Stream Deposits Research Articles

Geomorphic Development of the Alluvial Plain along the Lower Nagdong River, Korea

The Nagdong River Basin occuping the area of 24, 000km2 with 525km long trunk stream is located in the southeastern part of the Korean Peninsula. The alluvial plain along the lower Nagdong is divided into two areas, i. e. the down-reaches basin and the river-mouth delta.This study intends to clarify geomorphic characteristics and geomorphic development of this plain in relation to post-glacial sea-level change. The investigation is made up of micro-landform classification, stratigraphic analysis, and restoration of depositional environment.The main findings are as follows:1) There are buried valleys below the 50-70m thick alluvium in th delta, and below the 15-10m thick one at Woegwan located about 210km upstream from the river mouth.2) The alluvium of the delta is classified into the following six different layers in ascending order; Basal Gravel (BG), Lower Silt-Clay (LC), Middle Sand (MS), Middle Silt-Clay (MC), Upper Sand (US) and Upper Clay (UC). The LC and MC are considered to be the bottom-set bed deposited in the transgression phase. The MS and US are considered to be the foreset or topset bed formed in the regression phase.3) The above mentioned sequence of alluvial deposits shows that there are two large sealevel fluctuations in the post-glacial time in this plain as demonstrated in the plains of Central Japan (Ikeda, 1964).4) In the longitudinal profile, the LC and MC show a wedge-shape structure which is sharp upstream. These layers were traced up to 50km (LC) and 150km (MC) from the river mouth.5) In the cross-section of down-reaches basin, silty clay lenses are recognized on the both sides of main stream and are identified as back marsh deposits which formed in submerged environment during the transgression.6) Geomorphic development of this alluvial plain are summarized as follows:a) The down-reaches of Nagdong River was drowned in the postglacial transgression. This drowned valley was consequently filled continuously with stream deposits supplied from the neighbouring mountain sides and with flood deposits provided from the main Nagdong River sides. The area of water surface was gradually decreased. The back marshes such as Upo, Samolpo etc., are the latest remnants of water surface.b) The delta plain formed on the river mouth is made up of three different geomorphic units, that is, the main delta, the lower delta and the marginal marsh. In the bay, the main delta was formed by bifurication of main stream resulting in the formation of interdistributary islands. By contrast in the bay mouth which is affected by open sea, the lower delta consisting of beach ridge, swale, and barrier islands has been developed. The development of main delta provided the enclosure of water surface and resulted in the formation of marginal marsh.

Sedimentology of plio-pleistocene late orogenic deposits associated with intraplate subduction—the Upper Siwalik Subgroup of a part of Panjab Sub-Himalaya, India

This study provides a detailed sedimentologic documentation of the lithofacies, cyclicity, sandbody geometry, palaeoflow variability and depositional models of the youngest late orogenic deposits of the Siwalik molasse. These deposits represent one of the important examples of sedimentation in the zone of intraplate subduction of the Himalayan collision belt. Lithofacies analysis has resulted in the recognition of four conglomerate facies (massive, matrix-supported conglomerate; crudely stratified conglomerate; trough cross-stratified conglomerate; planar cross-stratified conglomerate), five sandstone facies (erosional scour and fill; pebbly sandstone; medium- to coarse-grained sandstone; fine- to medium-grained ripple drift laminated sandstone; bioturbated calcareous fine-grained sandstone), and six mudstone facies (parallel-laminated bioturbated mudstone; reddish brown massive mudstone; variegated mudstone; bluish-grey to violet mudstone; buff-coloured mudstone; pebbly mudstone). In addition, hard calcareous layers have been recognised as a separate facies state. The deduced cyclic sequences of the Tatrot and Pinjor Formations of the Upper Siwalik Subgroup show fining-upward characters from CS (coarse sandstone) to MS (mudstone) through FS (fine sandstone) and SS (alternations of sandstone and siltstone). The sandbodies of the Tatrot and Pinjor Formations reveal a sheet-like geometry and an internal dominance of inter-storey scour surfaces. The presence of intraformational debris, mud drapes, reactivation surfaces and a general fining upward of grain size indicates fluctuating hydrodynamic conditions with frequent channel abandonment and re-occupation, resulting in net vertical aggradation. The dominant style of sedimentation both in the channel and overbank facies is through vertical aggradation. Data on the palaeoflow variability indicates that the Upper Siwalik Subgroup can be subdivided into four and three sediment-vector sequences in the Khetpurali and Ghaggar sections, respectively. The palaeodrainage pattern in the Khetpurali section reveals a major influence of northerly drainage twice in the stratigraphic column. The high percentage of overbank deposits in individual cycles, the lack of vegetal debris, organic-poor palaeosol units, and abundant syngenetic CaCO 3 activity warrant comparison with the modern anastomosed deposits described from arid Central Australia. Some intervals of the sequences are comparable to the sublinear, subparallel drainages with local anastomosis of the modern piedmont zone of the Himalaya. The higher parts of the stratigraphic column indicate proximal braided stream deposits and finally proximal alluvial fan deposits. Syntectonic activity has controlled the deposition of the megasequences which are a response to episodicity in the activity of basin margin-basin margin or basin margin-intrabasin structures. The vertical stacking of the Siwalik megasequences appears to be episodic in character.

