Holbrook, John¹, Whitney J. Autin², Stephen Marshak³, Tammy M. Rittenour⁴ 
(1) Southeast Missouri State University, Cape Girardeau, MO 
(2) Department of Earth Sciences, SUNY College at Brockport, Brockport, NY 
(3) Department of Geology, University of Illinois, Urbana, IL
(4) Department of Geosciences, University of Nebraska, Lincoln, NE

ABSTRACT: The Enigma of Long-term Tectonic Control on Deposition Yet Low Cumulative Strain: Insights from Holocene Fluvial Strata in the Active New Madrid Seismic Zone

The notion that topography from minor displacement on basement faults affects depositional patterns is well accepted. Familiar examples commonly bear two seemingly contradictory elements. First, tectonic facies control commonly endures for millions of years, affecting thick rock sections. This demands evidence of significant coeval accumulated strain if any reasonable steady slip-rate is applied over the duration of deposition. Second, independent structural evidence usually indicates such coeval accumulated strain is negligible. Fluvial sediments of the Mississippi River provide insights. 
The Mississippi River traverses a hanging-wall anticline in the active New Madrid wrench-fault system that is expressed surficially as 10 m of topography over a 20 km valley reach. Uplift here has forced the Mississippi River to straighten on at least two occasions, resulting in a modest leaning toward smaller lateral-accretion elements, higher ratio of active to passive channel fills, and splay accumulation where sinuosity was tectonically reduced on the upstream side of the uplift. 
Close inspection of meander trends (derived from mapping) and paleoseismic data reveal two explanatory points. First, the recorded tectonic sinuosity changes require only about 1 m of topographic shift per episode, so minimal deformation is actually required to force the architectural variations preserved. Second, slip appears to be temporally clustered on millennial scales, meaning that slip is not constant, but is intermittent. That the tectonically induced facies architecture preserved in these Holocene fluvial deposits can derive from minimal and intermittent uplift explains how many analogous ancient deposits can preserve enduring tectonic facies control while accumulating minor coeval strain.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.