Paleocurrent Studies Research Articles

Ezeaku Formation Calcarenaceous Sandstone Facies at Abini, Cross River State, Nigeria

Mineralogical and paleocurrent studies of Amasiri Sandstone, a member of Ezeaku Group outcropping as a ridge in Abini were done to determine the quantitative values of component minerals for proper designation of the facies of sandstone and the direction of paleocurrent. Sampling, paleocurrent measurements, petrographic and X-ray diffraction (XRD) analyses were employed to achieve results. Three highly indurated beds of white fine – medium, angular - subrounded grained sandstones were observed. The basal and second beds had thin laminations, whereas the third was massive. The sandstone is found to be hybrid facies, it is not a quartz arenite neither a calcareous sandstone. Calcite (CaCo3) is the most abundant mineral ranging from 43% - 66%, with an average of 51.39%. Calcite is seen occurring as skeletal grains and sparry calcite, corroding, and replacing quartz grains. Quartz (SiO2) ranges from 3% - 37.6% with an average of 17.22%, occurring as detrital quartz and authigenic quartz overgrowth. Monocrystalline and polycrystalline quartz occur, with monocrystalline dominating. Feldspars such as albite, plagioclase, sanidine and orthoclase occur, with the potasssic feldspars dominating. Silica, calcite, and iron oxide were the observed cementing materials. The paleocurrent direction is in the SE. The angularity of the grains implies a source area with close proximity. Therefore, the source area could be inferred to be the Oban Massif or and the Cameroun Mountains.

Paleocurrents, Paleohydraulics, and Palaegeography of Miocene-Pliocene Siwalik Foreland Basin of India

Early Miocene-Pliocene Middle Siwalik Subgroup of Kuluchaur area, Uttarakhand, North India, comprises sheet-like, coarse to medium grained, cross bedded sandstone bodies and multistoreyed variegated mudstones. Paleocurrent study shows commonly unimodal and locally bimodal distribution and displays a high magnitude of resultant (R*=0.7337) oriented towards south-southwest (206° ± 42.27°). Palaeochannel morphological attributes suggest that the depositing river system was about 230 m wide and 4.5 m deep. These broad, shallow, and low sinuous channels with extensive flood plain flowed on a steeper slope (0.00043 degrees) with flow velocity of 60–140 cm/sec. It is visualized that the immature Middle Siwalik rocks were deposited by southward flowing braided rivers transverse to the Himalayas, predominantly in the form of overlapping alluvial fans similar to those depositing sediments at present in the Indo-Gangetic plains. Fairly consistent fluvial palaeoslopes, together with enormous thickness of sediments, are evidences of rapid subsidence of the basin. We conclude that the paleogeography of Indian subcontinent established at the onset of Miocene-Pliocene Siwalik sedimentation is continuing till today.

Provenance of Pindos Foreland Submarine Fans, Western Greece

The detrital grains and lithic fragments in sandstones from two cross-sections in the Pindos foreland reveal that the most probable sources are the Pindos, Koziakas (and probably and Vourinos) ophiolite complexes, as well as metamorphic sequences of the Pelagonian Zone. Abundant serpentinites and basaltic fragments, as well as detrital spinel grains and calcic plagioclases introduce supply from ophiolitic sources. Spinels show the Pindos ophiolite complex, and some of them are also related to the Vourinos ophiolitic complex. Epidote chemistry from Sardinia–Inachos samples show supply from the Koziakas ophiolite. Abundant petrographic evidence is strongly suggestive for a granitic source, as well as for a volcanic source of intermediate composition. Paleocurrent study of that area revealed that the most probable candidate plutonic body that also supplied the studied areas is the Varnountas plutonic body for the Mazia–Votonosi cross-section and the Pieria or the Verdikousa pluton for the Sardinia–Inahos cross-section. The analysed detrital muscovites are rather rich in the phengitic molecule, thus indicating a probable derivation from the Pelagonian Zone, where phengitic crystals commonly exist. Tourmaline, although not related to relevant crystals found in-situ, are most probably derived from the same geological area. Supply from the Pelagonian Zone is also suggested from paleocurrent data.

