Abstract Previously termed concave bank-bench deposits, point-bar-tail deposits, and distal point-bar deposits, counter-point-bar deposits have received little research attention, and their stratigraphy and sedimentology is poorly understood in modern and ancient river meander belts. The stratigraphy and lateral continuity of counter-point-bar deposits were studied in the modern Peace River Delta, northeast Alberta, Canada. Morphology of counter-point-bar deposits was identified as having concave-shaped scroll-bar and vegetation patterns, and they are always located immediately downriver from sandy point bars. Stratigraphy of counter-point-bar deposits, composed of 55 to 86% silt, were studied using a vibracorer to depths of 6.5 m and channel-bottom grab samples to depths of 20 m over a river distance of 1800 m. This contrasts with 95 to 100% sand in adjacent point-bar deposits. Like point bars, counter point bars are also lateral-accretion deposits, but are composed of inclined heterolithic strata. Counter-point-bar deposits can form in short-radius meanders, as well as in wide-radius broad meanders. Counter-point-bar deposits formed where channels impinge at low angles (10 to 40°) against bedrock valley sides or other erosion resistant sediment. Within the Athabasca River Delta, a different feature called an eddy-accretion deposit, was studied and compared to counter-point-bar deposits. Eddy-accretion deposits formed as short-radius, pronounced concave scroll-bar and vegetation patterns that notably arc up-valley. In contrast to the counter-point-bar deposits analyzed, these deposits are thicker, sandier, and not as laterally extensive for a channel of a given size. Eddy-accretion deposits formed where river flow impinges more directly against (40°–140°) a valley side or channel margin composed of bedrock or resistant sediment such as an oxbow-lake fill, abandoned-channel fill or, as in this case, a Holocene prodelta deposit. The eddy-accretion deposit studied is up to 500 m wide and 24 m thick, although probably closer to 34 m thick during near-bankfull discharge. It is composed of up to 80% sand.

Abstract The wave-dominated Holocene William River delta, located along the south coast of Lake Athabasca, northern Saskatchewan, is a lobate sand body 9 km long by 8 km wide by 15-22 m thick. Mean lake elevation is 209 m and fluctuates ± 1.5 m (1810-2000). Periodic strong winds, waves, and currents generated in deep water (50-100 m)with long fetches (50-110 km) shape the delta morphology and determine its internal sedimentary structure. Its surface is dominated by multiple arcuate sets of eolian-dune-capped beach ridges separated by peatlands. A one-kilometer-wide arcuate, subaqueous shelf is covered by multiple (up to 19) offshore bars and troughs in water depths to 5.5 m. Farther offshore, a 1 to 7° foreset slope plunges to a depth of 15 to 17 m. Lakeward from the toe of the foreset slope, gently sloping (0.0072 or 7.2 m/km) bottomset beds of thin sand lenses interfinger with lacustrine mud. Luminescence and radiocarbon dates suggest that the rate of delta progradation has declined an order of magnitude from an initial 1.4 m yr -1 (9800 to 7000 cal yr BP) to the present 0.16 m yr -1 , because of reduced fluvial sediment influx from a nearby depleting eolian dune field and exponentially larger volumes of sand required to maintain constant rates of linear expansion. Radar stratigraphic profiles to depths of 15-22 m along depositional dip and strike indicate lakeward-inclined erosional surfaces and lensoid sand bodies. Nested within these lenses are “J-shaped” reflections which represent progradational sequences deposited by annual to decadal storms. Several gently basinward-dipping (< 1°) erosional surfaces between 5 and 14 m deep are interpreted to have formed by severe millennial-frequency storms.

