Canyon Fan Research Articles

Quantifying the relative contributions of Miocene rivers to the deep Gulf of Mexico using detrital zircon geochronology: Implications for the evolution of Gulf Basin circulation and regional drainage

AbstractSediment routing from hinterland to the deep sea is complicated because it involves evolution of river drainage from source areas to coastal plains and sediment mixing on the shelf and slope by marine currents. Previous regional paleogeographic mapping in the Gulf of Mexico (GOM) has observed a >150 km offset between the middle Miocene paleo‐Tennessee fluvial axis and the associated deep‐sea fan depositional axis, indicating a complicated sediment pathway. We integrate new and published detrital zircon (DZ) U‐Pb age data from fluvial, shelf and deep‐sea deposits to examine the complex Miocene sediment routing system in the northern GOM. These data suggest an increase in sediment load derived from western North America (increased Western Cordillera terranes; <300 Ma zircon age component) from the early to middle Miocene in the deep‐water Green Canyon protraction area. The early Miocene Green Canyon area received sediments mainly from fluvial axes located directly updip: the paleo‐Mississippi River (44%–56%; characterized by Yavapai‐Mazatzal, Mid‐Continent and Western Cordillera sourced 1800–1600 Ma, 1500–1300 Ma and <300 Ma, respectively, and Grenville‐Appalachian sourced 1300–950 Ma and 500–300 Ma age components) and smaller rivers and tributaries draining the Appalachian Mountains (e.g. paleo‐Tennessee River, 18%–43%; mainly Grenville‐Appalachian sourced 1300–9500 Ma and 500–300 Ma age components). In contrast, the middle Miocene Green Canyon deep‐sea fan shows a strong DZ signal from the paleo‐Red River (38%; increased <300 Ma zircon age component), which requires input of additional sediment sources from west of the paleo‐Mississippi system. In addition, the paleo‐Tennessee River, which was a major middle‐Miocene sediment source for the central‐eastern GOM due to uplift and increased erosion of the Appalachian Mountains, is underrepresented (34%; decreased 1300–950 Ma zircon age component) in the middle Miocene Green Canyon fan. We suggest that two mechanisms combined to produce the increased middle Miocene input from western sediment sources and restriction of locally up‐dip Tennessee River sources: (1) regional drainage changes involving middle Miocene capture of the paleo‐Red River and its tributaries by the paleo‐Mississippi River, which at the same time lost some of its eastern tributaries owing to expansion of the paleo‐Tennessee and (2) eastward (clockwise) marine transport of western‐sourced sediment along the shelf or slope, which deflected the paleo‐Tennessee signal >150 km eastward to feed the deep‐sea fan further east, perhaps reflecting intensification of a precursor to the GOM Loop Current.

Morphological controls on delta‐canyon‐fan systems: Insights from stratigraphic forward models

AbstractMorphology is one of the principal driving forces governing the sedimentology and evolution of delta‐canyon‐fan systems. However, quantified and systematic studies of morphological controls are still very limited. This study applied hydraulic‐based stratigraphic forward modelling to investigate the impacts of morphological parameters on sediment budget partitioning and channel network of delta‐canyon‐fan systems on passive margins. A total of six sets of stratigraphic forward models are built using 47 initial bathymetries with six varying morphological parameters: shelf gradient; shelf width; slope gradient; canyon sinuosity; canyon depth; and basin gradient. The quantified relationships between morphological parameters and sedimentological parameters (dimensions, deposition/erosion volume/area, number of tributaries and slope channels) are investigated. The causes behind the relationships are explored by analyzing the response of qualified sedimentary features to morphological controls, such as grain‐size distribution, channel migration and sequence stratigraphic frame. The results suggest that low shelf gradient, low shelf width, high canyon gradient and low canyon sinuosity are beneficial for sediment budget partitioning into deep basins and turbidity currents to flow inside canyons. Low canyon depth also promotes sediment delivery but results in more channels on canyon flanks. For the delta, channel lateral migration increases then decreases with increasing shelf gradient due to the gradient threshold in determining channel sinuosity; delta size increases then stops increasing with increasing shelf width due to the shelf width threshold in determining the balance between accommodation and sediment supply. For the canyon, low canyon gradient, high canyon sinuosity and low canyon depth have similar effects on canyons, all resulting in significant channel translation and canyon widening but less canyon head retrogradation. This study improves the knowledge of controlling factors and sediment transport regime of a delta–canyon–fan system. Moreover, the observed relationships could provide semi‐quantitative guidelines to predict the dimensions of delta–canyon–fan systems and the distribution of hydrocarbon reservoirs.

