Braided Stream Deposits Research Articles

Sedimentology and tectonic significance of Wolfcampian (Lower Permian) conglomerates in the Pedregosa basin: Southeastern Arizona, southwestern New Mexico, and northern Mexico

A single interval of red chert-clast conglomerate and associated strata (RCC/CRCC interval) occurs within the Earp Formation (Pennsylvanian-Lower Permian) at many localities in southeastern Arizona, southwestern New Mexico, and northern Mexico; this interval records a middle Wolfcampian erosional event in the northern Pedregosa basin and adjacent shelf. The RCC and CRCC intervals are probably proximal and distal braided-stream deposits, respectively, consisting of conglomeratic longitudinal and transverse bar deposits; fluvial-channel sandstone; and interfluvial fine-grained sandstone, siltstone, and shale, and they are interstratified within a much thicker sequence of shallow-marine deposits. The RCC/CRCC interval is uniformly thin (average 5 m thick), yet it is characterized by sheet-like lateral continuity, indicating that deposition occurred on an extensive, low-lying surface on which fluvial channel migration and channel switching were important processes. Paleoflow directions were generally southeastward. Biostratigraphic evidence suggests that middle and lower Wolfcampian strata below the RCC/CRCC interval were beveled northward. Much clastic material of the RCC/CRCC interval may be composed of reworked residuum of the beveled strata, with additional contributions possibly from regions north and northwest of the Pedregosa shelf. Middle Wolfcampian emergence of the northern Pedregosa basin and shelf and accompanying deposition of the RCC/CRCC interval were probably caused by tectonics. Other less important factors, such as eustatic sea-level fluctuations, may have amplified the regression. Evidence favoring a tectonic cause of regression are synchronous timing of tectonism and synorogenic sedimentation throughout a wide area in the Ouachita foreland. The RCC/CRCC interval was probably deposited on a forebulge peripheral to the Pedregosa foreland basin. Stratigraphic evidence indicates that the southern Pedregosa basin evolved rapidly from a slowly subsiding platform to a deep foreland basin during early or middle Wolfcampian time. Subsidence of the Pedregosa foreland basin was presumably caused by downflexure under northward-verging thrust sheets in the Chihuahua region during the Ouachita orogeny. Downflexure of the Pedregosa foreland basin evidently caused flexural forebulging and subaerial exposure of large areas cratonward. Flexural models predicting deflection of lithosphere under isostatic thrust and sediment loads agree satisfactorily with the forebulge concept for the origin of the RCC/CRCC interval.

Alluvial fan facies in the miocene-pliocene coastal plain sands, Niger Delta, Nigeria

A 30 m thick alluvial fan developed in the lower part of the Coastal Plain Sand (Miocene-Pliocene) in southeastern Nigeria comprises four intergradational lithofacies: quartz pebble conglomerate, pebbly sandstone, interstratified pebbly sandstone and parallel to cross-stratified sandstone. The quartz pebble conglomerate facies consists of massive pebble packed beds set in a matrix of coarse to very coarse sand. Unordered poorly developed fabric with rare imbrication is characteristic. Deposition by debris flow on the proximal zone of the fan is interpreted for this facies. The pebbly sandstone facies is a structureless unit composed of pebbles dispersed through a medium- to coarse-grained sand with no significant pebble to pebble contact. Reversed distribution grading as well as cut-and-fill structures are common. Deposition by debris flow within the medial section of the fan is proposed. The interstratified pebbly sandstone and sandstone facies comprises relatively thin alternations of pebbly sandstone and horizontally stratified sandstone beds. Alternate deposition by sheet floods and/or debris flows in the distal part of the fan is suggested. Parallel- and cross-stratified fine to medium sand facies represents sheet flood deposits in the most distal part of the fan. These facies characteristics are similar to arid, semi-arid, and temperate alluvial fans. Fault scarp and/or non-tectonic base-level change may have provided the necessary relief for the formation of the fan. The absence of braided stream deposits associated with the alluvial fan appears to support the earlier contentions ∗ ∗ Le Blanc (1972) and Brown et al. (1973). that alluvial fan and braided stream deposits could be separate in space at any given time.

