Paleocurrent Indicators Research Articles

Mesozoic and late tertiary submarine fan sequences and their tectonic significance, Sumba, Indonesia

The Indonesian island of Sumba occupies a critical zone which separates a “normal” subduction complex (Java Trench System) from a continent-island arc collision zone (Banda Arc). Sumba is considered by some authors to be a microcontinental fragment, although in this context its pre-fragmentation origin is uncertain. Detailed studies of the stratigraphy of Sumba, however, throw light on the origin and subsequent geological history of the island and surrounding areas. On available evidence, the oldest outcropping rocks are of Cretaceous age and locally contain a tropical Tethyan marine fauna. They comprise dark-coloured, sometimes carbonaceous and often volcanogenic mudstones, sandstones, gravels and diamictites, pervasively intruded by andesitic and dacitic dykes and locally intruded by granodiorite plutons. These sediments are siliclastic and appear to have had a continental provenance. The Cretaceous rocks in many areas compromise part of a major submarine fan complex which, at least in south-central Sumba, prograded towards the southwest and south. Shallow marine and non-marine Paleogene sediments and volcanic agglomerates lie unconformably above the uplifted, tilted and eroded Cretaceous strata. These are overlain in turn by the widely outcropping Miocene Sumba Formation, an oceanic sequence with island-arc affinities. Accessible outcrops of the Sumba Formation comprise platform carbonates with abundant reef developments in west Sumba, and a major submarine fan—pelagic chalk complex in east Sumba. The fan complex, which extends beneath the southern Savu Sea north of Sumba, is dominated by large-scale slumps, olistostromes and thin- and thick-bedded turbidites. Turbidite sands and gravels invariably are volcaniclastic, containing components derived from a typical inner (volcanic) arc complex. Paleocurrent indicators are difficult to find in the Sumba Formation, thus it is uncertain at this stage whether the volcaniclastic turbidites were derived from the present volcanic arc or from an undetermined source which may since have subsided. The portion of the fan complex which underlies east Sumba, however, has undergone vertical uplift since its emplacement of between one and several kilometres. Uplift is actively in progress at present, possibly due to underthrusting by northward-moving Australian continental crust. In this context the island of Sumba comprises an uplifted, north-tilted and subaerially exposed part of the forearc basin of the Indonesian Arc. This uplift can be related to initial stages of continental collision.

Sandstone-Carbonate Repetitive Alternations: Butterfield Peaks Formation (Middle Pennsylvanian), Oquirrh Group, Central Utah: ABSTRACT

The Butterfield Peaks Formation is an impressively thick (up to 2,000 m, 6,600 ft, or more) example of alternating siliciclastic and carbonate deposition. In the Provo area, it exhibits several prominent facies of sandstone and carbonate, as well as intermediate gradations. The sandstones are quartzose, generally very fine and fine-grained, and are best separated into facies on the basis of sedimentary structures. These include tabular cross sets interpreted as eolian in part, and a variety of other marginal to shallow-marine facies. Trace fossils of the Cruziana facies occur in some units of most of these sandstones except the subaerial ones. They are also common in the carbonate rocks. The carbonate rocks can be separated into facies on the basis of composition and texture. The fossiliferous carbonate rocks, predominantly and contain diverse marine fauna dominated by brachiopods, bryozoans, and echinoderm ossicles. Rarer fossiliferous carbonate facies include spiculitic dark mudstone to wackestone, and fossil grainstone. In sandy carbonate rocks, the content of siliciclastic grains ranges from support of the rock (grading into sandstone with interstitial carbonate), to a sprinkling of silt grains in micrite or End_Page 496------------------------------ microspar. Much of the sandy carbonate is wackestone and packstone, some with poorly organized lamination probably resulting from wave action. Many sandy carbonate units lack internal structure or exhibit only bioturbation. In the Provo area, Butterfield Peaks Formation sandstone deposition appears to have been dominated by eolian processes and shallow marine currents related to the Pennsylvanian trade wind regime. Paleocurrent indicators (mostly sandstone foresets) show a strong unimodal pattern from north to south in present coordinates, which corresponds with expected Pennsylvanian trade wind directions from published paleomagnetic paleolatitude reconstructions. Subaerial and marginal-marine deposition seems to have been dominated by siliciclastic sand, with carbonate deposition limited, for the most part, to open marine environments. This contrasts with the Butterfield Peaks section 70 km (43 mi) south in the southern East Tintic Mountains. Much less siliciclastic sand is present there, both as sands one and as a component in carbonate rocks. Marginal marine carbonate facies, such as mudstones to wackestones with fenestral fabric and brecciated textures, are present. Apparently a less-abundant supply of sand allowed the development of these facies instead of overwhelming them, as seems to have been the case farther north. Despite the repetitive nature of the clastic-carbonate alternations, and widespread Middle Pennsylvanian cyclic deposits in other areas, stepwise Markov chain analysis suggests that true cyclic successions cannot be demonstrated mathematically in the Provo-area Butterfield Peaks Formation. The semi-ordered succession present appears to be a function of the interaction of glacio-eustatic sea level fluctuation, tectonically induced rapid episodic subsidence, high rate of carbonate production, and facies shifts between carbonate and clastics partly controlled by wind-influenced delivery of siliciclastic sand and silt. End_of_Article - Last_Page 497------------