Recognizing Allogenic Controls on the Stratigraphic Architecture of Ancient Alluvial Fans in the Western US

Alluvial fans are a significant part of the sediment routing system, forming distinctive steep, fan-shaped deposits of coarse-grained detritus where rivers lose flow velocity after exiting confined mountain drainages. Processes on the fan are influenced by both internal (autogenic) feedback cycles like channel avulsion and by external (allogenic) conditions such as climate and tectonics. These conditions in turn influence the stratigraphic architecture (i.e., the pattern of channel stacking and sizes) within the fan. Studying stratigraphic architecture of alluvial fans can, therefore, provide insight into controls on fan deposition. We employ UAV-based photogrammetric models to analyze the stratigraphic architecture of two well-exposed ancient alluvial fans in the western US – the Eocene Richards Mountain Conglomerate and the Cretaceous Echo Canyon Conglomerate. Both fans were deposited under relatively warm, wet climates and compressional tectonic regimes. We use a seven-fold hierarchy of bounding surfaces and associated lithosomes to describe alluvial fan architecture. First- through fourth-order surfaces and lithosomes represent bedform to channel-scale features influenced primarily by autogenic processes on the fan. Controls on fifth-order surfaces/lithosomes have historically been poorly understood, but probably represent fanhead trench migration and lobe construction. Sixth-order surfaces bound individual alluvial fans and seventh-order surfaces correspond to formation boundaries. These are controlled primarily by tectonics. The fifth-order architectural style of the deposits in our two study areas is significantly different and we use this difference to try to isolate a primary control on fifth-order alluvial architecture. Average width:height ratios of fifth-order lithosomes are nearly twice as high for Echo Canyon (112:1) than for Richards Mountain (64:1). This indicates that active channels on the Echo Canyon fan were more mobile than those on the Richards Mountain fan. We attribute this to a more seasonal climate and less vegetation during the deposition of the Echo Canyon Conglomerate. This would have increased lateral migration by destabilizing channels through increased sediment flux and flood events. Our results imply that fifth-order stratigraphic architecture of ancient alluvial fans may provide insight into allogenic processes related to paleoclimate. They also indicate risk of increased geologic hazards on alluvial fans where anthropogenic climate change increases future climate variability.

Rotating laser scan method to measure the transient free-surface topography of small-scale debris flows

We present an imaging method aimed at measuring the transient three-dimensional topography of rapidly deforming opaque surfaces. The imaging setup features a laser stripe scanned repeatedly across the surface using a spinning wheel with mirror facets. Acquisition rates of 20 scans per second are attained by spinning the wheel rapidly and imaging the resulting stripes using a fast camera. The algorithms adopted to capture the stripes, reconstruct the surface topography for each scan, and calibrate the camera and laser configuration are described. Error estimates for successive steps are evaluated and compared to results from repeat and independent measurements of the final deposit shape. The method is then used to acquire time-resolved measurements of small-scale debris flows. Specifically, we measure the free-surface topography of debris surges as they flow across an idealized canyon–fan transition. In addition to profiling the final deposit shape, the method successfully resolves the transient evolution of the free surface. Applied to textured surfaces, the method can also be combined with particle tracking velocimetry.

