Base-level Lowering Research Articles

The problem of reworked pollen and spores in marine sediments

Some pollen-and-spore assemblages found in ocean-bottom sediments contain many reworked plant microfossils. In the search for a method to separate the recycled pollen grains and spores from the primary assemblage recovered from Pleistocene and Recent ocean-bottom-core samples, it was found that morphologically distinct Tertiary and older pollen grains and spores accept Safranin O stain differently than do most of the Pleistocene or younger plant microfossils in the same assemblage. Variations in stain acceptance may be a reflection of exine-chemistry changes that took place during the reworking process. High percentages of reworked pollen and spores in sediments formed during periods of continental glaciation are thought to be the result of increased erosion resulting from base-level lowering. Large numbers of recycled plant microfossils in Pleistocene and even pre-Pleistocene sediments may be indicative of decreasing base levels associated with orogenic movements or former periods of continental glaciation. Secondary plant microfossils can offer important clues on the location of the source areas of marine sediments.

The application of palynology to oceanology with reference to the northwestern Atlantic

Investigation of over 250 samples from 16 cores collected chiefly off the eastern coast of the United States indicates that palynology can supplement micropaleontological and stratigraphical studies of marine sediments. Plant microfossils recovered from ocean bottom cores may provide information when other geological methods fail to yield results.High percentages of reworked pollen and spores in offshore cores are found at horizons where large numbers of cold-water Foraminifera have been recovered. The increase in the percentage of reworked plant microfossils at these levels is believed to be correlative with the lowering of base level associated with Pleistocene glaciation. In some of the cores collected from the northern portion of the study area the occurrence of non-reworked spruce and fir pollen together with high percentages of reworked plant microfossils lends additional support to the existence of a relationship between reworked grains and base level.Sediments collected on the continental shelf yield the largest variety of non-reworked pollen and spore types and the plant microfossil assemblages in these samples are believed to be the most reliable offshore indicator of onshore climatic conditions. In the ocean basin off the continental shelf smaller numbers of non-reworked plant microfossil forms are encountered because of apparently selective transportation, destruction, or both. The percentages of reworked pollen and spores in ocean basin cores may be used to separate sediments deposited under glacial conditions from those formed during inter- and postglacial periods.More detailed sampling and refinement of techniques in the study of continental shelf cores may yield detailed information on climate, as reflected by the vegetation on the land, position of ancient shore lines, current patterns and delimit areas of river deltas in the Quaternary and theoretically pre-Quaternary marine sediments. The relationship of base level to the percentage of reworked pollen and spores has application to the correlation of ocean bottom sediments and perhaps can even be applied to the correlation of pre-Quanternary continental sediments resulting from changes in base level due to orogeny.

Geomorphological observations on Sermersôq. A contribution to the geomorphology of S. Greenland

On the island of Sermersôq four erosion surfaces have been recognized, which are of regional importance in S Greenland. Their development is related to three cycles of erosion, interrupted by two glacial stages. The high-level erosion surface at altitudes above 1000 m is the remnant of an old peneplane, which was formed during the oldest erosion cycle and uplifted in late Cretaceous or early Tertiary times. This uplift initiated a second erosion cycle in the course of which a main glacial stage intervened. In this glacial stage the icelevel in the main valleys (the present fjords) acted as the effective base level of erosion and this resulted in the formation of stretches of an intermediate-level erosion surface, which now occur along the main fjords at altitudes between 400 m and 650 m. An important lowering of effective base level of erosion occurred when in the subsequent interglacial stage the sea-level resumed its role as the effective base level of erosion. This event initiated a third erosion cycle in the course of which the low-level erosion surface, between 100 m and 250 m altitude, and the strandflat, a coastal platform below 50 m, were formed. The third erosion cycle was interrupted by a second glacial stage, which on Sermersôq has the localized character of a mountain glaciation; it is during this second glacial stage that the present day fretted upland morphology of central Sermersôq originated. Post-glacial vertical movements causing the emergence of the strandflat induced a rejuvenation of erosion and consequently the incision of this coastal platform and valley floors by recent rivers.

