Alluvial Conglomerates Research Articles

About the dating of Upper-Pleistocene fluvial deposits in the arid zone of Morocco: comparative data of radiocarbon, optic stimulation luminescence and uranium/thorium methods

In South Morocco Quaternary studies, one of the major tasks is to date the so-called ‘Soltanian’ alluvial terrace, which occurs widely and lies for its most part beyond the 14C limit [4]. This terrace was well identified in the Atlantic Atlas [7], in the Souss Basin [1,2,9], on the northern border of the Anti-Atlas [8], and more recently at its southern border [6,10] and also at its eastern side [3]. However, dates were limited at ca 35 kyr, often at the upper or at the middle part of the deposits, lock of adequate dating methods; a subsequent important question was the disagreement between the dating results and the presence of a Prehistoric industry of Mousterian tools throughout the sediments [8,11]. A first response is given by Thorp et al. [6] with recourse at the OSL dating method applied to alluvial quartz grains, which give an age of 45.0 ± 3.7 kyr for the bottom silts of the Soltanian terrace of Agadir Tissint and by Weisrock et al. [10]: OSL age of 49.1± 4.3 kyr for silts in a comparable stratigraphic section, and U/Th age of 55.9+ 9.3–8.6 kyr for an underneath travertine. New dating results, by OSL and U/Th, of the basal units underneath the bottom silts (calcreted alluvial conglomerates at Agadir Tissint, other silts and travertine sequences at Oued Noun) will be soon available [5,6]. Another important challenge in the Atlantic coastal area is to make reliable the bottom of the Soltanian terrace and the base level of the high

The Andean orogenic front at Sierra de Las Peñas-Las Higueras, Mendoza, Argentina

The Sierra de Las Peñas-Las Higueras (Mendoza Province, Argentina, 32°15′S–32°45′S) presents one of the clearest and most continuous exposures of the Quaternary thrust front of the Precordilleran fold-and-thrust belt. It is characterized by an east-verging thrust that breaks the surface and causes Neogene sedimentary rocks to override Quaternary alluvial conglomerates. Monoclinal folds and progressive unconformities are characteristic of deformation in the upper part of the alluvial cover, indicating synchronous development of sedimentation and thrusting during the Quaternary. South of this range, ongoing deformation is by gentle warping of the piedmont alluvial plain, hiding blind thrusts at depth.

Evolution of the Minle and Chaoshui Basins, China: Implications for Mesozoic strike-slip basin formation in Central Asia

The Minle and Chaoshui Basins of northern China are pull-apart basins that originated during Jurassic to Early Cretaceous strike-slip and extensional deformation in the western North China block (Alashan), and the region to its southwest, known as the Hexi Corridor. The basins are separated by the Longshou Shan, which has been a positive topographic feature since Jurassic time. Lower Cretaceous successions in the Minle and Chaoshui Basins display similar rift-related, fining-upward motifs. These comprise alluvial conglomerates and sandstones that pass upward into lacustrine mudrocks, followed by a final fine-grained alluvial phase with extensive paleosol development. The upper part of each succession includes a second lacustrine interval, which is related either to a short-lived humid climatic period, or downstream damming of the exit for the fluvial systems of both basins. North-south–trending normal faults and rare synsedimentary rotation and slump structures affect the basin fills. The Minle and Chaoshui Basins are only two of a series of Late Jurassic–Early Cretaceous strike-slip–related basins in this region that nucleated on three basement blocks, Tarim, Qilian Shan, and Alashan. All of the basins are characterized by a nonmarine, alluvial-lacustrine fill, controlled by a combination of strike-slip and normal faults. From an analysis of the distribution, orientation, and motion sense of subsurface and exposed faults we conclude that three types of strike-slip–related basins are present: pull-apart basins (Minle, Huahai-Jinta, Chaoshui), transtensional basins (Southwest Badanjilin, northern Wuwei), and a third type that developed between divergent strike-slip faults of opposing motion senses. We term this type “extrusion fault-wedge basins” (Jiuquan, Bayanhaote, western Wuwei). The cause of Late Jurassic to Early Cretaceous strike-slip and extension in this region is interpreted to be the Lhasa block–Asia collision. Compression arising from this event is speculated to have caused eastward extrusion of the crust of the Hexi Corridor and Alashan. Deformation was accommodated by a 25° counterclockwise rotation of the Alashan with respect to the remainder of the North China block; thrusting occurred in the intervening Helan Shan. The extension in the Hexi Corridor occurred at the same time as vast foreland basins formed to the west (Tarim) and east (Ordos), and while extension affected much of eastern Mongolia and northeastern China.

