Meandering Fluvial Facies Architecture: Insights from the Ferron Sandstone of Utah

Anderson, Sloan T.; Kerr, Dennis R.

Detailed mapping of low sinuosity meandering channel-belts in the Ferron Sandstone has helped address several poorly understood meandering fluvial facies relationships. Documenting the fate of sands in the riffle between successive lateral accretion bars was a primary objective. Riffles, also known as crossovers, are well documented in coarse sand and gravel systems, however, their character in single thalweg fine sand systems is ill-defined. Well developed riffle facies could improve fluid flow between lateral accretion bars and increase reservoir volumes.

This study utilizes three-dimensional outcrops to evaluate channel pool, riffle, and lateral accretion bar evolution through the development of a channel-belt. Facies distributions within a channel belt were mapped with a differential GPS, as were adjacent floodplain and splay facies. A hierarchy of bounding surfaces was developed to distinguish pool and riffle facies from lateral accretion elements, and delineate superimposed channel-belts and storeys.

Lateral accretion bar elements are considered middle channel-fill sub-facies and are comprised of cosets of medium-scale (dune) trough cross bedding and small-scale (ripple) cross lamination with coset contacts merging with low angle (7°-12°) lateral accretion surfaces. Riffle elements are difficult to distinguish from pool elements, so both are grouped into a lower channel-fill sub-facies. The lower channel-fill sub-facies is coarser and less sorted than middle channel-fill, and contains medium-scale (dune) trough cross bedding, small-scale (ripple) cross lamination, and upper plane bed tabular lamination attributed to channel floor bedforms. Mudstone intraclasts and plant debris eroded from the cutbank aid in locating the pool. Pool-riffle transitions are marked by better sorting, smaller intraclasts, and less plant debris. Successive lateral accretion bars are connected via their common base atop the lower channel-fill. This is similar to the "string of beads model" proposed by Donselaar and Overeem (2008).

To assess the impact of facies architecture on permeability anisotropy, stratal oriented core plugs were collected from middle and lower channel-fill sub-facies. Results indicate a consistent order of magnitude decrease in permeability perpendicular to bedding planes and lateral accretion surfaces compared to stratal strike- and dip- parallel oriented plugs.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013