Sediment Lobes Research Articles

Glacial and nonglacial sediment contributions to Wisconsin Episode loess in the central United States

Research Article| October 01, 2000 Glacial and nonglacial sediment contributions to Wisconsin Episode loess in the central United States David A. Grimley David A. Grimley 1Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA Search for other works by this author on: GSW Google Scholar Author and Article Information David A. Grimley 1Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA Publisher: Geological Society of America Received: 08 Feb 1999 Revision Received: 07 Sep 1999 Accepted: 22 Nov 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (10): 1475–1495. https://doi.org/10.1130/0016-7606(2000)112<1475:GANSCT>2.0.CO;2 Article history Received: 08 Feb 1999 Revision Received: 07 Sep 1999 Accepted: 22 Nov 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David A. Grimley; Glacial and nonglacial sediment contributions to Wisconsin Episode loess in the central United States. GSA Bulletin 2000;; 112 (10): 1475–1495. doi: https://doi.org/10.1130/0016-7606(2000)112<1475:GANSCT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The relative contributions of various glacial and nonglacial sediments to Wisconsin Episode loess units along the lower Illinois and central Mississippi Valleys are estimated on the basis of a comparison of magnetic susceptibility and silt and clay mineralogy. A mathematical method of source area calculation, using four compositional parameters, was guided by current knowledge of the regional glacial history. On the basis of this technique, the Roxana Silt, along the Illinois and Mississippi River Valleys, is composed of significant Superior lobe sediment (35%–40%) as well as Wadena or Des Moines lobe sediment (about 35%). This can account for its high magnetic susceptibility, feldspar content, kaolinite content, and pink hue. Lower Peoria Silt contains about 25%–35% Lake Michigan lobe sediment with reduced contributions of the other sources. After the Mississippi River's diversion (20.4 ka), the supply of Superior, Des Moines, and Wadena lobe sediment was cut off from the Illinois Valley in favor of Lake Michigan lobe sediment (75%–80% contribution). This major source area shift accounts for higher dolomite and illite contents and a more yellow hue in approximately the upper two-thirds of Peoria Silt in the study area. In loess south of St. Louis, less pronounced compositional shifts occur because Superior lobe sediment was not cut off and because Des Moines lobe, Wadena lobe, and Missouri River sediments, having more intermediate composition, compose 40%–50% of the loess, thereby diluting other source area changes. Nonglacial sediment, from fluvial and periglacial sources, is estimated to compose 10%–40% of loess in both regions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cretaceous‐Tertiary tectonic history of the Fiordland margin, New Zealand

Integration of regional seismic reflection data with swath bathymetry and imagery data and onshore geology has revealed the complex sequence of tectonic events that have affected the development of the offshore Fiordland margin. There are no core or dredge samples to constrain the timing of these events, but comparison with recent work on the Tertiary evolution of the Australia‐Pacific plate margin provided a temporal framework for the subsurface interpretation. The integrated dataset allows the prediction of the age and likely lithology of the seismic sequences, the timing of tectonic events, and details of how structures along the Fiordland plate margin developed in response to changes in relative plate motion direction. We conclude that Cretaceous rifting in the Tasman Sea formed half‐grabens along Caswell High and beneath the Fiordland Basin. Up to several kilometres of Cretaceous to Early Eocene sediments, probably terrestrial and marginal marine sediments derived from the Campbell Plateau, are preserved in these grabens. A second phase of rifting in the Eocene, associated with the formation of the Southeast Tasman Sea, resulted in the separation of the Caswell High and Fiordland Basin block from the Campbell Plateau and the start of its journey northwest, northeast, and finally east to its present position adjacent to Fiordland. The rift sediments are overlain by a relatively uniform sequence, probably Late Eocene to Mid Miocene deep‐water carbonates. Strike‐slip faults developed along the continental shelf and slope in the Miocene, and increasing margin‐normal shortening in the Pliocene‐Pleistocene, led to the development of thrust faults along the margins of sediment lobes in the Fiordland and Milford Basins. The youngest sediments are up to several kilometres thick, probably Late Miocene‐Quaternary elastics derived from Westland and Fiordland.