Provenance of the Late Paleocene Sandstones of the Jaisalmer Basin, Western India

Abstract: Provenance of the late Paleocene sandstone of the Jaisalmer basin has been determined by petrographic and heavy minerals analysis supported by paleocurrent study. Petrography of the quartzose-arenite sandstone reveals an abundance of sub-angular to sub-rounded monocrystalline non-undulatory quartz and some amount of feldspar and rock fragments. The rock fragments are dominated by argillites (slate, phyllite) and limestone. The heavy minerals suite of these sandstones comprises of angular to sub-angular grains of magnetite, zircon, tourmaline, kyanite and staurolite. The paleocurrent analysis indicates bipolar paleocurrent pattern with the dominance of NW flow suggesting that the provenance was in the SE direction of the depositional basin. Q-F-L and Qm-F-Lt diagrams suggest for a provenance at the margin of the craton interior and transitional continental. It is envisaged that the basic igneous rocks of the Deccan basalt, low- to medium-grade metamorphic rocks of the Aravalli belt and Jurassic limestones present in the vicinity are the source rocks for the late Paleocene sandstones of the Jaisalmer basin.

Turbidite Systems in the Inner Forearc Domain of the Hikurangi Convergent Margin (New Zealand): New Constraints on the Development of Trench-Slope Basins

The development of trench-slope basins on subduction wedges is strongly related to subduction processes. Theseconfined basins form on the lower trench slope, and their edges consist of structurally controlled linear bathymetric highs. In this particular setting, sedimentary processes may be controlled both by tectonism and by sea-level changes. Thus, the study of the sedimentary record may permit evaluation of the spatial and temporal contribution of tectonic activity on the development and stratigraphic evolution of trench-slope basins. The recent evolution of the Hikurangi subduction wedge (North Island of New Zealand) is characterized by the uplift and subaerial emergence of the trench­slope break, which has provided good exposure of the Miocene Akitio trench-slope basin. Identification of the main sedimentary discontinuities, reconstruction of the geometry of the sedimentary units, and paleocurrent studies were undertaken in the Akitio basin in order to propose a stratigraphic and structural scheme for the evolution of this emerged trench­slope break. Deep-sea facies associations are representative of three main fine-grained gravity-driven systems that may develop during various stages of development of mature trench-slope basins. These are, from base to top, large submarine slides (olistostrome deposits), fine-grained sand-rich submarine fans, and low-gradient submarine ramps (sheet-like turbidites). This basin-fill megasequence constitutes a lowstand systems tract, which can be subdivided into basin-floor fan, slope fan, and progradingwedge complex. Slope facies associations are common and highlight the important contribution of slope processes to the basin sediment budget. Rapid shifts of facies and important paleoenvironmental changes, reflecting slope creation or modification, characterize five major discontinuities that are related to regional tectonic events (i.e., onset of subduction at 25 Ma, beginning of regional subsidence at 15 Ma) or local tectonic events (i.e., local uplift of structural edges of the Akitio trench-slope basin at 17.5 Ma and 16.5 Ma, acceleration of subsidence at 13.2 Ma). Moreover, we show that the timing of development of some lowstand system tracts (i.e., of slope fans and of prograding-wedge complexes) may also be controlled largely by changes in style and/or amplitude of tectonic activity.

Provenance of quartzarenite clasts in the Tetori Group (Middle Jurassic to Early Cretaceous), Japan: Paleogeographic implications

There are many strata containing quartzite clasts in the Japanese Islands, but the on-land geology of the Japanese Islands has no candidate for the source rocks of the quartzite clasts. This study deals with the provenance of quartzite clasts of the Tetori Group (Middle Jurassic and Early Cretaceous) distributed in northern central Japan. Quartzarenite clasts from conglomerate beds of the Tetori Group are texturally subdivided into three types; the most abundant is matrix-bearing quartzarenite (type 3), followed by poorly sorted feldspar-bearing quartzarenite (type 2) and by well-sorted pure quartzarenite (type 1). To identify the provenance of quartzarenite clasts in the Tetori Group characteristics of quartzarenite sequences distributed in North China and Korea were studied and compared with those of the Tetori quartzarenite clasts. Texturally and geochemically, the quartzarenite clasts of the Tetori Group are very similar to pre-Mesozoic quartzarenites distributed in the south central and east central Korean Peninsula. Considering the previous results of depositional environments, paleomagnetism and paleocurrent studies on the Tetori Group, this study suggests that during deposition of the Tetori Group, the northern part of central Japan was probably land-connected with the central region of eastern Korea.

