Six Degrees of Separation Across the Low-to-High-Accommodation Continuum in Fluvial Systems
Fluvial systems have long been subdivided into low- and high-accommodation systems based upon the degree to which channel belts are amalgamated. In low-accommodation systems, channels vertically scour into lower stories and form contagious sandy units composed of many truncated and amalgamated channel-belts. In high-accommodation systems, these belts are vertically separated by muddy flood-basin strata. These two distinctions are presumed to record a change in balance between the sedimentation rate and the rate of accommodation increase. Rivers can switch from a low-accommodation system, whereby floodbasins fail to aggrade more than a belt thickness prior to avulsive return of a channel to the same site, to a high-accommodation system, whereby they aggrade more than one channel belt prior to avulsive return. This binary description of fluvial-system accommodation states has served well for general interpretations and mapping of fluvial strata. Within these two accommodation states, however, are important change-overs in fundamental process that alter the preserved architecture of the fluvial section in significant ways that are not captured by the simpler binary model. Two fundamental distinctions in process are native to the low-accommodation system, and four are native to high-accommodation systems. Low-accommodation systems may be broken in to progressively stacked and iteratively stacked end members whereas, high-accommodation systems can be broken into 1) well-drained avulsive distributive, 2) well-drained bifurcating distributive, 3) poorly drained distributive, and 4) fluviolacustrine systems, reflecting progressing intensity of relative accommodation rate vs. sediment supply. These all have fundamentally different architecture, reflecting their differing origin that impact reservoir architecture, particularly in the size arrangement of channel-belts and the connecting units within the floodbasin deposits. The “progressively stacked” and well-drained avulsive distributary models are the most commonly used and are likely well over-subscribed for lack of consideration of the other four options.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014