Upper Messinian siliciclastic rocks in southeastern Calabria (southern Italy): palaeotectonic and eustatic implications for the evolution of the central Mediterranean region

The Messinian stratigraphy of eastern Calabria (southern Italy) is characterised by a threefold subdivision: (1) a pelite section with local limestone and gypsum, deposited in a restricted-marine environment, is unconformably, or disconformably, overlain by (2) coarse-grained alluvial conglomerate, which is in turn locally overlain by (3) a thin and discontinuous ribbon-shaped sedimentary body of sandstone and pelite, commonly displaying a shallow-marine to continental progradational trend. The basal unconformity/disconformity, coarse grain-size, and abrupt compositional-sedimentological change of unit 2 with respect to unit 1 can be explained as a response to tectonic instability and out-of-sequence thrusting in the Calabrian orogenic wedge, possibly induced by isostatic back-tilting of the wedge following the desiccation of the Mediterranean Sea. This mechanism could explain widespread late Messinian thrusting and syntectonic sedimentation along the Apenninic–Maghrebian orogenic belt. The uppermost Messinian continental to shallow-marine siliciclastic deposits of unit 3 crop out today at elevations of up to 300 m. Similar, age-equivalent sedimentary deposits can be traced along the Apennines and the Sicilian Maghrebides, thus, indicating that the Mediterranean area was flooded before deposition of the Trubi Formation, the base of which is traditionally regarded as marking the reestablishment of marine conditions in the Mediterranean region.

Anatomy and evolution of a Lower Cretaceous alluvial plain: sedimentology and palaeosols in the upper Blairmore Group, south‐western Alberta, Canada

ABSTRACTThe Lower Cretaceous (Albian) upper Blairmore Group is part of a thick clastic wedge that formed adjacent to the rising Cordillera in south‐western Alberta. Regional transgressive intervals are superimposed on the overall regressive succession. Alluvial conglomerates, sandstones and mudstones were deposited in east‐north‐eastward draining fluvial systems, orientated transverse to the basin axis. Five facies associations have been identified: igneous pebble conglomerate, thick sandstone, interbedded lenticular sandstone and mudstone, thick mudstone with thin sandstone interlayers, and fossiliferous sandstone and mudstone. The facies associations are interpreted as gravelly fluvial channels, sandy fluvial channels, sand‐dominated floodplains, mud‐dominated floodplains, and marine shoreline deposits, respectively.Five types of palaeosols are recognized in the upper Blairmore Group based on lithology, the presence of pedogenic features (clay coatings, root traces, ferruginous nodules, slickensides, carbonate nodules) and degree of horizonization. The regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution. In source‐proximal areas, palaeosol development was inhibited by high rates of sedimentation. In source‐distal locations, poor drainage resulting from high watertables, low topography and lower rates of sedimentation also inhibited palaeosol development. The best‐developed palaeosols (containing Bt horizons) occur in intermediate alluvial plain positions (tectonic hinge zone) where the floodplains were most stable due to a balance between sedimentation, erosion and subsidence rates. Extrapolating from the upper Blairmore Group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis. At the hinge zone, soil development is controlled primarily by climate and tectonics and their effect on sediment supply, whereas closer to the palaeoshoreline, relative sea level fluctuations, resulting in poor drainage, may have a more significant influence.

Relative sea level control of deposition in the Late Permian Newcastle Coal Measures of the Sydney Basin, Australia