The Triassic–Jurassic junction at Golspie, inner Moray Firth Basin

Synopsis The Dunrobin Bay Formation is redefined to include as its basal member the Dunrobin Pier Conglomerate which was previously assumed to be unfossiliferous and tentatively dated as Rhaetian (Late Triassic). Temporary exposures of organic-rich horizons within the deposit have yielded assemblages of palynomorphs dominated by pollen grains of the Circumpolles group. Kraeuselisporites reissingeri (Harris), Converrucosisporites luebbenensis Schulz, and Pilospora sp. cf. P. parva (Cookson and Dettmann) are among the more common associated forms. Although taxonomically impoverished, the overall composition of the palynoflora suggests a ?latest Rhaetian to Hettangian (Early Jurassic) age and accumulation in a freshwater environment. The conglomerate is interpreted as the deposits of braided alluvial streams on an alluvial fan derived from vegetated areas to the southwest. It is unconformable upon Triassic calcretes and silcretes which were formed in a hot, fairly arid climate. The hiatus between the two units represents a period of local erosion before deposition of the conglomerate during a period of increased rainfall.

Sedimentology of Basal Potsdam Sandstone in Adirondack Border Region, New York, Southeastern Ontario, and Southwestern Quebec: ABSTRACT

Field evidence supports the relatively widespread presence of nonmarine facies within the basal Potsdam formation of the Adirondack border areas of northern New York, southeastern Ontario, and southwestern Quebec. Detailed observations of areal extent, analysis of sedimentary structures and paleocurrent directions, and petrographic studies have been combined with the paleomagnetic determination of the temporal relationships of these strata to establish depositional patterns and facies trends within basal Potsdam units. Four distinct nonmarine lithofacies have been identified: massive matrix-supported conglomerate, stratified framework-supported conglomerate, conglomerate-arkose, and pebble conglomerate-arkose fining-upward sequences, interpreted to represent debris flows, proximal gravelly braided-stream deposits, intermediate to distal gravelly braided-stream deposits, and proximal sandy braided-stream deposits, respectively. Facies of eolian or possibly tidal, and shallow marine origin have also been identified at the base of the Potsdam sequence. Most basal Potsdam sediments are compositionally and texturally immature, derived directly from the crystalline detritus of the extensively weathered Precambrian surface and regoliths that may have locally developed on the craton. The desert-like environment of the Precambrian surface allowed for rapid transport and deposition of relatively unweathered sediments and the subsequent construction of a braided alluvial plain system. Field relations and evidence derived from consideration of the paleomagnetic properties inmore » five localities of fine-grained alluvium suggest that terrestrial Potsdam deposition in the Early and Middle Cambrian largely preceded the marine transgression that deposited the thick, shallow marine units typifying most of the Potsdam sequence.« less

Distinctive Vertical Sequence in Braided-Stream Deposits: ABSTRACT

Distinctive Vertical Sequence in Braided-Stream Deposits: ABSTRACT

Upward coarsening sedimentation units and facies lineages, Old Red Sandstone, Scotland

ABSTRACTProximal braided stream deposits of the Old Red Sandstone of central Scotland are characterised by a variety of upward coarsening units formed during downstream migration gravel bars. Transitional variations in thickness, grain size and structure of these units, and their associated sediments are used to establish facies and paths of facies change (lineages). Three facies lineages are recognised which are interpreted as recording the downstream changes the structure of sediments of three different types of braided river systems. Lineages differ from each other in proportion of sandstone, channel depth and the structure of upward coarsening units: lineage 1 represents alluvium formed in a river system which produced texturally mature gravels and which increased in depth downstream; lineage 2 comprises sediments deposited in shallow channels which become shallower downstream; lineage 3 represents a downstream change from conglomerate to sandstone and from an unchannelled to achannelled regime.Stream depth and ratio of sand to gravel exert fundamental controls on the structure of alluvium. Thick (thickness >50 m) sections showing vertical changes in clast size often comprise single facies lineage. Thick upward coarsening sequences are found with lineages 2 or 3 and upward fining with lineage 1 or 3.

