Frank G. Ethridge

Recent Developments in Fluvial Sedimentology

This volume brings together 36 of the manuscripts that were presented at the Third International Fluvial Sedimentology Conference hosted by Colorado State University in August, 1985. Fluvial systems and sedimentary rock sequences discussed range in age from Holocene to Precambrian and include many diverse areas. The principal objective of the volume was to document the recent developments in the application of facies analysis to the reconstruction of the architecture of fluvial systems. Reconstruction of architecture is an integral part of the overall process of basin analysis.

Architecture of channel-belt deposits in an aggrading shallow sandbed braided river: the lower Niobrara River, northeast Nebraska

Abstract Architecture of recent channel-belt deposits of the Niobrara River, northeast Nebraska, USA, records the response of a sandy braided river to rapid base-level rise. Up to 3 m of aggradation has occurred within the lower 14 km of the Niobrara River since the mid-1950s as a result of base-level rise at the confluence of the Niobrara and Missouri Rivers. Aerial photographs and channel surveys indicate that the lower Niobrara has evolved from a relatively deep, stable channel with large, bank-attached braid bars to a relatively shallow, aggrading channel with braid bars and smaller secondary channels. Architecture of channel-belt deposits associated with the recent aggradation has been defined using ground-penetrating radar (GPR) and vibracores. The channel-belt deposits exhibit a series of amalgamated channel fills and braid bar complexes (i.e., macroforms). Radar facies identified in the GPR data represent architectural elements of the braid bar complexes, large and small bedforms [two-dimensional (2-D) and three-dimensional (3-D) dunes], and channels. Individual braid bars appear to consist of basal high-flow and upper low-flow components. Preservation of the complete, high-flow bar geometry is generally incomplete due to frequent migration of smaller scale, secondary channels within the channel belt (i.e., braided channel network) at low discharges. The large-scale stratification of the braid bar deposits is dominated by cross-channel and upstream accretion. Elements of downstream accretion are also recognized. These accretion geometries have not been documented previously in similar sandy braided rivers. Braid bar deposits with low-flow modification (e.g., incision by secondary channels) are recognized in the deeper portions of the deposits imaged by GPR. Preservation of braid bars, with both high- and low-flow components, is a result of the rapid base-level rise and channel-bed aggradation experienced by the Niobrara River over the past 45 years. Recent avulsion of the river channel allowed preservation of the upper, low-flow component of the braid bar deposits (i.e., bar-top sequences). The relative abundance and stratigraphic position of the amalgamated channels and braid bar complexes within the channel-belt deposits constitute a “signature” of the recent base-level rise.

An Experimental Study of the Effects of Base-Level Change on Fluvial, Coastal Plain and Shelf Systems

ABSTRACT Several of the concepts of sequence-stratigraphic models were tested by experiments in a large flume. Using different gradients for successive runs, a physical model of a fluvial drainage basin, coastal plain, continental shelf, and continental slope was constructed. In all experiments, initial gradients of the coastal plain and shelf were identical. Systematic raising and lowering of the water surface simulated the changes in relative sea level (i.e., base level) that are similar to most sequence-stratigraphic models. Base-level changes significantly affected the shelf area but they had little effect on the fluvial drainage basin. During base-level falls, fluvio-deltaic progradation occurred. Fluvial aggradation occurred only during periods of base-level stillstand or rise. Because of the slow rate of head ward erosion of incised valleys, a significant time gap existed from the time base level first fell below the shelfbreak and the development of a cross-shelf bypass valley. During this lag time, deposition occurred on the exposed shelf to form a fluvial braid plain. Concurrently, channels developed at the shelfbreak and grew by headward erosion at various rates. Continuous incision and widening of these channels formed valleys. These valleys were not linked directly to the main drainage basin unt l ultimately one captured the flow from that drainage. At that point, other valleys were abandoned and ceased to grow. During the subsequent base-level rise, the single, large, cross-shelf valley filled with coarser sediments while the other valleys partially filled with finer suspended-load sediments. The main cross-shelf valley also supplied coarser sediments to the slope.

Fan-Delta Sedimentology and Tectonic Setting--Yallahs Fan Delta, Southeast Jamaica

Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation. The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gra ity processes resulting in patchy sediment-size distribution. Active coral growth occurs along the western delta margin. Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposi s are characterized by gravelly, proximal, braided-stream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of fan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.