Sedimentology of Upper Cretaceous Submarine Fan Strata, San Miguel Island, California, and Comparison to Selected Mainland Localities: ABSTRACT

Upper Cretaceous sandstone, mudstone, and conglomerate are exposed in an eastward-dipping homoclinal sequence at the western end of San Miguel Island. These strata were deposited in the inner, middle, and middle-fringe regions of a submarine fan during the Campanian and Maestrichtian Stages. Outer fan and basin-plain sedimentary rocks are absent, whereas thick sections of mudstone-dominated strata deposited in fan-fringe areas abound. The great thickness of the middle-fan fringe facies suggests that these rocks were deposited in an elongate basin, the long axis of which was oriented normal to the direction of sediment transport. The middle-fan fringe strata on San Miguel Island are characterized by rhythmically interbedded mudstone, siltstone, and sandstone commonly found in thickening-upward sequences. The mid-fan strata are distinguished from outer fan and basin-plain deposits by the facies occurrence along strike from mid-fan channelized sequences, by paleocurrent indicators which trend subnormal to channel orientations, and by large- and small-scale slumping subnormal to channels. The conglomerate clast suite on San Miguel Island is dominated by durable, but brittle, black dacites and plutonic rocks. Some black dacite clasts were shattered during transport to produce black sand grains that have given the sandstones a salt and pepper appearance. Upper Cretaceous submarine fan strata in San Diego County contain a much smaller percentage of black dacite clasts than on San Miguel Island, but both are overlain by the same Eocene submarine fan strata dominated by Poway rhyolite clasts. This relationship suggests that these rocks were deposited as two separate, but closely adjacent, submarine fans which received coarse clastic detritus from similar and overlapping volcano-plutonic source terranes. The Upper Cretaceous strata on San Miguel Island have been referred to the Jalama Formation as established on the mainland. However, the clast suites of the Jalama conglomerates in Santa Barbara County are dominated by metamorphic rocks which indicate a source terrane very different and separate from the volcano-plutonic dominated highlands that supplied detritus to the San Miguel Island and San Diego fans. Thus, the name Jalama Formation should not be used on San Miguel Island. End_of_Article - Last_Page 1684------------

Late Cretaceous Anastomosing Fluvial Systems, Northwestern Colorado: ABSTRACT

Basal Mesaverde deposits exposed near Rangely Dome in northwestern Colorado indicate a progradation of deltaic sandstones over the marine Mancos Shale. Overlying these deposits is a thick nonmarine sequence containing four major facies: (1) extensive organic-rich siltstones with laterally discontinuous coals, (2) elongate lenticular cross-bedded sandstones, (3) thin (0.3 to 2.5 m) ripple-stratified, sheetlike fine sandstones, and (4) thick (maximum 11 m) fine-grained sandstones with large-scale foresets and syndepositional slumping. Two fluvial facies models were developed on the basis of study of over 80 measured sections. Variations in the models are seen in the types of cross-bedded sandstones. Type 1 forms belts 80 to 130 m wide and up to 21 m thick. Major erosion surfaces delineate nearly symmetrical units 3 to 8 m thick. In contrast, type 2 lenticular sandstones belts are broader (600 m) and thinner (3 to 7 m). The component lenses are fewer in number, have a greater width to depth ratio, and commonly show evidence of lateral accretion. Individual units in type 1 belts show little vertical variation in grain size or scale of structures. Type 2 units commonly fine upward in both respects. The fine-grained sandstones flank the major belts and have paleocurrent indicators at high angles to the transport direct on of the coarser lenses. The coarse-grained sandstones were major channels and the fine-grained sandstones were crevasse splays. The thin splays were deposited along levees or in shallow flood basins. The thick splays with foresets are associated only with type 1 belts. They were Gilbert deltas which prograded into interfluvial lakes. The type 1 belts, major interfluvial lakes, and Gilbert splays were associated with greater subsidence rates. This model is comparable to recently described anastomosing fluvial systems. Our other model is similar to a low-sinuosity fluvial system. End_of_Article - Last_Page 616------------