Analysis of Facies Sequences and Paleoenvironmental Interpretation of Kyrock Sandstone (Pennsylvanian), Edmonson County, Kentucky: ABSTRACT

The Kyrock sandstone member of the Caseyville Formation consists of a sequence of interlayered conglomerate and distinctly cross-bedded coarse-grained sandstone. The Kyrock sandstone is dominated by conglomerate near its base, but it is a fining-upward sequence. In this study, the Kyrock sandstone is interpreted as a product of braided-stream deposition. The change from conglomerate-dominated to sandstone-dominated in vertical section is attributed to a transition from medial to distal braided-stream environments. Facies transitions were recognized in 21 measured sections in the Kyrock sandstone and classified according to the fluvial facies models of A.D. Miall. Markov chain analysis and cryptologic statistical techniques were used to determine significant facies relationships. These analyses indicate that the sequence is noncyclic with borderline rhythmic-nonrhythmic facies ordering. Therefore, the fluvial environment probably consisted of braided streams throughout the sequence. Comparison of facies relationships with braided-stream sequence models also indicates that the sequence was deposited by a braided stream. The lower part of the sequence closely resembles Miall's Donjek model, which typifies a distal braided stream. This transition from medial to distal probably resulted from a rise in base level in response to the Early Pennsylvanian sea encroaching on the Illinois basin.

Stratigraphy, Petrology, and Depositional Environments of Upper Cretaceous and Lower Tertiary Sabbath Creek Section, Arctic National Wildlife Refuge (ANWR), Alaska: ABSTRACT

A 9,387-ft (2,816.1-m) section of Upper Cretaceous-Lower Tertiary strata is exposed along Sabbath Creek in the northern ANWR of northeastern Alaska and represents a regressive depositional sequence. The entire section is divided into four lithologic units (A-D), each characterized by distinct depositional assemblages. Unit A, at the base of the section, consists of several coarsening-upward sequences of alternating thick organic-rich siltstones and fine-grained litharenites, representing deposition in subaqueous to lower delta-plain environments. Unit B stratigraphically overlies unit A and is characterized by multiple, mutually erosive, fining-upward sequences of fine to coarse pebble litharenites typical of point-bar sequences in a meandering stream environment (lower t upper delta plain). Unit C consists of multiple, poorly developed fining-upward sequences of dominantly clast- and matrix-supported pebble conglomerate interpreted as braided stream deposits. At the top of the section, unit D is characterized by multiple fining- and a few coarsening-upward sequences of organic-rich shale with minor amounts of medium to coarse litharenite and pebble conglomerate representing meandering stream deposition. The Sabbath Creek section is lithologically dissimilar to coeval units to the west. The Sagavanirktok Formation and Colville Group contain pyroclastic material and thick coal beds not seen in the Sabbath Creek section. Instead, this section is lithologically similar to the Moose Channel formation--a regressive, fluvial, deltaic sequence exposed in the MacKenzie delta area of northwestern Canada. Consequently, detailed interpretation of the Sabbath Creek section has important implications concerning the petroleum potential of the Arctic National Wildlife Refuge and offshore Beaufort Sea. End_of_Article - Last_Page 658------------

Jelly Bean"Conglomerate (Lower Permian): Record of a Forebulge in Southeastern Arizona?: ABSTRACT"

The most incongruous stratigraphic unit the Earp Formation (Pennsylvanian-Permian) is the Jelly Bean conglomerate (JBC), a unit rarely more than 5 m thick, but occurring over 15,000 km/sup 2/. The JBC consists mostly of clast-supported chert-pebble and limestone-clast conglomerate, litharenite, and pebbly sandstone, whereas most of the Earp Formation is marine limestone, siltstone, and shale. The JCB lies on eroded siltstone or limestone, and is capped conformably by siltstone. The JBC is probably a braided-stream deposit as indicated by presence of fluvial dunes and ripples, amalgamated bar and channel conglomerates, imbricated clasts, channeled underbeds, and lack of point bars. Paleocurrents were generally southward. The thinness and widespread occurrence of the JBC suggest a uniform, gentle paleoslope down which the streams flowed. Deposition of the JBC occurred at about the climax of the Marathon phase of the Ouachita orogeny in west Texas and northern Mexico. The age and location of the JBC, which fringes cratonic North America, indicate that it was related to the late Paleozoic convergence of North and South America, and may have resulted from flexural forebulging caused by thrusting in the Marathon orogene and associated sedimentation in a foredeep.