Outcrop‐scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California

The uppermost Quaternary deposits of the Hueneme and Dume submarine fans in the Santa Monica Basin have been investigated using a closed‐spaced grid of boomer seismic‐reflection profiles, which give vertical resolution of a few tens of centimetres with acoustic penetration to 50 m. Acoustic facies integrated with geometry define six architectural elements, some with discrete subelements that are of a scale that can be recognized in outcrops of ancient turbidite systems. In the Santa Monica Basin, the relationship of these elements to fan morphology, stratigraphy and sediment source is precisely known.The width of upper Hueneme fan valley has been reduced from 5 km since the last glacial maximum to 1 km at present by construction of laterally confined sandy levees within the main valley. The middle fan comprises three main subelements: thick sand deposits at the termination of the fan valley, low‐gradient sandy lobes typically 5 km long and &lt; 10 m thick, and scoured lobes formed of alternating sand and mud beds with many erosional depressions. The site of thickest lobe sediment accumulation shifts through time, with each sand bed deposited in a previous bathymetric low (i.e. compensation cycles). The lower fan and basin plain consists of sheet‐like alternations of sand and mud with shallow channels and lenses.Variations in the rate of late Quaternary sea level rise initiated changes in sediment facies distribution. At lowstand, and during the approximately 11 ka stillstand in sea level, the Hueneme Fan was fed largely by hyperpycnal flow from the Santa Clara River delta, depositing high sediment waves on the right hand levee and thick sandy lobes on the middle fan. At highstand of sea level, most turbidity currents were generated by failure of silty prodelta muds. In contrast, the smaller Dume Fan was apparently always fed from littoral drift of sand through a single‐canyon point source.

Braided stream aggradation on an alluvial fan margin: Emerald Lake fan, British Columbia

Evolving bar and channel patterns were observed at the distal margin of an active alluvial fan at Emerald Lake, British Columbia, during peak midsummer flows. At the fan margin, bed slope is 0.024, sediment is predominantly coarse gravel, flow is shallow and fluctuating, and sediment transport and deposition are dominated by chutes and lobes. Flow unconfinement at the exit of rapidly formed shallow scour channels typically results in deposition of sediment lobes 0.2 m thick and 10-250 m2 in area. Closely spaced deposition of a number of these sediment lobes results in aggradation of composite sediment sheets. One such sheet, monitored daily over a 15 day period, deposited 129 m3 of gravel over 710 m2 of adjacent marsh sediment, locally extending the distal fan margin by 39 m. Thickness of the aggraded bed varied up to 0.37 m, depending on surface topography. During active deposition, individual lobe deposits formed simple unit bars that partly projected above the water surface. These bars caused local flow division that, together with avulsion of the dominant channel, initiated a braided stream pattern. Complex braid bars composed of several annealed lobe remnants are gradually exposed as waning discharge becomes confined to adjacent chutes. The newly aggraded fan margin is mainly composed of massive to crudely stratified imbricated gravel with interstratified, discontinuous, centimetre-thick finer grained layers. High-angle cross-stratification was not observed. Since chutes and lobes dominate sediment transport and deposition in streams at the distal margin of this rapidly aggrading fan, it is likely that similar deposits should be present in many ancient alluvial fan sequences, but as yet have gone largely unrecognized.

