Origin Of Mounds Research Articles

Origin of mounds in the Pantanal wetlands: An integrated approach between geomorphology, pedogenesis, ecology and soil micromorphology

Vegetated mounds are an important geomorphological feature of the Pantanal, where the influence of floods dictates not only hydropedological processes, but also the distribution and ecology of the flora and fauna. This work aimed to identify factors and processes that influence the formation and spatial distribution of the mounds, which are commonly associated with termite activity. In order to characterize pedological processes, macro and micro morphological descriptions, satellite image interpretation, dating of the sandy sedimentary material using OSL and carbon dating using 14C AMS were carried out. This dating of the materials indicates that the sediments in which the soils were formed were deposited during the Pleistocene, while the carbonates are from the Holocene. The basin-like format of the laminar structures suggests that part of the more clayey material was deposited in lacustrine environments. The more humid climate in the Holocene intensified argilluviation, which at an advanced stage, led to a more pronounced textural gradient, reducing drainage and leading to ferrolysis and thickening of the E horizon. Besides pedogenic processes, more erosive flooding during the Holocene began reducing and rounding the landscape’s more elevated structures (paleolevees). In the final stage, these structures were occupied by termites to shelter from flooding. Thereafter, the bio-cementation action of the termite nests has increased the resistance of the vegetated mounds to processes of erosion.

Gas hydrate pingoes: Deep seafloor evidence of focused fluid flow on continental margins

Gas hydrates in the shallow subsurface form one of the largest reservoirs of methane in the global organic carbon cycle. Seafloor seeps and associated features represent the venting points of methane released from the shallow lithosphere to the hydrosphere and atmosphere. Here we document the discovery of seep-related seafloor mounds in the Kwanza Basin, offshore Angola, and employ high-resolution three-dimensional seismic analysis to unravel the subsurface plumbing system and the origin of mounds. Mounds with distinct morphologies and geophysical signatures illustrate different development stages associated with the formation and dissociation of shallow gas hydrate, linked to thermogenic fluid migration along salt diapir flanks draining deeply buried salt minibasins. The mounds are more than an order of magnitude larger than previously described submarine hydrate pingoes, and comparable to hydraulic pingoes commonly found in terrestrial periglacial environments, suggesting hydrate volumes of individual mounds up to 1.1 × 106 m3 (equivalent to 2.0 × 108 m3 of methane gas). The interpretation of seismically well-defined seep-related seafloor mounds brings new insight to the occurrence and development of concentrated near-surface gas hydrate accumulations and their relationship with thermogenic fluid migration and host sediment properties along continental margins.

Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States

The origin and significance of pimple mounds (low, elliptical to circular dune-like features found across much of the south-central United States) have been debated for nearly two centuries. We cored pimple mounds at four sites spanning the Ozark Plateau, Arkansas River Valley, and Gulf of Mexico Coastal Plain and found that these mounds have a regionally consistent textural asymmetry such that there is a significant excess of coarse-grained sediment within their northwest flanks. We interpret this asymmetry as evidence of an eolian depositional origin of these mounds and conclude they are relict nebkhas (coppice dunes) deposited during protracted middle to late Holocene droughts. These four mounds yield optically stimulated luminescence ages between 2400 and 700 yr that correlate with well-documented periods of eolian activity and droughts on the southern Great Plains, including the Medieval Climate Anomaly. We conclude vegetation loss during extended droughts led to local eolian deflation and pimple mound deposition. These mounds reflect landscape response to multi-decadal droughts for the south-central U.S. The spatial extent of pimple mounds across this region further underscores the severity and duration of late Holocene droughts, which were significantly greater than historic droughts.

The origin of deep-water, coral-topped mounds in the northern Rockall Trough, Northeast Atlantic

Mounds associated with the cold water coral Lophelia pertusa are widespread in the North Atlantic, although the factors controlling their distribution are not well understood. Here we examine a group of small, coral-topped mounds (the Darwin mounds) which occur at 1000 m water depth in the northern Rockall Trough, northwest of the UK. Individual mounds are up to 75 m in diameter and 5 m high, although some ‘mound-like’ targets seen on sidescan sonar have little or even negative relief. Some mounds are associated with ‘tail-like’ features, imaged as elongate patches of moderate backscatter up to 500 m long, elongated parallel to prevailing bottom currents. High-resolution sidescan images and seabed photographs show hundreds of coral colonies, each a metre or so across, on each individual mound. Many other organisms, mainly suspension feeders, occur in association with the coral. Piston cores from the mounds contain predominantly quartz sand with only scattered coral fragments, showing that bioclastic material is not a major contributor to mound building. A field of seabed pockmarks occurs immediately south of the Darwin mounds. On sidescan sonar data, pockmarks are low relief, circular depressions, typically around 50 m in diameter. The seafloor around the pockmarks consists of uniform, heavily-burrowed, muddy sediments and no specific biological communities, nor any sedimentological or photographic evidence for active seepage, were observed. The distribution of mounds and pockmarks suggests a gradual transition from mounds in the north to pockmarks in the south. This, combined with the lack of bioclastic material in the mound sediments, suggests that both mounds and pockmarks are created by fluid escape from below the seafloor. Mounds occur where fluids carry subsurface sand to the surface, where it forms mounds because bottom currents are not strong enough to disperse it. Pockmarks form where muddy material is eroded by fluid escape but dispersed by bottom currents. Despite the origin of mounds through fluid escape, we suggest that it is the elevated mound topography, rather than any fluid escape, that is advantageous to the corals. This is supported: (1) by the wide variety of suspension-feeding organisms that occur on the mounds, since all of these are unlikely to have a specialised seepage-related lifestyle, and (2) because corals and their associated community do not occur around pockmarks, where seepage has also occurred but elevated topography is absent.

