Francis P. Shepard

Nomenclature Based on Sand-silt-clay Ratios

ABSTRACT Following a canvassing of sedimentationists an attempt is made to standardize nomenclature of sediment types relative to sand, silt, and clay content. A triangle diagram with boundaries between types, which met with general approval, is submitted and compared with other systems which have been used for the purpose. The new system uses old well established names and has a simplicity and symmetry which make it easily remembered. The boundaries appear to be well located for description of sediments such as those that have been analyzed in large volume from the investigations of the northern Gulf of Mexico (API Project 51), but it is inadequate in describing well sorted sediments with median diameters near the boundaries of sand and silt or silt and clay. The nomenclature suggested applie only to sediment grade sizes so that other names should be used depending on other characteristics of the sediments. Furthermore, the nomenclature should not be applied to sediments containing large percentages of gravel.

Sea level and climatic changes related to late Paleozoic cycles

INTRODUCTION Numerous investigators in recent years have called attention to regularly recurrent sequences in the strata of the Pennsylvanian system in the eastern and central parts of the United States. Wanless, in association with J. M. Weller, has been engaged for several years in field studies of the Pennsylvanian in Illinois and surrounding states, for the Illinois State Geological Survey, and has traced many of the recurrent sequences or cyclothems[1][1] over wide areas, using them as an aid in correlation. Reconnaissance studies have also been made by him in Ohio, Kentucky, Indiana, and the northern mid-continent and Rocky Mountain regions. Shepard has suggested the basic idea as to the cause of the cycles developed in this paper and has contributed information from studies of recent sea level oscillations and the influence of these changes on continental shelf sediments. The authors have attempted to determine whether this peculiar type of sedimentation . . . [1]: #fn-3

Recent Sediments, Northwest Gulf of Mexico

All but one of the papers contained in this volume represent a symposium summarizing the results of work carried on in Project 51 of the American Petroleum Institute. This study of modern sediments along the northwest margin of the Gulf of Mexico contains 14 papers plus a consolidated bibliography. Paper titles are: Geologic framework of Gulf coastal province of United States; Sources and dispersion of Holocene sediments; Mississippi delta; Delta building and the deltaic sequence; Phytoplankton production in the Mississippi delta; Bays of central Texas coast; Sediments of Laguna Madre; Gulf Coast barriers; Sediments and history of Holocene transgression; Sedimentary patterns of microfaunas; Ecology and distributional patterns of marine macro-invertebrates; Rise of see level; Regional aspects of modern sedimentation; and Recent sedimentology.

Submarine Canyons: Multiple Causes and Long-Time Persistence

The actual investigation of submarine canyons as field work was begun about 50 years ago. A large amount of factual information has accumulated as result of operations of deep diving vehicles, first in the Pacific Coast canyons and more recently in the remarkable dives of the Woods Hole minisubmarine Alvin into East Coast canyons. Taking the results of these recent dives and combining them with earlier investigations, including much work done by the French in the Mediterranean as well as our extensive studies off California and Baja California, we can now say with some confidence that these amazing deep excavations into the sea floor off so many coastal areas can be explained. New methods such as side scanning have also given us a greater understanding of the exact character of submarine canyons, particularly in the Bay of Biscay. The development of multichannel sonar has greatly increased our knowledge of the nature of continental margins and hence their history. This has given us more insight into the history of canyon development, particularly off the east Coast where drilling for oil and gas has become so important. In the past we have seen a great variety of hypotheses for explaining submarine canyons. Unfortunately almost all of these have been based on information from a small selection of the canyons, usually from one area. From the new information, it is evident that canyons are of composite origin and that many of the hypotheses suggested in the past were partly correct but did not appreciate that coordination of other processes was required. Thus there is growing evidence that, in the history of many canyons, there was a period in which subaerial erosion was an important precursor, but that present features are predominantly the result of marine erosion. Those advocating turbidity currents as the unique cause of canyons failed to appreciate that debris flows down the incipient valleys, as ell as other types of landslides, could be an almost equally important factor in marine erosion. The great effect of biologic activity on the rock walls of incipient canyons has been almost completely neglected in explanations, and various types of currents such as those of the tides have been left largely out of the picture. Perhaps the most important feature absent in these various hypotheses has been the realization that canyons may well be the result of a long period of formation, much longer than the short episodes of Pleistocene glacial sea-level lowering features which commonly cut into hard crystalline rock. New information is showing that the canyons may date back to at least the Cretaceous.