Bioclastic sediment lobes on a supply dominated Upper Cretaceous carbonate platform margin, Montagna della Maiella, Italy

A thick bioclastic sediment wedge was deposited on the slope of the Maiella carbonate platform margin in the Late Campanian to Late Maastrichtian. The wedge consists of lobate depositional units (laterally and vertically convex structures). The complex internal geometries of the lobes combine characteristics of unidirectional sandwaves and the alternating point‐sources of deltas. Excellent outcrop permits a detailed documentation and discussion of the depositional processes.The sediment wedge constitutes a supersequence, which prograded along the platform margin as a result of high sediment supply and forced regression. Within the supersequence, a hierarchy of higher‐order stratigraphic units (sequence sets and sequences) are developed. The individual bioclastic sediment lobes are interpreted as systems tracts and parasequences of the sequences within the sequence sets.

Sediment dispersal patterns in a deep marine back-arc basin: evidence from heavy mineral provenance studies

Abstract Heavy mineral provenance studies can aid reservoir description in deep marine depositional systems by providing a clearer understanding of the lateral extent of discrete sediment bodies. In this study the lower Gustav Group (Lagrelius Point, Kotick Point and Whisky Bay formations) of James Ross Island, Antarctica, which was deposited in a deep marine slope-apron-submarine fan complex has been investigated. The Kotick Point and Whisky Bay formations crop out in two separate areas on the west coast of James Ross Island. The provenance of the group has been studied by both standard sandstone petrography and electron microprobe analysis of the heavy minerals’ garnet, pyroxene and Fe-Ti oxides. The sandstones are lithic to arkosic arenites derived from the adjacent Antarctic Peninsula magmatic arc. Although the general source area can be recognized, individual stratigraphical or depositional units cannot be distinguished on the basis of sandstone petrography. However, electron microprobe analyses of the detrital heavy minerals allows a more detailed discrimination of sediment provenance. In particular, analyses of the detrital Fe-Ti oxide minerals show that in the Whisky Bay Formation, the southern outcrop area, contains ilmenite, whilst the northern outcrop area contains titanomagnetite. In conjunction with the available biostratigraphical and palaeocurrent evidence, this can be interpreted in two ways. Firstly, if the two outcrop areas are direct age equivalents then they were derived from separate point sources, and deposited as discrete sediment lobes. Alternatively, if they are not directly time-equivalent, the depositional system may have switched laterally with time, with sediment derived from source rocks of varying composition. Either interpretation has implications for the understanding of the deep marine depositional system. Detrital opaque minerals are commonly source specific and should not be ignored during provenance studies.

High resolution stratigraphy and facies differentiation of the shallow marine Annot Sandstones, south‐east France

ABSTRACTHigh resolution stratigraphical analysis divides a rock succession into the basic genetic units of stratigraphy which are here termed small scale stratigraphical cycles. Each cycle records the sedimentological response to an episode of shallowing and deepening. Assuming that these changes in water depth reflect changes in the shoreline position, they can be considered as regressive/transgressive episodes. Each cycle comprises a regressive and transgressive facies tract which will be variably proportioned; in some examples a facies tract may only be represented by a hiatal surface of no deposition, erosion and/or bypass. In the Annot Sandstones of south‐east France, variations in facies types, proportions and associations can be demonstrated both laterally and vertically through the succession. First, it is demonstrated that facies variations occur within regressive or transgressive facies tracts as a function of the stratigraphical stacking pattern of the cycles (i.e. landward, vertical or seaward stacked); this is termed ‘vertical facies differentiation’. Second, the proportions of facies tracts and their constituent facies types within an individual cycle vary between more landward and more seaward palaeogeographical locations; this is termed ‘lateral facies differentiation'.The upper Eocene/lower Oligocene Annot sandstones outcrop in the Maritime Alps of south‐east France, within the thin skinned outer fold and thrust belt of the Alpine arc. The sandstones are well exposed in the area of the Col de la Cayolle on the north‐west margin of the Argentera Massif, where lithostratigraphical correlations are possible over 3·5 km in a NNW/SSE direction, perpendicular to the edge of the depositional basin. Traditionally, these outcrops have been interpreted as deep marine turbidite lobe sediments; this study reflects a significant reinterpretation of this succession as having been deposited in a shallow marine environment.Seven sedimentary sections were measured through the succession, which is divided into 10 small scale stratigraphical cycles. These cycles are described in terms of eight facies which are separated into their transgressive or regressive facies tracts. In eight of the 10 cycles, the regressive facies tracts reflect the progradation of storm influenced braid deltas over shelf muds and silts. In two of the 10 cycles, the regressive facies tracts reflect barrier inlet and wash‐over sands interfingering with back barrier deposits. These latter two cycles are located within landward stepping cycle sets; this is an example of vertical facies differentiation. Transgressive facies tracts locally reworked the upper surface of the regressive facies tract and also comprise barrier and back barrier deposits.The facies succession within each cycle varies according to its position with respect to the palaeoshoreline. The more landward portion of an individual cycle comprises a deltaic shoaling upward succession, culminating in coarse distributary channel conglomerates, overlain by a transgressive barrier/inlet system with extensive back barrier deposits. Beyond the delta front, the more seaward equivalent of individual cycles comprises an erosive base, with aggradational massive pebbly sandstones sitting directly upon offshore heterolithics; these sandstones are interpreted as hyperconcentrated fluvial efflux into the nearshore environment. This grades upward into offshore heterolithics and graded storm deposits representing the products of ravinement, which are then overlain by shelf mudstones. In summary, the more landward portions of cycles preserve predominantly regressive facies tracts, whereas the more seaward portions preserve aggradational to retrogradational strata of the transgressive facies tract; this is an example of lateral facies differentiation.