A Model for Flow in Meandering Streams

Erosion and deposition in streams and tidal channels depend on the divergence of the sediment‐flux field, and this is governed by the distribution of boundary shear stress. As a consequence, erosion arid deposition patterns in such systems are sensitive to spatial variations in boundary shear stress, which in turn, can be induced by the complex interplay between the flow and bed and bank topography. A method whereby variations in boundary shear stress can be calculated is presented herein for flows that are broad in relation to their depth and for which the bed slopes are small. The formal scheme employs a regular perturbation expansion around a zero‐order state that includes vertically integrated, topographically induced convective accelerations. Use of a zero‐order velocity field that includes these accelerative effects yields a model that is applicable to streams with typical bed slopes and channel curvatures, one that can be employed in the majority of situations of interest to geomorphologists, sedimentologists, and hydraulic engineers concerned with fluvial and estuarine systems. In order to verify the model, our calculations are shown to reproduce, with reasonable accuracy, the free‐surface topography and boundary shear stress distribution measured by Hooke (1975) when applied to a channel of the geometry that he investigated. The latter part of the paper describes how the interplay of the various components of flow through a curved channel with bar‐pool topography is affected by the parameters of the problem.

Lower Cantabrian basin margin deposits in NE Leon, Spain—a model for valley‐fill sedimentation in a tectonically active, humid climatic setting

ABSTRACTBasal Stephanian deposits of early Cantabrian age infill a deeply incised fossil topography at the western edge of the post‐Leonian basin of NE Leon. Seven low sinuosity palaeovalleys, trending NNW–SSE range in depth of incision from 25 to 450 m. The fill is composed of: (i) Red bed succession: a proximal alluvial valley‐fill association of debris flow and stream deposits in which seven mesosequences and two fining upwards megasequences are recognized on the basis of maximum clast sizes. (ii) Grey bed succession: two megasequences are distinguished. The lower megasequence comprises distal alluvial valley‐fill deposits (sheet sandstone, lacustrine, swamp deposits). The upper megasequence comprises a mid‐fan association of debris flow and interlobe/floodplain deposits.The palaeovalleys are considered in the context of previously described palaeovalleys and fill. These are categorized according to the degree of tectonic control and climatic setting.The lower Cantabrian examples may serve as a model for valley‐fill sedimentation in a tectonically active area and a humid climatic setting.

Permo-Triassic fluvial systems in the southeastern Karoo Basin, South Africa

A prominent clastic wedge of latest Permian to early Triassic age (Katberg Sandstone) prograded northwestwards into the main Karoo Basin from a southerly source area as a sedimentary response to renewed tectonism associated with the Cape Fold Belt. Proximal to distal relationships within this clastic wedge and its relationships with underlying and overlying formations reveal a variety of stream types which reflect changing tectonic and climatic conditions. Towards the close of Permian times, the depositional area of the southeastern Karoo Basin was crossed by meandering river channels cutting through earlier formed floodplain deposits (Balfour Formation). Tectonic rejuvenation of the source area in early Triassic times led to steeper gradients and a sharp increase in the supply of coarser grained detritus. As a result, alluvial fans developed in areas adjacent to the source terrane and the river channels became braided, depositing only sands (Katberg Sandstone) with muds and silts being carried down into the most distal parts of the floodplain (Burgersdorp Formation). Subsequent denudation of the source area gradually reduced slopes and allowed the source-ward shift of the distal facies until it eventually overstepped the earlier formed braided stream deposits. Accompanying the changes in tectonic conditions was a change in climatic which also influenced stream type. The late Permian to early Triassic period records a general change to warmer climatic conditions following the widespread glaciation of the early to middle Permian. Thus the lowest Beaufort Group sediments were probably deposited in warm temperate to humid conditions with later deposits being laid down under an increasingly arid regime.