Accumulation of the 400 m-thick Late Permian Newcastle Coal Measures of the Sydney Basin was controlled by changes in relative sea level. Three 3rd-order sequences, which constitute the coal measures, consist of 4th-order depositional sequences of fluvial conglomerate, coal, and small paralic/lacustrine deltas/crevasse splays, deposited on a coastal plain landward of a marine shoreline. Each 4th-order sequence was deposited during a single 4th-order relative sea level cycle. Following falls in relative sea level alluvial conglomerates derived from the New England Orogen filled incised valleys above sequence boundaries forming lowstand systems tracts. Sigmoidal conglomerates with ‘giant crossbeds’ were deposited as alluvial fill in compactional moats formed at the toes of abandoned paralic deltas. Alluvial sediments passed through the coastal plain directly to the marine shoreline causing the shoreface to prograde. Rising relative sea level, caused siliciclastic sedimentation to wane. During these hiatuses, in the transgressive systems tract, a rising water table stimulated peat mire growth blanketing the entire non-marine area. When the vertical accumulation of peat was outpaced by increasing rates of rising relative sea level, transgressing lagoons, interdistributary bays and lakes inundated the mires above maximum flooding surfaces. Continuing relative sea level rise in the highstand systems tracts caused paralic/lacustrine crevasse splays, crevasse subdeltas, and small deltas to prograde westwards into the Newcastle half-graben from a volcanic source on the Offshore Uplift. At this time of rising base-levels sediments were trapped on the coastal plain while the marine shoreface was starved. Falling relative sea level terminated paralic/lacustrine delta progradation and initiated exposure and erosion to repeat the cycle and initiate another 4th-order sequence. Two source areas supplied sediment into the Newcastle half-graben. An easterly source on the Offshore Uplift shed volcanic detritus into the Newcastle Coalfield via paralic/lacustrine deltas, and the New England Orogen shed volcano-lithic detritus via braided streams. The supply from the New England Orogen was switched on or increased by a fall in relative sea level while supply from the Offshore Uplift was switched off, reduced, or diverted, and vice versa during a rise in relative sea level. Increasing rates of 2nd-order falling relative sea level resulted in an upward change from predominantly marine shoreface and coastal plain sedimentation to predominantly fluvial sedimentation in the upper Newcastle Coal Measures.

Geochemistry of Carbonate Cements in Surficial Alluvial Conglomerates and their Paleoclimatic Implications, Sultanate of Oman

ABSTRACT Early diagenetic carbonate cements are a common feature of Quaternary alluvial conglomerates in Oman. Cements are formed in the vadose and, more commonly, phreatic zones from near-surface groundwaters. In drainage areas underlain by the Semail Ophiolite, groundwaters have Mg2+/Ca2+ ratios greater than two, and cements are often dolomite or high-magnesium calcite in addition to low-magnesium calcite. In drainage areas underlain by limestone, groundwaters have Mg2+/Ca2+ ratios of around one or less and cement mineralogy is nearly always low-magnesium calcite. The oxygen and carbon stable isotopic ratios of the cements vary widely, from -10.6 to + 3.0 PDB and from -10.0 to + 0.7 PDB, respectively. Cement d18O values principally reflect variation in rainfall d18 over a time scale of several thousand years. Rainfall and cement d18O values probably are inversely correlated with the mount of rainfall, which is related to the frequency and intensity of the Indian Ocean monsoon. Thus, cement d18O is potentially a proxy indicator of relative rainfall and monsoon activity. For each of three sampling areas, d13C is positively correlated to d18O. Cement d13C values are also related to rainfall amount because rainfall controls the plant population. Greater plant respiration of isotopically depleted CO2 to shallow groundwaters and burial of organic material in conglomerate deposits results in lower cement d13C values compared to periods of lesser plant activity.

Depositional and tectonic setting of the Archean Moodies Group, Barberton Greenstone Belt, South Africa.

The 3.22-3.10 Ga old Moodies Group, uppermost unit of the Swaziland Supergroup in the Barberton Greenstone Belt (BGB), is the oldest exposed, well-preserved quartz-rich sedimentary sequence on earth. It is preserved in structurally separate blocks in a heavily deformed fold-and-thrust belt. North of the Inyoka Fault, Moodies strata reach up to 3700 m in thickness. Detailed mapping, correlation of measured sections, and systematic analysis of paleocurrents show that the lower Moodies Group north of the Inyoka Fault forms a deepening- and fining-upward sequence from a basal alluvial conglomerate through braided fluvial, tidal, and deltaic sandstones to offshore sandy shelf deposits. The basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying Fig Tree Group dacitic volcanic rocks. Shoreline-parallel transport and extensive reworking dominate overlying deltaic, tidal, and marine facies. The lithologies and arrangement of Moodies Group facies, sandstone petrology, the unconformable relationship between Moodies strata and older deformed rocks, presence of at least one syndepositional normal fault, and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower Moodies Group was deposited in one or more intramontane basins in an extensional setting. Thinner Moodies sections south of the Inyoka Fault, generally less than 1000 m thick, may be correlative with the basal Moodies Group north of the Inyoka Fault and were probably deposited in separate basins. A northerly derived, southward-thinning fan-delta conglomerate in the upper part of the Moodies Group in the central BGB overlies lower strata with an angular unconformity. This and associated upper Moodies conglomerates mark the beginning of basin shortening by south- to southeast-directed thrust faulting along the northern margin of the BGB and suggest that the upper Moodies Group was deposited in a foreland basin. Timing, orientation, and style of shortening suggest that this deformation eventually incorporated most of the BGB into a major fold-and-thrust belt.