Wave-Dominated Shallow-Marine Sediments in the Lower Carboniferous of Morocco

ABSTRACT The Jebilet Paleozoic inlier near Marrakech, Morocco, contains a series of Lower Carboniferous outcrops that were previously grouped together as a single flysch formation. Two of these areas display a succession of at least 1100 m of exclusively shallow-marine sediments comprising conglomerates, sandstones, and mudrocks with abundant shell fragments and plant debris. The conglomerates contain exclusively intraformational clasts, and some occur as couplets with sandstone caps interpreted as storm deposits. In the sandstones, flat lamination and low-angle cross-stratification predominate and there are abundant wave-rippled surfaces. These sandstones are interpreted as shoreface and inner-shelf deposits. Mudrocks are less common but in places form thick units with delicately preserved fo sils; a tranquil, probably outer-, shelf environment is envisaged for these thicker mudrocks. The major sediment input was probably via a wave-dominated delta, but there is no evidence for active distributaries in the area described. Although some paleocurrent indicators, such as wave ripples, show a consistent orientation, there is insufficient evidence at present to fix the shoreline orientation. However, important thicknesses of wave-dominated shallow-marine sediments can now be clearly demonstrated in an area that also has thick turbidite successions of equivalent age. The larger scale paleogeographic implications must await further data.

The Orientation of Small Muddy Clasts in Turbidites: Paleocurrent Indicators on a Jurassic Carbonate Slope, Tunisia

The Orientation of Small Muddy Clasts in Turbidites: Paleocurrent Indicators on a Jurassic Carbonate Slope, Tunisia

Evidence for Deltaic Environment of Deposition for Aguja Formation (Upper Cretaceous), Southwest Texas: ABSTRACT

The Aguja Formation consists of approximately 650 ft (200 m) of claystones, calcareous concretions, sandstones, ironstone concretions, humate-bearing shales, and seams of humate material, in an area 8 mi (13 km) northwest of Big Bend National Park, Texas. Three informal members of the Aguja Formation are based on the varied lithology. The lower member is gradational with the underlying Pen Formation and is composed of approximately 200 ft (60 m) of yellow and light to dark-brown massively bedded claystones, lenticular beds of calcareous concretions, and thin to massive-bedded and cross-bedded channel sandstones. The lenticular beds of calcareous concretions and cross-bedded channel sandstones occur near the top of this member. Massive claystones dominate the lower member and contain a wide variety of forams, and a restricted fauna of gastropods and pelecypods. The relation of claystone to sandstone, within the lower member, reflects a coarsening-upward sequence and suggests a gradational change from a prodeltaic to a lower delta-plain nvironment. The middle member of the Aguja Formation is gradational with the underlying lower member. The middle member is composed of approximately 300 ft (90 m) of lenticular, thin to massive-bedded, and cross-bedded sandstones, dark-gray massive claystones, and interbedded claystones and sandstones. Lenticular beds of ironstone and ironstone concretions, humate-bearing shales, and seams of humate material also are present in this member. The wide variety of gastropods and pelecypods within the channel sandstones and vertebrate and wood remains in the massive claystones in the middle member suggest both marine- and brackish-water conditions typical of a delta-plain environment. The upper member of the Aguja Formation unconformably overlies the middle member and is composed of approximately 150 ft (45 m) of cross-bedded and massive-bedded sandstones, lenticular beds of limestone-pebble conglomerate, and minor amounts of claystone. Marine fossils are associated with some lenticular sandstones and include cephalopods, pelecypods, and gastropods. Vertebrate remains and petrified wood are present throughout the upper member. The upper member probably reflects a prograding upper delta-plain environment. Marine fossils associated with the lenticular sandstones may represent intermittent destructive phases during progradation. The Aguja Formation was deposited within a deltaic environment as is indicated by the geometry of the claystones, channel sandstones, and humate-bearing units. The lower member represents a prodeltaic to lower delta-plain environment. The source area for the Aguja was probably on the west, northwest, and southwest as is suggested by paleocurrent indicators in the channel deposits. End_of_Article - Last_Page 764------------