Petroleum Geology of Santa Rosa Sandstone (Triassic), Northeastern New Mexico: ABSTRACT

The Santa Rosa Sandstone (Triassic) occurs at depths of less than 2,000 ft (610 km) over most of northeastern New Mexico. Two major, presently unproductive heavy oil accumulations are known to exist in the Santa Rosa Sandstone in New Mexico: the Santa Rosa tar sands near the town of Santa Rosa in central Guadalupe County and a subsurface accumulation near the town of Newkirk in northeast Guadalupe County. The Santa Rosa Sandstone is 67-350 ft (20-107 m) thick in northeastern New Mexico. It overlies the Artesia Group (Permian) with regional angular unconformity and is subdivided into three regionally recognizable units: a lower sandstone unit 18-140 ft (5-43 m) thick, a middle mudstone unit 0-144 ft (0-44 m) thick, and an upper sandstone unit 7-150 ft (2-46 m) thick. The lower and upper units are blanket deposits of braided streams and consist mostly of fine to medium-grained porous sandstones and minor red mudstones. The middle unit is lacustrine, consisting chiefly of red mudstones and minor sandstones. Structures on the Santa Rosa Sandstone are mostly northwest to northeast-trending gentle folds superimposed on a southeast regional dip of 0.4°. The two heavy oil accumulations occur in the upper sandstone unit. Shows of asphaltic hydrocarbons occur in the lower unit. Stratigraphic and petrographic studies indicate that good reservoirs are widespread in the lower and upper sandstone units in northeastern New Mexico. The blanket geometry of the lower and upper sandstone units indicates that structure should play an important or even dominant role in the trapping of undiscovered hydrocarbons in the Santa Rosa Sandstone. End_of_Article - Last_Page 844------------

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.

Fluvial sedimentation on a quivering craton: Influence of slight crustal movements on fluvial processes, upper Jurassic Morrison formation, western Colorado plateau

One of the most important challenges facing the fluvial sedimentologist is identification of processes outside the stream channel that influence deposition of fluvial sediments. Detailed studies in the lower sequence of the Salt Wash Member (Morrison Formation, Upper Jurassic) demonstrate that crustal deformation at the site of deposition may considerably influence braided-stream processes. Late Jurassic crustal movements in the western part of the Colorado Plateau are interpreted largely from thickness variations and facies distribution, but other features such as vertical repetition of facies, coincidence with at least parts of present-day folds, and the geographic distribution of bedding parameters measured in the fluvial deposits, are also used as corroborating evidence of syndepositional tectonism. These features indicate that several of the large uplifts and basins in the region as well as some of the smaller folds within them were actively moving during deposition of the lower sequence. Tectonic activity altered the stream gradients, which in turn governed sinuosity, flow regime, energy levels, and sediment distribution. Cross-bedding studies indicate that reduced gradients within downwarped areas led to slight increases in sinuosity of the braided-stream channels and of the small sub-channels within them. The lowered gradients apparently resulted in a decrease in the depth of the channels and allowed the streams to flood more readily, producing abundant upper-flow regime horizontal laminations in the channel deposits. In addition, greater quantities of sediment containing higher proportions of sand were deposited in downwarped areas than in positive localities. The inability of the streams to transport bed load through downwarped areas indicates loss of stream energy. However, an increase in the quantity of upper-flow regime horizontal laminations in the same downwarped areas suggests that an increase in flow regime is not necessarily accompanied by an increase in energy levels, at least in regions of slight tectonic activity where the local configuration of the stream channels may change appreciably. Strata presently dip less than 2° throughout most of the region, and this relatively small amount of deformation reflects the combined effects of Late Jurassic, Cretaceous and Tertiary tectonism. This demonstrates that the amount of structural deformation at the site of deposition may appear to be insignificant, yet it can cause appreciable changes in the nature of braided-stream deposits.

