D I T E F O R M A T I O N S
NESTEROFF and HEEZEN (1963) have sought to apply their conclusion that no pelagic sediment accumulates between oceanic turbidites, to the problems of fiysch. The) selected the case of the "flysch /t helminthoides" of the Rivi6ra described b) LANTEAUME (1962). They suggest that in contrast to the opinion of Lanteaume the turbidites follow directly one on top of the other without pelagic sediment between Marine Geol., 2 (1964) 236-24(
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
243
If this line of reasoning is followed many turbidite formations call for re-interpretation. A brief review of the conclusions of several authors who have dealt with this problem of pelagic sediment in flysch will be given. But as the flysch described by Lanteaume is particularly complex, no choice will be made between the opposing views for that particular formation. A simpler case has been described by VAN HINTE (1963) in which an Alpine flysch apparently contains a pelagic stratum above each turbidite. The graded beds show a wide variety in composition and in maximum grain size. They end upwards in laminated clay covered by homogeneous marl. The latter two strata both contain pelagic Foraminifera, but the marl is much richer and also holds occasional ammonites and inocerams. Grading is absent in the marl. Burrowing is detectable both in the clay and in the marl. Plant remains are restricted to the clay. Under coarse beds the marl may be absent. Jointly these features are indicative of the pelagic character of the marl. To these arguments presented by Van Hinte one can add the significant fact that in spite of wide variation in the composition of the turbidites the marls are identical. NATLAND (1963) claims that on top of the turbidites in the Ventura Basin there is a thin stratum of pelagic material containing pelagic and benthonic fossils. He estimates that between 2 and 10 ~ of the basin fill is (hemi-)pelagic. BOUMA (1962), on the other hand, considered the flysch formation he studied in the French Rivirra (Peira Cava) to be almost devoid of pelagic sediments 1. Occasionally he found a marl intercalation at the top of what he termed the "peletic interval" (p.50) and he interpreted it as being of pelagic origin. For the same general area STANLEY(1963, p.786) reported that he was unable "to differentiate with certainty sediments which were deposited from the less-dense, slow-moving tail of the current from those accumulated by pelagic sedimentation". De Raaf in a discussion (SYMPOSIUM,1961, p.71) drew attention to unpublished results obtained by J. Brouwer (paleontologist of the "Royal/Shell Exploration and Production Laboratory", Rijswijk, The Netherlands) in analysing a number of pelitic intervals from turbiditic ultra-helvetic flysch sediments, from the Peira Cava flysch, and from the macigno of Italy. The sampled top of the intervals often proved to contain autochthonous foraminiferal assemblages. De Raaf (personal communication) further points out that since 1961 quantitative analyses were carried out on a number of these samples (a.o. Gurnigel flysch), which confirmed the autochthonous, deep-water character of the said microfaunas upon comparison with present-day normal bathyal to abyssal deposits containing Foraminifera. In contrast to the redeposited lower part of the pelitic intervals the upper parts show no indications of faunal mixture or sorting. On the strength of this evidence, combined in some cases with colour contrasts, De Raaf distinguishes in turbidite sequences true pelagic (or hemi-pelagic) muds and proposes for these the 1 Bouma informs the present author that he has no reasons for changing his former opinion and that a remark in a later joint paper (STANLEYand BOUMA,1964,p.62) is misleadingin this respect. Marine Geol., 2 (1964) 236--246
term pelagic pelites. The underlying mixed muds deposited by the tail of the turbidit, current he calls turbiditic pelites. In the latter fossils tend to be absent or scarce, witl allochthonous elements predominating 1. CONTESCU et al. (1963) described a turbidite formation from Romania witl pelagic lutites between the graywackes, increasing in importance in a down-curren direction. SLACZKA (1963) attributed a large part of two shaley turbidite formation: from Poland to pelagic deposition. MEISCHNER (1964) claims that the pelitic inter calations between bioclastic turbidites were formed in stagnant water by pelagic sedimentation. They are characterized by evenly divided nektonic and planktonic fossils. RAOOMSKI (1960) made a special study of this problem in some Polish ftysct formations. He concluded (p.124): "that at least part of the shale layers in flysch i~ deposited by turbidity currents. It is possible that the uppermost parts of some shak layers are pelagic deposits." In an earlier paper RADOMSKI (1958) showed that in th( shaley horizon of the Podhale flysch beds the lower part contains angular