Temporal and spatial controls on the alluvial architecture of an axial drainage system: late Eocene Escanilla Formation, southern Pyrenean foreland basin, Spain

AbstractIn young or currently active foreland basins of the world, along‐orogen variations in structural deformation and/or depositional environments are common elements of the later phases of basin development. The late Eocene Escanilla Formation of the South‐Central Pyrenean foreland basin represents an ancient drainage system in which such variability can be studied in detail using high‐resolution magnetostratigraphy combined with a more traditional field‐based approach. Downstream changes in the nature of the alluvial system were strongly influenced by the on‐going Eocene structural partitioning of the foreland basin as it began to become incorporated into the southward‐advancing South Pyrenean thrust system. Lower subsidence rates within these allochthonous ‘piggy‐back’ sub‐basins served to increase channel‐body interconnectedness of sheet‐like alluvial conglomerates, to inhibit the preservation of significant volumes of fine‐grained overbank material, and to promote the extensive development of pedogenic calcretes.During the phase of coastal progradation along the subsiding basin axis, a number of N‐S‐trending anticlines impeded the westward progradation of the alluvial system, producing a strong diachrony in the age of a Lutetian‐Priabonian‐aged deltaic system along the orogen. Fold growth across the western oblique ramp of the South‐Central Unit thrust system dramatically influenced middle to late Eocene drainage patterns and lithofacies distributions. Within the portions of the drainage system upstream of these active folds, the alluvial deposits were periodically ponded, allowing the deposition of micritic lacustrine limestones. Fluctuations in regional base‐level exerted control on the drainage system upstream into the alluvial drainage basin. Base‐level rises caused short reversals in the longer‐term westward regression of marine environments across the foreland basin, whereas base‐level falls produced widespread sheet conglomerate deposition.

Sedimentary record and climatic implications of recurrent deformation in the Tian Shan: Evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins, northwest China

Detailed stratigraphic, sedimentologic, paleocurrent, and subsidence analyses were conducted on Mesozoic nonmarine sedimentary sections of the south Junggar, north Tarim, and Turpan basins, Xinjang Uygur Autonomous Region, northwest China. These three basins have been foreland basins throughout the Mesozoic and Cenozoic eras, as demonstrated by asymmetrically distributed basinwide sediment accumulations, foreland-style subsidence profiles, and a variety of outcrop and subsurface facies data. Mesozoic paleocurrent indicators measured in the south Junggar and north Tarim basins, as well as Mesozoic sandstone compositions from both basins, indicate that the intervening Tian Shan has existed as a positive physiographic feature partitioning the two basins throughout Mesozoic and Cenozoic time. Paleocurrent, facies, and subsurface isopach data suggest that the Turpan basin was established as a discrete feature by the Early Jurassic period. The timing and style of depositional systems within the north Tarim Mesozoic depocenter, the south Junggar Mesozoic depocenter, and the central Turpan basin are remarkably similar. Upper Triassic strata of each basin consist of alluvial conglomerate and associated braided-fluvial sandstone and siltstone which fine upward into lower through Middle Jurassic, locally organic-rich, meandering-fluvial, and lacustrine strata. Upper Jurassic braided-fluvial red beds in each basin are overlain by a distinct pulse of uppermost Jurassicmore » alluvial conglomerate. Lower Cretaceous exposures consist of fine-grained red beds in north Tarim and Turpan and interbedded red and gray shale with local silty carbonates in south Junggar. Upper Cretaceous strata of the north Tarim and south Junggar basins are composed of alluvial conglomerate with associated braided-fluvial sandstone and siltstone. 94 refs., 17 figs.« less

