Fluvial Sediment Input Research Articles

Biogeochemistry of a tropical river affected by human activities in its catchment: Brantas River estuary and coastal waters of Madura Strait, Java, Indonesia

On a global scale tropical SE Asia is critical in terms of fluvial nutrient and sediment input into the ocean as well as in terms of human modifications of the coastal zone altering these inputs. In order to obtain information on the biogeochemistry of the Brantas River estuary and adjacent coastal waters of the Madura Strait in eastern Java, Indonesia, a densely-populated area which is strongly affected by human activities in the river catchment, we investigated water, suspended matter and sediments from estuarine and coastal waters and plants and soils from the catchment collected during the dry season. Water samples were analyzed for dissolved nutrients and phytoplankton abundance and composition. Suspended matter, sediment, plant and soil samples were analyzed for carbon, nitrogen, amino acids and stable carbon and nitrogen isotopes. Nutrient concentrations were high in the river and then decreased rapidly seaward. Runoff from agricultural soils may be a major nutrient source. Phytoplankton was dominated by diatoms and most abundant in the estuarine mixing zone. A fairly high phytoplankton abundance despite nitrate-depletion and an N/P ratio <4 in coastal waters indicates additional nitrogen sources. Biogeochemical characteristics and stable isotopes indicate the autochthonous origin of suspended organic matter (OM) during the dry season. Major part of the fluvial input appears to be trapped in the inner estuary. Similar information obtained from the analysis of sediments suggests seasonal differences in the quantity and origin of suspended sediments and OM transported by the river with high inputs into coastal waters during the rainy season, the period of peak river discharge. The amino acid composition in combination with stable isotopes indicates agricultural soils to be a major OM source during that time. A high proportion of amino acid-bound nitrogen in suspended matter and sediments and the presumed sources of dissolved nitrogen other than nitrate to sustain primary productivity in the coastal zone suggest that organic nitrogen may play a more important role for coastal food webs and the nitrogen cycle in tropical regions than previously thought.

Sea-floor sediment distribution in the Gulf of Mexico

Carbonate content, smear-slide analysis and diffuse reflectance spectrophotometry were utilized to determine modern sediment composition and distribution throughout the Gulf of Mexico. In all, 186 core top and grab samples distributed throughout the Gulf were analyzed. Reflectance spectra were taken from thick smear slides from the near ultraviolet, through the visible, and into the near infrared. The first derivatives of the percent reflectance data were subjected to factor analysis producing factors that grouped covarying first-derivative wavelengths. Factors were interpreted by comparison to first-derivative curves for known sediment components and minerals. Interpretation was aided by the mapping of both calcium carbonate content and smear-slide sediment classes. The most easily interpreted factor solution was produced by analyzing only the visible region of the spectrum and extracting seven factors which explained ∼98% of the cumulative variance. These factors, in order of their relative importance, are interpreted as (1) marl and calcareous clay, (2) glauconite, (3) kaolinite, (4) organic matter, (5) phosphorite, (6) hematite, and (7) goethite. Some factor maps are consistent with known sources of fluvial sediment input; for example, kaolinite is deposited off rivers draining the southeastern US. Other factor maps are related to the origin of the material in the factor, glauconite, for example, being confined to low sedimentation regions of the outer shelf. The most unusual observation concerns the distribution of hematite, which appears to be transported from the rivers of south Texas, primarily the Rio Grande, across the shelf then eastward downslope along the base of the Sigsbee Escarpment. This eastward transport seems to be explainable only by transport in bottom currents flowing along the base of the Sigsbee Escarpment.