sand grains whereas in the upper part only a few well-rounded grains are found. He explainec this relation by assuming that the upper stratum is truly pelagic and contains som~. eolian grains. KSIAZKIEWICZ(1961; and in: DZULYNSKI et al., 1959) noticed that it some cases the turbidite ends in pure clay a few centimeters thick covered by a cla) rich in pelagic and/or benthonic Foraminifera. The latter part of the clay is presumabl 3 of pelagic origin. He also points to the occurrence of fish, ammonites, belemnites and thin-shelled inocerams in the pelitic interval of flysch sequences. They evidentl) dropped from above and the horizon in which they are enclosed must be pelagic. UNRUG (1963) remarked that in the Istebna Beds, a Carpathian formation composed largely of fluxo-turbidites and slumps, there are many dark, graded siltstone beds, poor in microfossils and deposited by diluted turbidity currents. There is also an insignificant development of clayey shales of various colours rich in microfossils. These latter are true pelagic deposits occurring only rarely and in thin beds. THOMSON and THOMASSON (1964) describe an Upper Paleozoic formation of limestone turbidites in Texas. Well-marked beds of dark terrigenous shale of pelagic origin are distinguishable from the brown lime mudstones forming the tops of the turbidites. These various findings indicate that in most cases a fair part at least of the pelitic interval belongs to the turbidite, but that the upper part, usually insignificant in thickness, is pelagic. Obviously the pelagic part was first deposited in greater thick-
In order to retain Bouma's system of lettering for the strata in turbidites, which is meant for general use in all turbidite sequences including the present sea floor, it is here recommended by De Raaf and the author to designate the entire pelitic interval by the letter e. This was also done by Bouma in his graphic sections. Butit is suggested that one uses the symbols ep and e t where there is sufficientevidence to distinguish pelagic and turbiditic parts. In ancient turbidites there also occur crypto-crystalline limestone beds, presumably formed by precipitation, siliceous horizons, ash beds, slumps, etc., to which can be added possible pavements similar to those discovered by Nesteroff. It would be confusing to use a system of lettering for such scarce intercalations. Marine Geol., 2 (1964) 236--246
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
245
ness, but partly eroded when the flutings and tool markings were produced. It has also been compacted to a quarter of its unconsolidated magnitude. On the other hand, over large areas of some turbidite formations the pelagic sediments accumulated only in small thicknesses and were washed away every time by the next turbidity current, leaving no pelagic cover on the preceding turbidite 1.
CONCLUSION In summary, the conclusion here presented is that for both kinds of turbidite sequences, those of the recent ocean basins and those o f the ancient geosynclinal troughs, pelagic sediment is c o m m o n beneath the soles o f the turbidites, but is by no means ubiquitous. N o fossil pavements have yet been found, but it is to be expected that on close attention, especially underneath fine grained turbidites examples will be discovered. The present writer, although suggesting a different interpretation o f the facts discovered by Nesteroff, would still like to end with a tribute to this author for the very extensive and painstaking examination o f a huge collection o f samples. His d i s c o v e r y - - a m o n g other t h i n g s - - o f the pavement is also important for the study o f ancient sequences. It focusses attention on the value o f micro-fossils in discussing the origin o f various parts of ancient turbidite sequences.
REFERENCES BOUMA,A. H., 1962. Sedimentology of Some Flysch Deposits. A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168 pp. CONTESCtJ, L., JIPA, D. and MIHAILESCtJ,N., 1963. Turbidite in Flisul eocenului de Sotrile. Congr. Assoc. G~ol. Carpato-Balcanique, 5e, Bucarest, Travaux, pp.109-128. DZtJLVNSKI, S., KStAZrdEWICZ,M. and KtJENEN, PH. H., 1959. Turbidites in flysch of the Polish Carpathian Mountains. Bull. Geol. Soc. Am., 70 : 1089-1118. HARMArq,R. A., 1964. Distribution of Foraminifera in the Santa Barbara Basin, California. Micropaleontology, 10 : 81-96. HJULSTR/JM,F., 1934-1935. Studies of the morphological activity of rivers as illuminated by the river Fyris. Bull. Geol. Inst. Univ. Upsala, 25 : 221-527. KSIAZKmWICZ,M., 1961. Life conditions in flysch basins. Ann. Soc. G~ol. Pologne, 31 : 3-21. LAIqTEAUME,M., 1962. Contribution d l'l~tude g~ologique des Alpes maritimes franco-italiennes (Stratigraphie). Th~se, Univ. Paris, Paris, 409 pp. MEISCHNER,K. D., 1964. Allodapische Kalke, Turbidite in Riff-nahen Sedimentations-Becken. In: A. H. BOUMAand A. BROUWER(Editors), Turbidites. Elsevier, Amsterdam, pp.156-191.
