Network Of Valleys Research Articles

An introduction to the South Wales Pennant Formation: its origin, outcrop and conservation

In the South Wales coalfield the Pennant Formation is widely distributed and is responsible for controlling much of the landscape seen today. The typical scarp and slack features of the northern margin are complimented by central, relatively high plateau areas that are typically incised by substantial rivers and streams to form a complex network of valleys. The Pennant Formation of South Wales has been generally accepted as being composed of relatively monotonous sequences of coarse sandstone, with several thin uneconomic coal seams. This Pennant lithology is very different to that of the underlying Productive Coal Measures (Langsettian to middle Bolsovian) where the sequences are dominated by mudstones and thick coals deposited in extensive marginal swamp forests. The change was primarily due the northerly advancing Variscan causing uplift and erosion of the landmass to the south of the South Wales Coalfield Basin. Exposures of these strata are extensive and provide the ideal setting for the establishment of a UNESCO Geopark.

The Neogene to Recent Rallier-du-Baty nested ring complex, Kerguelen Archipelago (TAAF, Indian Ocean): stratigraphy revisited, implications for cauldron subsidence mechanisms

Abstract The Kerguelen Archipelago is made up of a stack of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres at various times since 30 Ma ago. In the SW, the Rallier-du-Baty Peninsula is mostly occupied by two silicic ring complexes, each with an average diameter of 15 km, comprising dissected calderas cross-cut by subvolcanic cupolas. Previous radiometric determinations yield ages ranging from 15.4 to 7.4 Ma in the southern centre, and 6.2 to 4.9 Ma in the northern one. The felsic ring dykes were injected by coeval mafic magmas, forming, successively, swarms of early mafic enclaves, disrupted synplutonic cone sheets, and late cone-sheets. After the emplacement and subsequent unroofing of the plutonic ring complexes, abundant and thick trachytic pyroclastic flows and falls were emitted from the younger caldera volcanoes, while hawaiite and mugearite lava flows were erupted from marginal maars and cones. Huge trachyte ignimbritic flows filled the glacial valleys in the central Peninsula, and capped lacustrine deposits and older lava flows, while related pumice falls are widespread throughout the archipelago. This powerful plinian eruption took place after the network of glacial valleys was established, but before the Little Ice Age that occurred during the last centuries. In the south of the peninsula, even younger trachytic formations are exposed, and fumarolic vents are still active. The growth mechanisms of a caldera-related ring complex can be explained as a repetitive sequence of two eruptive episodes. The first episode of hydrofracturing, induced by volatile exsolution within the evolving magma chamber, creates a vertical circular fracture zone, along which highly vesiculated magmas are emitted during explosive eruptive events occurring at the surface in a caldera volcano. It is followed by a second episode of cauldron-subsidence of a crustal block down to the degassed magma chamber, induced by pressure release. Downward movement of the crustal block favours the emplacement at shallow depths within the older caldera-filling formations, of discrete magmatic sheets characterized by a 16-km mean diameter and a 1-km mean thickness, corresponding to an average unit volume of 200 km 3 . Actually, the estimated volumes of the different igneous episodes within the Rallier-du-Baty nested ring complex vary from 60 to 900 km 3 , and correspond to the production during 15 Ma of about 2800 ± 850 km 3 of new materials and a net crustal growth of about 100 ± 30 × 10 3 m 3 per year.

Pleistocene sea-level changes, sapping processes and development of valley networks in the Apulia region (southern Italy)

The central area of the Apulia region is characterized by a network of valleys with peculiar features, locally called ‘gravine’ or ‘lame’. They are short, straight valleys, deeply incised in Plio-Pleistocene calcareous sandstones and in Mesozoic limestones. These characteristic valleys cut a staircase of marine terraces. A series of valley generations is recognisable, each one of them leading to the internal margin of a marine terrace representing its base level. Morphological features and hydrogeological conditions suggest that sapping processes were responsible for the development of the valley network. As the aquifer rests on seawater intruding from the nearby coastal area, sapping processes were enhanced during interglacial high sea-level stands so that each ancient coastline is marked by its own generation of valleys. However, the longest and deepest valleys formed at the Ionian side during OIS 7. This is due mostly to a fast sea-level rise accompanied by very humid climatic conditions that increased the hydraulic head at the springs and the intensity of sapping processes. Sapping valleys could be useful for regional correlation of marine terraces and as indicators of relict coastlines at present placed below sea level.

