Ancient Turbidite Research Articles

Recent and Ancient Turbidites and Contourites: ABSTRACT

Fossil turbidites have been recognized and described from many areas all over the world. A turbidite model, comprised of a fixed succession of sedimentary structures, was established a decade ago and seems to be usable, although some changes have been suggested. Turbidites are generally assumed to be deposited by turbidity currents, but the presence of these currents in the marine realm has not been definitely established. Submarine canyons presumably are the major, if not only, important transport route for moving shallow water material to deeper basins. Questions arise about the origin of turbidity currents when studying canyons in which gradual filling followed by sudden emptying has occurred. The material in the canyon head moves downward slowly, comparable to glaciers. Besides this slow sliding, traction currents and debris flow have been suggested. Where turbidity currents start, and if they absorb the slow moving canyon fill, are questions that cannot be answered yet. Other problems are the relation between fluxoturbidites, or grav ties, and turbidites, and the use of the terms proximal and distal turbidites. In comparing recent turbidites with ancient ones, many discrepancies appear, most of which can be eliminated by considering the influence of primary consolidation on sedimentary structures. Studies indicate that the use of electrical logging and seismic records do not allow detailed interpretation of deposits such as turbidites. The resolution of the records is not fine enough, although their application for basin analyses and overall trends is necessary. Recently a new genetic term, contourites, was introduced for sediments redeposited by contour currents. Recent and ancient contourites are compared with turbidites and only minor differences exist. A combination of parameters may allow a distinction between the two types and it is possible that both can be found in the same area. End_of_Article - Last_Page 1896------------

Rhythms in Deep-Sea Sediments from Gulf of Mexico and Caribbean: ABSTRACT

Rhythmic patterns observed in unconsolidated marine deposits in cores, collected from the western abyssal plain of the Gulf of Mexico and from the Beta Straits in the Caribbean, are based on sedimentary structures rather than on lithology. From the present knowledge of contourites, nepheolites, pelagites, and turbidites, it is believed that the silty clay intercalations from the Gulf cores, as well as the sandy intercalations from the Caribbean cores, can be interpreted best as incomplete turbidite sequences. This interpretation is based primarily on the incomplete sedimentary facies model as developed for ancient turbidites. The thin clay seams commonly found in recent deposits, as well as some other features not known in ancient turbidites, normally become invisibly thin from the effect of consolidation. End_of_Article - Last_Page 605------------

CONTROL OF SLOPE ON DEPOSITION FROM SMALL‐SCALE TURBIDITY CURRENTS: EXPERIMENTAL RESULTS AND POSSIBLE GEOLOGICAL SIGNIFICANCE

ABSTRACTThe experiments involved pouring aqueous starting suspensions of pumice sand and kaolinite silt into a submerged trough. The steepness of the bottom slope of the trough was varied from run to run. Heights of sand and silt were recorded at 10‐cm intervals away from source. The results: (1) support earlier suggestions that the ratio of sandstone to shale is a sensitive indicator of proximality in ancient turbidites, but do not support clearly the suggestion that the rate of decrease in that ratio away from source may indicate steepness of bottom slope; (2) suggest that the rate of decrease in sandstone thickness away from source in ancient turbidites may be inversely proportional to steepness of bottom slope.The possible application of the results to the geological record is discussed.

NON‐GREYWACKE “TURBIDITE” SANDSTONES IN THE WELSH GEOSYNCLINE

SUMMARYIn contrast with the commonly accepted notion regarding ancient turbidites, non‐greywacke sandstones are not uncommon in the typically graded turbidite facies of the Cambrian and Silurian sediments in north Wales. The sandstones are the arkosic and lithic types of PETTIJOHN (1957) or the feldspathic and lithic arenites of GILBERT (1954) and occur at the bottom of graded beds when the grain size tends to be above medium grade. Petrological features suggest that debris forming the sandstones in north Wales was not significantly modified during transportation and original provenance characters are well preserved.The occurrence of such sandstones implies that: (1) the current concept of ancient graded greywackes in the turbidite facies should be revised; (2) non‐greywacke sandstones in ancient turbidites are comparable in petrological features to recent deep‐sea sands; (3) these sandstones are important in connection with the origin of the clay matrix in greywacke.

