ABSTRACT: Sandbody Geometry in an Aggrading Flood Basin

FARRELL, KATHLEEN M., North Carolina Geological Survey, Raleigh, NC, and NORMAN D. SMITH, University of Illinois, Chicago, IL

In the avulsion belt of the Saskatchewan River (Cumberland Marshes, Saskatchewan), the composition and architecture of splay deposits is controlled by local sediment supply and flow conditions. An examination of several cored splays reveals two major sand facies set within contemporaneous finer-grained sediment: (1) a large-scale, sharp-based, cross-stratified sand, and (2) an interlaminated facies.

The interlaminated facies usually consists of disconformable sets of graded layers of sand, silt, clay, detrital plant debris, and mudball conglomerate. The composition and thickness of the graded layers reflect the local availability of quartz sand relative to other grain sizes, grain types, and the energy level of the flood waters. This facies occurs as extensive sheets beneath splay lobes and as narrow, wedge-shaped levee deposits alongside splay and trunk channels. It encloses or underlies the cross-stratified sand.

While all the splays include deposits of the interlaminated facies, the occurrence, geometry, architecture, and scale of the cross-stratified sand facies varies between sites, apparently as a consequence of the relative abundance of quartz sand available to the system. This facies occurs (1) as extensive, subsurface sand sheets beneath splay lobes, (2) as surficial, coalescing, elongate lobes, (3) as lobes within splay channels, and (4) as narrow, discontinuous, wedge-shaped sheets alongside splay channels.

Thus, splays within a flood basin range from (1) sand-starved splays built of graded interlayers of mudball conglomerate, detrital plant debris, and/or silt, to (2) sand-rich splays with a complex internal architecture. The difference between the two end members is probably a consequence of local sand supply.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)