New Insights into Archean Crustal Development from Geochronology in the Rainy Lake Area, Superior Province, Canada

Precise isotopic analyses of zircon, titanite, and rutile from units in the Rainy Lake area of northwest Ontario are combined with structural investigations to develop new stratigraphic interpretations in which lithotectonic sequences were juxtaposed by thrusting prior to steepening by wrenching. Ages for the oldest volcanic and plutonic rocks in the area range from $2725 \pm 2 Ma to 2728 +4/-3 Ma$. The age of metagray-wackes of the Coutchiching Group is bracketed between $2704 \pm 3 Ma$ and $2692 \pm 2 Ma$ by detrital zircon and a crosscutting intrusion. Alluvial conglomerate of the Seine Group is bracketed between 2696 +5/-3 Ma, the age of a clast, and 2686 +2/-1 Ma, the age of the undeformed Ottertail Lake Pluton. The Coutchiching Group is younger than structurally overlying metavolcanic rocks and is therefore allochthon-ous. These data suggest that sedimentation and deformation were controlled by collisional plate interactions and subsequent transpression at the subprovince boundary. Combined with dat...

Structural and sedimentary history of the Quaco Head area, southern New Brunswick

Remapping of the Quaco Head area of southern New Brunswick has revealed a more complex structure than had been previously documented. The structure is dominated by two major thrust faults that dip to the south and southeast at between 35 and 45°. The lower (younger?) thrust emplaces Precambrian basalts and tuffaceous siltstones (Coldbrook Group) over alluvial sediments of the Boss Point and Tynemouth Creek formations (Westphalian A–B). Resting unconformably on the upper surface of the basalt is a thin layer (≤ 10 m) of marine, basal Windsor Group limestone, disconformably overlain by alluvial conglomerates of inferred Hopewell Group age. This sequence is truncated by an older, southeast-dipping thrust fault, to the southeast of which lie Hopewell conglomerates overturned towards the west and northwest. The latter sequence probably forms the lower limb of a large recumbent fold. Folding and thrusting in the Quaco Head area may have begun during Westphalian B times, although major movement was probably post-Westphalian C and occurred contemporaneously with dextral strike-slip movement on the Minas geofracture. Following this Alleghenian–Variscan deformation, the area underwent deep erosion prior to the deposition of a thick sequence of Triassic alluvial sediments. The more southerly reverse fault was reactivated as a normal fault during a phase of late Triassic or Jurassic faulting along the Bay of Fundy that was probably related to early Atlantic rifting.

Oil Exploration in Nonmarine Rift Basins of Interior Sudan: ABSTRACT

In early 1975 Chevron Overseas Petroleum Inc. commenced a major petroleum exploration effort in previously unexplored interior Sudan. With the complete cooperation of the Sudanese Government, Chevron has acquired a vast amount of geologic and and geophysical data during the past 9 years. These data include extensive aeromagnetic and gravity surveys, 25,000 mi (40,200 km) of seismic data, and the results of 66 wells. This information has defined several large rift basins which are now recognized as a major part of the Central African rift system. The sedimentary basins of interior Sudan are characterized by thick Cretaceous and Tertiary nonmarine clastic sequences. Over 35,000 ft (10,600 m) of sediment have been deposited in the deepest trough, and extensive basinal areas are underlain by more than 20,000 ft (6,100 m) of sediment. The depositional sequence includes thick lacustrine shales and claystones, flood plain claystones, and lacustrine, fluvial, and alluvial sandstones and conglomerates. Those lacustrine claystones which were deposited in an anoxic environment provide oil-prone source rocks. Reservoir sandstones have been found in a wide variety of nonmarine sandstone facies. The extensional tectonism which formed these basins began in the Early Cretaceous. Movement along major fault trends continued intermittently into the Miocene. This deformation resulted in a complex structural history which led to the formation of several deep fault-bounded troughs, major interbasin high trends, and complex basin flanks. This tectonism has created a wide variety of structures, many of which have become effective hydrocarbon traps. End_of_Article - Last_Page 526------------

Reconnaissance of Tertiary structures along Nares Strait, Ellesmere Island, Canadian Arctic Archipelago

In the coastal area of Ellesmere Island between Copes Bay in the south and Cape Baird in the north, at least three phases of Tertiary structural deformation can be distinguished. These are: 1) Paleocene uplifting, 2) Eocene thrusting and 3) strike-slip faulting, probably not younger than Miocene.
 Uplifting is documented by remnants of a coarse, oligomictic, alluvial conglomerate of Tertiary age. In the southern and central part of the area the conglomerate is over-thrusted by Lower Palaeozoic rocks. Conjugate strike-slip faults are present on Darling Peninsula and northern Judge Daly Promontory. Sinistral displacement along the Judge Daly fault zone is estimated tentatively at 19 km.
 Thrusting and strike-slip faulting in the Nares Strait area probably correspond to the main phase of the Eurekan orogeny.