Pleistocene climatic variation and cave development

The climatic variation of the Pleistocene acted on karst terrains to change the conditions of cave development. These changes were the result of three major effects: Ice Contact Effects, Ice Proximity Effects and Global Effects. Ice Contact Effects involve ice directly, producing glacial quarrying, sediment redistribution, water budget alteration, isostatic rebound, temperature controlled weathering, geochemistry changes, and hydraulic conditions. Ice Proximity Effects work at a distance from glacial ice, and include periglacial conditions, aggradation by outwash, loess deposition and disruption of master streams. Global Effects work over all planet areas and involve custatic sea level change and variation in precipitation.

Paleogeographic controls of coal accumulation, Cretaceous Blackhawk Formation and Star Point Sandstone, Wasatch Plateau, Utah

Analysis of sequences, associations, and relationships of facies in the Upper Cretaceous Blackhawk Formation and Star Point Sandstone in the Wasatch Plateau, Utah, leads to inferences regarding the paleogeographic controls affecting the accumulation of coals in delta–barrier-island complexes. Progradation, lateral shifting, and abandonment of these complexes created four major landward-thinning tongues of the upper sandstone bed of the Star Point Sandstone that extend into the lowermost Blackhawk Formation. Each tongue of the Star Point Sandstone trends northwest to southeast, subparallel to the general orientation of the Late Cretaceous paleoshoreline, and represents accretion-ridge, distributary-channel, and mouth-bar sandstones. The accretion-ridge or barrier-island sandstones grade landward into coastwise lagoonal and back-barrier swamp deposits, which, in turn, merge with bayhead deposits of short-headed streams. These deposits grade laterally into delta-plain deposits that consist of major distributary channels which crosscut the coastwise lagoons. Two coal populations were formed in this paleogeographic setting: delta-plain and back-barrier coals. Delta-plain coals formed as thin to thick, discontinuous bodies. Back-barrier coals are thicker and more laterally continuous. They parallel depositional strike and accumulated in swamps on surfaces of infilled lagoons. Here, the swamps were sites of accumulation of thick coals and were influenced by abandonment of associated deltaic barrier-island complexes. Successive offlap events resulted in accumulation of the Hiawatha coal, not as a single continuous bed, but as a series of discontinuous, related coal bodies.

Volcanogenic Alluvial Fan Sedimentation, Puye Formation, New Mexico: ABSTRACT

The Pliocene Puye Formation of north-central New Mexico represents a rift-filling volcaniclastic alluvial fan sequence developed on the east flank of the Jemez Mountain volcanic pile. This coarse-grained sequence can be separated into two compositionally distinct members that are genetically unrelated. The basal Puye (here referred to as the Totavi Member) consists of a 25-m (82-ft) thick, clast-supported conglomerate with interbedded sandstone lenses. The Totavi is a coarse-grained braided stream deposit composed of reworked Precambrian terrane of the Sangre de Cristo Mountains and mafic lavas of the Taos basin. The sequence displays cross-bedded sandstones and conglomerates, and well-developed clast imbrication and long-axis orientation, which indicate a paleoflow direc ion to the southwest. The Totavi member is an axial-stream gravel deposit of the Rio Grande rift. The Totavi member is conformably overlain by a 100+ m (300+ ft) thick volcanogenic alluvial fan deposit here referred to as the San Ildefonso Member. This member represents the eroded and reworked detritus of the growing silicic volcanic complex of the Jemez Mountains. The San Ildefonso Member consists of four major interstratified lithofacies associations. These include: (1) clast-supported, massive and horizontally stratified conglomerates, with interbedded sandstones; (2) matrix-supported conglomerates; (3) laminated claystones and mudstones; and (4) primary pyroclastic deposits of both silicic and mafic composition. The clast-supported conglomerates and interbedded sandstones are typical coarse-grained, braided-stream deposits representing longitudinal bar and minor transverse bar accumulations. Internal stratification and clast imbrication points to an easterly flow direction. The matrix-supported units display a complete spectrum of types from clast-rich (debris flows) to clast-poor (mudflow) varieties. These deposits are sediment gravity flows developed in response to steep slopes, abundant unconsolidated volcanic detritus, and excessive loading by water uptake. A number of debris flows and mudflows are underlain by primary volcanic airfall material and were undoubtedly initiated by volcanism. The laminated claystones and mudstones are lacustrine deposits that resulted from temporary damming of the alluvial fan and ancestral Rio Grande drainage systems. The San Ildefonso Member contains at least 7 primary silicic air fall ash beds and 3 basaltic ash beds. These not only provide a time framework for sedimentation but can be used to delineate lateral facies changes. End_of_Article - Last_Page 537------------