Sediment Movement Along the Eastern Margin of the Cretaceous Western Interior Seaway, as Revealed by Cross-stratification in the Dakota Formation, Washington County, Kansas

A detailed paleocurrent study of Cretaceous Dakota Formation sandstones in Washington County, Kansas was undertaken to determine directions of sand movement and depositional environments along the eastern margin of the Western Interior Seaway. Azimuth measurements of cross-stratified units were plotted in “rose diagrams” and examined with respect to their geographic and approximate stratigraphic position. A general southwestward direction of sediment transport is indicated (244° mean azimuth). Although mean paleocurrent directions can differ markedly from outcrop to outcrop, most sandstones in the lower part of the Dakota Formation generally have unimodal paleocurrent directions. Some of these sandstones have channel-like geometry and are situated in paleovalleys, with their mean paleocurrent direction oriented subparallel to the channel axis. Fluvial deposition is indicated. Higher in the Dakota Formation unimodal and bipolar paleocurrent azimuths are oriented nearly perpendicular to the axis of the sandbody, suggesting tidally influenced deposition. Unimodal paleocurrents again dominate the highest part of the Dakota Formation, but with overall less directional variability than in the lower part of the unit. Sheetlike sandbody geometry is suspected and more open-marine deposition could be inferred, but fluvial depositional conditions also may be possible.

Holocene regression and the tidal radial sand ridge system formation in the Jiangsu coastal zone, east China

Abstract A radial tidal sand ridge system (RTSRS) spans more than 20 000 km 2 in the southwest Yellow Sea, is fan-shaped and stretches from Qianggang toward the sea. The marine RTSRS is located in a convergent–divergent tidal current field (CDTCF). Numerous drilling holes reveal an area of 3000 km 2 containing tidal sand strata in the Jiangsu coastal plain on land, adjacent to the marine RTSRS, which stretches eastward from Chengdong and is also a fan-shaped with a central angle of 130°. The tidal sand unit lies on a transgressive coastal barrier-lagoon unit with an erosional surface between them, and the latter unit is in turn lying on late Pleistocene stiff mud. The tidal sand unit is overlain by the littoral–tidal-flat unit and together they constitute the Holocene regressive succession. Paleo-current study reveals that a paleo-current pattern similar to the modern marine CDTCF was present in the Holocene sub-aerial tidal sand strata. Therefore, the sub-aerial tidal sand bodies are deduced to be an ancient RTSRS that was formed during the Holocene regression, and gradually exposed and changed into land as the Changjiang and Huanghe deltas prograded. Both sub-aerial and marine RTSRS should be formed in the same tidal depositional environment but in different development stages. Thus, the radial tidal sand ridges in southwestern Yellow Sea should have formed not during the Holocene transgression, as most previous studies have inferred, but in the process of Holocene regression. Mineralogical analyses demonstrate that the sediment source of the sub-aerial RTSRS initially came from the Changjiang and later predominantly from the Huanghe. Sea-floor erosion by tidal currents may also provide sandy material to form the RTSRS. Drilling cores and ground-penetrating profiles show that the paleo-valleys in the study area are much smaller than the present distributaries of the Changjiang River. Thus, the presumption of direct sand supply by a large river to form the RTSRS through its apical area is cannot be supported. After the sea level reached the present high stand, the Jiangsu coastline has been prograding seaward as the Changjiang Delta expanded and this progradation was accelerated when the Huanghe debouched in the study area. The nearshore part of RTSRS has been gradually changing into land while the entire RTSRS extends seawards. In the process of the development, the RTSRS apex changed from Chengdong to Qianggang. At present, the coastline is still moving seaward while the outer part of the RTSRS suffers from some erosion owing to reduction of sediment supply from both the Changjiang Delta and the abandoned Huanghe Delta.