Millennial-scale varnish microlamination dating of late Pleistocene geomorphic features in the drylands of western USA

Varnish microlamination (VML) dating is a climate-based correlative age determination technique used to correlate and date various geomorphic features in deserts. In this study, we establish a generalized late Pleistocene (18–74ka) millennial-scale microlamination sequence in fine-grained, fast-accumulating rock varnish for the drylands of western USA, radiometrically calibrate the sequence and correlate it with the δ18O record in the GISP2 Greenland ice core. We then use this climate-correlated varnish microstratigraphy to estimate surface exposure ages for radiometrically dated late Pleistocene geomorphic features in the study region. The VML dating of debris flow deposits on the Sehoo recessional shorelines of Lake Lahontan at the Jessup embayment of central Nevada yields a minimum-limiting age of 14.95–15.95ka, in good agreement with a calibrated 14C age of 15.22±0.12ka for the timing of the lake recession. The VML dating of a giant ejecta block on the rim of Meteor Crater in northern Arizona yields a minimum-limiting age of 49.15ka, closely matching a thermoluminescence (TL) age of 49±3ka and slightly younger than a recently updated cosmogenic 36Cl age of 56.0±2.4ka for the meteor impact event. The VML dating of distal Q2c fan surfaces on Hanaupah Canyon alluvial fan in Death Valley, California, yields a minimum-limiting age of 73.55ka, in accord with cosmogenic 36Cl depth-profile ages of 66+22/−14ka and 72+24/−20ka for the same fan deposits. The close agreement between the VML age estimates and the independently derived radiometric ages for these geomorphic features attests to the validity and reliability of millennial-scale VML dating.To further assess its potential in desert geomorphological research, we use the VML method to study alluvial-fan responses to millennial-scale climatic changes. The VML dating of a small tributary fan in Death Valley reveals two episodes of fan aggradation, one ceasing at 73.55–86.75ka during the dry period of the last interglacial (MIS 5a) and the other finishing at 66.15ka during the wet period of the last glacial (MIS 4). The VML and 36Cl dating of the distal Q2c fan surfaces on Hanaupah Canyon fan reveal two episodes of large-scale fan aggradation ended at 72+24/−20ka and 73.55ka during the wet period of MIS 4. Fanhead incision and associated within-channel or fantoe aggradation are found to take place during the relatively dry period of the glacial-to-interglacial climatic transition (12–24ka) and the Holocene interglacial dry period (0–12ka). These data indicate that, on the millennial to sub-Milankovitch timescale (~103–104years), fan aggradation is a discrete sedimentational process under various climatic conditions. Because fan aggradation is ultimately controlled by the intensity and frequency of precipitation events — which in turn are modulated by major climatic oscillations such as Heinrich events, Dansgaard/Oeschger (DO) events, and glacial/interglacial shifts — these major climatic changes could be the pacemaker of regionally contemporaneous large-area fan segmentation.

Well-balanced two-dimensional coupled modelling of submarine turbidity currents

Submarine turbidity currents may wreak havoc on deep sea facilities and also affect the formation of many morphological features, which are closely linked to hydrocarbon reserves. Yet most previous understandings are based on the analyses of geological and geophysical data or decoupled modelling. Here a two-dimensional coupled mathematical model for turbidity currents over erodible beds is developed. The governing equations are built upon the complete layer-averaged mass and momentum conservation laws. The well-balanced slope limiter centred scheme is deployed, which facilitates a reasonable balance between the flux gradients and the bed source terms. This renders the model applicable to both regular and irregular topographies. The coupled model is verified against both one- and two-dimensional experimental turbidity currents. When applied to turbidity currents near a submarine canyon–-fan transition and over submarine fans, coupled modelling is found to be critical, which incurs negligible increase in the computational cost. Computational case studies demonstrate that bed slope, sediment particle size and upstream boundary condition are key factors dictating the formation of submarine channel–levee morphology. Appropriate bed slope and sediment particle size as well as larger inlet Richardson number may favour formation of submarine channel–levee morphology.

Holocene Earthquakes and Late Pleistocene Slip-Rate Estimates on the Wassuk Range Fault Zone, Nevada

The Wassuk Range fault zone is an 80‐km‐long, east‐dipping, high‐angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0.7±0.1 mm/yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1 m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3‐m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range‐front fault has truncated and uplifted a remnant of the Penrod Canyon fan by >40 m since the surface was deposited ∼113 ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at >0.3–0.4 mm/yr for the Wassuk Range fault zone. Online Material: Tables summarizing radiocarbon and cosmogenic analyses, photos of rocks sampled for cosmogenic analyses, and figures summarizing cosmogenic exposure age estimates.