1 This also means that the organoglyphs (= organic sole markings) are re-excavated and do not show the surface relief of the basin floor at the moment the turbidity current arrived. Whether they were originally produced internally as burrowings or superficially as ploughings is a different question. Another deduction is that a significant percentage of the clay in a turbidite exposure was first deposited pelagically up-current from the point in question. How much of the clay has been carried by the current from its point of origin is difficult to say. It depends on the distance and current velocity, but also on the nature and thickness both of the pelagic deposit and of the generating slide. Marine GeoL, 2 (1964) 236-246
NATLAND,M. L., 1963. Paleoecology and turbidites. J. Paleontol., 37 : 946-951. NESTEROFF, W. D., 1961. La "sequence type" dans les turbidites terrig~nes modernes. Rev. G~ograpt Phys. Gkol. Dyn., 4 (2) : 263-268. NESTEROFF, W. D., 1963. Essai d'interpr6tation du m6canisme des courants de turbidit6. Bull. Sot G~oL France, 7 (4) : 849-855. NESTEROFF, W. D. and HEEZEN, B. C., 1963. Essais de comparaison entre les turbidites modernes e le flysch. Rev. G~ograph. Phys. G~oL Dyn., 5 (2) : 115-127. RADOMSKI, A., 1958. The sedimentological character of the Podhale ftysch. Acta Geol. Polon., 8 335-395. RADOMSKt, A., 1960. Remarks on sedimentation of shakes in flysch deposits. Bull. Acad. Polon. Sci. Skr. G~ol. G~ograph, 8 (2) : 123-129. RUSNAK, G. A., BOWMAN, A. L. and t)STLUND, H. G., 1963. Miami natural radiocarbon measure ments. 2. Radiocarbon, 5 : 23-33. SLACZKA, A., 1963. Observations on the sedimentation of hieroglyphic beds and variegated shale: from Dukla Unit. Ann. Soc. G~oL Pologne, 33 : 93-110. STANLEY, D. J., 1963. Vertical petrographic variability in Annot Sandstone turbidites: some prelimi nary observations. J. Sediment. Petrol., 33 : 783-788. STANLEY,D. J. and BOUMA,A. H., 1964. Methodology and paleographic interpretation of flysct formations: a summary of studies in the Maritime Alps. In: A. H. BOUMAand A. BROUWEr (Editors), Turbidites. Elsevier, Amsterdam, pp.34-64. SUNDBORG, A., 1956. The river Klar~ilven, a study of fluvial processes. Geq(raf. Ann., 38 : 127-316. SYMPOSIUM, 1961. Some aspects of sedimentation in orogenic belts. Proc. GeoL Soc. London, 1587 69-80. THOMSON, A. and THOMASSON, M. R., 1964. Sedimentology and stratigraphy of the Dimple Lime. stone, Marathon Region, Texas. In: The Filling of the Marathon Geosyncline. Guidebook 1964 Permian Basin Section - - Soc. Econ. Paleontologists Mineralogists, Publ., 64-9 : 22-30. TUREKIAN, K. K., 1964. The geochemistry of the Atlantic Ocean Basin. Trans. N.Y. Acad. Sci., Ser. 2 26 (3) : 312-330. UNRUG, R., 1963. lstebna beds--a fluxo-turbidite formation in the Carpathian flysch. Ann. Soc Geol. Pologne, 33 : 49-92. VAN HINTE, J. E., 1963. Zur Stratig~aphie und Mikropal~iontologie der Oberkreide und des Eozan~, des Krappfeldes (K~irnten). Jahrb. Geol. Bundesanstalt (Austria), Sonderband, 8 : 147 pp.
Marine Geol., 2 (1964) 236--246
Geological Institute, State University, Groningen (The Netherlands) (Received June 1, 1964) (Resubmitted August 27, 1964)
SUMMARY
Nesteroff discovered, among other interesting facts, that many oceanic turbidite are covered by a thin film of pelagic shells, and he attributed this to deposition fror the tail of the turbidity current. Any later pelagic deposit is supposed to have bee washed away by the following current, and is therefore missing. Nesteroff's findin is important in itself and throws new light on ancient turbidites. However, the presen author favours an explanation through winnowing of the shells from a pelagi stratum by the current that deposited the covering turbidite. This would mean tha the shell pavement does not mark the end but the beginning of a turbidite, and tha the upper part of the pelitic interval with pteropods and planktonic Foraminifer~ is a pelagic deposit. Several arguments are presented in favour of the latter view. A Nesteroff and Heezen are led by their interpretation to doubt similarly the presenc of pelagic strata between flysch turbidites, a brief survey is made of this subject. I shows that most of the ancient turbidite sequences also contain many beds with pelagic part towards the top of the pelitic interval. Up to the present, however, n~ shell pavement has been reported, but careful search will probably reveal example of such fossil pavements.