Permian Gondwana Sedimentation in Yellandu (Singareni) Coalfield, Andhra Pradesh, India with Notes on Regional Paleogeography and Tectonic History

Abstract The Yeliandu coalfield of Andhra Pradesh contains an 850 m Permian Gondwana rocks composed of Barakar, Barren Measures and Raniganj Formations. They are characterised by fining upward cycles. The sandstones are channel shaped, sheet-like and multistoried, and planar and trough cross bedded. The shales are massive to laminated and form thin lenses in the Barakar, but moderately thick sheets in the ovelying Barren Measures and Raniganj Formations. The coal seams are thin and lensoid. Paleocurrent studies suggest consistently northhwesterly dispersal direction areally and temporally. Evidence indicates that initially braided streams at the onset of Early Permian Barakar sedimentation became meandering through time into Barren Measures and Raniganj Formations of Middle and Late Permian, respectively. The close similarity in lithofacies depositional environment and dispersal direction of Permian rocks of Yellandu coalfields and those of neighbouring Pranhita-Godavari basin in the east. and their geological and tectonic setting suggest that the Yellandu coalfield was part of the Godavari basin until the Late Permian or Early Triassic.

Jonah Field, Sublette County, Wyoming: Gas Production from Overpressured Upper Cretaceous Lance Sandstones of the Green River Basin

Jonah field, located in the northwestern Green River basin, Wyoming, produces gas from overpressured fluvial channel sandstones of the Upper Cretaceous Lance Formation. Reservoirs exist in isolated and amalgamated channel facies 10-100 ft (3-30 m) thick and 150-4000 ft (45-1210 m) wide, deposited by meandering and braided streams. Compositional and paleocurrent studies indicate these streams flowed eastward and had their source area in highlands associated with the Wyoming-Idaho thrust belt to the west. Productive sandstones at Jonah have been divided into five pay intervals, only one of which (Jonah interval) displays continuity across most of the field. Porosities in clean, productive sandstones range from 8 to 12%, with core permeabilities of .01-0.9 md (millidarcys) and in-situ permeabilities as low as 3-20 µd (microdarcys), as determined by pressure buildup analyses. Structurally, the field is bounded by faults that have partly controlled the level of overpressuring. This level is 2500 ft (758 m) higher at Jonah ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.Grateful acknowledgment is made to Snyder Oil Corporation and Ron Finch, Integrated Petroleum Technologies, for figures and data used in this article. Thanks also to Floyd Bardsley for artwork.1Petroleum Consultant, 1511 18th Avenue East, Seattle, Washington 98112. 2Snyder Oil Corporation, 1625 Broadway, Suite 220, Denver, Colorado 80202. Send reprint requests to AAPG Publications Manager, P.O. Box 979, Tulsa, Oklahoma 74101-0979. Tcf = trillion cubic feet; Gcf = billion cubic feet; Mcf = million cubic feet; kcf = thousand cubic feet.

Paleomagnetic Studies of Carboniferous and Permian in the U.K. Southern North Sea: Core Orientation, Paleocurrent Analysis, and Diagenetic Application

The U.K. southern North Sea is a major gas province with production from Carboniferous and Permian reservoirs. It was a foreland basin in Carboniferous times uplifted to form a Lower Permian desert basin and subsequently deeply buried during the Mesozoic. Paleomagnetic methods have been used for burial analysis, core orientation, and paleocurrent studies. VRM is shown to be of only limited value for core orientation; problems relating to drilling effects and deflection by ChRM will be discussed. Detailed thermal demagnetization studies are most valuable. Blocking temperature spectra can be related to burial history curves, and ChRM directions isolated and compared with Carboniferous and Permian reference directions. ChRM is a valuable parameter for core orientation and thus paleocurrent studies. Examples will be shown from the Carboniferous and Lower Permian.

