Grain‐size variability within a mega‐scale point‐bar system, False River, Louisiana

Abstract

Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.

An Integrated Approach for Constraining Depositional Zones in a Tide-Influenced River: Insights from the Gorai River, Southwest Bangladesh

“…Normal gradation is a common element of fluvial point bar depositional models and is attributed to the progressive accumulation of finer overbank sediments over coarser thalweg sands as the channel migrates across the floodplain (e.g., Refs. [18,53,72,73]). Inherent to this model of formation is the notion that fluvial point bars are laterally mobile features, migrating over time through extension and down-valley translation (e.g., Refs.…”

Ontogeny of a subtidal point bar in the microtidal Venice Lagoon (Italy) revealed by three‐dimensional architectural analyses

“…Indeed, correlations between planform evolution (i.e. scroll‐bar pattern) and related point‐bar geometries were carried out for fluvial meanders (Pranter et al ., 2007; Ghinassi et al ., 2013, 2014; Ielpi & Ghinassi, 2014; Ghinassi & Ielpi, 2015; Yan et al ., 2017, 2019; Willis & Sech, 2018a; Ghinassi et al ., 2019a; Parquer et al ., 2019) along with models concerning along‐bend grain‐size distribution (Purkait, 2002; Frings, 2008; Smith et al ., 2009, 2011; Bhattacharyya & Bhattacharya, 2015; Ghinassi & Ielpi, 2015; Clift et al ., 2019; Hagstrom et al ., 2019). Comparable insights are conversely missing for intertidal meanders, whose planform evolution has been inferred only for short time intervals (Choi et al ., 2013; Ghinassi et al ., 2018b) and for which the spatial distribution of sediment grain size was mainly investigated with vertical, rather than along‐channel, trends (de Mowbray, 1983; Dalrymple et al ., 1990; Pearson & Gingras, 2006; Choi, 2011; Choi et al ., 2013; Choi & Jo, 2015).…”

Chironomidae Morphological Types and Functional Feeding Groups as a Habitat Complexity Vestige

“…For the solution analyzed concentrations of such elements as: Na, K, Mg, Ca, Fe, Mn, Cu, Zn, and Pb, the atomic absorption spectrometry (AAS) was used. Grain size composition of the ash samples, remaining after the solution was made in accordance with Clift et al (2019), was determined using a Mastersizer 3000 laser particle-size analyzer (Malvern). The relationship between the mean grain size and the sorting index (the so-called coordinate system) follows Mycielska-Dowgiałło and Ludwikowska-Kêdzia (2011).…”