Timing of occurrence of large submarine landslides on the Atlantic Ocean margin

Submarine landslides are distributed unevenly both in space and time. Spatially, they occur most commonly in fjords, active river deltas, submarine canyon–fan systems, the open continental slope and on the flanks of oceanic volcanic islands. Temporally, they are influenced by the size, location, and sedimentology of migrating depocenters, changes in seafloor pressures and temperatures, variations in seismicity and volcanic activity, and changes in groundwater flow conditions. The dominant factor influencing the timing of submarine landslide occurrence is glaciation. A review of known ages of submarine landslides along the margins of the Atlantic Ocean, augmented by a few ages from other submarine locations shows a relatively even distribution of large landslides with time from the last glacial maximum until about five thousand years after the end of glaciation. During the past 5000 yr, the frequency of occurrence is less by a factor of 1.7 to 3.5 than during or shortly after the last glacial/deglaciation period. Such an association likely exists because of the formation of thick deposits of sediment on the upper continental slope during glacial periods and increased seismicity caused by isostatic readjustment during and following deglaciation. Hydrate dissociation may play a role, as suggested previously in the literature, but the connection is unclear.

Conditions under which a supercritical turbidity current traverses an abrupt transition to vanishing bed slope without a hydraulic jump

Turbidity currents act to sculpt the submarine environment through sediment erosion and deposition. A sufficiently swift turbidity current on a steep slope can be expected to be supercritical in the sense of the bulk Richardson number; a sufficiently tranquil turbidity current on a mild slope can be expected to be subcritical. The transition from supercritical to subcritical flow is accomplished through an internal hydraulic jump. Consider a steady turbidity current flowing from a steep canyon onto a milder fan, and then exiting the fan down another steep canyon. The flow might be expected to undergo a hydraulic jump to subcritical flow near the canyon–fan break, and then accelerate again to critical flow at the fan–canyon break downstream. The problem of locating the hydraulic jump is here termed the ‘jump problem’. Experiments with fine-grained sediment have confirmed the expected behaviour outlined above. Similar experiments with coarse-grained sediment suggest that if the deposition rate is sufficiently high, this ‘jump problem’ may have no solution with the expected behaviour, and in particular no solution with a hydraulic jump. In such cases, the flow either transits the length of the low-slope fan as a supercritical flow and shoots off the fan–canyon break without responding to it, or dissipates as a supercritical flow before exiting the fan. The analysis presented below confirms the existence of a range associated with rapid sediment deposition where no solution to the ‘jump problem’ can be found. The criterion for this range is stated in terms of an order-one dimensionless parameter involving the fall velocity of the sediment. The criterion is tested and confirmed against the experiments mentioned above. A sample field application is presented.

Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

The northern Death Valley fault zone (NDVFZ) has long been recognized as a major right‐lateral strike‐slip fault in the eastern California shear zone (ECSZ). However, its geologic slip rate has been difficult to determine. Using high‐resolution digital topographic imagery and terrestrial cosmogenic nuclide dating, we present the first geochronologically determined slip rate for the NDVFZ. Our study focuses on the Red Wall Canyon alluvial fan, which exposes clean dextral offsets of seven channels. Analysis of airborne laser swath mapping data indicates ∼297 ± 9 m of right‐lateral displacement on the fault system since the late Pleistocene. In situ terrestrial cosmogenic 10Be and 36Cl geochronology was used to date the Red Wall Canyon fan and a second, correlative fan also cut by the fault. Beryllium 10 dates from large cobbles and boulders provide a maximum age of 70 +22/−20 ka for the offset landforms. The minimum age of the alluvial fan deposits based on 36Cl depth profiles is 63 ± 8 ka. Combining the offset measurement with the cosmogenic 10Be date yields a geologic fault slip rate of 4.2 +1.9/−1.1 mm yr−1, whereas the 36Cl data indicate 4.7 +0.9/−0.6 mm yr−1 of slip. Summing these slip rates with known rates on the Owens Valley, Hunter Mountain, and Stateline faults at similar latitudes suggests a total geologic slip rate across the northern ECSZ of ∼8.5 to 10 mm yr−1. This rate is commensurate with the overall geodetic rate and implies that the apparent discrepancy between geologic and geodetic data observed in the Mojave section of the ECSZ does not extend north of the Garlock fault. Although the overall geodetic rates are similar, the best estimates based on geology predict higher strain rates in the eastern part of the ECSZ than to the west, whereas the observed geodetic strain is relatively constant.