Mechanisms of anabranch avulsion within gravel-bed braided rivers: observations from a scaled physical model

Abstract The physical modelling of gravel-bed braided rivers, using a 1:20 Froude scale model, permits analysis of the types and relative occurrence of different avulsion mechanisms. Anabranch avulsion within braided rivers involves three main mechanisms: choking avulsion caused by blockage of one channel by a sediment lobe, constriction avulsion produced by deflection, confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends. Each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers. Three factors are found to control anabranch avulsions, namely flow discharge, sediment flux and floodplain topography. These are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium.

A meso-scale field investigation of channel change and floodplain characteristics in an upland braided gravel-bed river, New Zealand

Abstract Detailed data from active gravel-bed braided rivers over time periods greater than a decade is relatively sparse. The aim of this study is to establish a field site for examining channel change over an extended period (&gt; 10 years) and determining the main mechanisms which produce the braidplain morphology. The prototype is the Ashley River, Lees Valley, New Zealand, which is 2.5 km in length, has an average braidplain width of 400 m and slope of 0.012 m m −1 . The flow regime is dominated by low flows (average flow 3.15 m 3 s −1 ) with 2–3 winter flood events in excess of 25 m 3 s −1 per year. Planform information on this reach dates back to the 1860s with ten sets of maps/air photographs up to the present. Since 1989 the sedimentary and morphological characteristics of the braidplain have been carefully surveyed. There is a downstream decrease in sediment size which is accompanied by a reduction in bank height at the margin of the floodplain and a decrease in cross-section bed amplitude. Certain elements in the floodplain (vegetated bars, channels with coarse lag sediments and banks with armoured bank toes) persist while other features such as unconsolidated gravel banks and fine-gravel sheets do not. Channels are migrating towards the northern edge of the braidplain with old bars becoming vegetated on the southern margin. Channel patterns change most dramatically when the flood flows reoccupy old channel segments. Observations of channel change from flood to flood indicate that, although some channel segments switch dramatically, other segments are left intact and largely unaltered. Movement of small sediment lobes affected the flow at node-points within the channel (e.g. areas of flow divergence and convergence) causes avulsion. The dominant mechanisms of channel change are avulsion and bank notching/scalloping.

