Browsing by Subject "Geomorphology"

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Channel processes and products in subaerial and submarine environments across the Gulf of Mexico
(2019-12) Swartz, John Marshall; Mohrig, David; Gulick, Sean P. S.; Goff, John; Passalacqua, Paola; Covault, Jacob; Stockli, Daniel
This dissertation characterizes the geomorphology and associated stratigraphy of channel systems across the northwestern Gulf of Mexico. I focus primarily on fluvial processes and products found within the coastal zone and alluvial plain, and also on a submarine channel network strongly influenced by fluvial behavior. I first use high-resolution bathymetry data combined with academic and industry geophysical data to detail the evolution of a linked shelf-edge delta / slope fan formed by the ancestral Rio Grande in the western Gulf of Mexico. The most recent period of deposition and margin growth was associated with the evolution of a submarine channel network, but one where the uppermost, high-gradient channel segments were strongly depositional and the lower, low-gradient channels with evidence of increasing sediment bypass. Chapter three then examines the controls on drainage network formation within a terrestrial environment, namely the coastal plain from the Rio Grande to the Mississippi delta. I show that floodplain channel network position and geometry is initially controlled by the position of alluvial ridges formed by avulsion of the largest coastal rivers. This provides a new and unique model of how drainage basins evolve in depositional environments. In chapter four I study the morphodynamics of the lower Rio Grande over the last hundred years and illustrate an intermittent river that does not adjust either its channel cross sectional geometry or lateral mobility as it approaches the coast, in contrast to more commonly studied perennial rivers. Finally, the fifth chapter seeks to capture the adjustment of a coastal river to sea-level rise as captured by offshore stratigraphy formed during the Holocene transgression. I provide evidence that the river channel did not simply flood back as commonly proposed but rather shifted from lateral migration to vertical aggradation as it continued to feed delta growth farther downstream. This dissertation provides a variety of studies spanning traditional domains and disciplines and brings together archival historical data, field surveys, and modern aerial and marine geophysical techniques. This work helps illustrate the complex dynamics of depositional environments and provides future research directions for fluvial sedimentology and landscape evolution.
Connecting delta morphology, surface processes, and subsurface structure
(2022-08-08) Hariharan, Jayaram Athreya; Passalacqua, Paola; Hodges, Ben; Werth, Charles; Michael, Holly
Home to a disproportionate population relative to their areas, river deltas are critically important landscapes. Their locations at the interface of the land and sea make them particularly susceptible to sea level rise, while their vast extents limit characterization based on in situ observations alone. Consequently, remote sensing and numerical modeling methods and studies are needed to better understand the current processes occurring within these systems, as well as to estimate their future evolution. In this dissertation, a suite of remote sensing and numerical modeling methods are developed and applied to better understand current processes within deltas, and to project future changes. The first study is an assessment of the accuracy of discharge partitioning estimation from remotely sensed imagery of river delta networks. This analysis aggregates data from 15 site-specific studies to find that errors associated with graph-theoretic estimates of discharge partitioning are consistent across a diverse set of delta landscapes. In the second study, reduced-complexity modeling simulates the evolution and anthropogenic modification of idealized river deltas. Simulation of different hydrodynamic scenarios, and routing of passive particles through the landscape, enables characterizations of the impact that natural morphological differences, anthropogenic modifications, and different flow conditions have on material transport. The results suggest that material type exerts a first-order control over particle behavior, and human modifications to the landscape reduce hydrological connectivity. The third and fourth studies present reduced complexity modeling of deltaic evolution over hundreds of years to investigate the relationship between surface processes and subsurface form within deltaic environments in the context of their future evolution. In the third study, testing of different input sediment compositions and steady rates of sea level rise suggests that both variables influence surface morphology and subsurface connectivity. The fourth study considers the impact of sea level rise acceleration, and finds that the dynamic response of surface channels to an unsteady rate of sea level rise changes based on its magnitude and trajectory. These changes on the surface are mirrored to an extent in the subsurface, which can only be estimated from surface information if the sea level change is relatively steady for some period of time. The results of these studies provide guidance for both policy makers and managers of deltas, as it is clear that humans are significantly impacting the natural processes of these landscapes. Taken together, research conducted as part of this dissertation provides information about current processes and potential future evolution of delta landscapes.