INTRODUCTION
In two papers NESTEROFF (1961, 1963) has recorded significant facts about recen deep-sea turbidites. His results were obtained by the study of large numbers of deep sea cores collected by U.S.A. expeditions, especially those from the Lamont Geologi cal Observatory. Nesteroff shows that turbidites are more numerous in suitabh localities than hitherto realized, and that grading is detectable in practically all thes~ beds when studied carefully. Furthermore, Nesteroff has discovered that in many deep-sea turbidites th, Marine Geol., 2 (1964) 236-24~
THE SHELL PAVEMENT BELOW OCEANIC TURBIDITE$
~.J,
pelitic upper part consists of two zones, (1) the lower one mainly inorganic with more or less calcium carbonate detritus, (2) the upper part with occasional pelagic Foraminifera and pteropods added. Where covered by a fine-grained turbidite the sequence tends to end with a thin band of forams and pteropods one shell thick. But below coarser beds, presumably deposited from a somewhat faster current, this band is missing. Nesteroff concludes that in the latter case the shells have been washed away. The present paper is concerned with the origin of this thin band. The explanation offered by Nesteroff of the upper band is that the tail of the turbidity current was too slow to carry clay but brought along shells rendered extremely buoyant by organic remains or gas bubbles (see NESTEROFFand HEEZEN, 1963). From this he goes on to conclude that there is no pelagic deposit. As pelagic deposition to an amount of a few centimeters must have occurred in the interval between two successive currents, he draws the inference that the pelagic deposit has been washed away by the successive turbidity current. Nesteroff's finding is of much importance, also because it can throw light on the manner in which ancient turbidites were formed. The present author would like to suggest an alternative explanation, which also leads to a different interpretation of the evidence concerning ancient deposits. He believes pelagic sediment containing a few shells was deposited after the turbidity current had laid down a stratum of clay practically devoid of shells. The next current started to wash away the pelite but left the shells which it was unable to pick up. As a result it created a winnowed pavement of shells protecting what remained of the pelagic sediment. Nesteroff points to two items of supporting evidence for his explanation. He mentions cores taken in the Tongue of the Ocean, Bahamas, where several turbidites are characterised by some special material (oolites, corals, spicules of holothurians, etc.) that is missing in the under- and overlying beds. This contrast is attributable to different source areas. The very fine fraction of this special material is found right up to the top of the bed, and is therefore directly overlain by the next turbidite. This is taken as proof of the absence of a pelagic stratum, because pelagic material should not vary in composition from bed to bed. However, the fine material intermixed up to the top of the calcareous turbidites can be accounted for by the burrowing and ploughing activity of animals, a phenomenon emphasized in most of Nesteroff's descriptions. This activity would mix the upper parts of the turbidites with the gradually accumulating pelagic deposit. There is an argument for this interpretation because the radio-carbon dating of the coarse part ( R u S N A K et al., 1963) shows it to be older than the upper fine part beneath or above. This indicates that the coarse part consists of reworked older material, but that the fine part is largely contemporaneous. Nesteroff's first argument is therefore not considered to be convincing. His second argument, based on the occurrence of the shell band in deep water, will be examined later and will also be found weak.
Marine Geol., 2 (1964) 236-246
OBSTACLES TO NESTEROFF'S EXPLANATION
The explanation of the upper film of shells as representing the end of the underlyir turbidite, from which the pelagic cover has later been stripped, meets with obstacle The arguments can be brought under a few headings.
Density of the shells If the shells are deposited after the clay, their settling rate in stagnant water must 1: slower than that of the clay floccules. A rough estimate can be made of their densit as follows. If the equivalent diameter of the clay floccules is put at 5 IXand that of tb shells at 2 Ix with a true diameter of 0.5 mm, then the density of the shells includin the buoyant material cannot be more than 0.0001 greater than that of the surrounc ing water in order to settle more slowly than the clay. So close a balance in densit between shells and water is impossible. Moreover, such material would be extremel sensitive to all types of current action on the deep-sea floor and would tend to b moved about by the slightest disturbances and to accumulate in irregular patches.
The gas lift If gas bubbles play a part and if the very slow-settling shells are deposited by th, current in deep water, the bubble must have been in the shells when they were picke~ up. At the point of origin in smaller depth the bubble must have been so much large that it would have rendered the shell lighter than seawater. Conversely, if the shell wa,. picked up from the bottom by the current with a bubble that made it almost float the higher pressure at the point of deposition would compress the gas and increas~ the density of the shell. This would cause the shell to settle more quickly than th~ clay and drop out before, not after, the pelitic material. Evidently buoyancy owin~ to a bubble, as suggested by Nesteroff, is too sensitive to changes in depth to b~ invoked in this problem.
Organic matter The area over which the turbidite is finally spread out is much larger than that of the deposit in shallower water before the take-off. This means that the shells must have formed a bed or part of a bed much thicker than the shell pavement. The accumulation of this mass requires a long time and the organic matter would have disintegrated before the current picked the shells up. Moreover, most of the shells reachin~ the bottom are devoid of decomposable organic matter anyhow. This alternative suggestion for explaining a very low density of the shells is evidently also unsatisfactory.
Marine Geol., 2 (t964) 236-246
THE SHELL PAVEMENT BELOW OCEANIC TURBIDITES
L3~
Motive force o f the current
Nesteroff's hypothesis implies a final stage in which the turbidity current no longer carries clay but only organisms. But a current cannot continue to flow under the influence of gravity unless the density is greater than that of the surrounding clear sea water. Obviously a mass of water without clay and only weighted by some micro-
Stratum
~
'-~ -1 density Great
¢:......;. : . . . . . . . . ~ ' . :.-.:..:
S0.ro.