Lithofacies and Depositional Environments of Coaledo Formation, Coos County, Oregon: ABSTRACT

The middle to upper Eocene (Narizian) Coaledo Formation, well exposed along the southwestern Oregon coast, includes seven basic lithofacies types which constitute a complex marine-deltaic sedimentary sequence. The lithofacies are defined by lithology, fossils, bedding characteristics and sedimentary structures. Petrographic examination of Coaledo sandstones indicates a dominantly andesitic volcanic source with subordinate plutonic, metamorphic, and sedimentary sources. Paleocurrent indicators suggest that these source areas were located to the southeast, from the initial calc-alkaline vents in the southern ancestral Cascades and from the northern Klamath Mountains. Regional paleocurrent analysis also suggests open-ocean conditions to the west, offshore from a wide, swampy coastal plain. The pre-Coaledo units exposed in the Cape Arago area are interpreted to be delta foreset deposits. Coarsening-upward sequences recognized within the lower Coaledo member are interpreted as delta topset deposits, including subaqueous distributary channel, interdistributary tidal flat, lagoonal, and barrier-bar facies. The lower part of the middle member is characterized by deeper marine facies. The upper part of the middle Coaledo member represents prodelta deposition during a second progradational phase. Coarsening-upward sequences are recognized in the upper Coaledo member, again representing delta topset deposits. Uppermost Coaledo sands and overlying units show a gradual return to deeper marine conditions. Coaledo sediments unconformably ovelrie gently folded fore-arc sediments of the Tyee basin. Deposition of coal-bearing deltaic sediments far to the west of the axis of the basin suggests regional tectonic uplift along the flank of the basin, or progradational filling of the continental flank of the basin. End_of_Article - Last_Page 984------------

Fluvial and Deltaic Facies in the Sentinel Butte Formation, Central Williston Basin

ABSTRACT Fluvial facies in the Sentinel Butte Formation include meandering-channel, channel-plug, natural-levee, floodbasin, and swamp facies. The channel facies comprises elongate, tabular, fining-upward sand beds that exhibit gently dipping point-bar surfaces (sigmoidal cross bedding). Trough cross bedding in the lower part of the sand beds grades upward to planar and ripple cross bedding. Paleocurrent data indicate current directions generally parallel to bed trends. Channel-plug facies consist of organic silty clay filling arcuate, asymmetrically trough-shaped abandoned meanders. Levee facies grade upward from channel facies and consist of ripple cross-bedded sandy silt and silty clay. Floodbasin facies comprise dark-gray, organic, bioturbated silty clay, and grade upward to woody lignite f the swamp facies. Petrified tree stumps are present in levee, floodbasin, and swamp facies. Deltaic facies include distributary-channel, channel-plug, natural-levee, marsh, delta-front, prodelta, interdistributary-bay, and crevasse-splay facies. The channel facies consists of elongate, narrow, trough-shaped, fining-upward sand beds. Planar cross bedding grades upward to ripple and climbing-ripple cross bedding. Paleocurrent indicators are parallel to sand bed trends. Most channel deposits grade upward to organic, disturbed clay and lignite of the marsh facies, but others grade upward to ripple cross-bedded sandy silt and silty clay of the channel-plug facies. Small levee facies of silty clay border the upper part of the channel facies and grade downward to planar cross-bedded silty sand and silty clay of the delta-front facies. The delta-front facies overlies laminated, spar ely fossiliferous silty clay of the prodelta facies and grades laterally to fissile silty clay of the interdistributary-bay facies. Thin, lens-shaped beds of silty sand within the interdistributary-bay facies represent the crevasse-splay facies.