Abstract Ground penetrating radar (GPR) stratigraphic profiles of the classic cross-valley barrier and associated spits of Late Pleistocene Lake Bonneville, near Stockton, Utah, are used to infer transgressive depositional style and internal sedimentary structures. From onlapping patterns of radar reflections, which mimic subsurface stratigraphy, we reconstruct the following depositional sequence and style: (1) at the north end of the Rush Valley, the barrier formed by vertical accretion while keeping pace with hydro-isostatic-forced basin subsidence and/or slow lake-level rise; (2) a reorientation of the longshore transport pathway, induced by continued basin subsidence and/or a lake-level rise, produced a spit that prograded 2.5 km southwestward into Rush Valley. The NW-dipping radar reflections from the spit onlap SE-dipping reflections from the back-barrier, indicating that this spit was deposited after the barrier; (3) a final rise in lake level and/or basin subsidence again reoriented longshore transport and deposited the smaller upper spit. Radar reflections from the upper spit onlap the proximal eastern margin of the Stockton spit. This upper spit is the final landform deposited during the Bonneville highstand. The depositional sequence inferred from radar stratigraphy agrees with the transgressive hypothesis formulated in 1890 by G. K. Gilbert.

ABSTRACT The interpretation and reconstruction of subsurface environments is an important task faced by oil/gas exploration and academic scientists. Of the various geophysical techniques employed in such work, ground penetrating radar (GPR) is increasingly being used to image and assess both modem and ancient depositional environments. The central aim of this paper is to present GPR results delineating modern and ancient coastal and fluvial sequences. GPR investigations were conducted to examine the sedimentary structures at test sites in Washington, Utah, Alberta, and British Columbia. Along coastal Washington State, several GPR profiles, including a grid dataset, were collected. The grid data was displayed, using three-dimensional rendering software that greatly enhanced our ability to interpret the three-dimensional geometry. Results from the GPR study indicate a shingle-like accretionary depositiona! pattern ofbeach and upper shoreface reflections which dip seaward at about one degree of slope. Outcrop and GPR results in the Book Cliffs (Panther Tongue), Utah, show similar stratigraphy. A second comparison of GPR lines shows three modern braided fluvial deposits in Alberta and British Columbia, and an ancient fluvial channel in Utah. Results indicate a variety of depositional features. GPR profiles in modern fluvial environments can aid in the interpretation of ancient fluvial examples and valley-fills. GPR proved to be a very effective method for mapping subsurface stratigraphy and sedimentary facies of coastal and fluvial environments. Radar stratigraphic analysis, an emerging approach for interpreting sedimentary environments, was used. The results, based on modern analogues and ancient examples, will provide the oil and gas industry with additional insights regarding stratigraphy and specific characteristics of ancient reservoirs.

Abstract Modern meandering river point bar deposits formed in fluvial and tidally-influenced environments were investigated to explain why large-scale epsilon cross-stratification (ECS) is common in ancient fluvial rocks but appears to be absent in modern deposits. To resolve this problem several modern meandering systems were studied; the Athabasca upper delta plain in northeast Alberta, Canada; the mesotidally-influenced reach of the Willapa River, southwest Washington State, U.S.A.; and the lower Daule and Babahovo Rivers, Ecuador, which have micro- and mesotidally-influenced depositional conditions. As well, exposed point bar facies in 18-m-high cut-banks were examined in the fluvial, lower Liard River (Middle Holocene), NWT, Canada, and tidally-influenced Willapa River (Late Pleistocene), Washington State. Based on sedimentologic results obtained from these areas, a threefold lithofacies classification of point bar deposits is proposed: (1) fluvial sandy point bar facies, (2) low-energy fluvial and microtidally-influenced (upper estuary) point bar facies, (3) mesotidally-influenced point bar facies deposited in upper and middle estuary settings. The latter two facies are very similar to many reported ancient meandering river point bar rocks. The three lithofacies models are compared with four ancient examples of point bar rocks selected from Alberta, Canada, the Lower Cretaceous middle McMurray Formation (Athabasca Oil Sands), the Upper Cretaceous Judith River and Horseshoe Canyon Formations and the Paleocene Paskapoo Formation.