The response of turbidity currents to a canyon–fan transition: internal hydraulic jumps and depositional signatures

Turbidity currents often carve canyons into the continental slope, and then deposit submarine fans on lower slopes farther downstream. It has been hypothesized here that this slope decline can cause a turbidity current to (a) undergo an internal hydraulic jump near the canyon–fan transition, and (b) leave a depositional signal of this transition. These hypotheses are studied with a numerical model. Rapidly depositing turbidity currents need not undergo a hydraulic jump at a slope break. When a jump does occur, it can leave a depositional signal in terms of an upstream-facing step. A previous attempt to capture this signal failed because the current was treated as purely depositional. In the present model both sediment deposition and entrainment are included. An upstream-facing step appears when deposition dominates erosion. The step requires entrainment since the deposition rate is continuous through the jump, whereas the sediment entrainment rate is not. Therefore, the step is caused by enhanced net deposition due to reduced entrainment rate across the jump. Under certain circumstances, a single step can be replaced by a train of upstream-migrating cyclic steps, each separated by a hydraulic jump. The numerical model is verified against experiments, and then applied at field scale

Activity and composition of the benthic fauna in the Whittard Canyon and the adjacent continental slope (NE Atlantic)

We compared the sediment and its community on the Celtic continental slope (Goban Spur) with those in a branch of the nearby Whittard Canyon in search for evidence of canyon mediated transport of (labile) organic matter. We studied the megabenthos and macrobenthos biomass and taxonomic composition, measured in situ sediment community oxygen consumption and determined sediment concentrations of particulate organic carbon, phytopigments, and nucleic acids. While the sediment community and activity on the canyon fan was similar to that on the abyssal station near Goban Spur, the sediment within the canyon had relatively enhanced sediment community oxygen consumption rates and higher levels of phytopigments, particulate organic carbon and nucleic acids, particularly towards the canyon head. However, near-bottom sediment traps and transmissometre readings gave no sign of enhanced particle fluxes. Most likely this enrichment is supplied periodically through lateral transport as suggested by increased numbers of filter-feeding macrobenthos. Markedly higher concentrations of sedimentary pigments were found in a second branch of the Whittard Canyon illustrating the complexity of canyon systems and the need for a much more extensive study.

Sedimentary processes and facies of the waterlaid Anvil Spring Canyon alluvial fan, Death Valley, California

The Anvil Spring Canyon fan of the Panamint Range piedmont in central Death Valley was built entirely by water‐flow processes, as revealed by an analysis of widespread 2‐ to 12‐m‐high stratigraphic cuts spanning the 9·7 km radial length of this 2·5–5·0° sloping fan. Two facies deposited from fan sheetfloods dominate the fan from apex to toe. The main one (60–95% of cuts) consists of sandy, granular, fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5–25 cm thick oriented parallel to the fan surface. The other facies (0–25% of cuts) comprises 10‐ to 60‐cm‐thick, wedge‐planar and wedge‐trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3–28°. Both facies are interpreted as resulting from rare, sediment‐charged flash floods from the catchment, and were deposited by supercritical standing waves of expanding sheetfloods on the fan. Standing waves were repeatedly initiated, enlarged, migrated, and then terminated either by gradually rejoining the flood or by more violent breakage and washout. The frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50–250 cm thick across the active depositional lobe of the fan. Erosional intensity during washout of the standing wave determined whether early‐phase backset‐bed deposits or washout‐phase sheetflood couplet deposits were selectively preserved in a given cycle. Two minor facies are also found in the Anvil fan. Pebble–cobble gravel lags (0–20% of cuts) are present above erosional scours into the sheetflood couplet and backset deposits. They consist of coarse gravel concentrated through fine‐fraction winnowing of the host sheetflood facies by sediment‐deficient water flows. This reworking occurred during recessional flood stage or from non‐catastrophic discharge during the long intervals between major flash floods. This facies is common at the surface, giving rise to a ‘braided‐stream’ appearance. However, it is stratigraphically limited, present as thin, continuous to discontinuous beds or lenses that bound 50‐ to 250‐cm‐thick sheetflood sequences. The other minor facies of the Anvil fan consists of clast‐supported and imbricated, thickly stratified, pebbly, cobbly, boulder gravel present in narrow, radially aligned ribbons nested within sheetflood deposits. This facies is interpreted as representing deposition in the incised channel of the fan, a subenvironment characterized by greater flow competence resulting from maintained depth from channel‐wall confinement, and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel.