Tertiary Tectonics and Sedimentation in the Salin (Fore-Arc) Basin, Myanmar

Salin basin of central Myanmar is a tertiary fore-arc basin that extends over 10,000 mi{sup 2} and contains 30,000+ ft of siliciclastic rocks. In the western Salin basin, Tertiary deltaic and fluvial formations contain thousands of feet of lithic sandstones that alternate with transgressive shallow marine shales. Facies and paleocurrent studies indicate deposition by north-to-south prograding tidal deltas and associated fluvial systems in a semi-restricted basin. Presence of serpentinite and volcanic clasts in Tertiary sandstones may imply that the basin was bounded to the east by the volcanic arc and to the west by a fore-arc accretionary ridge throughout much of the Cenozoic. Salin basin is currently defined by a regional north/south-trending syncline with uplifts along the eastern and western margins. Elongate folds along the eastern basin margin verge to the east and lie above the reverse faults that dip west; much of Myanmar's present hydrocarbon production is from these structures. Analogous structures occur along the western margin, but verge to the west and are associated with numerous hydrocarbon seeps and hand-dug wells. These basin-bounding structures are the result of fault-propagation folding. In the western Salin basin, major detachments occur within the shaly Tabyin and Laungshe formations. Fault ramps propagatedmore » through steep forelimbs on the western sides of the folds, resulting in highly asymmetric footwall synclines. Stratigraphic and apatite fission track data are consistent with dominantly Plio-Pleistocene uplift, with limited uplift beginning approximately 10 Ma. Paleostress analysis of fault/slickenside data indicates that fold and thrust structures formed during regional east/west compression and are not related in any simple way to regional transpression as suggested by plate kinematics.« less

Sedimentation Pattern of a Rift-filling Unit, Tesuque Formation (Miocene), Espanola Basin, Rio Grande Rift, New Mexico

ABSTRACT The Miocene Tesuque Formation is a > 2,000m-thick sedimentary unit filling most of the Espanola basin, an asymmetrical basin which is part of the Rio Grande continental rift in north-central New Mexico (U.S.A.). It is composed of conglomerate, sandstone, mudrock, and limestone of nonmarine origin, with numerous interbedded ashfall-tuff layers. The sedimentation pattern of the Tesuque Formation was determined by the interaction of two distinct paleodrainage systems deriving from the eastern and northeastern margins of the Espanola basin during the rifting process. The two systems generated two lithosomes, which can be discriminated in outcrop by means of paleocurrent study, sandstone and conglomerate petrology, and sedimentary facies analysis. Their surficial expressions are called rovinces A and B. Province A has westward paleocurrents and a mixed plutonic and medium-to-high-grade metamorphic provenance. Province B has SSW-directed paleocurrents and significant volcanic and sedimentary detrital inputs. Both provinces represent deposition in a dynamic alluvial-fluvial-lacustrine environment. Nevertheless, sedimentary facies of province A are representative of an ephemeral environmental regime, whereas those of province B seem to represent a more perennial regime. The basin fill, as a whole, is organized in upward fining megasequences averaging about 200 m in thickness. The gradual passage from proximal to more distal environments of deposition is documented within several megasequences. The lower part of these megasequences is composed of braidplain deposits, whereas the upper (and finer) part is characterized by sandflat deposits, lacustrine carbonates and nodular calcretes. Although the eastern margin of the Espanola basin (contributing the detritus of the Tesuque Formation) represents the relatively unfaulted side of a half graben, the documented upward fining megasequences are comparable to similar megasequences derived from tectonically active basin margins. This suggests that sedimentation style on a shallow ramp along the tectonically inactive side of a half graben can be determined by tectonism of the master fault system along the opposite (deeper) side of the basin.

Paleocurrent Directions from Two-Dimensional Exposures of Cross Laminae in the Devonian Flysch of Maine

A common problem in paleocurrent analysis in flysch sequences arises when the strike and dip of cross laminae cannot be measured, even though a general paleoflow direction is evident from two-dimensional, cross-sectional exposures. A new method of paleocurrent analysis is described which makes use of these imperfectly exposed sedimentary structures. It is assumed that younging direction and the attitudes of bedding and fold axis are known; and that the cross laminae within a coset all appear to dip in the same direction. The line of intersection between the outcrop surface and the cross lamina of unknown attitude is measured in the field. A simple stereographic manipulation yields two numbers: an estimated paleocurrent direction corresponding to the tilt-corrected apparent dip direction of the cross lamina, and an uncertainty, normally in the range of $$\pm 60^{\circ}$$ and never greater than $$\pm 90^{\circ}$$. All paleoflow directions within this range correspond to potential cross laminae with acceptable initial dips. Data are plotted on a modified rose diagram. Although individual readings may give misleading results, a large sampling adequately replicates the vector mean paleoflow direction determined from true dip directions. This conclusion is based on (1) a field study of cross laminae in Devonian turbidites in Maine, which yielded comparable paleocurrent directions using the true dip and apparent dip methods; and (2) application of the apparent dip method to cross beds of known attitude intersected by random surfaces of inspection. The new method should prove valuable in areas where traditional paleocurrent studies have been difficult or impossible owing to a lack of three-dimensional exposures of cross laminae.