Origin of Chiwaukum Graben, Chelan County, Washington: ABSTRACT

The Chiwaukum graben in central Washington is bounded by the Entiat fault on the east and the Leavenworth fault on the west. The graben is largely underlain by the middle Eocene Chumstick formation, an unusually thick (> 5,800 m? or 19,000 ft?) sequence of sandstone, shale, conglomerate, and minor tuff. Five lithofacies are recognized in the Chumstick formation. They are interpreted as debris-flow and stream-generated alluvial fan deposits, braided and meandering stream deposits, and lacustrine deposits. Stratigraphic relationships within the basin are highly variable and include lateral facies changes, pinchouts, and interfingering of units. Basaltic dikes and sills intrude Chumstick sediments throughout the graben. Structural data from the Leavenworth fault zone show highly variable orientations of minor fault planes, and dominance of subhorizontal slickensides. The fault zone has an irregular trace, and contains braided and en echelon fault segments which separate lenses of conglomerate. The Entiat fault has a highly linear trace. Fault surfaces with slickensides of variable orientation, fractured and folded mylonites, and pods of fractured conglomerates within the Entiat fault zone indicate a complex history of faulting. Within the graben, folds, normal faults, and fracture zones trend northwest, parallel to the graben-bounding faults; strike-slip faults in the graben trend northeast. It is proposed that the Chiwaukum graben formed as a rhombochasm or possibly as a composite pull-apart basin. Consistent with this interpretation are the rhomb-shape of the graben; the thickness of basin fill; sedimentation patterns, including cyclic upward-coarsening sequences adjacent to uplifted fault blocks, and rapid facies changes; the presence of extension-generated(?) volcanism synchronous with sedimentation; and evidence for syndepositional deformation in the graben and on the basin margins. Orientations of structures in the graben are not consistent with a transtensile model; however, they may be the result of strike-slip and normal faulting in an asymmetrical shear zone. Strike-slip models for the origin of the Chiwaukum graben are consistent with Ewing's tectonic synthesis of the Pacific Northwest during the Eocene. End_of_Article - Last_Page 492------------

Microirghizites recovered from a sediment sample from the Zhamanshin Impact Structure

Microscopic glass spherules, called microirghizites, have been recovered from a stream deposit in the Zhamanshin crater. The microirghizites partly overlap the irghizites (tektites) and SiO2‐rich zhamanshinites (impactites) in composition, but exhibit greater variations in composition and have lower average SiO2 contents. A similar relationship has been observed between microtektites and tektites in the Australasian, Ivory Coast, and North American strewn fields. The compositions and oxide trends observed for the microirghizites, SiO2‐rich zhamanshinites and irghizites are rather similar to those observed for the Australasian microtektites and tektites. The major differences are the Al2O3 and FeO versus SiO2 trends, which could be explained by differences in amount of meteoritic contamination. The irghizites and microirghizites contain higher nickel contents (up to 0.41%) than the SiO2‐rich zhamanshinites and Australasian tektites and microtektites. The major oxides versus silica trends for the microirghizites support a previous conclusion that the impacting body was a chondrite meteorite and the bulk parent material was terrestrial. Although the irghizites may have been more contaminated by the impacting body than were the SiO2‐rich zhamanshinites, we find that the major differences in composition between the irghizites and SiO2‐rich zhamanshinites may be due to different parent materials rather than meteoritic contamination.

Volcanic dry avalanche deposits — Identification and comparison with nonvolcanic debris stream deposits

A “volcanic dry avalanche deposit” is defined as a volcaniclastic deposit formed as a result of a large-scale sector collapse of a volcanic cone associated with some form of volcanic activity. Avalanche transport occurred in response to the gravitational field, in a manner similar to the transport of nonvolcanic debris streams (e.g. Hsü, 1975). Such deposits are characterized by megablock structure — deformed and fractured large blocks up to several hundreds meters in diameter. A megablock preserves original layering, intrusive contacts or weathered surfaces of the source volcanic edifice. Surface topography of the deposit is characterised with hummocky relief. Ratios of fell height to travel distance for volcanic dry avalanche deposits are between 0.18 and 0.06. This range is similar but smaller than the value of 0.58 to 0.08 for nonvolcanic debris stream deposit. This similarity suggests similar transportation mechanisms. Excessive travel distances as defined by Hsü (1975), calculated for volcanic dry avalanche deposits, give values larger than for debris stream deposits of the same volume. The difference is explained by lower rigidity of the collapsing mass due to the existence of soft pyroclastic layers, alteration around the vent, development of fractures owing to new cryptodome intrusion, and boiling of supercritical fluid contained within the collapsed mass.

