Structures and facies associated with the flow of subaerial basaltic lava into a deep freshwater lake: The Sulphur Creek lava flow, North Cascades, Washington

The ca. 8800 ¹⁴C yrs BP Sulphur Creek lava flowed eastward 12 km from the Schriebers Meadow cinder cone into the Baker River valley, on the southeast flank of Mount Baker volcano. The compositionally-zoned basaltic to basaltic andesite lava entered, crossed and partially filled the 2-km-wide and > 100-m-deep early Holocene remnant of Glacial Lake Baker. The valley is now submerged beneath a reservoir, but seasonal drawdown permits study of the distal entrant lava. As a lava volume that may have been as much as 180 × 10⁶ m³ entered the lake, the flow invaded the lacustrine sequence and extended to the opposite (east) side of the drowned Baker River valley. The volume and mobility of the lava can be attributed to a high flux rate, a prolonged eruption, or both. Basalt exposed below the former level of the remnant glacial lake is glassy or microcrystalline and sparsely vesicular, with pervasive hackly or blocky fractures. Together with pseudopillow fractures, these features reflect fracturing normal to penetrative thermal fronts and quenching by water. A fine-grained hyaloclastite facies was probably formed during quench fragmentation or isolated magma-water explosions. Although the structures closely resemble those developed in lava-ice contact environments, establishing the depositional environment for lava exhibiting similar intense fracturing should be confirmed by geologic evidence rather than by internal structure alone. The lava also invaded the lacustrine sequence, forming varieties of peperite, including sills that are conformable within the invaded strata and resemble volcaniclastic breccias. The peperite is generally fragmental and clast- or matrix-supported; fine-grained and rounded fluidal margins occur locally. The lava formed a thickened subaqueous plug that, as the lake drained in the mid-Holocene, was exposed to erosion. The Baker River then cut a 52-m-deep gorge through the shattered, highly erodible basalt.     

The Effects of Varying Salinity on Ammonium Exchange in Estuarine Sediments of the Parker River,Massachusetts

We examined the effects of seasonal salinity changes on sediment ammonium (NH₄ ⁺) adsorption and exchange across the sediment–water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH₄ ⁺ relative to adsorbed NH₄ ⁺ in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH₄ ⁺ adsorption by reducing the exchangeable pool from 69% to 14% of the total NH₄ ⁺ in the upper estuary sediments that experience large (0–20) seasonal salinity shifts. NH₄ ⁺ dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH₄ ⁺ diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH₄ ⁺ profiles that fit measured profiles very well and described a seasonal pattern of NH₄ ⁺ flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH₄ ⁺ release from the sediments. Salinity-mediated desorption and fluxes of NH₄ ⁺ from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.     

Statistical treatment of fluvial dose distributions from southern Colorado arroyo deposits

Many of the small-scale arroyo systems found across southern Colorado contain well-preserved sedimentary records of prehistoric fluvial erosion and aggradation epicycles. In the following paper, we date a set of 50 ephemeral fluvial samples from four southern Colorado arroyo systems using a combination of single-grain and single-aliquot OSL techniques. Analysis of the sample D_e distribution characteristics reveals that these arroyo sediments were subjected to a diverse array of bleaching conditions prior to deposition. The use of appropriate burial dose estimation procedures is therefore deemed vital to ensuring that accurate age estimates are produced for each of these samples. In this study we apply the formal ‘age model’ decision procedures of Bailey and Arnold [Statistical modelling of single-grain quartz D_e distributions and an assessment of procedures for estimating burial dose. Quaternary Science Reviews, in press.] and Arnold [2006. Optical dating and computer modelling of arroyo epicycles in the American Southwest. D.phil. Thesis, University of Oxford, unpublished] to our fluvial sample dataset in order to enable a more objective selection of appropriate burial dose estimates. The resultant OSL chronostratigraphies are examined and discussed. These formal decision procedures yield sample ages that are stratigraphically consistent for 94% of the 50 fluvial samples examined. The resulting OSL ages also display a greater degree of stratigraphic consistency in comparison to those ages that would have been generated by simply applying a single type of age model to all samples.     

Carbonate sedimentation in a starved pull-apart basin, Middle to Late Devonian, southern Guilin, South China

ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle‐ to rhomb‐shaped basin filled with Devonian shallow‐ to deep‐water carbonates. This Yangshuo Basin is interpreted as a pull‐apart basin created through secondary, synthetic strike‐slip faulting induced by major NNE–SSW‐trending, sinistral strike‐slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N–S‐trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike‐slip faults; the NW–SE‐trending margins were related to parallel, oblique‐slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1–2, 3–5 and 6–9), reflecting three major phases of basin evolution. During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low‐gradient shelf. Only at the steep, fault‐controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull‐apart of the master strike‐slip faults during the late Givetian intensified the differential subsidence and produced a spindle‐shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north‐west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike‐slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform‐margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen‐depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep‐seated faults. Two ‘subdeeps’ were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian. This case history shows that the carbonate platform architecture and evolution in a pull‐apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.     

The verification of numerical models with multivariate randomized block permutation procedures

Summary Multivariate randomized block permutation procedures (MRBP) can be used effectively to verify numerical models. Compared to other statistical methods, MRBP shows several distinct advantages. First of all, MRBP operates in the same Euclidean analysis space as its input data. The root mean square error (RMSE) is discussed, since it is a natural choice as a distance measure between two data sets and is closely related to the distance measure on which MRBP is based. The RMSE by itself provides no basis for inferential comparisons, whereas MRBP is well suited for such deductions. Since MRBP is computationally economical and requires only a few case studies for meaningful comparisons, it is also useful for model development.With 3 Figures     

Predicting sediment flux from fold and thrust belts

The rate of sediment influx to a basin exerts a first-order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment-flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re-eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.     

An improved ‘Battjes’ method for predicting the probability of extreme waves

The Battjes method for predicting the 50 or 100-year design wave was developed to allow for the possibility that the highest wave in a 50 or 100 year period may occur during the second highest storm or even in lower storms. It uses the probability distribution of individual waves. It is first shown that a slightly different logical approach removes some of the problems encountered with the use of the method. It is then shown that it actually uses a different definition of return period to that used by the classic method because if two or more waves in a severe storm exceed H₅₀, then these are counted as separate events. A formula is developed which considers each storm as one event, but still takes account of the possibility of the highest wave in 50 years not coming from the most severe storm. Computation using this formula shows that it reduces H₅₀ by about 3% relative to the Battjes method.