Grain-size variability within a mega-scale point-bar system, False River, Louisiana PETER D. CLIFT* † , ELIZABETH D. OLSON*, ALEXANDRA LECHNOWSKYJ*, MARY GRACE MORAN*, ALLISON BARBATO* and JUAN M. LORENZO* *Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA (E-mail: pclift@lsu.edu) †Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA ABSTRACT Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocar- bon reservoirs. Standard point-bar models of how the internal sedimentol- ogy 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 (>25Á0 m thick and 7Á5 9 13Á0 km across) on the Mis- sissippi River (USA) was examined. The lithology and grain-size character- istics 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 read- ily after the establishment of a compound bar in the later stages of con- struction, 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 coarsen- ing-upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics. Keywords Electrical conductivity, grain size, meander, Mississippi, point bar. INTRODUCTION Mega-point bars Alluvial floodplains are a major class of conti- nental sedimentary depositional setting (Miall, 2006). Sediments preserved in such locations can form thick successions in the geological record where subsidence allows their long-term preservation [for example, the Old Red Sand- stone of the British Isles (Marriott & Wright, 1993); the Beaufort Formation of the Karroo Basin (Smith, 1990); the Indo-Gangetic Plain (Gibling et al., 2005); and the Rocky Mountain foreland basin (McCarthy et al., 1997)]. Mean- dering rivers form a significant sub-category of all such deposits, although their preservation potential may be less than braided rivers that have been argued to dominate the geological record (Gibling, 2006). Whether this is actually correct or not has recently been called into ques- tion (Hartley et al., 2015). Although the basic 1 © 2018 The Authors. Sedimentology © 2018 International Association of Sedimentologists Sedimentology (2018) doi: 10.1111/sed.12528