Submarine-fan development in the southern Chile Trench: A dynamic interplay of tectonics and sedimentation

Submarine fans are well developed in the southern Chile Trench, from 33°S to 41°S latitude. SeaMARC-II side-scan sonar and seismic reflection records image steep erosional escarpments, as much as 400 m in relief, extending seaward across the trench basin from the mouths of submarine canyons. The scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough. Fanlobes are depositional and constructional up-gradient (south) from the canyon mouths. They are composed of aggraded channel/levee complexes, smooth and conformable sediment drapes, and crescentic levees rimming the headwalls of erosional scarps. Fanlobes are carved and dissected by erosional processes down-gradient (north) from the canyon mouths. They exhibit amalgamated lag pavements, composite sediment lobes, longitudinal furrows, braided channels, and canyon-mouth bars. Thick, massive-to-laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes, whereas fine-grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes. The texture and composition of the sediment supply, the onshore climate, and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans. Stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled, high-amplitude seismic facies: reflective lenticular bodies, truncation and scour surfaces, planar-amalgamation, and sigmoidaccretion structures. The severe erosional dissection of down-gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin. Extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum. The trench fans may have been deposited on progressively steeper trench gradients, because the buoyant Chile Rise migrates northward as it subducts beneath the Andean continental margin. Fan distributary channels, axial channels, slump scars, and erosional gullies are largely localized along structural features. Normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high-velocity mainstreams of turbidity currents. Orthogonal fault sets within the deposits of the trench basin, which parallel the spreading and transform structures of the extinct Pacific-Farallon Rise and the Chile Rise, are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin. Uplifted thrust ridges, generally restricted to a narrow zone along the base of the deformation front, are dissected by distributary channels, and channel courses are locally deflected seaward of these propagating structures. Transform-oriented basement ridges, associated with strike displacements of the axial channel and vertical faults in the trench basin, may accommodate renewed strike-slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin.

Heterogeneity of Holocene Clinothems: Significance for the Exploitation of Ancient Slope Deposits: ABSTRACT

Inclined margin deposits (clinothems) of ancient carbonate platforms and reef complexes are known to be important hydrocarbon reservoirs and may function as conduits for the migration of oil from basin to reef. Recent work in the Tongue of the Ocean (TOTO), Bahamas, provides new insight into the depositional anatomy of Holocene clinothems. Observations from 94 dives in a research submersible in the TOTO indicates that modern carbonate slopes of 30-40{degree} are forming by the synsedimentary amalgamation of linear trains of sediment and talus blocks derived from the margin of the platform. Sand-sized sediment is transported downslope by various gravity induced mechanisms and deposited in topographic lows between linear ridges where it is rapidly lithified by submarine cementation. These sediment lobes are decimeters thick, and extend across and down the slope for a few meters and tens of meters, respectively. Repeated episodes of deposition and cementation result in the amalgamation of individual sediment packages that are linear in shape and discontinuous in both lateral and vertical dimensions along the slope. Progressive slope accretion of this nature will result in a heterogeneous texture and composition, further complicated by a complex internal facies geometry. Awareness of the internal heterogeneity of modern slope depositsmore » may be a critical factor when evaluating the potential of ancient slope deposits to act as conduits for the migration of hydrocarbons or to be economically viable reservoirs.« less

The mangrove-forming palm Nypa from the early Paleogene of southern Morocco. Paleoenvironment and paleoclimate

The first fruit of Nypa burtini (BRONGN.) ETTINGSHAUSEN from northwestern Africa and associated silicified woods of predominantly tropical-subtropical families are described from marginal marine Paleocene-Eocene (Thanetian-Ypresian) boundary beds south of the central High Atlas (Morocco). The fossils are allochthonous drift-wood on an E-W trending beach, or prelittoral sand spit. Nypa has been derived from a nearby mangrove biotope. It was situated in an estuary, or close to a sediment lobe in front of the river mouth. The associated woods had been transported down that river from a tropical forest in the hinterland . These data and a réévaluation of published paleontological and sedimentographic data (pollen, vertebrates, clay minerals, evaporites) strongly indicate a tropical humid climate at the southern rim of the central High Atlas during the late Paleocene and early Eocene. Most probably an and season existed. A northeastern trade wind system is inferred.

Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California

The south-central Monterey Bay shelf is a high-energy, wave-dominated, tectonically active coastal region on the central California continental margin. A prominent feature of this shelf is a sediment lobe off the mouth of the Salinas River that has surface expression.High-resolution seismic-reflection profiles reveal that an angular unconformity (Quaternary?) underlies the entire shelf and separates undeformed strata above it from deformed strata below it. The Salinas River lobe is a convex bulge on the shelf covering an area of approximately 72 km2 in water depths from 10 to 90 m. It reaches a maximum thickness of 35 m about 2.5 km seaward of the river mouth and thins in all directions away from this point. Adjacent shelf areas are characterized by only a thin (2 to 5 m thick) and uniform veneer of sediment. Acoustic stratigraphy of the lobe is complex and is characterized by at least three unconformity-bounded depositional sequences. Acoustically, these sequences are relatively well bedded. Acoustic foresets occur within the intermediate sequence and dip seaward at 0.7° to 2.0°.Comparison with sedimentary sequences in uplifted onshore Pleistocene marine-terrace deposits of the Monterey Bay area, which were presumably formed in a similar setting under similar processes, suggests that a general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequences are interpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposits formed during interglacial highstands and/or during early stages of falling sea level. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities that separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions.The internal stratigraphy of the Salinas River lobe thus suggests that at least several late Quaternary regressions and transgressions may be recorded under the present shelf. This record may represent the last major eustatic cycle of sea level, an interval not observed in uplifted onshore Pleistocene marine terraces.

Dynamic Coupling of Sedimentation and Convergence Tectonics in Peru-Chile Trench and Outer Andean Margin: ABSTRACT

The convergence rate and sediment supply to the trench control the evolution of trench deposits as well as the subduction processes of accretion and erosion along the adjacent margin. South of 41/degrees/S latitude, where Pleistocene cordilleran glaciation was severe, turbidity current deposition and unchannelized, producing sheeted basin deposits. Between 41/degrees/S and 33/degrees/S, trench fans (/approximately/ 20 km wide) exhibit both depositional and erosional morphologies in response to dynamic tectonism within a prevailing axial gradient. An outboard axial channel transports massive quantities of remobilized sediments to the north. Subsurface lenticular bodies seen on seismic reflection profiles represent buried channel deposits. SeaMARC-II records show complex dispersal patterns and erosional features on the fans, extensional structures on the descending plate, and anastomosed accreted ridges on the inner trench wall. Lithofacies include channel (amalgamated laminated to massive sand), levee (rhythmic thin-bedded graded sand and silt), and basin (low-energy graded and laminated silt) deposits. At 33/degrees/S, San Antonio canyon feeds an axial sediment lobe at the base of a 1400-m vertical discontinuity in the subducting slab. The canyon captures littoral sands and represents the last major source of sediment supply. North of 33/degrees/S, the trench consists of small basins ponded within block-faulted depressions on themore » oceanic plate. Lithofacies include contourite (winnowed silt and sand laminae) and basin deposits. Large offsets in the descending plate, a steep inner trench wall and the lack of slope basins indicate the northern Chile margin is undergoing subduction erosion.« less

Lake Tapps tephra: An early Pleistocene stratigraphic marker in the Puget Lowland, Washington

The rhyolitic Lake Tapps tephra was deposited about 1.0 myr ago, shortly after culmination of the early phase of the Salmon Springs Glaciation in the Puget Lowland. It is contained within sediments that were deposited in ponds or lakes in front of the reteating glacier. An herb-dominated tundra existed in the southern Puget Lowland at that time. Lake Tapps tephra is most likely the product of an eruption that in part was phreatomagmatic. It forms an early Pleistocene stratigraphic marker across the southern sector of the Puget Lowland and provides a link between Puget lobe sediments of the Cordilleran Ice Sheet and sediments deposited by Olympic alpine glaciers.

Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy

The depositional bodies of the Chile Trench (trench fans, the axial channel, sheeted basins, ponded basins, and axial sediment lobes) control the spatial distribution of modem lithofacies in the basin. Sheeted basins (south of 41°S) are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope. Trench fans (41°S−33°S) are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The axial channel follows the northward gravitational gradient, draining the distributary networks of trench fans into the longitudinal transport system. Down-gradient (northern) fan lobes are severely dissected by erosional processes; evidently, periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments. Channelized basins in the canyon-mouth areas yield to sheeted basins (depositional surface maintains an axial gradient) or ponded basins (depositional surface is strictly flat) in inter-canyon areas. Tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization, or reverse the gradient and induce sediment ponding. A large, margin-parallel sediment lobe is built at the base of a high axial escarpment near 33°S, where the axial channel crosses a transverse discontinuity at the convergent plate boundary. Five lithofacies are defined by Q-mode factor analysis of sediment textures and hydro-dynamic structures in 27 cores from the Chile Trench. The Channel facies (thick, amalgamated sand, massive to laminated or cross-bedded) is deposited by high-energy processes within the coarse-grained bedload of turbidity currents; it forms in distributary and axial channels. The Levee facies (rhythmically bedded, internally structureless, graded sand and graded silt) is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover; it forms on channel flanks, although constructional levees are not always present. The Basin-1 facies (more complete Bouma sequences, both upper and lower flow regime structures) forms in ponded basins where flows are confined by a high-relief, seaward trench wall. The Basin-2 facies (graded and laminated silt, lower flow regime structures) forms in low-energy environments, such as interchannel areas, distal basins, trench walls, and elevated topographic features. The Contourite facies (silt and sand laminations winnowed from hemipelagic muds and distal turbidites) is best developed in sediment-starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls. The trench wedge records a coarsening-upward sequence as the oceanic plate migrates toward and into the trench during plate convergence, and becomes more proximal to sediment sources along the base of the continental margin. Near canyon mouths, prograding trench fans drive the axial channel seaward into the trench wedge, and the coarsening-upward sequence is truncated by a time-transgressive erosional unconformity. Abandoned axial channel deposits are carried landward beneath prograding fans to record a fining-upward sequence above the basal unconformity. Channel migration and lobe aggradation may produce fining- and coarsening-upward sequences on depositional fan lobes, but sequences on the erosional lobes are fragmented by numerous truncation surfaces.

Eocene Sediment Dispersal Pattern Records Asymmetry of Laramide Green River Basin, Southwestern Wyoming: ABSTRACT

Provenance and paleocurrent data from synorogenic fluvial sandstones can be used to constrain theories about the timing and structural style of Laramide foreland uplifts and associated basins. The Green River basin of southwestern Wyoming is a large ellipsoidal basin bounded by uplifts with diverse orientations and basement rock compositions. Sandstone from the main body of the Eocene Wasatch Formation in the Green River basin was sampled along the south and west flanks of the Rock Springs uplift. Petrographic examination and paleocurrent measurements reveal two main facies. The first facies is rich in feldspar and metamorphic rock fragments derived from the Wind River Mountains to the north. The second facies is dominated by quartz and sedimentary rock fragments, reflecting a source in the Uinta Mountains to the south. Distribution of these facies indicates that a sediment lobe extends 15 km into the basin from the Uinta Mountains. Another sediment lobe originates from the Wind River Mountains and extends approximately 100 km south into the basin. This pattern suggests that the topographic axis of the depositional basin was an east-west-trending trough about 15 km north of the Uinta Mountains. The position of the basin axis is inferred to reflect greater subsidence nearmore » the Uinta Mountains. The asymmetric distribution of the two facies of the Wasatch Formation thus supports the model of basin subsidence caused by thrust-loading and indicates the Uinta Mountains were the dominant active thrust block bounding the Green River basin during the early Eocene.« less

