Evolution of Fluvial Meander Belt Deposits With Implications for the Completeness of the Stratigraphic Record

Abstract

The completeness of the stratigraphic record has been widely discussed, yet the gaps in the rock record are often readily over-looked when interpreting paleoenvironments. The fragmentary nature of the stratigraphic record is particularly evident in fluvial deposits, which are characterized by a hierarchy of depositional units deposited and eroded over a wide range of time scales and sedimentation rates. In this study we evaluate stratigraphic preservation in meander belt deposits by determining a history of bar evolution, the total area deposited (MA), and the preserved bar area (PA) passed into the depositional record. Two datasets from the modern Mississippi River and one from the Cretaceous McMurray Formation of Alberta are considered. An understanding of completeness strikes directly at the question of what preserves in the stratigraphic record of modern environments and the degree and manner to which models of modern systems can be used to predict and interpret reservoir architecture in ancient strata. Two study areas in the Mississippi River valley were chosen to capture different levels of uncertainty in our analysis: the New Madrid Seismic Zone (NMSZ: New Madrid, MO) and 25km upstream of the confluence with the St. Francis River at Helena, AR. At the NMSZ, a series of radiocarbon (RC) and Optically Stimulated Luminescence (OSL) dates document the age of abandoned channel fills and scroll bars, respectively. Cross-cutting relationships, scroll bar patterns and RC/OSL dates (at NMSZ only) were used to reconstruct paleochannel migration patterns. These evolution patterns of the meander belt were used to interpret seismically constrained scroll patterns of the ancient McMurray Formation; the evolutionary history of a series of meander belt elements, including point bars, counter point bars and abandoned channel fills is characterized. Reconstructed paleochannel migration pathways are deduced, which enable calculations of MA, PA and percent of bar preserved (PA/MA). The analysis reveals that total bar preservation is typically < 50%; youngest bars are unsurprisingly more complete. Removal of early products of bar growth results from downstream erosion and development of increased sinuosity. At the scale of a meander belt, older bars are more likely to be reworked through continued channel migration. The analysis provides a link between meanderbelt processes and the stratigraphic complexity typically encountered in the stratigraphic record.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016