Effects of triple junction migration and glacioeustatic cyclicity on evolution of upper slope morphologies, offshore Eel River Basin, northern California

The upper continental slope of the Eel River Basin is affected both by uplift and seismicity associated with the northward-migrating Mendocino Triple Junction (MTJ) and by glacioeustatic fluctuations. As a result, seismically imaged slope sequences vary dramatically along strike. The southern slope is dominated by the Humboldt Slide, a stack of nine deformed sequences. Older, undeformed sequences thicken landward, while slide sequences maintain a constant travel-time thickness. Onset of deformation was abrupt. Within the slide, reflector amplitudes alternate: high-amplitude reflectors are wavy to shingled, and suggest increasing deformation with depth. Low-amplitude reflectors are sub-parallel to wavy throughout. High- and low-amplitude sequence ‘couplets’ suggest repetitive deposition of contrasting lithologies, a response we ascribe to late Pleistocene glacioeustatic cyclicity. If each ‘couplet’ represents a ∼100 kyr late Pleistocene sea-level cycle, then Humboldt Slide deformation began ∼450 ka. This is approximately coeval with onset of deformation on the adjacent shelf ∼500 ka, previously attributed to uplift and seismicity associated with northward encroachment of the MTJ. We believe migration of the MTJ triggered the slide, and is the cause for continuing deformation today, by sediment creep along internal glide planes. North of the Humboldt Slide, we interpret an upper slope anticline as a seaward extension of the Little Salmon Fault Zone (LSFZ) previously identified landward. Both faulting and folding extend to the seafloor, indicating that deformation continues. An adjacent buried anticline suggests that deformation has also jumped northward within the LSFZ on some parts of the upper slope. North of the LSFZ, the slope is dominated by downslope-trending channels and gullies. These features are v-shaped, vertically stacked, and increase in depth downslope. They are mostly infilled; their physiographic expression is generally subdued by sediment draping. Channel fills generally consist of basal high-amplitude reflectors, overlain by reflectors of lower amplitudes. Several large channels on older surfaces are concentrated near the northern end of the seismic coverage; smaller channels are more evenly distributed along the margin on younger surfaces. Channels also extend increasingly landward on progressively younger surfaces, indicating migration in that direction with time. These incisions are clearly inactive during highstands, as at Present; slope deposition today is dominated by draping of hemipelagic sediments. Channels form and migrate landward through headward erosion during base-level changes, when both shoreface and fluvial sediment sources are more proximal to the upper slope. The increasing lateral distribution of channels through time supports an increase in shoreface erosion north of the LSFZ. A possible explanation is a known decrease in fluvial sediment input from the north, combined with MTJ-related uplift in the south. Along-strike contrasts in upper-slope sequence morphology in the southern Eel River Basin are clearly a function of both regional tectonism and proximity to sediment sources, influenced by base-level changes. Humboldt Slide sequences, while deformed, are continuously influenced by systematic lithological variations occurring as a result of glacioeustatic cycles. Effects of base-level changes similarly control the distribution and fill of incisions on the northern slope. All of these observations reinforce previous seismic investigations of the offshore Eel River Basin shelf, and confirm that the competing effects of tectonism and glacioeustacy on the preservation of continental margin stratigraphy can be differentiated.

Mediterranean moisture source for an early-Holocene humid period in the northern Red Sea.

Paleosalinity and terrigenous sediment input changes reconstructed on two sediment cores from the northernmost Red Sea were used to infer hydrological changes at the southern margin of the Mediterranean climate zone during the Holocene. Between approximately 9.25 and 7.25 thousand years ago, about 3 per thousand reduced surface water salinities and enhanced fluvial sediment input suggest substantially higher rainfall and freshwater runoff, which thereafter decreased to modern values. The northern Red Sea humid interval is best explained by enhancement and southward extension of rainfall from Mediterranean sources, possibly involving strengthened early-Holocene Arctic Oscillation patterns and a regional monsoon-type circulation induced by increased land-sea temperature contrasts. We conclude that Afro-Asian monsoonal rains did not cross the subtropical desert zone during the early to mid-Holocene.

In situ acoustic and laboratory ultrasonic sound speed and attenuation measured in heterogeneous soft seabed sediments: Eel River shelf, California