Controls on accumulation in modern and ancient wet eolian systems
(1994) Crabaugh, Mary Margaret Carr, 1959-; Kocurek, Gary
Wet eolian systems are those in which the water table is shallow and the floors of the interdune areas are within the capillary fringe. Sediment accumulation within a wet eolian system occurs as a result of a relative rise of the water table, and these accumulations are characterized by the presence of both dune and interdune-flat deposits. Surfaces capping the accumulations form under conditions of falling or static water table and are the result of erosion or sediment bypassing, respectively. Together an accumulation and its bounding surface are termed a wet eolian sequence. The Middle Jurassic San Rafael Group of eastern Utah is interpreted as the product of a coastal wet eolian system. Four complete sequences are correlated along a 365 km transect of eastern Utah stretching from near Dinosaur National Monument to Bluff. The areal extent of these sequences suggests that the controls on relative change in the level of the water table were regional in nature, and related to relative sea-level fluctuation caused by eustasy, regional and local subsidence. The rate of relative sea-level rise balanced against the rate of sediment supply controlled the depositional facies that form the accumulation. However, the rate of sediment supply was not an independent variable, and appears to parallel the rate of relative sea-level rise. The probable cause of this relationship is that as the sea transgressed, it delivered sediment to the coastal areas upwind of the San Rafael Group. Because both marine sequences and wet eolian sequences accumulate with a rising relative sea level and form bounding surfaces on the relative sea level fall, they can be tied into the same sequence stratigraphic framework. The gypsum dune field at White Sands National Monument is a modern example of a continental wet eolian system. The dune field is characterized by dunes separated by damp interdune areas migrating to the northeast under the influence of southwesterly Spring winds. Accumulations observed in shallow trenches in the interdune areas are composed of inclined packages of cross-stratified dune and flat-bedded interdune strata. The average inclination of the upper bounding surface of the interdune accumulations is a tenth of a degree, which reflects the angle at which the dunes are climbing under the influence of a rising water table. The water table at White Sands is experiencing a relative rise as a result of sediment compaction and subsidence within the tectonically active Tularosa Basin. Lateral variations in the angle of climb result from temporal and spatial variations in the depth of scour. Temporal variations are the result of fluctuations in the water table level controlled by annual changes in precipitation. Spatial variations in the depth of scour reflect variability in the size of the interdune areas. In larger interdune areas, any point on the surface is exposed to the erosive effects of the wind for a longer period of time than in a smaller interdune area, thus resulting in a greater scour depth.
Deposition and stratification of oblique dunes, South Padre Island, Texas
(1981-12) Weiner, Stephen Paul; McGowen, J. H.; Scott, Alan Johnson.
Oblique dunes have orientations that are intermediate between those of transverse and longitudinal dunes. The oblique dunes studied are reversing dunes which undergo no net annual migration when associated with normal meteorological patterns. From April 1980 through September 1980, the dunes migrated up to 65 feet (19.8 m) northwestward under the influence of prevailing onshore winds. High velocity northerly winds (November 1980 through February 1981), associated with the passage of winter frontal systems, caused the dunes to rapidly migrate 65 feet (19.8 m) southward. Volumes of sand transported by these strong winds were commonly reduced by accompanying rainfall. In October 1980 and March 1981 neither wind direction was dominant, and frequent changes in wind direction caused many of the dunes to become flattened. Hurricanes, which strike the area in late summer, have had no lasting effects on the dunes. Three major stratification types were observed in trenches and on etched surfaces. Translatent strata were deposited by wind ripples; grainfall deposits accumulated when saltating grains settled on leeward slopes of the dunes, and grainflow cross-strata were developed by avalanching on leeward slopes. Preservation of these stratification types occurred in zones of net deposition, predominantly leeward of the dune crests. Strata deposited during the summer wind regime dip northeast, whereas the winter strata dip in a southerly direction. The winter deposits are best preserved in the central cores of the dunes. This suggests that either the high velocity winds of the initial winter frontal systems destroy large volumes of the summer deposits, or that the dunes migrate southward, under the influence of dry northerly winds, during droughts. Oblique dune deposits should be difficult to discern in the rock record, because they may contain aspects of either transverse or longitudinal dunes. It is likely that some ancient oblique dunes have been mistakenly described as other dune types in the literature.
