Overburden characterization and post-burn study of the Hanna IV, underground coal gasification site, Wyoming, and comparison to other Wyoming UCG sites

Analysis of 21 post-burn cores taken from the Hanna IV UCG site allows 96 m (315 ft) of overburden to be subdivided into four local stratigraphic units. The 7.6 m (25 ft) thick Hanna No. 1 coal seam is overlain by a laterally discontinuous, 3.3 m (11 ft) thick shaley mudstone (Unit A') in part of the Hanna IV site. A more widespread, 30 m (90 ft) thick well-indurated sandstone (Unit A) overlies the A' unit. Unit A is the roof rock for both of the Hanna IV cavities. Overlying Unit A is a 33 m (108 ft) thick sequence of mudstone and claystone (Unit B), and the uppermost unit at the Hanna IV site (Unit C) is a coarse-grained sandstone that ranges in thickness from 40 to 67 m (131 to 220 ft). Two elliptical cavities were formed during the two phases of the Hanna IV experiment. The larger cavity, Hanna IVa, is 45 x 15 m in plan and has a maximum height of 18 m (59 ft) from the base of the coal seam to the top of the cavity; the Hanna IVb cavity is 40 x 15 m in plan and has a maximum height of 11 m (36 ft) from the base of the coal seam to the top of the cavity. Geotechnical tests indicated that the Hanna IV overburden rocks were moderately strong to strong, based on the empirical classification of Broch and Franklin (1972), and a positive, linear correlation exists between rock strength and volume percent calcite cement. There is an inverse linear correlation between rock strength and porosity for the Hanna IV overburden rocks. 28 refs., 34 figs., 13 tabs.

Crevasse splays from the rapidly aggrading, sand bed, braided Niobrara River, Nebraska: effect of base level rise

Base‐level rise of ≈2·35m on the Niobrara River has resulted in aggradation of the channel belt and a recent avulsion. Overbank areas have become flooded by rising groundwaters, and more than eight crevasse splays have formed between 1993 and 1997. Two crevasse splays, situated on the west and east sides of the Niobrara, have been studied using ground‐penetrating radar (GPR), shallow boreholes and topographic surveys. The vibracores and GPR profiles provide a nearly three‐dimensional view of the architecture of crevasse splay deposits. The east splay was initiated in the winter of 1993/94 and has expanded to cover an area ≈200m by 1000m, with sediment up to 2·5m thick. The west splay, which was initiated by the opening of a crevasse channel through a levee in the autumn of 1995, covers an area ≈150m by 250m, with up to 1·2m of sand deposited in a single year. The Niobrara splays are sand dominated and characterized by bedload deposition within channels, 5–30m wide and 0·5–2m deep, with the development of slipfaces where splays prograde into standing bodies of water. Sedimentary structures in cores include horizontal lamination, ripple lamination and sets of cross‐stratification. There is a slight tendency for splays to coarsen up, but individual beds within the splays often fine up. The abundance of crevasse splays on the Niobrara River contrasts with other braided river floodplains. In the Niobrara, crevasse splay formation followed aggradation within the channel belt, which occurred in response to base‐level rise. The link between crevasse splays, channel aggradation and base‐level rise has important implications for the interpretation of ancient braided river and floodplain sequences. It is suggested that crevasse splay deposits should be an important component of aggrading fluvial sediments and, hence, should be preserved within the rock record. In this case, the aggradation and crevassing have been tied to a rise in base‐level elevation, and it is suggested that similar deposits should be preserved where braided rivers are affected by base‐level rise, for instance during transgression and filling of palaeovalleys

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

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

Gradual avulsion, river metamorphosis and reworking by underfit streams: a modern example from the Brahmaputra River in Bangladesh and a possible ancient example in the Spanish Pyrenees

Book synopsis: Understanding of rivers and their sediments, both as modern systems and as ancient counterparts in the geological record, has progressed steadily but markedly over the past several decades, with contributions by practitioners in diverse fields of geosciences and engineering. This book contains 31 papers, with authors from 13 countries, who participated in the Sixth International Conference on Fluvial Sedimentology held in Cape Town, South Africa, in 1977. True to the nature of these quadrennial conferences, the papers in this book discuss a broad range of fluvial subjects that include the character of bedforms and sediment transport in river channels, morphological and sedimentological features of modern fluvial environments, modern and ancient avulsions, internal and external controls on the behaviour of river systems, and the facies and architectural organization of alluvial deposits

Experimental distributive fluvial systems: Bridging the gap between river and rock record

A debate has called into question as to which fluvial channel patterns are most widely represented in the stratigraphic record, with some advocating that distributive fluvial systems (DFS) predominate and others that a broad diversity of fluvial styles may become preserved. Critical to both sides is the adequate recognition of original channel planform from geological outcrops separated from their formative processes by millions or even billions of years. In this study the river and rock record are linked through experimentally created DFSs with both aggrading channel beds and floodplains. This approach allows depositing processes and deposited strata to be studied in tandem. Proximal areas comprise coarse, amalgamated channel-fills with scarce fine-grained floodplain material. The overall spread of sandbody dimensions become far more varied in medial stretches, with an overall reduction in mean width and depth. In these areas channel-fills may be sand-rich or mud-rich and, following avulsion, all channels are covered by floodplain sediment. Channels, levees and splays form discrete depositional bodies each with varying aspect ratios; a novel breadth of deposits and morphologies in aggrading experiments largely concurrent with proposed trends indicative of DFSs. The proportion of floodplain material increases distally, resulting in decreased interconnectedness of distal channel-fills. Muddy floodplain sediments significantly change DFSs behaviour and subsequent stratigraphic architecture by enhancing bank stability and reducing avulsion through the filling of floodbasins. The laboratory methods utilised here open up the possibility of controlled experimentation on the effects and mechanisms of DFSs sedimentation, which is important since the modelled stratigraphic trends are rarely so tractable in ancient geological outcrop belts