Silcretes of Central Australia

THE widespread occurrence in central Australia of silcrete which there caps numerous mesas and forms extensive stony pavements on the dissection slopes below them has long awaited a satisfactory explanation. A recent recording1 of the nature and disposition of certain lateritic and desertic soils, taken in conjunction with the location of major drainage divides and the pattern of endoreic streams to the Lake Eyre depression2, throws new light on the problem and indicates the following sequence of pedologic and geomorphic events : (1) A regional development of Lateritic Red Earths occurred across what is now the watershed of eastern and northern Queensland. The lateritic weathering was accompanied by the usual loss of silica in the drainage water, one part going to the Pacific Ocean and the Gulf of Carpentaria and the other to the precursors of the present-day ephemeral and endoreic streams, the Georgina, Diamantina, Barcoo Rivers and Cooper's Creek, the courses of which then extended up to 200 miles further south-west. (2) The silica which moved south-westerly was deposited in a large array of rocks and softer deposits of widely different chronology, including variably weathered sandstones, shales and quartzites, and calcareous, gypseous, saline and alunitic formations. To-day these materials are to be found beneath the silcrete caps of the mesas, and the later group at least indicate aridity and/or restricted drainage at the time of their deposition. (3) The large, gently sloping silcrete surface so formed was disrupted and eroded following the development of the down-warped Lake Eyre basin. This depression and its compensating upwarp to the west and south caused differences in elevation of the order of 1,000 ft. with the lowest point below sea-level. This provided a new, internal, and very low base level for erosion and caused rejuvenation and down-cutting of the streams coming from the north-east and reversal of the drainage in the silcrete area to the west and south of the newly formed depression. The combination of earth movement and erosion caused the break-up of the extensive silicified surface into numerous mesas and associated larger areas of fragmented silcrete represented to-day by the pavement on the Stony Desert Tableland Soils.

STRUCTURE, CLIMATE, AND BASIN LAND FORMS IN ARIZONA AND NEW MEXICO1

THE geomorphic history of structural basins in Arizona and New Mexico has been studied mainly by Kirk Bryan, his co-workers and students. In 1926 Bryan published his preliminary findings on the San Pedro Valley in Arizona as an abstract.2 Though a short paper and without illustrations, it is important because it contains ideas on geomorphic history that found further and more precise expression in later work on structural basins in New Mexico. Bryan, in his study of the San Pedro Valley, regarded the differential movements that depressed the trough and elevated the bordering mountain blocks as having ended with the post-Gila deformation at the close of the Pliocene period. Subsequent stability allowed the development of a broad erosional surface on the basin fill. Renewed incision by streams and the establishment of successively lower base levels led to the formation of lower erosional surfaces. Bryan did not publish a map or diagram of these erosional levels, and his paper is important chiefly because its suggestion of a correlatable tier of terraces and erosion platforms was used extensively as the fundamental explanation of the geomorphology of basins in New Mexico. Since Bryan's paper, the San Pedro Valley has received little attention from geomorphologists; but in New Mexico the geomorphic history of several basins has been examined