Downslope sediment transport processes and sediment distributions at the East Breaks, Northwest Gulf of Mexico
(1997) Piper, James Nolan; Behrens, E. W. (Earl William), 1935-
High-frequency 3.5-kHz acoustic reflection data and piston cores collected during 19 research and student-training cruises were used to interpret sediment distribution and downslope sediment transport processes involved in the formation of the two lobes of the East Breaks slide. The distribution of depositional systems was mapped by developing an acoustic facies classification system from 28,000 kms of high-resolution 3.5-kHz acoustic reflection data. Bathymetry and piston core data were also used in interpretations. Fourteen acoustic facies were recognized. Previous investigations of the near-surface geomorphology of the East Breaks slide interpreted this site as having been formed by a single mass-wasting event at the terminous of a late Wisconsinan shelf-edge delta. According to these interpretations, after initiation, this slide/slump propagated downslope and divided into two major lobes as it encountered a salt diapir-induced bathymetric high. This study shows that the two lobes are genetically separate. The formation involved the two major types of downslope sediment transport mechanisms that dominate the northern Gulf of Mexico: gravity-driven slide/slump/debris flows and turbidity currents. The upper part of the western lobe was produced by mass wasting, and the eastern lobe was formed by channelized turbidity currents
The effects of vegetation on island geomorphology in the Wax Lake Delta, Louisiana
(2014-05) Smith, Brittany Claire; Moffett, Kevan B.; Mohrig, David
Understanding how deltas build and maintain themselves is critical to predicting how they will respond to perturbations such as sea level rise. This is especially an issue of interest in coastal Louisiana, where land loss is exacerbated due to subsidence and decreased sediment supply. Feedbacks between ecology and geomorphology have been well documented in tidal environments, but the role of vegetation in delta morphodynamics is not well understood. This study investigates spatial and temporal correlations between vegetation succession and sediment accumulation at the Wax Lake Delta in Louisiana. I established a 2500 m long transect along the western levee of Pintail Island, capturing the full range of island elevations and the transition from bare sediment to herbaceous plants and trees. Shallow (50-100 cm deep) sediment cores taken along this transect were analyzed for particle size, organic matter content, and bulk density, and dated using ²¹⁰Pb. The resulting sedimentation rates and composition trends over time were compared to remote sensing-based analyses of temporal changes in island topography and flooding frequency derived from historical Landsat images. We found that the topography of Pintail Island has developed from a non-systematic arrangement of elevations to a discrete set of levees and intra-island platforms with distinct vegetation types, designated as high marsh, low marsh, and mudflat habitat. This elevation zonation is consistent with alternative stable state theory as so far applied to tidal salt marsh systems. At all but the youngest sampling site, sediment cores showed a significant decrease in organic matter content and a significant increase in grain size with depth. The total organic matter contribution to vertical growth was not sufficient to account for all the elevation change required to achieve the differentiation from low marsh to high marsh deduced from the time-lapse Landsat imagery analysis. Mineral sediment accumulation rates suggested that elevation growth was accelerating or holding steady over time, in contrast to theory suggesting rates should slow as elevation increases. These results provide an empirical foundation for future mechanistic models linking mineral sedimentation, organic sedimentation, vegetation succession, elevation change, and flood frequency in the delta.
Geomorphology and hydrogeology of the Edwards Plateau karst, central Texas
(1983) Kastning, Ernst H.; Baker, Victor R.
The Edwards Plateau is one of the largest continuous karstic regions of the United States; yet, its geomorphic evolution has previously received little systematic study. The objectives of this investigation are to (1) describe the physical characteristics of karstic features, (2) determine which geomorphic and hydrogeologic controls and processes have governed their development, and (3) relate genesis of karst spatially and chronologically to the geomorphic evolution of the Edwards Plateau. Caves provide a record of the evolution of karst and the development of major carbonate aquifers. Some controls on speleogenesis are pervasive; nevertheless, many caves exhibit characteristics that suggest a strong influence of local factors. Lithostratigraphic factors have influenced the development of chambers and passages. Solutional conduits were guided by variations in calcitic and dolomitic content, thickness of strata, and frequency of bedding-plane partings. This is particularly true of the Glen Rose Formation, limestone beds of the Edwards Group, and the Gorman Formation, the three principal cave-forming units. Caves were developed preferentially along fractures associated with regional