Shallow Seismic Reflection Images of Modern Point Bar Deposits, False River, Louisiana

Abstract

The internal structure of a point bar complex can be more heterogeneous than previously understood from study of a limited range of modern systems. For example, unit bars, which are known to exist in both braided and meandering river deposits, characteristically show internal stratigraphic dip opposite to that of the surrounding point bars. Shallow (0–30 m) shear-wave seismic reflection data were collected from the head and tail sections of the point bar complex associated with of an abandoned (~c. 1720) meander loop of the Mississippi River, False River, Louisiana. Shotpoints and single, horizontal-component geophones are located every meter and both hammer-impact and impulsive surface shear sources were deployed. Core/ log sites occur on average every 50 to ~100 m along each transect. In combination with well logs (electrical conductivity, hydraulic profiling tool, gamma ray) and core analysis, these 2-D and preliminary 3-D surveys supply unprecedented near-surface characterization of the sedimentological features of point bars for a large river. Three 2-D seismic transects (150 to ~ 200 m long) reveal images of internal structure of a large-scale, Holocene point bar system. Seismic-to-well correlation confirms the presence of upper bar, lower-bar and interbedded heterolithic units. Location within a point bar, with respect to the direction of channel flow of a meandering river, is a factor in the heterogeneity of the bar and its petrophysical characteristics. Position within the point-bar controls clay/sand ratios and the thicknesses of structural layers within the bar; these distinguishing features suggest that location within the point bar can be inferred from sedimentary characteristics. For the seismic transect at the head of the point bar we interpret that a unit bar lies beneath a major reorientation surface. On the Mississippi River, active, nearby (~30 km) analogous mid-channel bars appear as large, >300-m-wide deposits. Preliminary analyses confirm expectations that the point bar tail shows better reflectivity, and a higher ratio of silt- and clay-to-sand compared to the point bar apex and head, with additional fining associated with meander loop abandonment.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017