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.

Basin and Range-Age Reactivation of the Ancestral Rocky Mountains in Texas Panhandle: Evidence from Ogallala Formation: ABSTRACT

The Ogallala Formation (Neogene) is a widespread syntectonic alluvial apron that was shed eastward from the Rio Grande rift and related uplifts in Colorado and New Mexico during Basin and Range extension. In the Texas Panhandle, the Ogallala completely buried Ancestral Rocky Mountain (Pennsylvanian) structures. Renewed movement on these older structures during the Neogene influenced the thickness and facies distribution of the Ogallala. The Ogallala thickens into the Palo Duro, Dalhart, and Anadarko basins. Major distributary channels on Ogallala alluvial fans coincide with the axes of these basins, whereas major interchannel areas overlie intervening uplifts. Second-order structures subtly influenced the unit as well. For example, in the Carson basin, a Pennsylvanian rhomb graben along the Amarillo uplift, the Ogallala is over 250 m (820 ft) thick compared with 90 m (275 ft) in adjacent areas. Within the Palo Duro basin, local highs controlled the distribution of thin, interchannel fl od-basin and lacustrine deposits. Thicker, braided-stream channel deposits follow local lows. Later movement on the Amarillo uplift broadly folded the Ogallala. The southern high plains surface subtly reflects basement structure, with topographic highs overlying basement highs, suggesting post-Ogallala deformation within the Palo Duro basin. The Amarillo uplift is approximately perpendicular to the Rio Grande rift and parallel to the direction of Basin and Range extension. Thus, the stress field that produced the rift may have caused strike-slip movement and reactivation of the Carson basin along the Amarillo uplift. End_of_Article - Last_Page 459------------

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------------

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.

Minturn and Sangre de Cristo Formations of Southern Colorado--A Prograding Fan-Delta to Alluvial-Fan Sequence Shed from Ancestral Rocky Mountains: ABSTRACT

The Middle Pennsylvanian Minturn Formation and the Pennsylvanian and Permian Sangre de Cristo Formation of the northern Sangre de Cristo Range form a 4,000-m (13,000-ft) thick, progradational sequence of fan-delta and alluvial-fan deposits. This sequence was deposited along the western margin of the central Colorado trough during faulting and uplift of the late Paleozoic Uncompahgre highland of the Ancestral Rocky Mountains. The Minturn Formation is composed mostly of sandstone and shale deposited by fan deltas that prograded into the central Colorado trough. The Minturn of the northern Sangre de Cristo Range is divisible into (1) a turbidite-bearing facies, (2) a limestone-bearing facies, and (3) a red-bed facies. The turbidite-bearing facies is interpreted as deposits of fan deltas that prograded onto the sea bottom below wave base. The limestone-bearing facies is interpreted as deposits of fan deltas that prograded onto a shallow sea bottom above wave base. Turbidites and shallow-marine limestones, although they make up only a minor part of the Minturn Formation, are mutually exclusive deposits that serve to distinguish the two facies. In the limestone-bearing facies, sandstones containing deltaic fore ets overlie thin shallow-marine limestones and are considered diagnostic of that facies. Both facies contain thick intervals of sandstone and shale interpreted as deltaic and alluvial deposits. Where it onlaps the Uncompahgre highland, the lower part of the Minturn Formation contains quartzose and arkosic red beds of probable alluvial origin. The continental Sangre de Cristo Formation conformably overlies the Minturn Formation basinward, but it unconformably overlies Minturn Formation and Precambrian basement near the Uncompahgre highland. The Sangre de Cristo Formation contains (1) a sandstone facies deposited on the distal surfaces of alluvial fans, and (2) a conglomerate facies (Crestone Conglomerate Member) deposited on the proximal surfaces of alluvial fans. The sandstone facies consists of fining-upward cycles of red conglomeratic sandstone and siltstone interpreted as braided-stream deposits. The conglomerate facies consists of poorly sorted conglomerates interpreted as debris-flow and mudflow deposits, and sorted conglomerate and sandstone interpreted as streamflow and sheetflow deposits. End_of_Article - Last_Page 854------------

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------------

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------------

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------------