structural elements, including the Devils River Uplift, Llano Uplift, San Marcos Arch, and Balcones Fault Zone. Locally, faults have modified passages. Evolution of flowpaths within major aquifers has governed the morphology, distribution, and orientation of caves. Patterns of eaves vary markedly and include isolated cavities, interconnected cavities, single conduits, branching conduits, and network, anastomotic, and spongework varieties of mazes. These suggest a diversity of hydrogeologic controls and, in many places, a strong influence of the evolving topography. Most caves of the plateau are relict features, abandoned by groundwater as dissection of the area progressed. They are situated well above floors of valleys and represent early stages of speleogenesis. However, where streams have incised deeply along the Balcones Escarpment and at some places in the interior of the plateau, caves are well integrated, at grade with surficial streams at baselevel, and characteristic of later stages of development. Climatic changes in central Texas during the Quaternary are reflected in siliclastic sediments and speleothems in caves. Ample evidence suggests that caves were severely flooded during periods of colder and wetter climates
Geomorphology of a coastal sand-bed river : Lower Trinity River, Texas
(2012-12) Smith, Virginia Burton; Mohrig, David; Groat, Chip; Johnson, Joel; McKinney, Daene; Parker, Gary
The lower Trinity River in Texas flows 180 river kilometers from Livingston Dam to Trinity Bay. Like many sandy coastal rivers the lower Trinity is geomorphically active. Within this 180-km reach, the river exhibits three styles of channel geometry and kinematic behavior that have been characterized using aerial photographs spanning the past 60 years, as well as bathymetric surveys and field work completed over the past 5 years. The three channel zones are connected to spatial change in properties of the sediment transport field. The upstream zone is defined by channel-bed incision, relatively small and coarse-grained bars, and relatively low rates of lateral channel migration. These properties of the upstream zone are connected to the discharge of water with effectively no bed-material load from Livingston Dam. Eventually the channel flow scours enough sediment from the channel bed and sidewalls to reestablish the predicted transport capacity for sand in the river, marking the transition to the central zone. This zone is defined by the largest bars and channel bends with the highest rates of lateral migration that persist downstream until the transport of sand and gravel is influenced by the backwater hydraulics connected with the shoreline at Trinity Bay. This downstream river zone is characterized by very small point bars, the deepest flows at most discharges, and lower rates of channel migration. Studying the connections and transitions between these three river zones leads to a more complete understanding of the coevolution of river geometry and profile, channel kinematics, and downstream change in sediment transport in the coastal zone.
Geomorphology of megafans
(2020-12) Sounny-Slitine, Moulay Mohamed Anwar Adounia; Latrubesse, Edgardo; Luzzadder-Beach, Sheryl; Beach, Timothy; Arima, Eugenio Y; Hudson, Paul F; Ramos Scharrón, Carlos E
Megafans, and more broadly alluvial fans, are landforms developed at topographic fronts through fluvial and colluvial processes. Unlike other landforms that are part of the common lexicon (e.g., mountains, hills, deltas, etc.), megafans are one of the least known and understood landforms. In general, a megafan is a large fan-shaped landform with an extent of greater than 103 km2 built by a river as it exits a topographic front. Due to the limited understanding of megafans, they are popularly referred to as ‘inland deltas.’ In scholarship, megafans thought to behave like large alluvial fans. This work’s aim is to further the understanding of megafan geomorphology through the study of morphometrics and processes controlling their evolution. This study produces a novel global geodatabase of megafans populated with morphometrics of megafans and their contributing source basins. The study proposes a new definition of megafans that are based on metrics. This is achieved through a global survey of megafans and alluvial fans, which has an innovative approach to utilizing both satellite imagery and elevation models to map and analyze megafans in a system-wide integrative approach, including the source basin of the landform. Furthermore, I address a number of research questions about megafans, which include: What makes a megafan different than an alluvial fan? Is megafan formation driven by avulsion or distributive processes? What role does the source basins play in megafans sizes and shapes? And in what ways does climate influence how active the development of megafans are? Through this work, I demonstrate that megafans are a unique landform that can both be avulsive and distributive in nature, that the source basin has a large influence on how they form, and that globally larger megafans are mostly inactive and relic landforms of past climatic regimes. I also take a closer look at some major regions of the world influenced by megafans, including the Chaco Plain, The Sahel, Himalayan Forelands, Okavango Delta, and the Pantanal.