The Formation of Mima Mounds in the Kenya Highlands: A Test of the Dalquest-Scheffer Hypothesis

Earth mounds up to about 2 m in height and 50 m in diameter occur in densities up to about 50 per ha in many locations in western North America. Often termed Mima mounds because of their conspicuous development on Mima Prairie, Thurston Co., Washington, these mounds are known from the Puget Sound lowlands, the central valley of California (where the terrain is commonly known as hog-wallows), coastal marine terraces and cismontane valleys of Southern California, the Columbia and Snake River Plateaus of the interior Pacific Northwest, various valleys and plateaus of the Rocky Mountain region, and other isolated locations (Scheffer, 1947; Price, 1949). Mounds of similar form, usually termed prairie or pimple mounds, occur from Saskatchewan to Minnesota and south through Iowa and Missouri to Arkansas, eastern Oklahoma, and the coastal plain of Louisiana and Texas (Price, 1949; Ross et al., 1968; Mollard, 1982). Numerous hypotheses have been suggested for the origin of these mounds, most involving physical processes of soil deposition and erosion (Krinitzsky, 1949; Newcomb, 1952; Scheffer, 1947; Vitek, 1978). The most highly regarded biological hypothesis, formulated by Dalquest and Scheffer (1942), is that these mounds are the gradual product of the backward displacement of soil that results from outward tunneling of pocket gophers from sites representing territory centers and the locations of permanent nest and food storage chambers. According to this hypothesis the essential descriptive features of Mima mounds are that they (1) are composed only of textural materials that such animals can move, (2) are restricted to thin or poorly drained soils that limit tunneling to a shallow surface zone, (3) are confined to flat or moderately sloping land, (4) are nearly circular in outline regardless of slope, (5) tend to be evenly spaced because of the strong territoriality of pocket gophers, and (6) occur only within the historical range of these animals (or forms with similar characteristics). While the above features are not absolutely inconsistent with hypotheses of origin of mounds by depositional and erosional mechanisms, no hypothesis of this type predicts that Mima mounds will possess all of these characteristics and limitations. Mound areas that have been examined in detail in western North America conform to these descriptive criteria (Scheffer, 1947; Mielke, 1977; ,Cox, 1983). Mima mounds are so far known only from areas west of the Mississippi River. Pocket gophers occupy only limited ranges east of the Mississippi River, Geomys bursarius entering parts of Wisconsin, Illinois, and Indiana from the west, and G. pinetis occurring in parts of Alabama, Georgia, and Florida. Scheffer (1958) suggested that one test of the hypothesis of mound formation by fossorial rodents would be a search for Mima-type mounds in other parts of the world, especially those where animals with characteristics similar to those of pocket gophers are found. In Kenya, East Africa, earth mounds up to 6 m in diameter and 1.5 m in height have been described at elevations of 1,950-3,600 m on Mt. Kenya, and the occupation and possible formation of some of these mounds by fossorial rodents mentioned (Fries and Fries, 1948; Coe, 1969; Jarvis and Sale, 1971). We have recently observed such mounds in several grassland areas at elevations of 2,250-2,440 m on Mt. Kenya, in the Aberdare Mountains, and along the eastern edge of the Mau Escarpment; in July 1981 we studied in detail two such areas near Nyahururu, Kenya, in the Aberdare highlands (Cox and Gakahu, 1983). All of these localities lie at elevations above those at which species of fungus-gardening termites (genera Macrotermes and Odontotermes), which are definitely known to produce large mounds, have been reported (Hesse, 1955; Glover et al., 1964; Pomeroy, 1977). All lie within the geographical and altitudinal range of the rhizomyid mole rat, Tachyoryctes splendens (Riippell), a rodent convergent in morphology and behavior with pocket gophers (Rahm, 1971; Kingdon, 1974). The site examined in greatest detail, a fenced pasture 1.3 km E Nyahururu, elev. 2,375 m, exhibited nearly circular mounds up to 2.0 m high and 17.8 m in diameter; the mean density of mounds was 30.5 per ha (Cox and Gakahu, 1983). Analysis of distance-to-nearest-neighbor relationships (Clark and Evans, 1954) gave a dispersion index, R, of 1.22, indicating a significant tendency toward uniform spacing of mounds. The mounds were underlain by a horizontal laterite rock hardpan that varied in depth from a mean of 38.6 cm at mound edges to a mean of 24.6 cm at the centers of intermound basins. At the second February 1984 149

Some Topographic Features Formed at the Time of Earthquakes and the Origin of Mounds in the Gulf Plain. Wm. H. Hobbs

Previous articleNext article FreeReviewsSome Topographic Features Formed at the Time of Earthquakes and the Origin of Mounds in the Gulf Plain. Wm. H. Hobbs C. W. W.C. W. W. Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 16, Number 5Jul. - Aug., 1908 Article DOIhttps://doi.org/10.1086/621538 Views: 7 PDF download Crossref reports no articles citing this article.

Some topographic features formed at the time of earthquakes and the origin of mounds in the Gulf plain

Some topographic features formed at the time of earthquakes and the origin of mounds in the Gulf plain