Sedimentology of the debris‐flow‐dominated Warm Spring Canyon alluvial fan, Death Valley, California

Facies analysis of widely distributed exposures of the 32·6 km2 and 8·1‐km‐long Warm Spring Canyon fan, central Death Valley, shows that it has been built principally by debris‐flow deposits. These deposits were derived from a mature Panamint Range catchment mostly underlain by Precambrian mudrock, quartzite and dolomite. Stacked, clast‐rich and matrix‐supported debris‐flow lobes of slightly bouldery, muddy, pebble–cobble gravel in beds 20–150 cm thick dominate the fan from apex to toe, accounting for 75–98% of most exposures. Interstratified with the debris flows are less abundant (2–25% of cuts), thinner (5–30 cm) and more discontinuous beds of clast‐supported and imbricated, pebble–cobble gravel deposited by overland flows and gully flows. This facies formed by the surficial fine‐fraction water winnowing of the debris flows primarily during recessional flood stage of the debris‐flow events. Two other facies associations make up a small part of the fan. The incised‐channel tract consists of a 250‐m‐wide clast‐supported ribbon of irregularly to thickly bedded, boulder, pebble, cobble gravel nested within debris‐flow deposits. This channel fill is oriented generally perpendicular to the Panamint range front. It formed by extensive erosion and winnowing of debris flows deposited within the incised channel, into which all water discharge from the catchment is funnelled. The limited presence of this facies only straddling the present incised channel indicates that this channel overall has maintained a consistent position on the fan except for slight lateral shifts, some caused by strike‐slip offset. Fault offset temporarily closed the upper incised channel, causing recessional debris‐flow mud to be ponded behind the dam. The other local facies assemblage consists of subrounded to rounded, moderately sorted pebble gravel in low‐angle cross‐beds that slope both basinwards and fanwards. This gravel was deposited in beachface, backshore and shoreface barrier‐spit environments that developed where Lake Manly impinged on the Warm Spring fan during late Pleistocene time. These deposits straddle headcuts into, and were derived from, erosion of the debris‐flow deposits. Wave energy sorted finer sediment from the shore zone, concentrated coarser sediment and rounded the coarse to very coarse pebble fraction by selective reworking.

Contemporary channel-levee systems in active borderland basin plains, California Continental Borderland

Long-range large-scale side-scan (GLORIA) information, other seismic reflection profiling studies, and data from cores in the California Continental Borderland, have defined active levee-channel systems extending basinward from the lower fan of Hueneme-Mugu Submarine Fan, Redondo Submarine Fan, and Santa Cruz Canyon Fan in Santa Monica, San Pedro and Santa Cruz Basins, respectively. The Holocene distributaries have been created by a series of turbidity current events. These distributaries range in length from 10–25 km, and are wide (2–5 km) and low-relief (1–10 m) in their distal parts. They are also active conduits for nepheloid flows. Distributions of sedimentological parameters typically mimic the pattern of levee-channel systems. Organic carbon and biogenic carbonate content roughly outline the systems. Channels are incised in the upper to middle fan areas, and become constructional leveed channels in the lower fan and basin plain as the channel gradient adjusts to maintain a graded profile. Thus sediment gravity flows are generally confined to channels in the upper fan zones, but deposit both channelized and over-bank deposits on the lower fan and basin floor. The deposits show that the canyon-fan activity has continued during a rising sea level phase. It is evident that canyon headward erosion rates have been equal to or greater than the transgression rate, and that the canyon-fan systems have remained linked with their sediment sources. Frequency of events was probably higher, and volumes of the events were often larger, during the glacially lowered sea level episodes. However, turbidity currents of sufficient volume to reach the basin floors continue to occur at century or multi-century intervals. As one progresses headward in each system, the number of flows per length of core increases. Small flows that do not pass beyond the distributaries are much more frequent, and may be decadal in frequency, or even more frequent in the Santa Monica Basin system. These California borderland basins are probably typical of narrow active margins where rate of lateral sea level transgression is less than or equal to the rate of canyon headward erosion. The canyons maintain connections with sediment sources during sea level rise, and the systems therefore are active during the entire sea level cycle. Thus sediment supply is not a simple function of eustacy. This contrasts with the simplified sequence model developed on passive margins where canyons turn off as sea level rises.