The fluvial and pyroclastic deposits of the Cagayan Basin, Northern Luzon, Philippines—an example of non‐marine volcaniclastic sedimentation in an interarc basin

ABSTRACT The Cagayan basin of Northern Luzon, an interarc basin 250 km long and 80 km wide, contains a 900 m thick sequence of Plio‐Pleistocene fluvial and pyroclastic deposits. These deposits are divided into two formations, the Ilagan and Awidon Mesa, and three lithofacies associations. The facies, which are interpreted as meandering stream, braided stream, lahar, and pyroclastic flow and fall deposits, occur in a coarsening upward sequence. Meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits; lahars and ignimbrites. The coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent Cordillera Central volcanic arc. Uplift of the arc resulted in the progradation of coarser clastics further into the basin, the development of an alluvial fan, and migration of the basin depocentre away from the arc. The coarsening of the pyroclastic deposits reflects the development of a more proximal calc‐alkaline volcanic belt in the maturing volcanic arc. The Cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins. This is because the tectonic and volcanic processes which controlled sedimentation in the Cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins.

New fossil discoveries from the Miocene of Nepal include a hominoid

We present here preliminary findings of recent field studies on Miocene Lower Siwaliks deposits of southwestern Nepal. Twenty-four vertebrate taxa are now represented from over 50 localities. The fauna is largely aquatic, being dominated by fish, turtles, crocodiles and aquatic snakes. Lithologies indicate deposition in sluggish streams and associated environments. Biostratigraphy indicates correlation between the composite Nepal fauna and Lower Siwaliks faunas of India and Pakistan, a conclusion corroborated by a palaeomagnetic study showing an age of 9–10 Myr for some of the fossil sites. A hominoid specimen, found in Tinau Khola, is 9.0–9.5-Myr old. It is similar to several specimens of small Miocene hominoids from Pakistan and India, and helps fill the geographical gap between hominoids collected previously in northwestern India and southern China.

Depositional Processes in Beaverhead Formation, Southwestern Montana and Northeastern Idaho, and their Tectonic Significance: ABSTRACT

The Upper Cretaceous to Paleocene(?) Beaverhead Formation is a thick sequence of interbedded and intertonguing synorogenic conglomerates, sandstones, and limestones located at the juncture of a northeast-southwest-trending foreland uplift and the northwest-southeast-trending thrust belt in Beaverhead County, Montana, and Clark County, Idaho. In the vicinity of Lima, Montana, the conglomerates carry two distinct clast assemblages, one dominated by well-rounded quartzite clasts derived from Precambrian and early Paleozoic rocks to the west, and the second by locally derived, angular to rounded limestone clasts of Mississippian to Jurassic age. Based on clast imbrication data by Ryder and Scholten in 1972, the latter assemblage has long been thought to represent a deposit sh d radially from the southwest end of the foreland uplift. Recent observations on the details of depositional facies, clast composition, fining-away sequences, and the structure of deformed strata beneath the Beaverhead unconformity, however, suggest that the limestone conglomerates represent a complex of deposits with sources both in the thrust belt and the foreland terrane. Where deposits from these two uplifts can be distinguished, they display markedly different sequences of sedimentary structures and fabrics attributed to significantly different modes of deposition. In the Antone Peak area the limestone conglomerate of the Beaverhead lies unconformably on rocks deformed solely by foreland deformation. The sequence is characterized by cycles up to 10 m (30 ft) thick beginning with laterally extensive lenses of sandy, clast supported, well stratified but poorly sorted cobble and boulder conglomerate grading upward into cross-bedded pebble conglomerate, pebbly sand, and flat laminated and rippled sandstone. Such sequences are characteristic of perennial braided stream deposits. Clast composition, distance from the thrust belt, coarse clast size, and the nature of the unconformity dictate a foreland source for these conglomerates. In contrast, the limestone-rich conglomerates near Dell display a dominance of matrix-supported conglomerate and pebbly mudstones incised by steep walled channels filled with well-stratified, better sorted, clast-supported conglomerate lenses interbedded with thin discontinuous lenses of flat laminated coarse sandstone. These features are consistent with those observed on modern debris-flow-dominated alluvial fans in the Basin and Range province of the Western United States. Ubiquitous recycled sheared quartzite clasts strongly suggest a thrust belt origin for these conglomerates. Recognition of the link between depositional style and source terrane in the Lima area may provide a powerful tool for distinguishing different deposits of compositionally similar conglomerate. With careful mapping of these deposits and precise dating by pollen and fossils, the chronologic relationships of the various deposits may be established, enabling us to better understand the timing of the two uplifts. In addition this relationship suggests a general model that can and should be tested in other parts of the Cordillera where synorogenic deposits are found in both foreland uplift and thrust belt settings. End_of_Article - Last_Page 1340------------