We compared in situ and laboratory velocity and attenuation values measured in seafloor sediments from the shallow water delta of the Eel River, California. This region receives a substantial volume of fluvial sediment that is discharged annually onto the shelf. Additionally, a high input of fluvial sediments during storms generates flood deposits that are characterized by thin beds of variable grain-sizes between the 40- and 90-m isobaths. The main objectives of this study were (1) to investigate signatures of seafloor processes on geoacoustic and physical properties, and (2) to evaluate differences between geoacoustic parameters measured in situ at acoustic (7.5 kHz) and in the laboratory at ultrasonic (400 kHz) frequencies. The in situ acoustic measurements were conducted between 60 and 100 m of water depth. Wet-bulk density and porosity profiles were obtained to 1.15 m below seafloor (m bsf) using gravity cores of the mostly cohesive fine-grained sediments across- and along-shelf. Physical and geoacoustic properties from six selected sites obtained on the Eel margin revealed the following. (1) Sound speed and wet-bulk density strongly correlated in most cases. (2) Sediment compaction with depth generally led to increased sound speed and density, while porosity and in situ attenuation values decreased. (3) Sound speed was higher in coarser- than in finer-grained sediments, on a maximum average by 80 m s −1. (4) In coarse-grained sediments sound speed was higher in the laboratory (1560 m s −1) than in situ (1520 m s −1). In contrast, average ultrasonic and in situ sound speed in fine-grained sediments showed only little differences (both approximately 1480 m s −1). (5) Greater attenuation was commonly measured in the laboratory (0.4 and 0.8 dB m −1 kHz −1) than in situ (0.02 and 0.65 dB m −1 kHz −1), and remained almost constant below 0.4 m bsf. We attributed discrepancies between laboratory ultrasonic and in situ acoustic measurements to a frequency dependence of velocity and attenuation. In addition, laboratory attenuation was most likely enhanced due to scattering of sound waves at heterogeneities that were on the scale of ultrasonic wavelengths. In contrast, high in situ attenuation values were linked to stratigraphic scattering at thin-bed layers that form along with flood deposits.

The late Cenozoic evolution of coastal water bodies in Northern Kwazulu-Natal, South Africa

Abstract Three large coastal water bodies (St Lucia, Sibaya and Kosi) of markedly different contemporary morphology occur on the eastern margin of the northern KwaZulu-Natal coastal plain, South Africa. This paper examines their Late Cenozoic evolution and investigates the controls on their evolution that have produced the observed differences in morphology. All three systems origins were due to Mio-Pliocene low sea-level still stands allowing rivers to scour channels into Cretaceous and Palaeocene sedimentary sequences. During the Last Interglacial (oxygen-isotope stage 5e), sea level was approximately 5 m above present level and the morphology of the three systems was similar. An offshore barrier archipelago partially isolated a back-barrier environment that comprised intertidal sand-flats, localised reefs and tidal channels. The area around Bazaruto Island, Mozambique, provides a modern analogy. River rejuvenation and associated incision up to and during the Last Glacial Maximum (16 000–18 000 BP) caused a minimum of −40 m scour which is still reflected in the bathymetry of Lake Sibaya and Kosi. Accumulation of barrier sands partly against pre-existing Interglacial barrier remnants during the Holocene marine transgression (16 000–4500 BP, Ramsay, P.J., 1995. 9000 Years of sea-level change along the South African coastline, Quaternary Int. 27, 1–5.) was responsible for lagoon isolation, although the extent and nature of separation from the sea varies markedly between the systems. The northern part of Lake St. Lucia was closed from the sea during the late Holocene, probably during a sea-level fall from a Holocene high stand of ca. 3 m, at 3000 BP. The Kosi system had a late Pleistocene to early Holocene outlet situated at Bhanga Nek, Lake Nhlange, but this inlet mouth closed approximately at 3000 BP. The present mouth was established by 1600 BP. Lagoonal conditions within Lake Sibaya terminated ca. 5000 BP when coastal barrier dune formation caused complete impoundment and enabled the lake water level to rise to 20 m above mean sea-level. Lake St. Lucia's large catchment has meant that fluvial sediment input has kept pace with the Holocene transgression, whereas the localised drainage associated with Sibaya and Kosi has resulted in a negligible fluvial sediment input allowing for deep lacustrine conditions to develop. Lagoonal segmentation is present to some extent in each system as a result of mobile coastal sediments and their reworking by short-period wind waves. The differential stages of lagoonal segmentation in the three systems can be explained by (a) the presence of cohesive sediment and bedrock around St Lucia which reduce shoreline mobility, and (b) the great water depths in Sibaya compared to Kosi which require longer periods of infilling before segmentation is complete