Recognition of Barrier Environments

The vertical succession of sedimentary structures and textures at Galveston Barrier Island, Texas, is identical with vertical successions in two ancient barrier complexes, one in the Lower Cretaceous of Montana and the other in the Lower Jurassic of England. Within both Holocene and ancient examples, there is a gradation upward from (1) irregular interlaminations of siltstone and claystone at the base, through (2) burrowed and generally structureless sandstone, to (3) low-angle and microtrough cross-laminated sandstone, terminating in two of the examples in (4) structureless and rooted sandstone. This sequence represents deposition in (1) lower shoreface, (2) middle shoreface, (3) upper shoreface-beach, and (4) eolian environments, respectively. Analyses of quartz size and content of the Holocene and ancient barriers yield textural and compositional parameters that are environmentally sensitive. Plots of these parameters demonstrate that each of the environments may be distinguished on the basis of thin-section analyses. Consequently, full diameter cores, which show sedimentary structures, may not be necessary for precise environmental interpretation in the subsurface. Indeed, thin sections of sidewall cores may yield significant and reliable environmental interpretations in barrier sandstones. Textural and sedimentary structural similarities between Galveston Island and the ancient examples permit a general model of barrier sedimentation to be developed.

Laramide tectonics and humid alluvial fan sedimentation, NE Uinta Uplift, Utah and Wyoming

ABSTRACT Laramide uplift and erosion of the Uinta Mountains are recorded in a 10-km-long outcrop of the Lower Eocene Wasatch Formation, on the Utah-Wyoming border. This 750-m-thick package of interbedded sandstones and conglomerates is dominated by a coarsening then fining-upward megasequence 650 m thick that records the growth and abandonment of a humid alluvial fun system during a major cycle of uplift and unroofing of the thrust-bounded northern flank of the Uinta mountains. Grain size, thickness, and lateral extent of channel-complex deposits increase upward in the lower 400 m of the sequence, reflecting construction and northward progradation of the fan. Grain size and channel-complex thickness decrease upward in the upper 250 m of the sequence, reflecting gradual reduction of both sedime t yield and sediment caliber during postorogenic lowering of source-area relief. Within the megasequence, coarsening-up sequences 10-100 m thick built mainly of channel-complex deposits reflect progradation of fan lobes, punctuated by periodic fan-head avulsions. These medium-scale sequences in turn comprise small-scale fining-up cycles 1-10 m thick that reflect such fluvial processes on the fan as bar building, discrete flood events, and the filling and lateral migration of braided-stream channels. Both the medium-scale and small-scale sequences are commonly underlain by paleosols. Evidence of debris flows or other mass-movement processes is conspicuously lacking. Above the main megasequence a second, thinner megasequence containing lacustrine mudstones and wave-reworked conglomerates indicates that the fan persisted as a locus of coarse clastic deposition during the first of a series of lacustrine transgressions that began as ratios of sediment flux to subsidence rate decreased toward the end of the Early Eocene. Partial inundation of the fan during this transgression transformed it into a fan delta.

Depositional setting of the Salt Wash Member of the Morrison Formation, Southwest Colorado

ABSTRACT The Jurassic Salt Wash Member of the Morrison Formation, southwestern United States, is host to extensive uranium and vanadium mineralization. The ores occur in sandstones deposited in a complex fluvial system. The depositional geometry of the member is that of a wedge-fan. Detailed facies analysis of an area of the mid-paleoslope part of the member (the Slick Rock Uranium District) documents three major sandstone facies interbedded within a sequence of mudstones, siltstones, and thin sandstones. A large-scale trough cross-bedded facies, characterized by broad and generally thin trough crossbeds which rest locally on giant crossbeds as much as 6 m thick and 50 m wide, is the dominant sandstone facies. Paleocurrent azimuths are unimodal and have low consistency ratios indicating little variation in paleocurrent direction. This facies represents deposition in northeast-flowing, low-sinuosity trunk streams. The low-sinuosity streams were confined between leveed banks and probably had low-braiding indexes. Course change was primarily accomplished by avulsion. The second most abundant sandstone facies is primarily characterized by medium- to small-scale trough crossbedded sandstone within lensoid, fining-upward cycles. A progressive vertical decrease in the scale of structures and ariable paleocurrent orientations are characteristic of these depositional units. This facies represents the point bars of meandering streams that, according to paleocurrent and net sand map evidence, acted as tributaries to the low-sinuosity trunk channels. The third sandstone facies, interpreted as crevasse-splay deposits, contains coarsening-upward sandstone packages with variable structures and a distinctive convex geometry. The mudstone, siltstone, and thin sandstone facies averages 50 percent of the member and is subdivided into four subfacies. Oxidized red mudstone with numerous thin, laterally persistent, sandstone and siltstone interbeds represents well-drained floodplain deposits. Less abundant are depositional units composed of calcareous, oxidized silty mudstone with minor sandstone. These units represent sedimentation in shallow ephemeral lakes. Deeper water carbonaceous, gray-green mudstones deposited under anoxic conditions and clay plugs, representing abandoned channel fills, are rare in the district. Detailed analysis of sedimentary facies, sandstone distribution, and regional facies relationships leads to the conclusion that the Salt Wash Member in the Slick Rock District is the product of sedimentation on a fan apron marginal to an alluvial fan or coalesced fan complex. Sandstone thicks that transect the district represent depositional axes of this fluvial system.