Hydro-geomorphic dynamics in the Makgadikgadi Okavango Zambezi Basin, Northern Botswana
(2018-12-05) Bean, Robert Allen; Crews, Kelley A.; Geography and the Environment; Beach, Timothy; D'Odorico, Paolo; Kocurek, Gary A; Meyer, Thoralf; Young, Kenneth R
The Makgadikgadi-Okavango-Zambezi (MOZ) basin of Northern Botswana is examined through a series of connected studies at different spatio-temporal scales. With an aim of using different techniques to look at changes and drivers of change in hydrology where the MOZ terminates (Mababe, Ngami, Makgadikgadi), this work looks at dynamics from three perspectives in three chapters: 1) a historical approach (chapter) that examines changes in hydrology and causal factors over the last 200 years; 2) an examination of the soils and their origins (parent materials) in the terminal basins; 3) a case study using Lake Ngami to monitor seasonal changes to surface water over the last 20 years. For the historical chapter, reports and surveys including photographs were used and digitized to complete a catalog of changes to hydrology. Soil pits were excavated at 31 locations and to further investigate provenance elemental analyses, radiocarbon dating, and carbon isotope analyses were conducted. For analysis of change in the areal extent of Lake Ngami Google Earth Engine was used to review and calculate indices from complete Landsat and Sentinel archives. Results from the historical approach provide new photographic evidence combined with longitudinal stream profiles that shed light on the floods of 1925, previously thought to be the largest in this area. Soils that were radiocarbon dated confirm with other studies that 3000 ybp Lake Ngami was much larger and some massive shift in the system caused the regular seasonal inflows to the lake to cease. Similar lacustrine deposits were not found in excavations from either the Makgadikgadi or Mababe basins at topographic lows where deposits would have been expected if these were all filled simultaneously through a connected system. Water indices created for Lake Ngami over the last 20 years were comprised of over 100 scenes from 3 sensor platforms. Key findings include a direct correlation between water indices derived from atmospherically-corrected Landsat-8 and those derived from Sentinel-2 that were not atmospherically corrected. This finding allows for more complete time-series to be constructed in the future without the need for further processing as previously indicated in the literature.
Integrating geologic and SRTM data to identify geomorphologic landforms in the Eastern Amazon River Valley
(2014-08) Clause, Vincent Anthony; Latrubesse, Edgardo
Geography and the Environment
Isolating lithologic controls on landscape morphology in the Guadalupe Mountains, New Mexico and Texas
(2017-05-04) Marino, Emily Bradshaw; Johnson, Joel P.
In many geomorphic studies, lithologic contrasts are often acknowledged as important for landscape form, but are otherwise ignored in attempts to infer tectonic forcing or climatic control from topography. It remains difficult to separate the effects of tectonics, climate, and lithology due to the limitations of commonly used landscape evolution models. Tectonic inactivity and relatively little spatial variability in climate make the Guadalupe Mountains of Texas and New Mexico an ideal site to isolate and investigate the effects of lithology on topography. To assess the control of lithology, I compared topographic metrics including channel steepness index, channel concavity, and topographic relief in different mapped lithologic units across the region. Topographic metrics were calculated using elevation data extracted from USGS 10m Digital Elevation Models. These metrics were grouped spatially based on 23 regionally mapped lithologic units, including abundant limestone and dolomite with some evaporites, sandstone, and shale. To better evaluate the different rock units, I used published unit descriptions to develop a simple, semi-quantitative system to estimate the relative durability rating (RDR) of each rock unit. This rating system accounts for rock type and other rock properties such as relative bed thicknesses or spatial heterogeneity. RDR values were found to be correlated with unit-averaged channel steepness for each of the 23 lithologies in the region. Channel steepness shows a moderate correlation with RDR (R² = 0.44; Kendall’s 𝜏 = 0.52), demonstrating quantifiable control on landscape form. However, concavity does not show a significant correlation (R² = 0.016; Kendall’s 𝜏 = 0.13). Stratigraphic relationships among units suggest that thick, resistant reef deposits exert the main lithologic control on overall channel forms in the region. Less resistant units stratigraphically below these reef deposits generally have higher than expected steepness given their RDR. Units at the bottom typically have high concavity values as well. The opposite is true for weaker units stratigraphically above the resistant reef formations, which have lower steepness and higher convexity. The contrasting influence of strong units above or below weaker units and their observed effects on channel form should improve our ability to infer rock properties from topography, and to predict the evolution of landscapes with lithologic variability.
Joints in igneous rocks
(2009-03) Barker, Daniel S.
Late Quaternary geomorphic evolution of the Colorado River, Bastrop and Fayette counties, Texas
(1977) Looney, Robert Michael; Baker, Victor R.