Alluvial to Marine Facies Transition in the Antler Overlap Sequence, Pennsylvanian and Permian of North-Central Nevada

Autochthonous sedimentary rocks of the Antler overlap sequence indicate regional subsidence of north-central Nevada during Pennsylvanian and Early Permian time as an inland sea gradually transgressed over coarse alluvium eroded from remnants of the Antler orogenic highlands. Alluvial conglomerates and sandstones of the Lower Atokan (Middle Pennsylvanian) Battle Formation (up to 240 m thick) rest unconformably on folded and faulted lower Paleozoic strata that were deformed by emplacement of the Roberts Mountains allochthon during the Antler orogeny. Paleocurrents and stratigraphic relations suggest that the Battle Formation represents deposits of several paleotributaries of a single major paleovalley draining to the southwest. The Battle Formation and overlying limestones constitute a conformable transgressive sequence. In ascending order, the lithofacies and corresponding depositional environments of the Battle Formation are: 1) massive clast-supported conglomerate deposited in proximal portions of a braided-stream system, 2) very sandy conglomerate with rare foreset cross-stratification deposited in the middle reaches of a braided-stream system, 3) cross-stratified conglomeratic sandstone deposited in the distal part of a braided-stream system, 4) fine conglomerate and fine-grained, commonly muddy, sandstone deposited near the terminus of a braided-stream system, and 5) interbedded calcareous sandstone, calcareous conglomerate, and mudstone of tidal and deltaic origin. The Battle Formation is overlain by Pennsylvanian carbonates (Etchart Formation, Highway Limestone, and Antler Peak Limestone) deposited in shallow-marine and/or marginal-marine environments. Paleontologic evidence indicates roughly contemporaneous deposition of 1) open-marine carbonates within a shallow inland sea, 2) mixed terrigenous-carbonate sediments around the margins of the shallow sea, and 3) conglomerates and sandstones in alluvial environments farther inland.

Stratigraphy of Northeastern Cady Mountains and Its Implications for Cenozoic Volcanic Evolution of Mojave Desert: ABSTRACT

The Cenozoic rocks of the northeastern Cady Mountains can be divided into two separate sequences--an unnamed volcanic succession in the eastern Cady Mountains, and the volcanic and sedimentary strata of the Hector Formation in the northern Cady Mountains. In the eastern Cady Mountains, approximately 3,280 ft (1,100 m) of volcanic flows, flow breccias, domes, and laharic breccias overlie a basement complex of Mesozoic quartz diorite and quartz monzonite. These volcanic rocks have a continuous compositional variation from basaltic andesite to rhyodacite, although dacites and rhyodacites predominate. The age of this sequence is probably Oligocene and/or Miocene. In the northern Cady Mountains, approximately 2,395 ft (730 m) of strata assigned to the Hector Formation (Oligocene? to Miocene) lie unconformably on a basement complex of Mesozoic quartz monzonite and metamorphic rocks. The upper part of the Hector Formation also unconformably overlaps the volcanic rocks of the eastern Cady Mountains. The Hector Formation consists of alluvial sandstone and conglomerate plus lacustrine and paludal sandstone, siltstone, mudstone, and limestone. Laharic breccias, ash-flow and air-fall tuffs, and andesitic to basaltic lava flows are interbedded with the sedimentary rocks. Geochemical data indicate that the volcanic rocks from both areas are part of a calc-alkalic suite. The model proposed by various authors for the Cenozoic tectonic evolution of the western United States suggests that there should have been a transition from andesitic-type volcanism to basaltic-type volcanism, concurrent with the termination of subduction and the initiation of regional faulting. No such volcanic transition, however, can be identified within the northeastern Cady Mountains, nor has one been clearly identified in other late Oligocene to Miocene volcanic sequences within the Mojave Desert region. If this type of transition did take place within the Mojave Desert, it must have occurred after 18.6 ± 0.2 m.y.B.P. This is the date obtained from the youngest dated volcani rock that is unquestionably part of the calc-alkalic suite observed in the northeastern Cady Mountains. End_of_Article - Last_Page 1695------------