Mississippian pyroclastic flow and ash-fall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkansas

Two pumiceous vitric-crystal tuffs, the Hatton Tuff Lentil and Beavers Bend tuff, occur in the deep-marine Mississippian Stanley Group. These widespread rhyodacitic tuffs range in thickness from 7 to 40 m and are separated by tens of metres of non-tuffaceous quartzose and feldspathic turbidite sandstone and shale. The tuffs consist of varying proportions of ash-sized embayed quartz crystals, plagioclase crystals (oligoclase to andesine), relict shards, volcanic dust, and altered flattened pumice fragments. Each pyroclastic unit consists of two or more tuff lithologies, including a thick lower unstratified pumiceous vitric-crystal tuff with a density-graded crystal-rich base overlain by thin-bedded pumiceous tuff and an upper massive fine-grained siliceous vitric tuff. These tuffs were probably formed by highly explosive eruptions of vesiculating acidic magma from a vent or fissure that produced incandescent avalanches of pyroclastic debris and accompanying ash clouds. The hot turbulent suspensions were rapidly quenched by sea water to form steam-inflated density slurries that flowed into the Ouachita basin. Pyroclastic flows created thick, density-graded, pumiceous vitric-crystal tuff. Numerous smaller density slurries following the main flow in rapid succession deposited the overlying bedded pumiceous tuff. Toward the end of each volcanic eruption, continuous settling of fine ash formed thick, fine-grained upper vitric tuff. Isopach maps of tuff thicknesses, an isopleth map of pumice sizes, logarithmic plots of crystal size versus distance, paleocurrent indicators, and Late Mississippian paleogeography suggest a southern volcanic source that may have been part of a magmatic arc formed at a continental margin during plate convergence between the North American plate and a southern continental(?) plate.

Paleocurrents in part of the Franciscan Complex, California

A folded sequence of Upper Cretaceous Franciscan turbidite graywackes east of San Jose forms the core of the Diablo antiform and structurally underlies Franciscan melange. Measured paleocurrent indicators at 201 outcrops are mostly cross-bedding, parting lineations, and ripple crests. Paleocurrent directions were recovered by stereonet rotations about local fold axes. General flow was toward the southwest, but in detail it bends in individual turbidite channels, and divergence between channel and leveeoverbank flow is discernible. The strata are interpreted as channeled midfan deposits of a subsea fan.

Possible Strike-Slip Faulting in the Southern California Borderland: Reply

The Eocene Poway Conglomerate on San Miguel and San Nicolas Islands and age-equivalent turbidite sands on Santa Rosa Island appear to be parts of a bathyal fan deposit with an apex and source lying east of the halfway point between Santa Rosa and San Nicolas Islands. Palinspastic reconstruction along a proposed East Santa Cruz Basin fault system with 120 to 160 km of right slip aligns this bathyal point source with the compositionally equivalent Eocene river and shallow marine gravels of the type Poway Conglomerate in the San Diego area. Paleocurrent indicators in the Miocene San Onofre Breccia on Santa Cruz Island and the geometry of schist-bearing strata in the subsurface of the Santa Barbara Channel can be explained by right slip of the southern part of Santa Cruz Island from the southeast along the same fault system. Basement rock types in the borderland place no constraints on the proposed reconstruction, although they do constrain models involving large-scale westerly drift. The ridge and basin bathymetry of the central borderland is consistent with a strike-slip tectonic regime.

Did The Worm Turn? Deformed Burrow as a Slump Movement Indicator

ABSTRACT Analysis of a deformed burrow in a slump sheet within turbidites of the Coastal Franciscan sequence of northern California leads to a slump movement direction different from that yielded by paleocurrent indicators or slump folds.

Reliability of Cross-stratification Types as Paleocurrent Indicators in Fluvial Rocks

ABSTRACT Comparison of planar and trough cross-stratification directions from several fluvial formations and modern ephemeral streams shows significant differences in the distribution pattern of the two types of cross-stratification. Trough cross-stratification of all sizes is unimodal, coincident with the channel direction, and has a small degree of scatter. In contrast, planar cross-stratification generally is either unimodal with wide scatter or bimodal with subequal modes at 50° to 60° to both sides of the channel direction. Trough cross-stratification is thus a reliable and precise paleocurrent indicator, requiring relatively few measurements to reconstruct the true channel direction. In contrast, planar cross-stratification must be used with caution, and a relatively large number of measurements are needed to reconstruct the paleocurrent system.

