FALSE RIVER

Predicting Point Bar Lithological Heterogeneity

Point Bar Stratigraphy

Point bar complexes are the building blocks of stratigraphic units in meandering river sequences (Hartley et al., 2017). As such they make up a significant part of the sedimentary record of many fluvial-dominated, intra-continental or marginal basins (e.g., Old Red Sandstone of the British Isles). Such depositional settings can be vast (100's km across), so that where they are found to be hydrocarbon bearing, their scale makes them of considerable economic interest. Point-bar deposits in particular can make for excellent reservoirs because they commonly comprise substantial volumes of well-sorted sands that in certain circumstances can make productive hydrocarbon reservoirs. Understanding the complexity of point bar interiors is important to efforts to extract hydrocarbons from the sequences or use them as water aquifers because porosity and permeability, which are controlled by lithology, can vary quickly with depth, as well as laterally making the efficient placement of wells problematic. This is particularly true of the Cretaceous bitumen sands of Alberta, Canada where thick sequences of meandering river sediments of the McMurray Formation have been the target of hydrocarbon exploration and production

Predicting Heterogeneity

Within a given point-bar deposit there is a broad degree of lithological heterogeneity that extends beyond the primary controls of downstream fining and thickening of the IHS. Finer-grained fractions are thought to be deposited in the waning stage of flood events (Bridge and Diemer, 1983), as well as within tidal settings. Although significant IHS used to be exclusively linked to tidally influenced fluvial environments, deposits of 10's of meters are still viable in dominantly fluvial settings (Geehan et al., 1986; Verrien et al., 1967) so that a tidal mechanism for explaining muddy IHS is mostly not applicable, and certainly not through much of the lower Mississippi.

History of False River

The accretion history of the False River Point bar is only tightly constrained in age by the historically known cut-off date of 1720, but the building of the point bar can also be understood through interpretation of the scroll bar topography which has been defined in detail thanks to LiDAR mapping. This shows 12 clear surfaces where the point bar abruptly changed its direction of accretion, each likely following a flooding event that significantly changed the direction of current flow in the channel.

Site J2: Drilling in the early bar apex revealed a relatively thin IHS sequence with a large relatively homogeneous sequence of sands in the lower bar section. In general porosity and permeability were good in the lower bar. End member modeling of grain size data from cored sediment samples showed good sorting and a general coarsening down-section.

Site B3: Drilling at the terminal apex, deposited just before cut-off in 1720 revealed sharp changes in grain size towards the end of point bar construction. While Site B1 showed a high porosity, high permeability section, the youngest site, B3 has a very thick IHS with many interbedded clays and silt layers within the sand. In practise there is no homogenous sand sequence at the base of the point bar deposit at this site. We attribute this to slower current velocities prior to point bar cut-off.

Site W1: In the SE part of the point bar drilling was able to sample the downstream part of the section which was expected to be the finest grained sediment in the area. While drilling at Site W1 reveals a very thick IHS section and many fine-grained interbeds of clay and silt throughout the section, we were able to identify significant volumes of sand-rich deposits in the NW part of the area. These were deposited during the middle part of the bar accretion and prior to the onset of a compound geometry, resulting in a high quality reservoir. In general the porosity and permeability of the point bar sands decreased sharply after the onset of the compound geometry.

Summary

Investigations of the point bar at False River confirmed that the largest point bar complexes are more complicated in their interior architecture than those developed in small rivers. In particular, the transition from a simple geometry to a compound geometry appears to be important in defining large-scale change in the thickness of the IHS, as well as generally in causing a fining of grain size especially in the downstream parts of the bar. In the next stage of research on this topic we attempt to date the age of formation of this point bar and assess the duration over which one of these complexes is formed. We further use this as a high resolution record of development of composition of sediment in the Mississippi river over the last 2000 years to test models of sediment buffering in large drainages.

References

Clift, P.D., Olson, E.D., Lechnowskyj, A., Moran, M.G., Barbato, A., Lorenzo, J.M., 2018. Grain-size Variability within a Mega-scale Point Bar System, False River, Louisiana. Sedimentology. doi:10.1111/sed.12528.
Neubeck, N., Carter, A., Rittenour, T., Clift, P.D., 2023. Climate and anthropogenic impacts on North American erosion and sediment transport since the Last Glacial Maximum: Evidence from the detrital zircon record of the Lower Mississippi Valley, USA. GSA Bulletin. 10.1130/b36565.1.