Slope and Deep-Sea Fan Facies and Paleogeography of Upper Cretaceous Chatsworth Formation, Simi Hills, California

The Upper Cretaceous Chatsworth Formation is a sand-rich deep-sea fan deposit consisting of thick channelized sequences and interchannel deposits typical of middle fan facies. It has poorly developed outer fan lobes and slope facies. Petrographic and x-ray diffraction (XRD) data indicate the arkosic deposits of the Chatsworth Formation were derived from an uplifted granitic terrane. Paleocurrent studies indicate northward sediment transport and an east-trending slope. Regional comparisons of the Simi Hills with the Santa Monica and Santa Ana Mountains suggest these terranes were once contiguous and covered part of a deep-sea fan system that comprised much of the southern California area. Subsequently, these terranes have been offset along the Malibu-Cucamonga and Elsinore faults, and rotated in response to the opening of the Los Angeles basin and to other adjustments in the tectonic framework of the area. The Chatsworth Formation is thought to be part of a forearc-basin deposit similar to the Great Valley sequence and not part of the Salinian borderland block. The original geographic position of the forearc basin was to the south of the Sierra Nevada Mountains and the Great Valley sequence.

Laramide Sedimentation and Tectonic Development Along Southern Margin of Wind River Range, Wyoming: ABSTRACT

Surface observations along the southern margin of the Wind River Range indicate that motion on the Wind River and Continental faults controlled depositional patterns and lithologic character of early Eocene syntectonic sediments. The stratigraphic sequence consists of alluvial plain, lake margin, deltaic, and alluvial fan units. Subsequent movement on the Wind River fault warped this sequence into a monocline 2 mi (3.2 km) long. This structure dies out to the northwest and southeast along the Wind River fault and is overlain by undeformed middle Eocene sediments. Clast composition and paleocurrent studies of post-early Eocene conglomerates indicate that there were additional sources for coarse clastics formed by subsequent uplift in the Precambrian core of the range. Tectonic implications of these interpretations are: (1) early motion on the Wind River fault controlled the margin of Eocene Lake Gosiute and generated a distal sediment source to the east; (2) last motion on the Wind River-Continental fault system was episodic and principally over by the end of early Eocene; (3) the Continental fault, now a post-Miocene collapse End_Page 531------------------------------ feature, was a tear fault with a vertical component up on the north; (4) the Wind River fault is a zone consisting of segments that moved separately; and (5) there are at least two unmapped fault zones along which the core of the Wind River Range was uplifted. These interpretations suggest that compression in the Wyoming foreland continued significantly later than early Eocene. An imbricate thrust model is proposed to accommodate these observations and interpretations. End_of_Article - Last_Page 532------------

Heavy Mineral Shadows, a New Sedimentary Structure Formed Under Upper-Flow-Regime Conditions: Its Directional and Hydraulic Significance