Ancient and modern flood plain and channel morphology was mapped in the late Quaternary alluvial valley of the Colorado River between Utley and La Grange, Texas. The analysis of N.A.S.A.-generated color aerial infrared photography, SKYLAB remote sensing imagery, and aerial panchromatic photography revealed nine assemblages of fluvial channel patterns. The paleochannels occur on multiple flood plain and terrace levels and are associated with deposits with variable textures, sedimentary structures, and lithologic composition. Quantitative geomorphic analysis showed that bankfull width for the late Quaternary Colorado River varied from 450 m to 200 m, meander wavelength from 575 m to 1730 m, and sinuosity from 1.3 to 3.6. The channel adjustments from low sinuosity to high sinuosity streams was accompanied by a decrease in meander wavelength and bankfull width. Sedimentological analyses show corresponding changes in grain size from sand and gravel to sand and silt transporting streams. The nine channel assemblages of the Colorado River reflect changes in runoff and sediment load characteristics from upstream catchment areas. These runoff and sediment load changes correlate with an alternating arid-humid climate in central Texas during the late Quaternary. The resulting hypothesis is that channel 7 is pre-Wisconsinan; channels 6, 6A, and 6B are Wisconsinan; channel 5 is a dry period at the beginning of the Holocene; channel 4 is a humid Holocene phase; and channel 3 to the modern channel are the most recent Holocene fluctuations. Sedimentary structures and paleo-hydrologic implications indicate that channels 7 and 6 were laid down by broad, shallow braided streams, Channels 6A and 5 were either braided or coarse meander belt fluvial systems. The small channel width, meander wavelength, and low sinuosity indicates that channels 6B and 4 operated as fine grained meander belt fluvial systems. Channels 3 and 2 are similar to the modern Colorado River which is a bed-load (high bed-load/discharge ratio) stream transporting coarse sand and pebble-to-cobble gravel
Morphodynamics and geometry of channels, turbidites and bedforms
(2011-12) Peyret, Aymeric-Pierre Bernard; Mohrig, David; Kocurek, Gary; Kim, Wonsuck; Lake, Larry W.; Fulthorpe, Craig
The evolution of landscapes and seascapes in time is the result of the constant interaction between flows and topography. Flows change topography, which in turn change the flow. This feedback causes evolution processes to be highly non-linear and complex. When full analytical derivations of the co-evolution of topography and flow are not possible without oversimplifications, as is the case in river bends, recent large topographical datasets and modern computers allow for correlations between horizontal (planview) and cross-sectional geometry of channels. Numerical analysis in the Mississippi and Trinity rivers indicate that the type of correlation between river radius of curvature and bankfull channel width depends on the migration behavior of the river. In other cases, channel topography may only have a second-order effect on its own evolution, as is the case for fully depositional turbidity currents, and the evolution of aeolian field topography may only be a function of this topography. I show that in these situations, changes in topography may be decoupled from details of the flow field and modeled very easily with a good accuracy.
Morphologies and controls on development of Pliocene-Pleistocene carbonate platforms : Northern Carnarvon Basin, Northwest Shelf of Australia
(2013-08) Goktas, Pinar; Fulthorpe, Craig Stephen, 1954-; Austin, James Albert
The detailed morphologies, evolution and termination of Neogene tropical carbonate platforms in the Northern Carnarvon Basin (NCB) on the passive margin of the Northwest Shelf of Australia reveal information on the history of local oceanographic processes and changing climate. Cool-water carbonate deposition, dominant during the early-middle Miocene, was superseded by a siliciclastic influx, which prograded across the shelf beginning in the late-middle Miocene during a period of long-term global sea-level fall. The resulting prograding clinoform sets, interpreted as delta lobes, created relict topographic highs following Pliocene termination of the siliciclastic influx (Sanchez et al., 2012a; 2012b). These highs created a favorable shallow-water environment for subsequent photozoan carbonate production. A composite, commercial 3D seismic volume allows investigation of the temporal and spatial evolution of the resulting Pliocene-Pleistocene carbonate platforms. Initiation of carbonate development, in addition to being a response to cessation of siliciclastic influx and the existence of suitable shallow-water substrate, was also influenced by the development of the warm-water Leeuwin Current (LC), flowing southwestward along the margin. Four flat-topped platforms are mapped; each platform top is a sequence boundary defined by onlap above and truncation below the boundary. Successive platforms migrated southwestward, along-strike. Internally, platforms have progradational seismic geometries. The mapped platform tops are large (≥ 10 km wide). Evidence of karst (e.g., v-shaped troughs up to 50m deep and ~1 km wide and broader karst basins up to 20 km2 coverage area) on platform tops suggests episodic subaerial exposure that contributed to the demise of individual platforms. The most recent platform, platform 4, is unique in having interpreted reefs superimposed on the progradational platform base. The base of these reefs now lies at ~153 m and the reefs may therefore have developed post-LGM (~21 Ka). The reefs subsequently drowned, with drowning possibly aided by turbidity associated with formation of adjacent sediment drifts and weakening and strengthening LC during the late Pleistocene. The progressive drowning and termination of platforms from northeast to southwest along strike may result from differential compaction of the deltaic substrate or differential tectonic subsidence caused by the collision at the Banda Arc between the Australian and Pacific plates
Oxbow Lake sedimentation along the lower Guadalupe River, Texas
(2011-05) Leitch, Maraigh Brianna 1984-; Hudson, Paul F., 1968-; Butzer, Karl W; Latrubesse, Edgardo; Doolittle, William
Texas Gulf Coastal Plain Rivers are subject to a dynamic climatic and hydrologic regime. A common feature of Gulf Coast meandering rivers in this region are oxbow lakes. These oxbow lakes exhibit a complex connectivity with the active channel and slowly infill by floods that deposit sediment. Thus, the oxbows serve as an archive of a flood regime throughout time for that particular watershed. This paper reports the results of geomorphic study of flood sediments from two oxbow lakes along the lower Guadalupe River in south-central Texas. Grain size analysis and magnetic susceptibility measurements of the cores, supported by radiocarbon dating and hydrologic data, provide records of the flooding regime. The Guadalupe drains an area of 13,196 km2 in south-central Texas, including the Edwards Plateau and the Gulf Coastal Plain. Sediment data from a recent and older oxbow lake were examined in the context of Guadalupe River hydrology to were analyzed establish a relationship between climatic controls and sedimentation, with the intent of providing inference on paleohydrology of the lower Guadalupe River. In particular, this study examines sedimentary deposits through recent and paleo-oxbow lakes, to compare differences in the sedimentary record of a historical oxbow with the dynamics of a recent oxbow as a modern analog.