Depositional Environments of Lower Portion of Pottsville Group (Pennsylvanian) in Southern Ohio: ABSTRACT

Seven sedimentary facies are recognized within the lower members of the Pottsville Group (Pennsylvanian) in southern Ohio which define an overall onlap-offlap sequence. The facies succession, as defined by stratigraphic lithofacies analysis, begins with coarse clastic braided and meandering stream deposits suggesting an aggrading coastal plain depositional environment. The braided-stream facies comprises medium to coarse-grained quartzarenite and pebble to cobble conglomerate and disconformably overlies Lower Mississippian formations. Contained large-scale planar and some trough cross-bedding yield strongly unimodal (west) paleocurrent directions. The overlying meandering stream facies consists of multistoried fining-upward sequences of sandstone, minor conglomerate, silt tone, shale, and coal. Lacustrine and interdistributary bay facies are interpreted to represent a transitional environment from a low-lying coastal plain to a northward transgressing marine embayment. Varvelike laminae of mudstone and claystone of the lacustrine facies grade upward to marine-fossil bearing interdistributary bay and fine delta front facies. Subsequent southerly progradation of a delta system is indicated by interdistributary bay, delta front, crevasse splay, and distributary channel facies. Fossil plant assemblages suggest that the lower Pottsville Group, in southern Ohio, is entirely Morrowan (Namurian C-Westphalian A) New River Formation age-equivalent. End_of_Article - Last_Page 421------------

Synorogenic Sedimentation Associated with Development of Paris-Willard Thrust System, Wyoming-Idaho-Utah Thrust Belt: ABSTRACT

Depositional environments, facies distribution, and provenance analyses of Upper Jurassic through lowermost Upper Cretaceous strata in western Wyoming, eastern Idaho, and northeastern Utah suggest that episodic tectonic activity along the Paris-Willard thrust system strongly influenced deposition during early development of the Wyoming-Idaho-Utah thrust belt and associated foreland basin. Synorogenic conglomerates present at various stratigraphic levels in these strata reveal periods of rapid uplift. In general, the synorogenic units contain proximal cobble-boulder conglomerates (braided stream) which grade downslope into distal pebble conglomerates and coarse-grained sandstones (meandering stream). Periods of relative tectonic quiescence and/or less rapid uplift and eros on are represented by interbedded finer grained fluvial, lacustrine, and marine deposits. During the Late Jurassic, erosion of incipient highlands prior to thrusting resulted in eastward progradation of beach/barrier sandstones represented by the Stump Formation. Initial intensive thrusting followed in the latest Jurassic and earliest Cretaceous, with the newly formed highlands shedding proximal braided stream cobble-boulder conglomerates and more distal meandering stream pebble conglomerates and sandstones of the Ephraim Formation into the subsiding foreland basin. Continued subsidence coupled with a decrease in clastic input due to subdued uplift, resulted in establishment of extensive lacustrine systems and deposition of the Peterson Limestone. Renewed movement on the Paris-Willard thrust system then gave rise to the proximal conglomerates and distal sandstones and muds ones of the Bechler Formation. The overlying lacustrine Draney Limestone and marginal lacustrine Smoot Formation represent another period of continued basin subsidence with little or no uplift. The Wayan and laterally equivalent Bear River Formations represent, respectively, near-source fluvial and shallow beach-marine deposition following Smoot/Draney accumulation. Wayan strata were deposited on a meandering stream flood plain and are indicative of slow uplift and erosion in the source area. Bear River strata consist of a beach sandstone unit underlain and overlain by transgressive marine shales. Alternate deposition of transgressive and regressive facies resulted from either eustatic changes in sea level or differential uplift and erosion of the Paris-Willard highlands. End_Page 1354------------------------------ Bear River marine shales grade upward into marine shales of the Aspen Formation, deposited when the Mowry sea transgressed across the thrust belt region. Lack of significant tectonic activity during this time is suggested by the paucity of sand in the Aspen Formation. Aspen strata pass upward through marginal marine strata into a thick sequence of meandering stream deposits of the Frontier Formation that were derived from erosion of subdued Paris-Willard highlands. Subsequent transgression of the Greenhorn sea westward covered the entire region, producing extensive Frontier marginal marine sandstones and marine shales. Renewed intensive uplift in the source area caused rapid eastward progradation of the Greenhorn sea shoreline and concurrently deposited cobble conglomerates in northeastern Utah. Coarse detritus was deposited in eastward-flowing braided streams near the source area (northeastern Utah) and in meandering stream channels farther eastward (western Wyoming). The Niobrara sea subsequently covered much of the region, and synorogenic sedime tation related to the Paris-Willard thrust system was thus completed. End_of_Article - Last_Page 1355------------