ABSTRACT Distinct heavy mineral accumulations, named here as heavy mineral shadows, were observed on a current, lineated bedding plane in the quartzose Whirlpool Sandstone (Silurian) near Georgetown, Ontario. The shadows are irregular in form but show one consistent feature: in plan, along lines parallel to the current lineation, one side of a shadow is defined by an abrupt boundary between the heavy mineral-rich (dark) area and adjacent quartz-rich (light) area, while the other side is characterized by a gradual transition into the light area. Comparison with paleocurrent directions determined from nearby sedimentary structures and with grain imbrication a few millimeters below the bedding plane on which they occur, indicates that the paleoflow was from the quartz-rich areas towards th sharply defined side of the heavy mineral shadows. A set of flume experiments was undertaken with a mixture of 97% (by weight) quartz sand and 3% chromite sand as the bed material. Stream power during each run was within the range of upper-flow regime plane-bed stability. With increasing stream power, the chromite sand behaved as follows: (1) heavy mineral shadows were stable, identical in form to those described above moving, intermittently beneath a streaky, faster-moving quartz-sand bed load; (2) shadows were completely destroyed, and the chromite sand formed a diffuse layer beneath the quartz sand in transport; and (3) chromite sand was concentrated across the crests of very low standing-wave bed forms with wavelengths of 0.4 m and amplitudes as small as 1.5 min. Although the data available is too limited to allow anything more than speculation on their origin, the recognition of heavy mineral shadows provides a new tool for paleocurrent studies. Flume experiments also suggest that its presence on bedding planes may have paleohydraulic implications.

Uranium Deposits in Channel Curves of Salt Wash Sandstone Member of Morrison Formation (Late Jurassic), Utah: ABSTRACT

The Salt Wash Member of the Morrison Formation in southeastern Utah is characterized by fluvial sandstone in distinct paleochannels. The deposits are part of a large delta-fan with an apex in the vicinity of the western Grand Canyon. The exhumed channel fills, which excellently expose directional sedimentary structures, were deposited by streams of slight to moderate sinuosity, with occasional sharp bends. Such curves were loci of more conspicuous entrapment and burial of carbonaceous material. Later these areas became highly reducing environments where uranium ores precipitated from ground water percolating through permeable interconnecting channels. Major uranium deposits are more likely to occur in areas where such conditions existed. To determine areas with strong randomness of paleocurrent directions, the vector summation method was applied to the analysis of 2,638 paleocurrent measurements. Because this method is a sensitive measure of dispersion, it was possible to outline areas displaying high and low vector strength values. Areas of high scatter commonly yield low values and polymodal distribution, and areas with low scatter are characterized by high values and unimodal distribution. Thus, areas of high channel sinuosity were successfully outlined. Major known uranium deposits occur most commonly in areas of low vector strength values. Such areas were compared with those containing known ore deposits and those considered favorable for future discoveries, and the results support the conclusion. Although the ve tor method has been used in many paleocurrent studies, this is the first time it has been applied in conjunction with other geologic criteria to delineate and select areas favorable for uranium exploration in sandstone deposits. The results show an important relationship among low paleocurrent vector strength values, carbonaceous material, channel curves, and uranium occurrences in the sandstone channels. End_of_Article - Last_Page 1696------------

Turbidite-Deltaic Complex, Piru Gorge Sandstone Member of Ridge Rouge Formation (Pliocene), Ridge Basin, California: ABSTRACT

The Piru Gorge Sandstone member of the Ridge Route Formation (Pliocene) was deposited as a turbidite-deltaic complex within the tectonically active Ridge basin. In cross section, the Piru Gorge Sandstone is 190 m thick and over 5 km long. The sandstone consists of (1) a lower turbidite sequence, overlain by (2) multistacked channel-levee cycles, and (3) interchannel deposits. These sequences are arranged into several megasequences, each up to 60 m thick. The lower turbidite interval is up to 10 m thick and consists of facies B, C, and D of Mutti and Ricci Lucchi. The interval is interbedded with, and grades laterally into, lacustrine shale. The channel-levee sequence is up to 30 m thick and consists of crossbedded channel sandstone, slump folded strata, and levee deposits arranged in fining- and thinning-upward cycles. The sequence makes up the middle and upper parts of each megasequence. Interchannel deposits are interbedded with, and laterally transitional with, the channel-levee sequences. The deposits consist of thick intervals of organic-rich, bioturbated, mudcracked mudstone and sandstone that locally have rootlets and animal tracks. Paleocurrent, thickness, and petrographic studies indicate the Piru Gorge Sandstone was derived primarily from the north-northwest with minor contributions from the east and west. Petrographic data indicate the arkosic sandstones were derived from granitic source terranes. End_of_Article - Last_Page 949------------

A clast fabric paleocurrent study of the Late Devonian Keepit conglomerate, northeastern New South Wales

A clast fabric paleocurrent study of the Late Devonian Keepit conglomerate, northeastern New South Wales