Shoreline change and fine-grained sediment input: Isle of Sheppey Coast, Thames Estuary, UK

Abstract The northern coast of the Isle of Sheppey comprises actively-eroding cliffs up to 50 m high cut into the London Clay. From 1867 to 1998 the average cliff-top recession was about 1 m a −1 , while the maximum recession was 1.9 ± 0.08 m a −1 . From 1897 to 1998, this provided at least 4.5 × 10 5 t a −1 of fine-grained sediment to the Thames Estuary system and southern North Sea. This sediment input due to erosion is comparable in magnitude with fluvial sediment input from the Thames. The sinks for this material are unclear, but it is likely that the Essex and Kent estuaries and marshes have historically been important in this regard. Given climate change and accelerated sea-level rise, estuaries will again act as sinks, and it is prudent to maintain these supplies of sediment so that these estuarine systems can adjust to this forcing. To maintain sediment supply, shoreline management and estuary management needs to recognize more explicitly the importance and scale of fine-grained sediment supply and transport, including any implications for statutory land use planning.

7Be as a tracer of flood sedimentation on the northern California continental margin

Sediment inventories of the cosmogenic radionuclide 7Be ( t 1/2=53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined 7Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of 234Th and 210Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995–1997 monitoring period, 7Be was present (2–35 dpm cm -2) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of 7Be in shelf sediments after the floods. 7Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redistributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimentary systems of the northern California continental margin. Ratios of the 210Pb sediment-accumulation rate (100 yr average) to the 7Be deposition rate (1–2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3–30 depositional events complete the most recent stratigraphic record. This observation is consistent with the magnetude and frequency of fluvial sediment input, as Eel River floods with return periods of 3–33 yr (3% of the time of record) have supplied >80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

Modern accumulation rates and a sediment budget for the Eel shelf: a flood-dominated depositional environment

The northern California continental margin is periodically impacted by geologically significant storms, which have a marked influence on terrigenous sediment supply, flood deposition, and long-term accumulation of fine-grained sediment on the Eel shelf. Accumulation of Eel River muds on the adjacent shelf was investigated using 210Pb and 137Cs geochronologies, in order to understand the fate of sediment discharged by the Eel River and to relate patterns of net sediment accumulation (100-yr time scale) to sediment dynamics. 210Pb data demonstrate that modern accumulation of river mud occurs from the 50-m isobath seaward. Across-shelf accumulation rates decrease from maximum mid-shelf values of 0.6–1.7 g cm −2 yr −1 to values of 0.2–0.4 g cm −2 yr −1 at the shelf break, with a spatially weighted mean of 0.5 g cm −2 yr −1 (0.4 cm/yr) for the entire shelf. 210Pb xs sediment-depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity, which are produced by flood deposition. In some cores, particular 210Pb xs activity intervals may be associated with major Eel River floods of 1955, 1964, and 1974. It is postulated that, because of the coincidence of high-river-flow events and southerly winds during cyclonic winter storms, net northward transport allows for preferential deposition of fine-grained sediment north of the river mouth. Over the past ∼100 years, fluvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter, evident by both the spatial distribution of 210Pb accumulation rates and by clay-rich flood layers partially preserved in shelf deposits. A fine-grained sediment budget for the dispersal system, based on hydrological data and 210Pb geochronologies, demonstrates that a maximum of ∼20% (3×10 9 kg/yr) of the mean annual supply of fluvial mud (14×10 9 kg/yr) is trapped on the shelf. The results of this study demonstrate that: (1) short-term sedimentation processes associated with floods can influence sediment accumulation on longer time scales; and (2) a major fraction of fine-grained sediment supplied to tectonically active margins by flood-prone mountainous rivers bypasses narrow continental shelves.

Fluvial sediment inputs to upland gravel bed rivers draining forested catchments: potential ecological impacts

Abstract. As identified by the detailed long-term monitoring networks at Plynlimon, increased sediment supply to upland fluvial systems is often associated with forestry land-use and practice. Literature is reviewed, in the light of recent results from Plynlimon sediment studies, to enable identification of the potential ecological impacts of fluvial particulate inputs to upland gravel bed rivers draining forested catchments similar to the headwaters of the River Severn. Both sediment transport and deposition can have significant impacts upon aquatic vertebrates, invertebrates and plants.