Reliability of Cross-Stratification Types as Paleocurrent Indicators in Fluvial Rocks: ERRATUM

Reliability of Cross-Stratification Types as Paleocurrent Indicators in Fluvial Rocks: ERRATUM

Possible Strike-Slip Faulting in the Southern California Borderland

Research Article| February 01, 1974 Possible Strike-Slip Faulting in the Southern California Borderland D. G. Howell; D. G. Howell 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar C. J. Stuart; C. J. Stuart 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar J. P. Platt; J. P. Platt 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar D. J. Hill D. J. Hill 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar Geology (1974) 2 (2): 93–98. https://doi.org/10.1130/0091-7613(1974)2<93:PSFITS>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. G. Howell, C. J. Stuart, J. P. Platt, D. J. Hill; Possible Strike-Slip Faulting in the Southern California Borderland. Geology 1974;; 2 (2): 93–98. doi: https://doi.org/10.1130/0091-7613(1974)2<93:PSFITS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The Eocene Poway Conglomerate on San Miguel and San Nicolas Islands and age-equivalent turbidite sands on Santa Rosa Island appear to be parts of a bathyal fan deposit with an apex and source lying east of the halfway point between Santa Rosa and San Nicolas Islands. Palinspastic reconstruction along a proposed East Santa Cruz Basin fault system with 120 to 160 km of right slip aligns this bathyal point source with the compositionally equivalent Eocene river and shallow marine gravels of the type Poway Conglomerate in the San Diego area. Paleocurrent indicators in the Miocene San Onofre Breccia on Santa Cruz Island and the geometry of schist-bearing strata in the subsurface of the Santa Barbara Channel can be explained by right slip of the southern part of Santa Cruz Island from the southeast along the same fault system. Basement rock types in the borderland place no constraints on the proposed reconstruction, although they do constrain models involving large-scale westerly drift. The ridge and basin bathymetry of the central borderland is consistent with a strike-slip tectonic regime. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Poway Fan and Submarine Cone and Rifting of the Inner Southern California Borderland

Eocene and Oligocene sedimentary rocks of the northern Channel Islands, San Nicolas Island, Santa Monica Mountains, Simi Hills, and northwest Peninsular Ranges of southern California include conglomerate beds characterized by the distinctive Poway clast suite of gray and red siliceous tuffs. The beds containing Poway clasts in the offshore islands and the Santa Monica Mountains are separated from beds containing similar clasts in the Peninsular Ranges by blueschist and greenschist of Santa Catalina Island and the Los Angeles basin; these schists are basement rocks unrepresented in the clasts. The offshore islands taken together and the Santa Monica Mountains–Simi Hills are interpreted as being two separate blocks or small plates which broke away from the Peninsular Ranges ∼16 m.y. ago. These blocks originally comprised the upper plate of a large thrust fault (the southern California subduction zone of Hill, 1971) over blueschist and greenschist. Boundaries of these blocks are identified in part by the narrow belt of Miocene San Onofre Breccia along their edges, in part by basement contrasts in wells in the Los Angeles basin, and in part by the 400-fm isobath on tne western side of Santa Cruz basin. The island block drifted relatively westward for 3 m.y. during Miocene time at a rate of ∼5.3 cm per yr, thereby opening up the inner basins of the southern California borderland. If a palinspastic translation of the Santa Monica Mountains 90 km east is combined with a 155- to 160-km eastward translation of the island plate, the paleocurrent indicators of the Poway-bearing sequence radiate from a point east of the Santa Ana Mountains near Lake Elsinore. Northwest, west, and southwest from this point, the percentage of conglomerate decreases and, in the marine rocks, water depth at the time of deposition increases. The sedimentary body thus defined is called the Poway fan and submarine cone, analogous to the Mississippi delta and cone but now fragmented by Miocene rifting. The palinspastic restoration also places a north-trending lower Miocene shoreline in the Santa Monica Mountains on a line with a similar shoreline in the Santa Ana Mountains, and it places lower Miocene rocks of shallow-marine facies in the western Santa Monica Mountains adjacent to rocks of similar age and facies on Santa Cruz Island. Presumably, the drifting of these small blocks represents a brittle separation of Peninsular Ranges basement and attenuation of the underlying blueschist and greenschist accompanying east-west spreading of the underlying lithosphere. The east-west relative motion necessary to explain the distribution of these small Poway-bearing microplates suggests that the motion was entirely within the North American plate and not a consequence of the northwest-southeast relative motion between the Pacific and North American plates. The southern California borderland may have formed as an intracontinental interarc basin as the East Pacific Rise and the West Coast subduction zone were annihilated at the western edge of the continent, and the northern boundary of the borderland may represent an intracontinental transform structure analogous to the Garlock fault as postulated by Davis and Burchfiel (1973).

