Derald G. Smith

Inclined heterolithic stratification—Terminology, description, interpretation and significance

Abstract Parallel to sub-parallel strata possessing original (depositional) dips occur within both lithologically “homogeneous” and “heterogeneous” units of water-lain, siliciclastic sedimentary sequences. Most such inclined strata form as a result of the lateral growth of “active”, large-scale “bedforms” such as point bars or Gilbert-type deltas. The confusing diversity of terms previously used to describe inclined stratified deposits is reviewed. Virtually all these terms, including epsilon-cross-stratification and its derivatives are unsatisfactory because they are non-descriptive and/or communicate an overt genetic bias. The names Inclined Heterolithic Stratification (IHS) and Inclined Stratification (IS) are proposed as replacements. To facilitate comparison of IHS deposits a “standard” descriptive nomenclature is also proposed. IHS may occur as solitary sets or show vertical or lateral stacking forming cosets. Co-directional laterally stacked sets constitute an imbricate coset. Composite sets are those in which IHS sequences gradationally overlie inclined-stratified lithofacies units (typically sandstones). Individual inclined units comprising IHS sets may be either normally graded or (more commonly) consist of two distinct lithological members ar anged as a coarse-to-fine couplet. Inclined units are separated by inclined surfaces indicative of non-deposition or erosion. Published examples of modern and ancient IHS deposits are known or inferred to occur in a variety of environments, but the overwhelming majority are products of point-bar lateral accretion within meandering channels of freshwater rivers, tidally influenced rivers and creeks draining intertidal mudflats. Descriptions are given of the most characteristic and important (from an interpretation standpoint) physical features of point-bar IHS deposits and their probable modes of origin. Deposits predominantly composed of sand and mud layers arranged as coarse-to-fine couplets are emphasized. Factors thought to control the formation and preservation of sand-mud couplets in the tidally influenced river point-bar depositional environment are described and their probable effects evaluated. Several potentially useful criteria for the differentiation of ancient freshwater versus tidally influenced river point-bar IHS sequences are discussed. The significance of IHS deposits for: (1) palaeoenvironmental and palaeogeographic interpretation; (2) reconstruction of palaeochannel morphological characteristics; and (3) economic geology is outlined. Future recognition of tidally influenced river point-bar IHS in the rock record should furnish valuable information regarding shoreline proximity, possible palaeotidal ranges etc. Much additional work is required on IHS deposits of modern point bars in general, and tidally influenced river point bars in particular, before satisfactory process-response depositional models of their formation can be developed.

Effect of vegetation on lateral migration of anastomosed channels of a glacier meltwater river

A series of experiments were performed on bank materials of anastomosed channels in flood-plain silt deposits in the Alexandra Valley in Banff Park, Alberta, to determine the effect of vegetation roots on bank erodibility and lateral migration of channels. Underground roots from the dense growth of meadow grass and scrub willow provide the reinforcement of bank sediment and a riprap-like protection of channel banks from river erosion. Results from the experiments suggest that in cool environments with aggrading river conditions where overbank deposition of silt, clay, and fine sand dominate the valley fill, vegetation roots are able to rapidly accumulate and decay very slowly, thus affording protection to banks from erosion in deeper parts of the channels. Experiments were performed with a specially designed erosion box, used as a means to simulate natural erosion conditions and measure the influence of vegetation roots in reducing bank erosion. Results indicate that the bank sediment with 16 to 18 percent by volume of roots with a 5-cm root-mat for bank protection, typical of the area, had 20,000 times more resistance to erosion than comparable bank sediment without vegetation. Assuming five severe erosion days per year, potential lateral channel migration would amount to 4.2 cm per year. Such resistance, due to vegetation, accounts for the remarkable stability of channels during the last 2,500 yr in the Alexandra Valley.