T
B
Fig.1. Distribution of micro-organisms in typical oceanic turbidite sequences. (After NESTEROFF, 1961.) In the terrigenous deposits (T) micro-organisms occur at three levels: a stratum in the sandy part; scattered through the top part of the lutite; and as a covering stratum. According to the present author the latter is a winnowed concentrate. In the bioclastic deposits (B) they are met with throughout, but there is a concentration at the top. According to the present author the latter is either a winnowed concentrate or the true pelagic part.
organisms with very small excess density forms a liquid virtually equal in density to clear water. Hence it would have no potential energy to sustain the flow. In other words, the mechanism invoked to supply the stratum o f shells, a turbidity current weighted by very light shells only, is unrealistic. No cover to the shell band The thickness o f the supposed pelagic bed, covering the shell band, should vary according to the length o f the interval between the currents. One would expect to encounter cases in which the pelagic cover was too thick to have been entirely removed, thus leaving a stratum above the shell band. Nesteroff does not record having f o u n d this situation. Marine GeoL, 2 (1964) 236-246
Thickness of the shell band There is no logical explanation for the close balance between the available amoul of shells and the extent of the turbidite resulting in a band about one shell thick. An even if the right amount should be present to cover the turbidite, it still remains enil matic why it was spread so evenly over the entire surface. One would expect to fin all transitions in a normal frequency distribution between core samples of beds wit only an occasional shell on top to those with a stratum several shells thick. The fa4 that the band is nearly always a rather complete stratum of just one shell thick forrr good evidence for winnowing, and against deposition from a current.
No pteropod supply Deposits rich in pteropods are rare and occur only near Cuba and on the Mid-AtlantJ Ridge. Hence, the great majority of turbidity currents coming down a submarin canyon or down a continental slope would pass no source with sufficient concentr~ tion of these shells.
Solution of the shells Nesteroff points out that on deep abyssal plains after the shell band has been lai down, it is attacked by solution so that the shells are damaged. They are protecte, in his opinion from complete removal by the cover of pelagic mud gradually accumu lating on top. He claims that true pelagic sediment in such deep localities is free tz lime. This forms his second argument for the turbiditic origin of the band. If this i correct and pelagic sedimentation of clay can save a band of shells from solutior it would also seem possible for planktonic shells, dropped individually, to be thu protected. This would imply that even on the deep abyssal plains separate shells ca: accumulate provided sufficient clay is supplied. The absence of pteropod ooze, deposit poor in protective clay in depths exceeding 2,000 or 3,000 m, is no proof tha the shells cannot survive in greater depth when intermixed with clay. In fact, we kno~ that abyssal water fails to dissolve away all calcium carbonate, for in the Atlanti at 6,000 m the average content of samples is about 10 ~. Whether the shells are claim ed to have been emplaced together, or one by one, does not make much differenc to the problem of preservation.
The band not destroyed by burrowing Most of the beds described by NESTEROFF(1961) show plenty of burrowing activit' in the finer grained parts. This has not impaired the sharpness of the thin sheet o shells at the top, although according to his hypothesis the bed was present befor, the burrowing started. Even if some of the above arguments are not forceful, the combination appear
Marine Geol., 2 (1964) 236-24t
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
241
to render the deposition of the shell bed by the tail of a turbidity current most improbable.
ALTERNATIVE EXPLANATIONS
An alternative explanation could be deposition from the current that deposited the covering turbidite. The shells should then be considered the coarsest fraction. But no cases are known of ancient turbidites with all the coarse grains forming a pavement. They are always scattered through a certain thickness. Some of the arguments opposing Nesteroff's view also go against supply by the following current, namely that there is no known source in shallower water from which so much pteropod material could be derived and that there is no obvious reason why the pavement should usually be just one shell thick. If the two possibilities of supply either by the first current (Nesteroff) or by the second current (see above) are discarded, then the only remaining source is autochthonous, and for pelagic organisms this means pelagic sedimentation. The explanation by direct formation of the pavement due to organic sedimentation without clay is likewise unsatisfactory. For if a bottom current prevented clay deposition the pteropods would be dissolved at depths exceeding 5,000 m. Moreover, there should be many instances of greater thickness than one shell. Finally, burrowing would have mixed clay with the shells and destroyed the band. Direct pelagic sedimentation being excluded, there appears to remain but one possibility: that of gradual supply intermixed with clay, followed by removal of some clay by a current leaving a pavement of winnowed shells. If the logic of this reasoning is admitted, it is worth trying whether or not this alternative explanation of the pavement stands up better to the criticism raised against Nesteroff's hypothesis. In the hypothesis of winnowing, the density of the shells is assumed to have remained constant and normal. The turbidity current was nowhere free of clay and its density was distinctly greater than that of clear water. The bed should always lie directly below the covering turbidite. It should generally be one shell thick because from that stage of development onwards the underlying material is protected and the winnowing must come to a natural end when the pavement is created. No external supply of pteropods is required. The bed post-dates the burrowing and was therefore not mixed with the underlying material. The shorter radio-carbon age of the upper part of the sequence is logically explained. This view also accounts for the difference between the lower barren turbiditic part of the pelitic layer as opposed to the upper part with the same shells as in the covering pavement. In opposition to this view it can be claimed that the slowly deposited pelagic clays must show some cohesion and that this should make them resistant to erosion. Hence, if the following turbidity current is fast enough to be able to remove the clay, it should also be competent to wash away the pteropod shells. This argument would equally oppose Nesteroff's hypothesis in which the pelagic clay is removed by the Marine Geol., 2 (1964) 236-246