Glacial tunnel valleys in the Eastern Goldfields of Western Australia cut below the Late Paleozoic Pilbara ice sheet

Open-cut gold mines near Kalgoorlie in the Eastern Goldfields region of Western Australia, expose a Late Paleozoic (Carboniferous–Permian) network of glacially-eroded valleys filled with tillite and shale. Valleys are incised into Archean and Proterozoic strata of the low relief Western Australian Shield and buried by Tertiary and Quaternary strata. Subsurface data identify a poorly-integrated network of narrow (∼200 m wide), deeply-incised (>80 m) and steep-sided channels that connect broader sub-basins up to 3 km wide. Lowermost infill strata within valleys consist of bouldery diamictites containing abundant striated clasts of far-travelled lithologies and are identified as tillites deposited by a Late Paleozoic ice sheet (Pilbara Ice Sheet) that covered the West Australian Shield. Tillites are blanketed by shales containing Early Permian palynomorphs. The form and sedimentary infill of the buried valley network is comparable to subglacially-cut ‘tunnel valleys’ that formed below the wet-based margins of Pleistocene continental ice sheets and a similar origin is proposed for those at Kalgoorlie. The significance of this finding is that hitherto, little was known of the Late Paleozoic Gondwanan ice sheet that grew on the Western Australian Shield and which contributed substantial volumes of sediment to adjacent sedimentary basins. The preservation of a relict Late Paleozoic glacial topography near Kalgoorlie indicates that post-Permian erosion of the region was minimal and cannot be reconciled with denudation models which propose uniform post-Permian uplift and deep stripping across the shield.

Topographic Irregularities on the Base Zuni Supersequence Boundary and their Initial Cretaceous Sediment Fill, Central Texas

ABSTRACT The Cretaceous-preCretaceous unconformity, i. e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the erosional out-crop edge of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent to the Cretaceous outcrop edge indicate that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides. Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated in a sub-humid or drier climate as a downfilling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation, a conglomeratic sandstone, onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational meandering river complex during sea level rise.

Palaeogeographic implications of the Messinian surface in the Valencia trough, northwestern Mediterranean Sea

Sparker (3000 J and 8000 J) and multichannel seismic reflection profiles across the Valencia trough show a Messinian unconformity incised by numerous valleys. The main feature of this surface is a large valley that generally underlies the present Valencia valley and is deeply entrenched into the Miocene deposits. The size of this palaeo-valley ranges from 0.5 km wide and 15–100 m deep at its western end, to 1.6–2.8 km wide and 200–250 m deep downstream. An important tributary system is observed, with a main canyon (6–8 km wide and 150–200 m deep) draining the Ebro margin, as well as many other smaller valleys draining the Catalan and Balearic margins. Downstream, other tributaries underlie the present canyons of the Catalan margin. The location of the tributary system is controlled by the Early Miocene rift structures. The relief of the Messinian surface is affected by post-Miocene deformation that results from salt diapirism, extensional faulting and related volcanism. Late Neogene to Quaternary volcanic edifices cut the Messinian surface and coincide with large residual magnetic anomalies. Lower Pliocene to Quaternary salt diapirism in the abyssal plain north of Menorca has created a series of structural highs. Between these highs are deep interdiapiric troughs or basins that have become sediment depocentres during the Plio-Quaternary. The complex network of erosional valleys from the Valencia trough continental margin demonstrates that the valley system in the basin was not related to the refilling of the Mediterranean, but to the Iberian and Balearic margin palaeodrainage that developed during the Messinian desiccation. The presence of at least three erosional unconformities suggests that there were alternating periods of flooding and retreat of Atlantic water during Messinian time. The Messinian subaerial margin with erosional valleys contrasts sharply with the Pliocene-Quaternary marine margin with progradational turbidite systems.

Late Mississippian-Pennsylvanian Orogenic Movements of West Canadian Platform and Adjacent Areas--Their Role in Sedimentation and Hydrocarbon Accumulation: ABSTRACT

Important, though not too evident, structural changes took place near the end of the Paleozoic Era along the western edge of the North American craton. They roughly correspond to the orogenic phases in geosynclinal areas all around the continent: Appalachian orogeny, Antler orogeny, Cariboo The style was block-faulting (with predominantly normal faults) forming platform edges and grabens, surrounded by shallower troughs. The result is a network of rift valleys along the End_Page 646------------------------------ western edge (north-south) and across the platform (east-west), which was itself broken into tilted blocks. Those grabens extending across the continental margin and normal to it, are aulacogens, opening into the geosyncline to the west. All these fractures are rejuvenated, Precambrian basement features. The Liard Plateau in northeastern British Columbia, the Peace River basin in northwestern Alberta, and the Big Snowy trough in central Montana are part of this system, which exhibits analogies with the Ancestral Rockies area farther south (e.g., Paradox-Eagle-Laramie basins, and the Anadarko basin). Sediments are a mixture of easterly derived fine clastics (shales), coarser clastics with possible sources both from the craton and the Cordilleran belt, and some locally formed carbonate rocks. Chemical sediments, reefal buildups, and coarse, locally derived clastics, are unknown in the British Columbia, Alberta, and Montana area. The thickness of this Mississippian-Pennsylvanian series is usually 100 to 200 m (maximum 700 m). Excellent hydrocarbon source rocks and reservoirs are present, but their extent and quality are erratic. The petroleum habitats are further complicated in many places by the fact that the whole structural system has been reactivated during the Cordilleran orogeny. Some of the north-south oriented features became reverse faults, whereas most of the east-west aulacogens have undergone some strike-slip movement. In both situations, drape-over folding has resulted. Some earlier formed hydrocarbons may have remigrated and dispersed. Analysis of the remodeled structural pattern in conjunction with the upper Paleozoic reservoir geometry is the main challenge of present exploration on this ancient edge of the North American craton. End_of_Article - Last_Page 647------------