Sediment transport to the Laptev Sea?hydrology and geochemistry of the Lena River

This study focuses on the fluvial sediment input to the Laptev Sea and concentrates on the hydrology of the Lena basin and the geochemistry of the suspended particulate material. The paper presents data on annual water discharge, sediment transport and seasonal variations of sediment transport. The data are based on daily measurements of hydrometeorological stations and additional analyses of the SPM concentrations carried out during expeditions from 1975 to 1981. Samples of the SPM collected during an expedition in 1994 were analysed for major, trace, and rare earth elements by ICP-OES and ICP-MS. Approximately 700 km3 freshwater and 27 times 106 tons of sediment per year are supplied to the Laptev Sea by Siberian rivers, mainly by the Lena River. Due to the climatic situation of the drainage area, almost the entire material is transported between June and September. However, only a minor part of the sediments transported by the Lena River enters the Laptev Sea shelf through the main channels of the delta, while the rest is dispersed within the network of the Lena Delta. Because the Lena River drains a large basin of 2.5 times 106 km2, the chemical composition of the SPM shows a very uniform composition. In contrast, smaller rivers with more restricted catchment areas exhibit significant differences.

Oxygen and carbon isotope study of the Holocene oyster reefs and paleoenvironmental reconstruction on the northwest coast of Bohai Bay, China

The northwest coast of Bohai Bay supports large areas of Holocene Crassostrea gigas (oyster) reefs. Their huge size and the fact that the shells are composed of consecutive microgrowth layers, consisting of primary calcite, make them an ideal object for a stable isotope study. The δ 18O and δ 13C compositions of the oysters reveal a number of interesting features. The most important are annual cycles related to seasonal variations, the regional δ 18O (water) versus salinity ratio, and a stop in shell secretion below a water temperature of 11.5 ± 1.5 °C. A positive correlation (covariance) between δ 18O and δ 13C values is present in most specimens. The δ 18O and δ 13C values of single specimens are believed to be mainly controlled by water temperature and the occurrence of phytoplankton blooms and decomposition. However, correlation among oysters of different ages indicate that the δ 18O and δ 13C values are mainly dependant of three variables: water temperature, salinity and phytoplankton blooms/decomposition. Between ca. 6500 yrs cal B.P. and 2300 yrs cal B.P. the salinity in Bohai Bay increased from less than 20%. to 30%.. Based on geological data combined with ecological data from C. gigas, we suggest that precipitation decreased during this period, while the input of fluvial sediments increased. The sediment load of rivers increased markedly after ca. 4000 yrs cal B.P., possibly due to a change in climate. In our study area this resulted in the development of the Oyster Plain.

Physical sedimentation processes on a mid-proterozoic (1800 Ma) shelf: the reynolds range group, arunta block, central Australia

Abstract Shallow-marine siliciclastic and carbonate deposits belonging to the 1800 Ma old Reynolds Range Group, central Australia, represent a transgressive sequence deposited on a low-gradient shelf, that was affected by longshore currents, tidal currents, storm wave action, and fair-weather wave action. The sediments comprise a 1100 m thick sequence in which three main depositional facies associations can be distinguished; (i) a near-shore and inner shelf quartz-arenite facies association is overlain by (ii) a middle shelf mudstone facies association which contains lenses of (iii) a carbonate facies association. The quartz-arenite facies association mainly occurs along the northeast side of the range, and grades into the mudstone facies association towards the southwest. This suggests that the palaeo-shelf extended towards the southwest, which is in accordance with a concentration of southwest-directed palaeo-current directions within the quartz-arenite facies. A sudden increase in thickness of quartz-arenite in the Mount Thomas area, suggests this area was the locus for fluvial sediment input. Vertical and lateral stratigraphic transitions between the facies associations are generally gradational, which may reflect coexistence of all facies on the shelf and small, net, relative sea level changes. The quartz-arenite facies association contains tide-influenced, bidirectional, small-scale trough cross-bedded, tabular cross-bedded and parallel-laminated sets that were deposited on the shoreface. These are generally overlain by large-scale trough and tabular cross-bedded sets, which were deposited on the inner shelf and record a southeast-directed longshore current. Numerous truncation surfaces indicate that frequent storms eroded the shoreface and carried large volumes of sediment onto the shelf. The mudstone facies association consists of alternating mudstone beds and generally thin, sheetlike quartz-sandstone beds that lack grading and traction structures. They represent storm-induced suspension deposits laid down on the middle shelf (inferred water depths $ 100 m ). Longshore and tidal currents appear to have had little influence in this environment. Stromatolite bioherms, and rippled marls and grainstones constitute the carbonate facies association which formed on the middle shelf in shallow marine areas of low siliciclastic sediment input. Ripples in storm sands throughout the mudstone facies association and a lack of turbidites, reflect the shallow depths and low-gradient of the shelf slope. A general lack of tidal deposits and the non-barred nature of the palaeo-coastline, indicate a small tidal range (0–4 m). The unidirectional southeast-directed longshore current, appears to have only influenced sedimentation on the inner shelf which suggests it resulted from prevailing (westerly) winds. The described facies in the Reynolds Range Group occur in strongly deformed and highly metamorphosed rocks, and the paper serves as an example that even from such tectonised rocks valuable sedimentological information can be extracted.