PLEISTOCENE GEOLOGY OF PART OF CENTRAL SOUTH DAKOTA1

The area around Pierre, South Dakota, is bisected by the trench of the Missouri River, which forms a boundary between a region mostly covered by glacial deposits that overlie Pierre shale on the east and a region directly underlain by shale on the west. The earliest Pleistocene deposits are remnants of stream alluvium derived from the region to the west. These are now perched on some of the highest knobs in the area and bear witness to an inversion of topography caused, in part, by a lowering of base level when the Missouri River trench was formed marginal to the glacier of the Illinoian age. In the Wisconsin age, the glacier of the Iowan sub-age crossed the Missouri trench and in one place extended 20 miles farther west. The westward extension of each successive glacier was less than its predecessor. The glacier of the Tazewell sub-age reached the Missouri River, and in the course of its retreat built two massive end moraines in the area. Deposits of this substage cover most of the area east of the Missouri. Till of the Cary substage is restricted to the northeast corner of the area; the drift border of the glacier of the Mankato sub-age lies about 15 miles east of the area investigated in detail. Glacial drift of the several substages of the Wisconsin is locally mantled with as much as 30 feet of loess. The Tazewell-Cary interval is represented in this loess by a buried, immature soil profile.

STREAM GRADIENTS AND MONTEREY SEA VALLEY

The long profile of a mature stream approximates a smooth curve with ever decreasing gradient downstream. This profile curve may be called concave upward, or simply concave. Accurate surveys, mostly of streams in central or northwestern Europe or in the United States, show numerous major or minor deviations from simple smooth profile curves. Most major deviations can be correlated with obvious immaturity. Minor deviations can almost all be correlated with ascertained or probable differences in rock resistance, debris size, or other specific geologic or geomorphic circumstances. In a single region, gradients of mature streams are roughly inversely proportional to a function of discharge. One or more logarithmic curves can be drawn that will more or less closely approximate any given mature profile. General lowering of base level is found to produce a distal profile convexity. Probably such a convexity flattens and loses its initial prominence as it gradually extends upstream, unless localized on resistant rock as a distinct nickpoint. The Monterey sea valley off California appears on a map to be the approximate seaward continuation of Salinas land valley, but its head is cut into alluvium deposited by Salinas River and adjacent streams, its irregular upper profile is nearer to an exceptionally steep straight line than to a logarithmic curve, and the compound Salinas-Monterey long profile shows an extremely pronounced break or nick on weak sediments at the shore line. Therefore it is considered improbable that the Monterey submarine canyon is the product of subaerial rejuvenation of the Salinas or any other land river. Most other sea valleys with steep gradients appear to be essentially similar to the Monterey; the Congo canyon is ambiguous and may be exceptional.

Geomorphic History of the Carlsbad Caverns Area, New Mexico

The upland surface of the Guadalupe Range and three younger erosion surfaces in the Pecos lowland record progressively lowered base levels during the Pliocene and Pleistocene. Structurally, transverse consequent drainage in the range is attributed to superposition from a Pliocene (Ogallala) cover uplifted by block-faulting. Carlsbad and other caverns antedate the present topography and are believed to be related to a pre-Ogallala cycle of erosion.

Recognition and significance of multiple erosion surfaces

The question is raised as to whether multiple erosion surfaces that have been described in a number of recent papers actually exist, and, if so, whether they have been correctly interpreted as products of repeated partial peneplanations. Analysis of the criteria used for determining such surfaces indicates that some are not adequate. Particularly, the use of ridge crests, isolated hills, and spurs as markers of former erosion levels is unsafe. Too little correlation has been made between surface form and rock resistance. Analysis of what happens after an old-age surface is rejuvenated indicates that stream-bed grading is very rapid relative to inter-stream degradation, so that the “treppen” concept of stair-step-like benches with bordering scarps retreating across a region following successive rejuvenations of a master stream cannot ordinarily be valid. Multiple erosion surfaces at small vertical intervals are difficult to explain as products of sub-aerial erosion working toward intermittently lowered base levels, but they are entirely expectable results of another process that has been given too little attention. Progressive burial of surfaces planed by the waves of an intermittently advancing sea, or of pediment surfaces bordering a mountain range in an arid land, followed by uplift and stripping of the blanketing sediments, is not only a competent cause of the production of multiple surfaces but so inevitably leads to their production that such an explanation should receive first consideration where multiple surfaces can be definitely proven. Application of this principle to the interpretation of the physiography of the Front Range of the Rocky Mountains is suggested.