Quaternary slip history of the Bare Mountain fault (Nevada) from the morphology and distribution of alluvial fan deposits

Analyses of alluvial fan sedimentation around Bare Mountain, Nevada, indicate differential slip along the Bare Mountain fault during the Pleistocene and Holocene. Analyses show that the ratios of fan area to sediment source area and the degree of Holocene sedimentation at the fan heads can be correlated with dip and displacement of the fault. The well-developed bajada and Holocene deposits near the fan toes along the southwestern flank of Bare Mountain contrast with the location of Holocene deposits near the fan heads and significantly smaller individual fan lobes on the eastern flank of the mountain (adjacent to the Bare Mountain fault). In addition, ratios of fan area to source area along the eastern flank of the mountain decrease from north to south, which we interpret to be caused by increased Quaternary slip (from north to south) along the fault, with relatively little Holocene throw at Tarantula Canyon and maximum throw near Wildcat Peak. The dip of the fault also changes from north to south, with relatively shallow dip angles (45° to 50°) at Tarantula Canyon and steeper (∼ 70°) dips near Wildcat Peak. Geometric constraints dictate that, for any given increment of horizontal extension, vertical offsets will be smaller on those segments of the fault that have shallower dip angles. On the basis of these observations, we conclude that the recent slip rate of about 0.02 mm/yr derived from trenching studies in the Tarantula Canyon fan should be considered a minimum value that may not represent the slip rate of the Bare Mountain fault as a whole.

Bryant Canyon Fan System: An Unconfined, Large River-Sourced System in the Northwestern Gulf of Mexico

The high-relief sea floor of the northwestern Gulf of Mexico slope is pockmarked by salt-rimmed minibasins and dissected by channels and canyons. Analysis of multichannel seismic reflection data reveals that Bryant Canyon and its fan are an unconfined, fluvially sourced system associated with the ancestral Mississippi River. Bryant Canyon eroded an extensive bypassing cut on the slope and delivered sediments to the unconfined area of the base of slope where they formed a fan. The canyon incorporates several minibasins and shallow salt structures. The canyon cross sections are easily recognizable in the minibasins, but are partially or completely obliterated by salt tectonism where the canyon incised the shallow salt structures. Salt appears to have encroached upon the can on after abandonment and restored the sea floor to its precanyon, low-relief profile. We interpreted three depositional units in the Pleistocene section of the Bryant Canyon Fan. The lower units are characterized by erosional surfaces or channels and mass-transport deposits. The top unit consists of low- to moderate-amplitude reflections with moderate continuity, suggesting relatively uniform deposition. The widespread occurrence of erosional surfaces and channels in the lower units may suggest extensive canyon activity during sea level falls. Decreased depositional energy from lower units upward indicates abandonment of the canyon or eastward shift of the ancestral Mississippi River. The Bryant Canyon Fan system contains several potential reservoir facies, including turbidites, trapped in high-relief minibasins during periods of less extensive canyon activity, that may exhibit good reservoir quality and massive sand occurring in canyon fill. Channel deposits and distal, unchannelized turbidites of the fan also may contain reservoir-quality sand. Mass-transport deposits with moderate- to high-amplitude reflections and levee deposits close to channels probably contain sand.

Morphology and genesis of carbonate soils on the Kyle Canyon fan, Nevada, U.S.A.