Evolution of a Fluvial Clastic Wedge, Abo Formation (Wolfcampian), Sacramento Mountains, New Mexico: ABSTRACT

Outcrops of the Abo Formation (Wolfcampian) in the northern and central Sacramento Mountains of south-central New Mexico record the evolution of fluvial systems within a major clastic wedge. This wedge was derived from the Pedernal uplift and prograded westward into the Orogrande basin. Regionally, the Abo Formation thickens from 60 m (197 ft) in the central Sacramentos, to over 545 m (1,788 ft) in the northern portion of the range, a distance of approximately 29 km (18 mi). Superposed on the regional pattern are local thickness variations of up to 91 m (300 ft). These occur within distances as small as 0.4 km (0.25 mi), and represent local tectonic and paleotopographic controls on sedimentation. Clast-supported cobble conglomerates, associated with laminated mudstones and discontinuous coarse arkosic sandstones, are present near the base of the Abo Formation. The conglomerate lenses range from 0.5 to 20 m (1.5 to 66 ft) thick, and from 15 m (49 ft) to several kilometers wide. They exhibit poor stratification, occasional clast imbrication, and near horizontal to concave-upward erosional bounding surfaces. Locally, nodular CaCO3 occurs in beds or scattered throughout the associated mudstones. The central portion of the sequence is characterized by abundant conglomeratic medium to very coarse-grained arkosic sandstone lenses interbedded with laminated mudstones. These lenses range from 0.25 to 10 m (0.8 to 33 ft) in thickness, and 10 to 110 m (33 to 361 ft) in width. Commonly, several lenses will coalesce, forming laterally extensive belts. Both erosional and depositional (lateral accretion) bounding surfaces are well developed within individual lenses. Low-angle, large-scale trough cross-bedding is the dominant stratification type within these units, however, small to large-scale tabular foresets occur, as do planar, ripple, and small-scale trough stratification. Occasionally, individual units fine upward in grain size and/or scale of structures. This sandstone-rich horizo varies laterally in thickness, and grades upward into a mudstone-rich horizon at the top of the Abo Formation. The upper interval of the Abo sequence contains infrequent fine to coarse-grained arkose lenses enclosed in laterally extensive red mudstones. Paleosols and caliche horizons are present, but not common, in the mudstones. The basal conglomeratic horizon of the clastic wedge represents braided stream deposition at the distal end of arid alluvial fans associated with the tectonically active Pedernal uplift. Caliche and paleosols developed in the overbank mud drapes and levees of these deposits. The conglomerates grade into arkosic sandstones, reflecting the intense weathering and erosion of the granitic core of the uplift. The multilateral sandstone lenses characteristic of the mid portion of the Abo Formation were deposited in low-sinuosity, coarse-grained (bed-load) meander belts; however, higher sinuosity, mixed-load channel and crevasse splay deposits also occur locally. Finally, as the Pedernal source area was buried by its own debris, thick, broad flood-plain deposition predominated. Sandstones ass ciated with the red mudstones were deposited in shallow ephemeral channels and crevasse splays. End_of_Article - Last_Page 550------------

Paleozoic Lithofacies in Southwestern Sinai and Their Depositional Environments: ABSTRACT