Recognition of Seismic Facies, Genetic Elements, and Depositional Environments in Lowstand System Tracts, Pletmos Basin, Offshore South Africa: ABSTRACT

Postrift Cretaceous strata in the Pletmos basin comprise 67 cyclic depositional sequences that were deposited in response to third-, fourth-, and perhaps fifth-order sea level oscillations. Seismic facies characteristics, identified by analyzing 200 seismic profiles totaling 7000 km, depict an extensive spectrum of lowstand lithogenetic elements and inferred clastic depositional systems. Mid-Valanginian to late Hauterivian lowstand deposition was generally restricted to submarine channel infill and mounded and/or sheetlike marine fill lapping out onto type 1 unconformities. Deposition took place during falling sea level during valley and submarine canyon incision. The rarity of lowstand progradation reflects sea level rises that exceeded sediment supply. Late Hauterivian to middle Aptian areally restricted prograding wedges were deposited in response to apparent equilibrium between fluvial lowstand sediment input, basin subsidence, and slow sea level rise. The wedges clearly exhibit coastal onlap onto type 1 unconformities and pinch out updip against a relict shelf edge. High-angle, oblique clinothems with associated toplap are common and indicate active lowstand deltaic progradation. Clinothems may baselap onto (1) channelized slope fans typified by marine onlap and chaotic reflections, (2) mounded basin-floor fans, or (3) the basin floor itself. Detached mounded basin-floor fans distal from lowstand wedges are numerous throughout themore » sequences. Excellent seismic data quality and moderate to shallow depths of burial permitted resolution of the genetic elements of lowstand system tracts in this unique tectonic setting.« less

Fluvial Depositional Systems of Carrizo-Upper Wilcox in South Texas: ABSTRACT

In the Rio Grande embayment of south Texas, the Carrizo-upper Wilcox interval (Eocene) consists of two sand-rich coastal plain fluvial depositional systems that grade basinward into several deltaic complexes. The bedload channel system is dominated by multi-story, multi-lateral, fluvial, channel-fill sandstones. This system is typically > 90% sandstone. Shales are thin and laterally discontinuous, the remnants of abandoned channel fills. Bedload channel sandstones dominate the major fluvial axes and form the depositional framework of the interval. The mixed alluvial system consists of a more typical suite of coastal plain facies. Mixed-load channel-fill sandstones tend to be isolated and surrounded by overbank shales and thin sandstones. Crevasse splay and lacustrine f cies occur in the flood-plain area. Total-interval isopach patterns, sandstone geometries, and depositional systems distributions indicate that fluvial sediment input was converging upon the embayment from the west, northwest, and north. Economically, the Carrizo-upper Wilcox of south Texas has a threefold significance. The updip Carrizo sandstone is a major source of fresh groundwater, includes several large oil fields, and also contains deposits of uranium minerals. The downdip upper Wilcox trend is an area of active hydrocarbon exploration. End_of_Article - Last_Page 1465------------