Distinguishing between Beach and Dune Sands

ABSTRACT The comparison of adjacent pairs of beach (largely foreshore) and dune sands from 74 localities of world-wide distribution has shown that it is generally possible to distinguish between the two, particularly if there are predominant onshore winds. The dune sands are usually rounder than the adjacent beach sands, have a larger silt content, and this silt has a higher content of heavy minerals. The beach foreshores, on the other hand, usually contain more shells and other calcareous organisms than the dunes. There is also some reason to believe that mica content is greater in beach foreshores than in adjacent dunes. The sand of beach berms appears to be intermediate in most respects between that of the foreshores and dunes. The areas where beaches and dunes are most difficult to distinguish are those where there are longshore, extremely variable, or offshore winds. In such localities it is thought that the dune sands are blown back onto the beaches causing intermixture and that in some cases the dunes are derived from sources other than from the adjacent beach. The best results in separating the two environments have come from beaches of the west coast of the United States and Baja California where onshore winds predominate. It is suggested that the chief reason for the differences between dune and beach sands is that the wind picks up from the beaches more of the rounder grains than of the flat and angular grains. There is no reason to believe that the grains are rounded appreciably by the wind in transit to the dunes. Nor does there appear to be any evidence from these comparisons that the dune grains become frosted. The greater quantity of heavy minerals in the dune silt fraction may be caused by removal by the wind of the light minerals.

DELTA-FRONT VALLEYS BORDERING THE MISSISSIPPI DISTRIBUTARIES

Shallow discontinuous valleys crease the upper portion of the continental slopes in the region where the Mississippi Delta has built across the shelf. These valleys attest to the instability of the forward-growing slope despite its average inclination of only 1 per cent. Mass movements probably of the earthflow type appear to be the cause of the valleys rather than turbidity currents because deposits from valley floors show no concentration of sand by currents, and many of the valleys are not located at points where large discharge of sediment-laden water could produce turbidity currents. Furthermore, depressions and hills along the courses of the valleys and their short length differentiate them from the long continuous slightly incised valleys which are outer continuations of submarine canyons and which are generally ascribed to turbidity currents. The delta-front valleys shift position as the slopes are built forward.

Phosphorite deposits on the sea floor off southern California

Dredging operations on banks, on escarpments, and on walls of submarine canyons off southern California have shown that nodular phosphorite is the most abundant type of rock in these nondepositional environments. Approximately one-third of all the rock recovered is phosphorite. Petrographic and megascopic examination reveals that the nodules are largely formed by direct precipitation, but that they enclose some replaced material. Examination also shows that the phosphorite was probably deposited essentially in situ . Miocene Foraminifera have been identified in the nodules from many of the stations, but Recent or Pleistocene faunas have also been found in some of the phosphorite. Whereas the enclosed Miocene fauna suggests a Miocene age for most of the nodules, significant nonpaleontological data indicate that this fauna has probably been reworked into more recent deposits before enclosure in the nodules.

Physiography and Sedimentary Processes of La Jolla Submarine Fan and Fan-Valley, California