Sedimentation in anastomosed river systems; examples from alluvial valleys near Banff, Alberta

ABSTRACT Three anastomosed river systems are described. Each reach consists of an interconnected network of low-slope, narrow and deep, straight to sinuous, stable channels that transport coarse sand and gravel. Channels are separated by levees and wetlands composed of silt/mud and vegetation. Gravel-bed braided channels occur upstream from each anastomosed system, joined by a transitional reach comprising stable, elongate, silt islands within braided channels. The three anastomosed reaches have formed upstream from elevating base levels caused by deposition of alluvial fans across trunk valleys. Rapid aggradation of floodplain alluvium is confirmed by buried volcanic ash layers. Channel migration is inhibited by root-stabilized banks which, combined with rapid vertical aggradation, results in production of stringer-like, coarse-grained channel deposits surrounded by overbank fines in stratigraphic cross sections. Although it is unlikely that such small base-level controls (alluvial fans) could produce extensive anastomosed deposits, other mechanisms such as glacial moraines, isostatic rebound, or marine transgressions could provide plausible controls for yielding important contributions to the stratigraphic record.

Preboreal oscillation caused by a glacial Lake Agassiz flood

The Preboreal oscillation (PBO) has been attributed to increased meltwater, but the source of the meltwater and causative mechanism of the PBO has remained elusive. Here we attribute the source to a massive meltwater discharge event from an abrupt drainage of glacial Lake Agassiz, Canada, via the Mackenzie River into the Arctic Ocean. A maximum volume of 21,000 km 3 was discharged over a 1.5–3 yr period with a peakdischarge of 0.500 Sverdrups (Sv), equivalent to a 6 m rise in the Arctic Ocean (or 0.062 m rise in global sea level). The flood occurred at about 11,335 cal yr BP, and was followed by a B0.042 Sv flow until 10,750 cal yr BP when the southern outlet of Lake Agassiz reopened and diverted drainage to the Mississippi River system. We estimate that only 2–4% of the flood water would have frozen into sea ice within the Beaufort region, but coupled with increased river ice production during winter, and thicker pack ice growth throughout the Arctic Ocean, a thicker, longer lasting and more extensive packice may have been flushed through Fram Strait. The thicker and more extensive packice, and freshened sea surface, may have triggered the PBO by increasing albedo, and generating a low salinity anomaly upon melting in the North Atlantic, thus decreasing the formation of North Atlantic Deep Water. r 2002 Elsevier Science Ltd. All rights reserved.

Anastomosing river deposits, sedimentation rates and basin subsidence, Magdalena River, northwestern Colombia, South America

Situated in a tectonically active foreland basin, the Magdalena River consists of vertically accreting, levee-confined channels and adjacent extensive wetlands, which are interpreted as an anastomosing river sedimentary system. Equivalent rates of basin filling and subsidence average 3.8 mm yr−1 based on 18 14C dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement. Located in a savanna-tropical climate, anastomosing river deposits of the Magdalena are remarkably similar to the anastomosing deposits of the upper Columbia River in a temperate-cold climate in western Canada, suggesting that climate is not a controlling factor of anastomosis. The geometry of anastomosing channel-fills in the Magdalena consists of stratigraphically non-uniform, low sinuous, narrow stringers of sand up to 30 m thick by 600 m wide, a width-depth ratio of 20. Thin (1–2 m) off-channel crevasse-splay sand sheets extend laterally up to 10 km distance. When buried, both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps. While there are few modern anastomosing river systems as compared to braiding and meandering, there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion. Such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided-river deposition models.

Seismic geomorphology and sedimentology of a tidally influenced river deposit, Lower Cretaceous Athabasca oil sands, Alberta, Canada

The bitumen of the Lower Cretaceous McMurray Formation in Alberta arguably represents one of the most important hydrocarbon accumulations in the world. In-situ development relies on heat transfer through the reservoir via horizontal steam injection wells placed 4 to 6 m (13–20 ft) above horizontal producers near the base of the sandstone reservoirs. Given this technology, understanding the distribution of the resource is paramount for a successful development program. Sedimentary facies provide a direct control on bitumen distribution and recovery. Most facies models developed to describe and predict sedimentary units of the McMurray Formation consider fluvial, estuarine, and/or deltaic depositional settings. In-situ development, however, requires a particularly high-resolution sedimentologic interpretation. High-quality three-dimensional seismic reflection data and extensive drill cores from acreage located approximately 50 km (31 mi) south of Fort McMurray provide important insights into the sedimentologic organization of reservoir and nonreservoir deposits in the upper one third (40 m [131 ft]) of the reservoir interval. Geomorphologic characteristics of the strata observed in seismic time slices reveal that a fluvial depositional setting was prevalent. Ichnologic and palynologic data, as well as sedimentary structures suggestive of tidal processes, indicate a marine influence in the upper reaches of a fluvial system characterized by channels that were 390 to 640 m (1280–2100 ft) wide and 28 to 36 m (92–118 ft) deep. The complex stratigraphic architecture consists of a mosaic of large-scale depositional elements, including abandoned channels or oxbow lake fills, point bars associated with lateral accretion, point bars associated with downstream accretion, counter point bars, and sandstone-filled channels. Reservoir deposits are primarily associated with point bars and sandstone-filled channels.