Radiographic Investigation of Laminated and Ripple Cross-Laminated Flysch Sandstones, Ouachita Mountains, Oklahoma

ABSTRACT The radiographs of horizontally laminated and ripple cross laminated sandstones from the Paleozoic flysch of the Ouachita Mountains show the very details of their internal structures. The pattern of horizontal laminae inside flute casts is related to the flow direction and, therefore, can be used as a paleocurrent indicator. The current ripples are of three dimensional, truncated, and mostly nonclimbing type, indicating deposition from a traction current rather than from a suspended load. All samples studied document the conformity of paleocurrent directions deduced from internal structures and those measured on sole marks. This means that sole marks were cut by the same currents that deposited the horizontally and cross-laminated sand. The number of smaller and larger changes in the flow regime, such as the multiple alternation of laminae and the change from horizontal lamination into current ripples, evident on radiographs of a single bed, document that periods of sedimentation alternated with periods of non-sedimentation and erosion. There is an open question whether these breaks in sedimentation resulted from fluctuations within one turbidity current or from the long-term activity of different sedimentary processes.

Elongate Concretions as Paleochannel Indicators, Tongue River Formation (Paleocene), North Dakota

Both linear and tabular sand bodies occur in the nonmarine Tongue River Formation (Paleocene) of western North Dakota. The tabular sand bodies have sharply scoured bases, become finer grained upward, and the vertical sequence of sedimentary structures shows a decrease in the flow regime upward. These tabular bodies are overlain by gray lignitic silt and clay, and are interpreted as high-sinuosity stream deposits. Linear sand bodies are usually straight and have deeply channeled bases and flat tops. Vertical changes in grain size show no consistent pattern, but the upper parts of many bodies become finer grained upward. Sedimentary structures are about 60 percent planar (omikron) cross-stratification, 25 percent horizontal or low-angle planar laminae (plane bed) with parting lineation, and 15 percent small-scale ripple cross-stratification. Paleocurrent indicators are parallel to the axes of the linear bodies, which are interpreted as being low-sinuosity stream deposits. Elongate concretions as much as 5 m wide and a few hundred meters long occur in the linear bodies. The concretions consist of cal-cite-cemented sandstone, and usually occur at or near the tops of the bodies. The long axes of the concretions are parallel to both paleocurrent indicators and the long axes of the sand bodies, and so the concretions can be used to map paleochannel patterns.

Elongate Concretions as Paleochannel Indicators, Tongue River Formation, North Dakota: ABSTRACT

Two types of sand bodies occur in the nonmarine Tongue River Formation (Paleocene) of western North Dakota, linear and tabular. The tabular sand bodies have sharply scoured bases, become finer upward, and the vertical sequence of sedimentary structures shows a decrease in the flow regime upward. These bodies are overlain by gray lignitic silts and clays, and they are interpreted as high-sinuosity stream deposits. Linear sand bodies are the most common type in the Tongue River Formation. They are usually straight and have deeply channeled bases and flat tops. Vertical changes in grain size show no consistent pattern, but the upper parts become finer upward in many places. Sedimentary structures are about 60% planar (omikron) cross-stratification, 25% horizontal or low-angle planar laminae (plane bed) with parting lineation, and 15% small-scale ripple cross-stratification. Paleocurrent indicators parallel the axes of these bodies, which are interpreted as low-sinuosity stream deposits. Elongate concretions, up to about 15 ft wide and hundreds of feet long, occur only in the linear bodies. They consist of calcite-cemented sandstone, and they usually occur at or near the tops of the bodies. They help to protect the bodies from erosion so that in places the bodies exist as exhumed paleochannel systems that form an inverted topography. The axes of the concretions parallel both paleocurrent indicators and the axes of the sand bodies. The concretions are clearly visible on air photos and can easily be used to map paleochannel patterns either on the air photos or in the field. End_of_Article - Last_Page 958------------