The depositional environments of La Jolla canyon, fan-valley, and fan are well known from closely spaced sounding lines, deep-diving vehicle observations, numerous undisturbed box cores, and continuous reflection profiles. The narrow rock-walled canyon changes seaward at 300 fm (549 m) to a wider valley cut into the compacted clayey sediments of a fan, and bordered by discontinuous leveelike embankments. The fan-valley merges gradually into the relatively flat floor of San Diego trough. Numerous dives into the fan-valley have shown precipitous walls along the outside of the bends of the winding channel. Slumping is taking place actively from these walls and large slump blocks of clay are common on the floor. Small scour depressions around isolated erratics suggest the ero ive effect of relatively weak currents in some places but, for the most part, the muddy floor seems to have been little disturbed in recent years. Diagonal tension cracks cut the floor locally. Box cores show that most of the sediment deposited on the valley floor in the past few thousand years consists of poorly sorted clayey silt, underlain by discontinuous layers of well-sorted fine-grained sand with a few coarse sand grains, gravel, and mud balls. Sand layers occur in 94 percent of the valley axis cores, of which 26 percent are graded; 59 percent have parallel laminations; and 41 percent have current-ripple cross-laminations. Sand layers are less common in the cores from levees and from the small discontinuous terraces along the sides of the fan-valley. Cores from the open fan have less and finer grained sand. In all these environments the sand shows no consistent or systematic grain-size variation with increasing water depths. Some of the coarsest sediments, including g avel and mud balls, are found in sand farthest from shore and at the greatest depths. The character of the sand and the finding of shallow-water Foraminifera indicate the probability that sand has been carried from the coastal area along the valley axes and spilled over the levees onto the open fan. However, there is little evidence of recent high-velocity, high-density turbidity currents, because, in general, the covering mud layer is distinctly separated from the underlying sand deposits, and therefore does not suggest deposition at the terminus of a turbidity current. Also, the discontinuous character of the sands and series of laminae with heavy mineral concentrations indicate introduction by a traction type of pulsating current, such as has been seen during vehicle dives, and also has been measured in the few available current-meter records. The locally precipitou fan-valley walls and outcrops of gravel, and the sand layers on the levees and open fan, may be the product of stronger currents that moved down the valley during earlier more pluvial periods, when greater quantities of sediment entered the canyon heads. Possible confirmation of this idea comes from the available C-14 dates in plant layers, which suggest that deposition in the past few thousand years may have been considerably slower than that indicated for the Pleistocene. The finer sediments may be largely the result of slow downslope movement of slightly higher density muddy waters coming from the coastal areas. Continuous reflection profiles have shown that the inner La Jolla fan has only a thin cover of unconsolidated sediments overlying the folded and faulted Miocene-Pliocene rocks. The outer fan and adjacent San Diego trough contain a thick section (more than 1,000 m) of Quaternary sediments with probable buried older channels and possible thick lenses of sand sediments.

Late Quaternary Sea-Level Studies in Micronesia: CARMARSEL Expedition

The CARMARSEL Expedition to Guam and the Caroline and Marshall Islands has provided substantial evidence of a difference between islands in tectonically active and inactive belts. Guam has clear evidence of coral reefs that have been elevated above the present sea level, whereas the 33 islands visited in the apparently more stable belt to the east of Guam failed to show any elevated reefs. The low terraces found commonly in the Caroline and Marshall Islands all proved to be cemented rubble ramparts, with flat tops at about present high tide level. We were unable to find any coral or Tridacna in growth position, criteria we believe are necessary for postulating higher than present relative sea level. Dates on the rubble suggest formation of many of these ridges about 2500 to 3000 B.P. We conclude that there was no higher than present Holocene stand of sea level in the Caroline and Marshall Islands we visited, but that sea level was at least near to present level at the time of formation of the rubble ridges.

Currents in submarine canyons

Abstract Earlier work indicated that currents move alternately up and down the floors of submarine canyons with greater average speeds and longer duration downcanyon. Now we find there are roughly synchronous movements up to at least 34 m above the canyon floors. The speeds decrease somewhat with height above the canyon floors and upcanyon flows are more significant. The patterns of up- and downcanyon flow at adjacent stations usually can be matched, indicating that the currents are related to internal waves. Crosscanyon flow occurs, particularly during periods of strong crosscanyon winds and usually has a definite sequence of repetition related to the tidal cycle. The strongest and longest transverse flows were in the broadest floored canyon. During storms with strong onshore winds, there are violent downcanyon flows that have carried current meters with them and eroded the floors, but the speeds are unknown.