Pleistocene to recent geomorphic and incision history of the northern Rio Grande River Gorge, New Mexico : constraints from field mapping and cosmogenic 3He surface exposure dating
(2017-08) Clow, Travis Wellington; Behr, Whitney M; Helper, Mark Alan
Relationships between river incision, aggradation, and widening are investigated in a ∼5 km reach of the northern Rio Grande River Gorge near the confluence with the Red River in New Mexico using detailed geomorphic mapping and cosmogenic 3He surface exposure dating. This wide (exceeding 1.5 km) and deep (∼240 m on average) stretch of the river exhibits a unique set of cohesive, stacked toreva blocks, incoherent landslides, rockfalls, and slumps developed within ∼3-5 Ma Servilleta Basalts and intercalated weak Pliocene Santa Fe Group gravels. Toreva blocks are best developed along the eastern side of the river — they exhibit coherently-dipping, patinated tops that can be reconstructed to the gorge walls, but with toes that are truncated or buried by later deposits. Located below these landslide features is a flight of fill and fill-cut terraces spanning 6 levels at elevations of 60, 45, 29, 25, 15 and 7 m above the modern river grade, on average. Terraces at 29 m can be correlated across and along the river axis, whereas others are more locally preserved. All terraces exhibit well-defined boulder levees and risers constructed from alluvium sourced from upstream, and rounded and sculpted basalt clasts on their treads. Minimum 3He surface exposure ages of multiple samples from each terrace indicate Qt6 was likely abandoned at 63.4 ± 9.8, Qt5 at 30.0 ± 4.8 ka, Qt4 at 23.8 ± 3.7 ka, Qt3 at 22.7 ± 3.6 ka, and Qt2 at 19.0 ± 3 ka, however the potential for pre- depositional inheritance is evident. Terraces are the youngest preserved deposits in the gorge and record at least three aggradation-incision geomorphic cycles over the past ∼60 ka. If initiation of gorge incision was coeval with capture of the San Luis Valley of southern Colorado at ∼440-800 ka, average incision rates prior to the for- mation of Qt6 were 0.26-0.52 mm/yr. This incision was likely coincident with toreva block formation and substantial gorge widening, as the toreva blocks and large-scale incohesive landslide deposits predate terrace development. Landsliding controls the width of the gorge along this stretch, leading to a 2-2.5x increase in width compared to immediately up- and down-river. These landsliding events were likely primarily driven by undercutting of basaltic bedrock along weaker gravel horizons. Average incision from ∼60 ka to present day appears to have been faster, with maximum average rates of ∼1.0 mm/yr from Qt6-modern river. Gorge narrowing is observed during this time period, with only minor widening accommodated by slumping and incoherent landsliding that post-date terrace treads. This period of incision was punctuated by aggradational events that roughly correlate with regional climatic events and/or late Pleistocene MIS climate cycles that increase in amplitude towards present day, with terraces incised into and abandoned during transitions from glacial to interglacial climate. Rates of surface uplift from dynamic topography and/or active slip rates along basin bounding normal faults are over an order of magnitude too small to explain the observed incision rate increase over time. These increasingly rapid incision rates mirror the phenomena seen in other western US river systems since the Pliocene.