Turbidites in a Glacial Sequence: A Study from the Talchir Formation, Raniganj Coalfield, India

Ancient turbidites have mostly been described from flysch sequences. This paper deals with an occurrence of non-flysch turbidites from ancient glacial-lake sediments belonging to the Talchir formation (upper Carboniferous) of the Raniganj coalfield, India. The turbidites occur, punctuating a long sequence of glaciolacustrine shales and siltstones, as sandy inter-beds, usually 40-50 cm. thick, having a graded base (often sole-marked) and laminated top with slump structures and parallel, current-ripple, and convolute laminations. The glaciolacustrine sediments also contain varve-like rhythmites containing current-generated structures like current-ripple laminations and sole marks in the sand laminae. These are also believed to have been deposited by turbidity currents. A number of pebbly mudstones, interbedded with the turbidites and the rhythmites, have been interpreted as deposits made by high-density turbidity currents. Available evidence shows that the oversteepened and overloaded front of an outwash de...

Sedimentary characteristics of samples collected from some submarine canyons

Oriented rectangular cores of 20.3 × 30.5 cm and 45.7 cm high have been collected in a number of submarine canyons off southern California (U.S.A.) and off the southern tip of Baja California (Mexico) for a detailed study of their sedimentary structures. By applying several methods, mainly X-ray radiography, to vertical slices of the sediment samples, an abundance of information was obtained. The clayey shelf sediments in southern California reveal few primary sedimentary structures, which are only slumped and reworked by burrowing organisms. Sediments from the axis of the submarine canyons are mainly coarse and contain parallel lamination, current-ripple lamination, and sometimes small-scale graded bedding; slump phenomena and locally some reworking by animals are less important structures. A short distance outside the canyon axis the samples are much finer in grain size; they show fewer primary structures and more secondary ones. Sometimes the whole sample is completely disturbed by burrowing organisms, as is the case in a sample collected from a canyon terrace. One sample was collected from a gully on the apron of the La Jolla Fan Valley. This is similar to the facies model of ancient turbidites with regard to the type and succession of the sedimentary structures present. A box core from a small basin in the middle of the Gulf of California was found to be entirely reworked by organisms.

Paleocurrents and Oceanography: ABSTRACT

The rationale of paleocurrent studies suggests that directional current structures identify the provenance, transport direction, basin architecture, and paleoslope of sedimentary sequences. Numerous examples from fluvial deposits have reinforced this view. In marginal marine environments, directional current structures are fashioned by a combination of tidal, wave, and wind-driven currents which flow randomly with respect to slope and sediment source. Directional structures formed by these currents are oriented perpendicular or parallel to both slope and sediment source. The resulting structures, therefore, reveal only the direction of transport of the last current to act upon the sediments prior to burial. The direction of flow of bottom-scouring currents in continental shelves and deep ocean basins is determined by changes in density, temperature, and salinity, by wind action, and by the rotation of the earth. Such changes tend to drive currents in random directions in the ideal case, but because of the earth's rotation, most ocean currents flow parallel to topographic strike. Resulting directional current structures of shelf and deep marine sediments cannot define paleoslope or provenance, but they do define topographic strike. Because the direction of flow of ocean currents depends on many variables, several combinations of current systems can be recognized, including converging systems, diverging systems, and stratified systems. Where two currents converge, one system will override the other system because of differences in density. Converging directional data have been reported from ancient turbidites, indicating that similar converging current systems existed in the past. These data also suggest that many turbidite directional criteria were formed by bottom scouring ocean currents. End_of_Article - Last_Page 346------------

The shell pavement below oceanic turbidites

Nesteroff discovered, among other interesting facts, that many oceanic turbidites are covered by a thin film of pelagic shells, and he attributed this to deposition from the tail of the turbidity current. Any later pelagic deposit is supposed to have been washed away by the following current, and is therefore missing. Nesteroff's finding is important in itself and throws new light on ancient turbidites. However, the present author favours an explanation through winnowing of the shells from a pelagic stratum by the current that deposited the covering turbidite. This would mean that the shell pavement does not mark the end but the beginning of a turbidite, and that the upper part of the pelitic interval with pteropods and planktonic Foraminifera is a pelagic deposit. Several arguments are presented in favour of the latter view. As Nesteroff and Heezen are led by their interpretation to doubt similarly the presence of pelagic strata between flysch turbidites, a brief survey is made of this subject. It shows that most of the ancient turbidite sequences also contain many beds with a pelagic part towards the top of the pelitic interval. Up to the present, however, no shell pavement has been reported, but careful search will probably reveal examples of such fossil pavements.