next current with the band of shells left intact. However, this objection cannot bea much weight. The high resistance of clay to erosion is a geological dogma, base, mainly on Hjulstr/Sm's diagram. (HJULSTROM, 1934-1935, p.298). But SUNDBORq (1956) later pointed out that it only holds for firmly consolidated clays and that n, evidence is available for fresh deposits in quiet water. The present writer is carryin out tests on this problem and these indicate that in sea water unconsolidated clay i eroded by a current of about 30 cm/sec. According to SUNDBORG (1956, p.197 this is the velocity at which a bed of lignite grains of 0.2 mm (density 1.25) is eroded Assuming that forums and pteropods filled with watery clay have a bulk densit somewhat greater than 1.25, then clay can be eroded by a current unable to carr' away the shells. The thickness of the layer that would yield a pavement of shells could b, estimated if the concentration of the shells in the pelagic clay underneath it wet, known. No data are available but probably the winnowing of a few centimeters woult suffice. In abyssal deep-sea cores the upper part is usually almost watery in compo sition and it is reasonable to suppose that erosion requires only a sluggish current. But the turbidites covering the shell pavements are proof in themselves that current able to carry fine sand in suspension is unable to dislodge the shells. We als( know from ancient turbidites that currents depositing the same type of materia have formed flutings by erosion of the underlying mud. These two facts taken togethe provide a strong case for the possibility that the shells can be winnowed out. The most difficult feature to account for on the basis of the view here presente( is the often observed gradual change in colour and carbonate content through th~ pelagic part of the sequence. Perhaps burrowing and ploughing activity durint sedimentation is the answer. It would be interesting to know whether the pavement shells are aligned like thl Foraminifera at the base of a recent turbidite from the Santa Barbara Basin as note~ by HARMAN (1964). But it would not help to choose between winnowing or depositint because in both cases a current is involved. In passing it may be remarked that Nesteroff compares the grain size of calca reous detritus to that of clay and quartz, determined on deep-sea cores. However we do not know the size and composition of the clay floccules in the deep-sea whict may have held quartz dust enclosed. Hence, conclusions on settling rates must b~ viewed with circumspection.
P E L A G I C S T R A T U M 1N A N C I E N T T U R B I D I T E F O R M A T I O N S
NESTEROFF and HEEZEN (1963) have sought to apply their conclusion that no pelagic sediment accumulates between oceanic turbidites, to the problems of fiysch. The) selected the case of the "flysch /t helminthoides" of the Rivi6ra described b) LANTEAUME (1962). They suggest that in contrast to the opinion of Lanteaume the turbidites follow directly one on top of the other without pelagic sediment between Marine Geol., 2 (1964) 236-24(
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
243
If this line of reasoning is followed many turbidite formations call for re-interpretation. A brief review of the conclusions of several authors who have dealt with this problem of pelagic sediment in flysch will be given. But as the flysch described by Lanteaume is particularly complex, no choice will be made between the opposing views for that particular formation. A simpler case has been described by VAN HINTE (1963) in which an Alpine flysch apparently contains a pelagic stratum above each turbidite. The graded beds show a wide variety in composition and in maximum grain size. They end upwards in laminated clay covered by homogeneous marl. The latter two strata both contain pelagic Foraminifera, but the marl is much richer and also holds occasional ammonites and inocerams. Grading is absent in the marl. Burrowing is detectable both in the clay and in the marl. Plant remains are restricted to the clay. Under coarse beds the marl may be absent. Jointly these features are indicative of the pelagic character of the marl. To these arguments presented by Van Hinte one can add the significant fact that in spite of wide variation in the composition of the turbidites the marls are identical. NATLAND (1963) claims that on top of the turbidites in the Ventura Basin there is a thin stratum of pelagic material containing pelagic and benthonic fossils. He estimates that between 2 and 10 ~ of the basin fill is (hemi-)pelagic. BOUMA (1962), on the other hand, considered the flysch formation he studied in the French Rivirra (Peira Cava) to be almost devoid of pelagic sediments 1. Occasionally he found a marl intercalation at the top of what he termed the "peletic interval" (p.50) and he interpreted it as being of pelagic origin. For the same general area STANLEY(1963, p.786) reported that he was unable "to differentiate with certainty sediments which were deposited from the less-dense, slow-moving tail of the current from those accumulated by pelagic sedimentation". De Raaf in a discussion (SYMPOSIUM,1961, p.71) drew attention to unpublished results obtained by J. Brouwer (paleontologist of the "Royal/Shell Exploration and Production Laboratory", Rijswijk, The Netherlands) in analysing a number of pelitic intervals from turbiditic ultra-helvetic flysch sediments, from the Peira Cava flysch, and from the macigno of Italy. The sampled top of the intervals often proved to contain autochthonous foraminiferal assemblages. De Raaf (personal communication) further points out that since 1961 quantitative analyses were carried out on a number of these samples (a.o. Gurnigel flysch), which confirmed the autochthonous, deep-water character of the said microfaunas upon comparison with present-day normal bathyal to abyssal deposits containing Foraminifera. In contrast to the redeposited lower part of the pelitic intervals the upper parts show no indications of faunal mixture or sorting. On the strength of this evidence, combined in some cases with colour contrasts, De Raaf distinguishes in turbidite sequences true pelagic (or hemi-pelagic) muds and proposes for these the 1 Bouma informs the present author that he has no reasons for changing his former opinion and that a remark in a later joint paper (STANLEYand BOUMA,1964,p.62) is misleadingin this respect. Marine Geol., 2 (1964) 236--246
term pelagic pelites. The underlying mixed muds deposited by the tail of the turbidit, current he calls turbiditic pelites. In the latter fossils tend to be absent or scarce, witl allochthonous elements predominating 1. CONTESCU et al. (1963) described a turbidite formation from Romania witl pelagic lutites between the graywackes, increasing in importance in a down-curren direction. SLACZKA (1963) attributed a large part of two shaley turbidite formation: from Poland to pelagic deposition. MEISCHNER (1964) claims that the pelitic inter calations between bioclastic turbidites were formed in stagnant water by pelagic sedimentation. They are characterized by evenly divided nektonic and planktonic fossils. RAOOMSKI (1960) made a special study of this problem in some Polish ftysct formations. He concluded (p.124): "that at least part of the shale layers in flysch i~ deposited by turbidity currents. It is possible that the uppermost parts of some shak layers are pelagic deposits." In an earlier paper RADOMSKI (1958) showed that in th( shaley horizon of the Podhale flysch beds the lower part contains angular sand grains whereas in the upper part only a few well-rounded grains are found. He explainec this relation by assuming that the upper stratum is truly pelagic and contains som~. eolian grains. KSIAZKIEWICZ(1961; and in: DZULYNSKI et al., 1959) noticed that it some cases the turbidite ends in pure clay a few centimeters thick covered by a cla) rich in pelagic and/or benthonic Foraminifera. The latter part of the clay is presumabl 3 of pelagic origin. He also points to the occurrence of fish, ammonites, belemnites and thin-shelled inocerams in the pelitic interval of flysch sequences. They evidentl) dropped from above and the horizon in which they are enclosed must be pelagic. UNRUG (1963) remarked that in the Istebna Beds, a Carpathian formation composed largely of fluxo-turbidites and slumps, there are many dark, graded siltstone beds, poor in microfossils and deposited by diluted turbidity currents. There is also an insignificant development of clayey shales of various colours rich in microfossils. These latter are true pelagic deposits occurring only rarely and in thin beds. THOMSON and THOMASSON (1964) describe an Upper Paleozoic formation of limestone turbidites in Texas. Well-marked beds of dark terrigenous shale of pelagic origin are distinguishable from the brown lime mudstones forming the tops of the turbidites. These various findings indicate that in most cases a fair part at least of the pelitic interval belongs to the turbidite, but that the upper part, usually insignificant in thickness, is pelagic. Obviously the pelagic part was first deposited in greater thick-
In order to retain Bouma's system of lettering for the strata in turbidites, which is meant for general use in all turbidite sequences including the present sea floor, it is here recommended by De Raaf and the author to designate the entire pelitic interval by the letter e. This was also done by Bouma in his graphic sections. Butit is suggested that one uses the symbols ep and e t where there is sufficientevidence to distinguish pelagic and turbiditic parts. In ancient turbidites there also occur crypto-crystalline limestone beds, presumably formed by precipitation, siliceous horizons, ash beds, slumps, etc., to which can be added possible pavements similar to those discovered by Nesteroff. It would be confusing to use a system of lettering for such scarce intercalations. Marine Geol., 2 (1964) 236--246
THE SHELLPAVEMENTBELOWOCEANICTURBIDITES
245
ness, but partly eroded when the flutings and tool markings were produced. It has also been compacted to a quarter of its unconsolidated magnitude. On the other hand, over large areas of some turbidite formations the pelagic sediments accumulated only in small thicknesses and were washed away every time by the next turbidity current, leaving no pelagic cover on the preceding turbidite 1.
CONCLUSION In summary, the conclusion here presented is that for both kinds of turbidite sequences, those of the recent ocean basins and those o f the ancient geosynclinal troughs, pelagic sediment is c o m m o n beneath the soles o f the turbidites, but is by no means ubiquitous. N o fossil pavements have yet been found, but it is to be expected that on close attention, especially underneath fine grained turbidites examples will be discovered. The present writer, although suggesting a different interpretation o f the facts discovered by Nesteroff, would still like to end with a tribute to this author for the very extensive and painstaking examination o f a huge collection o f samples. His d i s c o v e r y - - a m o n g other t h i n g s - - o f the pavement is also important for the study o f ancient sequences. It focusses attention on the value o f micro-fossils in discussing the origin o f various parts of ancient turbidite sequences.