Sandstone Composition and Depositional Environment

The detrital mineralogy of sandstones is affected significantly by depositional environment. The imprint of the environment is recorded as the relative abundance and size of individual detrital species. Differences between environments within any one basin are accompanied by concomitant changes in detrital-sediment composition. Petrographic analyses of Holocene and ancient sedimentary sequences reveal that the environmentally produced compositional changes are process dependent, and are interrelated closely with textural variations. Fluctuations in rock-fragment content among environments indicate that the relative survivability of unstable detrital grains can influence the overall composition of sandstones. A total of 300 samples of varied grain sizes was selected from contrasting environments in three Holocene sedimentary sequences: (1) the Galveston barrier complex, Texas; (2) the lower alluvial valley of the Mississippi River; and (3) the Mississippi delta. In each sedimentary sequence, bivariate and multivariate analyses of petrographic data demonstrate the environmental sensitivity of both composition and texture. A comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus. Four ancient sedimentary sequences, of different age and geographic location, w re selected to complement the Holocene sequences: (1) Wilcox (Eocene) fluvial rocks, Texas; (2) Wilcox (Eocene) deltaic rocks, Texas; (3) Lower Jurassic (Toarcian) marine rocks, England; and (4) Lower Cretaceous Muddy Sandstone marine, transitional, and nonmarine rocks, northeast Powder River basin, Montana and Wyoming. Petrographic data from each sequence demonstrate that the composition of recent detrital sediments and ancient sandstones is environmentally sensitive. Composition is also sensitive to textural changes. In sediments with a wide range of grain sizes, the relation between the abundance and size of detrital monocrystalline quartz is curvilinear. Compositional and textural data derived exclusively from thin-section analyses can be used jointly to segregate depositional environments within a single sedimentary basin, providing no source changes occur, and providing that certain basic sampling restrictions are adhered to. Thus, small samples such as sidewall cores and cuttings may be used in the delineation of depositional environments. Such small samples are available in abundance in the p troleum industry and, if analyzed petrographically, they can render invaluable assistance in the search for stratigraphic traps.

Depositional History of the Lamotte Sandstone of Southeastern Missouri

ABSTRACT Interpretations of provenance and depositional environments of the Upper Cambrian Lamotte Sandstone provide a new model of basal Paleozoic sedimentation in the Ozark region. Detailed examination of sedimentary structures and lithologies in exposures and cores, as well as compositional and textural analyses of 110 thin sections, provide the data on which the model is based. The Lamotte can be divided into three major sandstone lithologies: a feldspathic type containing significant amounts of polycrystalline quartz, K-feldspar, plagioclase, and plutonic rock fragments derived from local granites; a lithic type containing significant amounts of volcanic rock fragments derived from local felsites; and a quartzitic type dominated by well rounded monocrystalline quartz grains derived from a quartz-rich source located northwest of the Ozarks. The feldspathic and lithic detritus was shed off the ancestral St. Francois Mountains and deposited on alluvial fans and in braided fluvial systems in topographic lows. The bulk of the quartzitic detritus was transported eastward by braided streams, resulting in the construction of an alluvial plain which thickened to the east, burying the flanks and lower portions of the ancestral St Francois Mountains. Local deposits of interbedded lithologic types resulted from mixing of locally derived detritus with quartzose detritus in braided fluvial settings. An Upper Cambrian sea eventually transgressed the region, resulting in contemporaneous deposition of alluvial, marginal marine and shallow marine facies (including local fan-delta developme t). The marginal marine deposits constitute a conformable transition zone between the Lamotte and the overlying Bonneterre formation in the study area.

Bathymetry and sediment dispersal dynamics along the Yallahs fan delta front, Jamaica

Abstract The shallow offshore Yallahs fan delta serves as a model for the interaction of littoral processes and bathymetry and how they influence sedimentation on steep submarine slopes and in deep marine basins in tectonically active areas. Bottom reflection profiles show that the Yallahs delta front is characterized by two bathymetric zones, a narrow island shelf and a steep slope dissected by the heads of at least three submarine canyons. The refraction and diffraction of waves across these canyons and shelf control sediment distribution and the alternation of erosional and depositional beaches. A dominantly westward longshore drift transports coarse-grained sediments until they are trapped in the heads of the submarine canyons or temporarily stored on steep, unstable slopes between the canyons. Mass gravity processes presumably move these sediments downslope where they are deposited as turbidites in the Yallahs Basin and out onto the Colombian Abyssal Plain.