Tunnelling through the andes : Kennett, R M Tunn Tunnlg, V11, N6, july–Aug 1979, P23–26

The article describes tunnelling construction methods used in an irrigation project in southern Peru designed to take water from the Andes over a distance of 240 km to western desert plains. The schemie required the construction of 37 tunnels varying in area from 11.9 sq M to 16.8 sq M and from under 1 km to 14.9 km long with a total of 103 km. With the greater part of the tunnelling taking place at a height of 3700 M above sea level, major problems concerned accessibility, earth tremors, and equipment performance at this altitude. Much of ht etunnelling has required excavation through unstable conditions with rapid changes from lava and sandstone to volcanic sand, shales and alluvial conglomerates. Details are given of construction methods and equipment used. (TRRL)

Tectonic control of sedimentation at the margin of a Carboniferous depositional basin in Kirkcudbrightshire

Synopsis The stratigraphy and sedimentation of the Lower Carboniferous rocks between White Port and Kirkbean, Kirkcudbrightshire have been re-investigated. The sediments fall into two distinct associations (i) light hued, fine-grained carbonate-rich rocks deposited by normal shallow marine and fluviatile agencies and (ii) red beds dominated by coarse conglomerates and deposited predominantly under sub-aerial conditions. Within the red bed association there are at least two large-scale cycles of deposition commencing with coarse alluvial conglomerates, fining upwards into subordinate shallow marine fossiliferous sediments and then coarsening upwards into alluvial conglomerates again. The origin and preservation of these cycles is related to penecontemporaneous activity on a large basin margin fault, possibly aided by late–Caledonian upward movements within the Southern Uplands.

Sedimentary Structures in Devonian Fanglomerates, Western Norway: ABSTRACT

Coarse Devonian terrestrial sediments were deposited in six distinct basins preserved along the western coast of Norway 100-200 km north of Bergen. The brown- to red-colored alluvial conglomerate, breccia, and sandstone, loosely termed Old Red Sandstone, were derived from nearby surrounding source areas. The basins are similar tectonically and sedimentologically to the Triassic Newark basins, in that they were formed after a major geosynclinal orogeny (Caledonian), consist of thick redbed sequences with plant and fish remains, are bounded by normal faults with apparently large displacements, contain mafic intrusives and extrusives, and have been subjected to post-depositional folding, faulting, and uplift. However, the Norwegian analogues are more limited in area and in lithologic variety. The Solund and Buelandet-Vaerlandet Norwegian Devonian basins contain well-exposed sedimentary structures, textures, and fabrics. About 5,000 m of coarse conglomerate, with scattered, interbedded lens-shaped bodies of sandstone, crops out in the Solund basin. The conglomerate represents a broad coalesced alluvial fan complex deposited into an eastwest-trending half-graben (40 × 20 km). The southerly source area, composed primarily of Caledonian igneous and metamorphic rocks, was a rapidly eroding highland dissected by youthful streams transporting coarse debris northward to the fans. Uniform orientation of sedimentary structures and fabric throughout the basin indicates a relatively steep northward-sloping depositional gradient. The Solund conglomerate beds are generally massive, structureless bodies, with a prominent, anisotropic imbricated fabric. Flat stratification is the most common feature of sandstone lenses within conglomerate beds; primary current lineation and cobble-boulder trains are typically associated. Cross-strata are either large-scale singular plunging troughs, festoon troughs, or planar or tabular small-scale types. Ripple-drift bedding is prominent locally, as are asymmetrical current ripple markings and sand waves. Erosive features include channel scours and preserved undercut banks of stream channels. Evidence of rockslide deposition is found locally. Fining-upward cycles of sandstone-conglomerate are characteristic of many of the sandstone lenses. The stratigraphic evidence suggests dep sition in braided stream channels, primarily under conditions of the upper-flow regime, on a humid-temperate coalescing alluvial fan complex. End_of_Article - Last_Page 543------------