Glacial Lake Agassiz: The northwestern outlet and paleoflood

Valley morphology and sediment in the Fort McMurray region of Alberta indicate that a catastrophic flood discharged down the lower Clearwater and Athabasca river valleys 9900 yr B.P. Geomorphic and chronologic evidence suggests that glacial Lake Agassiz (Emerson phase) was the probable water source. As the flood incised a drainage divide located near the Alberta-Saskatchewan border, the level of glacial Lake Agassiz decreased by 46 m, discharged 2.4 x 106 m3/s for at least 78 days, and stabilized at 438 m above sea level in the Lake Wasekamio area. At that time water entered the Arctic Ocean via glacial Lake McConnell and the Mackenzie River, rather than the Gulf of Mexico via the Mississippi River, as previously thought. Such a large influx of fresh water (8.6 km3/h) into the Arctic at the close of the last glaciation may have had an abrupt, major influence on northern climate.

Ground penetrating radar: antenna frequencies and maximum probable depths of penetration in Quaternary sediments

Abstract Ground penetrating radar (GPR) experiments were carried out in a gravel pit near Brigham City, Utah, USA,to determine maximum probable depths of penetration for 25, 50, 100 and 200 MHz antennas. We have found that this sedimentary field environment (quartzose-rich, thick, inclined gravel strata) is the most appropriate site known and available for the experimental objectives. With a 1000 V transmitter, 25 MHz antennas are capable of detecting stratigraphy to 52 m and possibly 57 m deep. Excessive signal losses for the 50, 100 and 200 MHz antennas occur at depths below 47, 37 and 28 m, respectively, preventing effective detection of stratigraphic interfaces. From 250 different field experiment sites, we suggest that these profiles represent the maximum probable GPR depths that can be confidently interpreted from any Quaternary unconsolidated sediments. A comparison of results shows a linear trend between different antenna frequencies and the maximum probable depth of penetration, suggesting that the 12.5 MHz antennas can detect strata to 66 m deep. Results obtained using the 25 MHz antennas indicate that at least 52 m of inclined strata, assumed to be foreset facies of gravel, occur beneath the gravel pit floor.

Evidence for eight great earthquake-subsidence events detected with ground-penetrating radar, Willapa barrier, Washington

A new approach to detect Holocene subduction-zone earthquakes combines the results from ground-penetrating radar (GPR), Vibracores, and accelerator mass spectrometry (AMS) dates from a barrier spit located west of Willapa Bay, southwest Washington. GPR data show a 10-m-thick facies of beach sand within which we identify, and Vibracores confirm, beach-parallel, wave-eroded, buried scarps mantled with multiple beds of magnetite. The eight GPR-detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subsidence during earthquakes associated with the Juan de Fuca plate subducting under the North American plate. Of these scarps, four have been AMS dated at 300, 1110, 2540, and 4250 (radiocarbon) yr B.P. No datable material has yet been found for the other four radar-detected scarps, but we interpolate and extrapolate dates of 1800, 3400, 5000, and 5800 yr B.P.

William River: An outstanding example of channel widening and braiding caused by bed-load addition

The lower William River in northwestern Saskatchewan, Canada, presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load. A relatively narrow and deep single-channel stream as it flows northward to Lake Athabasca, the river picks up a 40-fold increase of bed load over a 27-km reach as it encounters a large dune field just south of the lake. As a result of the large infusion of eolian sand, the channel develops a thoroughly braided pattern while undergoing a 5-fold increase in width and a 10-fold increase in width/depth ratio.