REFERENCES BOUMA,A. H., 1962. Sedimentology of Some Flysch Deposits. A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168 pp. CONTESCtJ, L., JIPA, D. and MIHAILESCtJ,N., 1963. Turbidite in Flisul eocenului de Sotrile. Congr. Assoc. G~ol. Carpato-Balcanique, 5e, Bucarest, Travaux, pp.109-128. DZtJLVNSKI, S., KStAZrdEWICZ,M. and KtJENEN, PH. H., 1959. Turbidites in flysch of the Polish Carpathian Mountains. Bull. Geol. Soc. Am., 70 : 1089-1118. HARMArq,R. A., 1964. Distribution of Foraminifera in the Santa Barbara Basin, California. Micropaleontology, 10 : 81-96. HJULSTR/JM,F., 1934-1935. Studies of the morphological activity of rivers as illuminated by the river Fyris. Bull. Geol. Inst. Univ. Upsala, 25 : 221-527. KSIAZKmWICZ,M., 1961. Life conditions in flysch basins. Ann. Soc. G~ol. Pologne, 31 : 3-21. LAIqTEAUME,M., 1962. Contribution d l'l~tude g~ologique des Alpes maritimes franco-italiennes (Stratigraphie). Th~se, Univ. Paris, Paris, 409 pp. MEISCHNER,K. D., 1964. Allodapische Kalke, Turbidite in Riff-nahen Sedimentations-Becken. In: A. H. BOUMAand A. BROUWER(Editors), Turbidites. Elsevier, Amsterdam, pp.156-191.
1 This also means that the organoglyphs (= organic sole markings) are re-excavated and do not show the surface relief of the basin floor at the moment the turbidity current arrived. Whether they were originally produced internally as burrowings or superficially as ploughings is a different question. Another deduction is that a significant percentage of the clay in a turbidite exposure was first deposited pelagically up-current from the point in question. How much of the clay has been carried by the current from its point of origin is difficult to say. It depends on the distance and current velocity, but also on the nature and thickness both of the pelagic deposit and of the generating slide. Marine GeoL, 2 (1964) 236-246
NATLAND,M. L., 1963. Paleoecology and turbidites. J. Paleontol., 37 : 946-951. NESTEROFF, W. D., 1961. La "sequence type" dans les turbidites terrig~nes modernes. Rev. G~ograpt Phys. Gkol. Dyn., 4 (2) : 263-268. NESTEROFF, W. D., 1963. Essai d'interpr6tation du m6canisme des courants de turbidit6. Bull. Sot G~oL France, 7 (4) : 849-855. NESTEROFF, W. D. and HEEZEN, B. C., 1963. Essais de comparaison entre les turbidites modernes e le flysch. Rev. G~ograph. Phys. G~oL Dyn., 5 (2) : 115-127. RADOMSKI, A., 1958. The sedimentological character of the Podhale ftysch. Acta Geol. Polon., 8 335-395. RADOMSKt, A., 1960. Remarks on sedimentation of shakes in flysch deposits. Bull. Acad. Polon. Sci. Skr. G~ol. G~ograph, 8 (2) : 123-129. RUSNAK, G. A., BOWMAN, A. L. and t)STLUND, H. G., 1963. Miami natural radiocarbon measure ments. 2. Radiocarbon, 5 : 23-33. SLACZKA, A., 1963. Observations on the sedimentation of hieroglyphic beds and variegated shale: from Dukla Unit. Ann. Soc. G~oL Pologne, 33 : 93-110. STANLEY, D. J., 1963. Vertical petrographic variability in Annot Sandstone turbidites: some prelimi nary observations. J. Sediment. Petrol., 33 : 783-788. STANLEY,D. J. and BOUMA,A. H., 1964. Methodology and paleographic interpretation of flysct formations: a summary of studies in the Maritime Alps. In: A. H. BOUMAand A. BROUWEr (Editors), Turbidites. Elsevier, Amsterdam, pp.34-64. SUNDBORG, A., 1956. The river Klar~ilven, a study of fluvial processes. Geq(raf. Ann., 38 : 127-316. SYMPOSIUM, 1961. Some aspects of sedimentation in orogenic belts. Proc. GeoL Soc. London, 1587 69-80. THOMSON, A. and THOMASSON, M. R., 1964. Sedimentology and stratigraphy of the Dimple Lime. stone, Marathon Region, Texas. In: The Filling of the Marathon Geosyncline. Guidebook 1964 Permian Basin Section - - Soc. Econ. Paleontologists Mineralogists, Publ., 64-9 : 22-30. TUREKIAN, K. K., 1964. The geochemistry of the Atlantic Ocean Basin. Trans. N.Y. Acad. Sci., Ser. 2 26 (3) : 312-330. UNRUG, R., 1963. lstebna beds--a fluxo-turbidite formation in the Carpathian flysch. Ann. Soc Geol. Pologne, 33 : 49-92. VAN HINTE, J. E., 1963. Zur Stratig~aphie und Mikropal~iontologie der Oberkreide und des Eozan~, des Krappfeldes (K~irnten). Jahrb. Geol. Bundesanstalt (Austria), Sonderband, 8 : 147 pp.
Marine Geol., 2 (1964) 236--246