Sedimentology (2018) doi: 10.1111/sed.12528 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:
[email protected]) †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.