The physical and chemical properties of soils formed in an arid climate on calcareous alluvium of the Kyle Canyon alluvial fan, southern Nevada, were studied in order to infer the rates and relative importance of various soil-forming processes. These studies included field and microscopic observations and analyses of thin sections, major oxides, extractable iron, and clay minerals. The results are interpreted to reflect five major pedogenic processes: (1) The calcic horizons and calcretes of Kyle Canyon soils form by precipitation of CaCO 3, derived from eolian dust and alluvium, as clast coats, matrix cement, and massive layers. (2) The A and uppermost B horizons are essentially dust-derived, for they contain large amounts of detrital material not present in the alluvial parent material, and their major-oxide content is similar to that of modern dust. (3) Clay particles are translocated from A into B horizons. (4) Iron-bearing minerals in the near-surface B horizons are slowly oxidized. (5) Carbonate and aluminosilicate grains are both displaced and replaced by pedogenic CaCO 3; the silica released by replacement of aluminosilicates may be locally precipitated as amorphous or opaline silica and (or) incorporated into newly formed palygorskite and sepiolite. Rates of soil development at Kyle Canyon are approximate due to uncertainties in age estimates. Some soil field properties change at rates that are similar to rates for soils formed in rhyolitic parent material near Mercury, Nevada. The rate of accumulation of CaCO 3 (3–5 g m −2 yr −1) at Kyle Canyon is an order of magnitude faster than that near Mercury, but is comparable to rates calculated for soils in southern New Mexico and Utah.

Sediment Transport processes on an active submarine fault escarpment: Gulf of Corinth, Greece

Sediment transport processes on the escarpment of a listric fault in the tectonically active and terrigenous sediment-dominated Corinth graben in Greece are studied using 3.5 kHz and airgun profiles and sidescan sonar images. The whole of the slope area is affected by gravitational mass movements: slumps, debris/mud flows and liquified and turbidity flows. In canyon fan systems, large volumes of sediment are transported through slumping, debris/mud and turbidity flows into the basin. Here, they build coalescing fans which consist of intercalations of debris flow deposits and turbidites. In delta front-slope-fan apron systems, sediments are transported radially downslope through slumping, debris/mud flows, liquified flows and high-density turbidity flows into the basin where they build coalescing fan aprons. These consist of massive homogenized deposits (fluxoturbidites). In shelf slope systems retrogressive rotational slumping is the dominant instability feature. However, due to the sharp increase in the slope gradient caused by the slumping, debris and turbidity flows are triggered. The major factors responsible for triggering gravitational mass movements in the region are earthquakes, high sedimentation rates off the river mouths and heavy rainfalls and high seas. These mass movements appear to occur at least once every 2 yrs as indicated by the failure of submarine cables.

Reservoir Geometry and Trapping Mechanisms, Lindsey Slough Gas Field, Southern Sacramento Basin: ABSTRACT

Multiple reservoir units, numerous unconformities and stratigraphic pinch-outs, and extensive faulting make the Lindsey Slough field area (T5N, R2E, MDBM) geologically intriguing and economically attractive. The petroleum geology class of California State University, Northridge, undertook a group mapping project in November 1985, to delineate producing pools, determine reservoir geometries and trapping mechanisms, and identify potential exploration and development locations. More than 12 separate pools, producing from at least one of the 13 reservoir zones, are present. Reservoirs are of two main types: (1) extensive sheet sandstones deposited in delta-front and shelf(.) environments, and (2) lenticular channel sandstones deposited in submarine canyon, slope, and submarine fan environments. Among the sheet sandstones, the upper and lower Petersen members of the Upper Cretaceous Starkey formation are especially productive, with pay zones exceeding 100 ft. Both members pinch out to the southwest, along the paleoshelf edge. Of the lenticular sandstones, reservoir quality and thickness appear to be greatest in delta-mouth slope channels that fed sand basinward to submarine fans. Net pay in the K-1 and related upper Cretaceous channels exceeds 200 ft in some wells. Middle-fan channels of the Upper Cretaceous channels exceeds 200 ft in some wells. Middle-fan channels of the Upper Cretaceousmore » Winters Fan are locally productive, and a small amount of gas has been produced from thin, poorly sorted sandstones within the Paleocene Martinez Submarine Canyon.« less