The Dry Valleys of Barbados: A Critical Review of Their Pattern and Origin

The major erosional feature on the emerged coral reefs of Barbados consists of a network of valleys. These valleys are generally dry, and the main problem discussed is that of their origin and subsequent desiccation. A secon- dary problem is provided by the courses of many valleys which have sections at right-angles to past and present shore- lines. Of the various hypotheses reviewed, that of surface run-off after heavy storms receives the most support. The possibility of climatic change is discussed and the contributory role of karstification considered. The valley pattern is judged to be a consequent one, its peculiarities resulting from primary depressions on the reefs. THE rock outcropping over six-sevenths of Barbados is a coralline limestone often referred to as the Coral Cap. In the north-east of the island this has been breached to reveal the underlying sandstones and clays of the Scotland district (Fig. i.) The Coral Cap has never been overlain by other strata and its surface descends west and south from elevations of over 350 m near the escarpment in a series of terraces. The origin of these terraces has been explained by K. J. Mesolella (i967), whose stratigraphical studies reveal an ordered sequence of fore reef, reef crest and back-reef deposits across each terrace. This indicates that the terraces are separate raised reefs, rather than wave-cut platforms on a single tilted reef or the product of a series of step faults. E. T. Price (1958) concluded that the declining silica/aluminium ratios with height on the limestone soils indicate a uniform rate of uplift. Mesolella et al. (i969) confirm this for the lower terraces by Thorium-Uranium dating, and they find an overall association of the height spacing of reefs with the time spacing of radiation maxima predicted by considering the changing tilt of the earth and the precession of the equinoxes. An almost uniform rate of uplift of 0-3 m per thousand years is indicated. This steady uplift has separated the reefs formed by the succession of high warm interglacial sea-levels in the Pleistocene. There has been some folding during the uplift (Fig. i) and the large-scale relief of the Coral Cap, with anticlinal domes and synclinal vales, faithfully reflects this. Two types of erosional feature are common on the limestone. The first consists of small dolines, usually discrete, predominantly circular in form and seldom greater than I5 m in depth, many containing sinkholes and many others ponds. The second and more striking features are the valleys which radiate from the high ground around the Scotland district and from the Golden Ridge dome. They are often incised to depths of 30 m and their sides slope com- monly at more than 30° and rarely less than I5°. The valley pattern is pinnate with some trellising. When analysed with respect to past and present shorelines, the trend of the former shown by the raised fringing reefs, 79 per cent of the total length makes angles greater than 6o0° to the shore, 7-3 per cent is between 30° and 6o0° and 13'7 per cent is less

The hemp cultivators of Uttarakhand and social complexity (with a special reference to the Rathis of Garhwal)

Uttarakhand is dotted with a network of valleys that lends itself to the development of many small distinct communities, each valley being a micro-region, both in human and geographical terms, whose inhabitants form a largely self-contained economic and cultural entity. A noteworthy example of such a micro-region is the little known Rath (Rāṭh) area, drained by the Ayar in Garhwal. The inhabitants of this area are called Rathis (Rāṭhī-s, Rathi-s), predominantly traditional bhang-(bhāṅga, hemp, Cannabis sativa) cultivating folks in Uttarakhand. In pre-Colonial Uttarakhand, the hemp producers were a prosperous community due to their near monopoly in hempen business and enjoyed due social status, which accompanies prosperity. However, with the introduction of machine-made cloth by the British, which, contrasted to bhangela (bhaṅgelā, hempen cloth), was cheaper and fashionable, the hempen fabric lost market and its producers suffered economically and socially. Responding to the demand of the new socio-political ideology prevailing under the British, the hemp-producing community organised themselves accordingly. Based on the principle of ‘self-organization’, the present study purports to unfold as to how a simple society transforms into a complex one.