Quantifying the influence of surface processes on subsurface geometry in deltaic environments
(2019-05-07) Hariharan, Jayaram Athreya; Passalacqua, Paola
River deltas are densely populated and dynamically changing environments located at the boundary between land and sea. Population demands for water as well as rising sea levels are increasingly threatening aquifer water quality in deltaic regions. The rate at which aquifer contamination by salt water or other contaminants occurs is dictated, in part, by the arrangement of sediment within the subsurface. In this work, we examine the heterogeneity of the subsurface from a structural vantage to better understand how surface processes and geometry are linked to subsurface architecture. The numerical model, DeltaRCM, is applied to simulate delta evolution under a variety of input conditions. The resulting model outputs simulate 800 years during which the growing delta generates a subsurface volume that is over 40m deep. Surface channel properties and behavior, such as channel depths and channel planform decay rates are measured. Similarly, the structure of the sand bodies in the subsurface domain is evaluated. These different types of analyses, surface and subsurface, are ultimately compared to take a first-look at how channel properties in a deltaic environment may relate to subsurface structure and form. Broadly, expectations about channel trends and subsurface structure from the field of geomorphology are supported. Channel depths decrease with distance from the inlet, and as the input sand proportion increases. Similarly, the channelized fraction of the delta surface increases with higher input sand fraction values. In the subsurface, different types of channel behavior on the surface correspond to different structures. The sand bodies are larger when the surface channels are shallower and more mobile. In addition, the spatial continuity within strike sections (sections taken perpendicular to the inlet channel) increases with channel depth. Comparisons of the modeled subsurface with stochastically re-arranged replicates have confirmed the assertion that surface processes create unique subsurface structures. When the input proportion of sediment contains at least 40% sand by volume, the average size of the subsurface sand bodies follows a power-law relation with respect to surface channel depths and the average channelized fraction of the delta platform. The range of spatial entropy (disorder) also increases with channel depth. Within models, with increasing distance from the inlet both channel depths and spatial entropy ranges decrease. Changing the input sediment proportions over the course of the delta evolution provides mixed results. Some channel parameters like channel depth are indistinguishable from steady input cases, while others are influenced by the initial topographic setup. In the subsurface, variable sediment input proportions create vastly different sand body geometries depending on the rate of variation of the input sand proportion. When the input sand proportion is gradually increased, the average sand body size becomes very large; however when the sand input is abruptly increased, the mean sand body value is less than a steady sand input analog.
Quantitative seismic geomorphology of a confined channel complex, southern Atwater fold belt, Gulf of Mexico, U.S.A.
(2010-08) Morgan, Jessica Leanne; Wood, Lesli J.; Steel, Ronald; Fisher, William L.
The structures along the Atwater Fold belt form important deep-water hydrocarbon traps in the northern Gulf of Mexico. The purpose of this study is to map and quantify the morphology, sedimentology and architecture of Plio-Pleistocene basin floor fan systems outboard of the Poseidon Minibasin, located along the Atwater deep-water fold belt (mid-Miocene to Pliocene), and apply that information to determine the temporal and spatial nature of the fill and its implications as a reservoir analog. The data set includes ~2200 km sq. of 3D seismic data, along with information from several wells. Wireline logs show the Tertiary age deposits outboard of the Sigsbee Escarpment to be several hundred feet thick, sharp-based, dominantly coarse-grained (sandy) but fining up cycles composed of sandy basin floor fans, mass transport complexes and leveed channels developed in a confined setting within deep-water “valleys.” The largest valley formed in five main stages: initiating from narrow channel incision, widening through lateral incision and sidewall slumping, straightening, and finally flooding and infilling. The valley system is ~20,000 feet across and ~ 1,400 feet deep, with what look like well-developed levees ranging from 700 to 1300 feet at their thickest point extending ~19000 feet away from the channel. This system is underlain by a ~700 foot thick mass transport complex and overlain by younger, low sinuosity leveed channel systems. Both of these systems appear to have been sourced by large submarine drainages, originating from a shelf edge sediment source system to feed the rugose slope with deep-water channel pathways uninhibited by salt wall inflation at the time of valley deposition. Major phases of salt thrusting along the southern edge of the Atwater were contemporaneous with the formation of these large, through-going valley system, which appear to be associated with the period of sheet thickening and development of monoclinal basinward dip related to rafted mini-basin docking. Well log signatures show evidence for armored clay drapes along the valley margins as well as a flattening of lateral accretion packages toward the distal end of the system. The flattening of these packages seems to signal proximity to the fan terminus, which would serve as an important indicator of spatial extent of plays in deep-water.