Major breaks in sedimentation, accompanied by well-developed paleosols, have been successfully used to subdivide the 600 m (1,968 ft) of Paleozoic sandstone and shale sequence of southwestern Sinai, Egypt, into five smaller facies association units (i.e., formations). The lowest unit (the Araba Formation) is dominated by 1 to 10 m (3 to 32 ft) thick coarsening-up sequences of parallel-bedded, varicolored, fine-grained arkosic sandstone and muddy sandstone with abundant Skolithos burrows and in places Cruziana traces. This is the deposit of a low-energy prograding sandy coastal plain complex that grades upward into a thin, fining-up channel-overbank deposits with poorly developed paleosols. The overlying Naqus Formation scours deep into the Araba, and is characterized by l nticular, coarse to medium-grained, cross-bedded quartz sandstone with only a few clayey intervals. Well-rounded vein quartz and quartzite pebbles are scattered in the lower half, but form lenses in upper half. Crossbeds are common. The Naqus is interpreted as alluvial fan-braided stream deposits. A 15 to 20 m (49 to 66 ft) thick, conspicuous dark brown, ferruginous shale, ochre-yellow dolomitic sandstone and fossiliferous gray siltstone sequence, persistent all along the Qabeliat valley, overlies the Naqus and represent lagoonal deposits laterally equivalent to the shallow marine shale-dolomite sequence of the Um Bogma Formation farther north. The upper few meters of this unit developed into a paleosol. Basal fluvial channel sands of the succeeding Ataqa Formation cut into the Um Bogma aleosol, and grade upward into the fossiliferous green-red marine shales and subordinate sandstones in shoaling-upward sequences. Laterally, these shallow marine beds grade into coastal swamp deposits of carbonaceous shale and coal. Excellent paleosols developed at the top of the Ataqa Formation which in turn is deeply channeled by a thick succession of fining-upward, lenticular fluvial channel-sand-overbank paleosol facies of the Budra Formation. Southward in the Qabeliat valley, a parallel-bedded sequence of thick green shales and thin brown sandstones, both nonfossiliferous, intervenes in the middle of the Budra Formation and represents ephemeral lake deposits related to the fluvial system. Although the repetition of facies associations in the Paleozoic sequence of southwestern Sinai oints to the repetition of events, each lithofacies shows characters of its own sufficient to assign it to a specific environment. Marked asymmetry in the facies sequences suggests spasmodic character of the events. End_of_Article - Last_Page 423------------

Evidence of Multiple Late-Wisconsin Floods from Glacial Lake Missoula in Badger Coulee, Washington

AbstractCatastrophic floods from glacial Lake Missoula entered the Pasco Basin in south-central Washington and backflooded its marginal valleys. Badger Coulee, one such valley, contains beds of fine-grained slackwater sediment deposited by these floods. The slackwater sediment contains two ash layers of the Mount St. Helens set S tephra, about 13,000 yr old. The ash was deposited on a ground surface developed atop slackwater sediment deposited during preash flooding. Evidence of the former ground surface includes the reworked ash, inferred trace fossils, stream and debris-flow deposits, slopewash and/or eolian sediment, and colluvium at the ash horizon. These features and the ash were buried by slackwater sediment deposited during postash flooding. Nonflood, subaerial deposits are not present atop other beds. Instead, beds commonly are reversely graded across “contacts,” suggesting that multiple beds were continuously deposited. The exposed beds thus record at least two late-Wisconsin floods, one preash, the other postash. The pre- and postash floods may be correlative with earlier-reported floods thought to have occurred 17,500-14,000 and 14,000–13,000 yr B.P., respectively.

The Orustdalen Formation of Brøggerhalvøya, Svalbard: A fan delta complex of Dinantian/Namurian age

The Orustdalen Formation of Broggerhalvaya is well exposed on two coastal sections and is 200–250 m thick. It consists dominantly of siliceous conglomerates and sandstones with subordinate shales and impure coal. One horizon low in the sequence yielded spores of uppermost Dinantian/lowermost Namurian age. Three sedimentary fades have been recognised. The fluvial channel facies (interbedded conglomerates and cross-stratified sandstones) are interpreted as braided stream deposits with flow directions to south and west. A shale facies, sometimes with drifted plant remains, is interpreted as overbank in origin. Highly quartzose, medium-to-coarse-grained, cross-stratified sandstones make up the reworked facies where palaeocurrents are bimodal or indicate movement to the NE. The Formation is thought to represent sediments shed from an early Carboniferous fault scarp eroding’a siliceous Lower Palaeozoic source terrain. The proximity of a nearby coastline led to the construction of fan deltas where fluvial deposits were reworked by waves and/or tides. Early diagenetic events include the local development of pyrite, kaolinite and calcite cements. Reddening’probably occurred during Middle Carboniferous times beneath an exposed land surface. Following burial, pressure solution and quartz cementation eliminated porosity at depths greater than 1000 m.