Distribution of Petroleum Reservoirs Relative to Allocycles and Autocycles, Upper Portion of the Cherokee Group (Middle Pennsylvanian, Desmoinesian), Mid-Continent Region, U.S.A.: ABSTRACT

Sequences of mud rocks, lenticular sandstones, coals, and thin carbonates form autocycles and allocycles in the upper portion of the Cherokee Group. Autocycles delineated in eastern Kansas and northern Oklahoma are relatively local in extent, while allocycles are traceable over the entire region. All autocycles delineated in this study are embedded within the regressive portions of allocycles. Petroleum-bearing sandstones consist of shoestring-shaped and thin sheetlike units in thicker sedimentary lobes. These lobes were deposited as deltaic complexes, which included fluvial and distributary channel sands, interdistributary muds, crevasse splay sands and muds, flood-basin muds, delta-front sands, and predeltaic muds. Delta lobes prograded across the margins of the Middle Pennsylvanian epeiric sea during times of eustatic stillstand or regression. When lobes were abandoned, waves and currents winnowed their upper portions, leaving thin sheetlike lenses of sand. These reworked sands along with marine muds above regressive deltaic sequences form the transgressive parts of autocyclothems. The transgressive parts of allocyclothems, generally consisting of marine shale, resulted from sea level rises that rapidly shifted shorelines far northeastward, moving siliciclastic sources away from the study area. The positions of reservoir-containing deltaic complexes were determined by strandline positions at various sea levels. Extent of eustatic sea levelmore » changes appears to have been the major mechanism that controlled the distribution of petroleum reservoir and source units. In addition, sea level changes probably were a significant factor in the nature of diagenetic alterations that affected reservoir properties.« less

Response of the mid shoreface of the southern mid-Atlantic Bight to a “northeaster”

An instrumented, bottom-mounted, tripod deployed off Duck, North Carolina, at a depth of 8 m provided time Series of pressure, benthic currents, suspended Sediment concentrations and bed-level changes prior to and during a typical northeast storm. A Strong jet-like southerly-setting current generated by the northeaster was accompanied by downwelling and Strong bottom agitation by wind waves. A total bed-level change of over 15 cm was recorded. After a phase of bed erosion, the bed accreted rapidly. Side-scan imagery supports the inference that offshore or alongshore migration of quasi-discrete sediment lobes may have produced the observed pulse-like accretion.

Offshore Transport and Sand-Body Formation: Evidence from a Steep, High-energy Shoreface, Southeastern Australia

ABSTRACT Elongate, shore-parallel sand bodies lie on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia. These sand bodies are 10 to 30 m thick, extend almost continuously for 40 km parallel to the coast, and display a pronounced convexity on the sea floor in profile. Sediment sampling and detailed seismic reflection and side-scan sonar profiling provide evidence that the upper part of these large lobate sand bodies consists chiefly of sand transported downslope from the upper shoreface and surf zone. The sand bodies consist entirely of marine sand probably derived locally from retreating embayed beaches and in part from sandstone cliffs. Channels and sediment lobes oriented normal to the shoreline and internal structure indicate growth by seaward progradation, and surface morphology indicates coalescing of lobes and damming of bed-load sediment against bed-rock ridges in water depths of 70 to 80 m. The precise mechanism, or mechanisms, of seaward transport on the shoreface is unresolved. We surmise that during storm events, disturbance by large waves and seaward transport by downwelling bottom currents result in sand movement onto and across the sand-body surface at depths of 40 to 80 m. Concurrent modification of the sand bodies by shore-parallel processes is suggested by textural trends, bed forms, and the overall alongshore continuity of the sand bodies.