Holbrook, John1, Franca Oboh-Ikuenobe2
(1) Southeast Missouri State University, Cape Girardeau, MO
(2) University of Missouri, Rolla, Rolla, MO
ABSTRACT: Is Climate the Real "Rainmaker" in Fluvial Lowstand Deposition? A Look at the Concept of Base-Level Buffers and Buttresses and the Comparative Role of Sea Level and Climate in Architecturally Complex "Lowstand" Fluvial Sandstone
Fluvial sequence stratigraphic models generally argue lowstand sandstone records continuous stacking and intensive amalgamation of channel belts during slow and progressive base-level rise immediately following falling-stage incision. Three fluvial sandstone layers in the Cretaceous Dakota Group reveal a prima fascia fit with generalized lowstand models as they each correlate to respective lowstand wedges, overly sequence boundaries, and record intensive channel-belt amalgamation. Closer inspection, however, reveals that these deposits have a complex internal architecture reflecting deposition during numerous aggradation/incision cycles of similar amplitude instead of a singular base-level rise. Likewise, these cycles cannot record sea-level variability as they occur up to 300km from the contemporary strand, increase in amplitude up dip, and exceed the fluctuation range of contemporary sea level. These cycles instead appear to record intensive reworking of fluvial sandstone over long durations in response to climatic variability. The upper and lower elevations gained by these cycles remained constant, and are interpreted to be constrained by the highest and lowest longitudinal river profile generated by climatic variability during the life of the system (the buffers), and the position of the contemporary sea-level buttress to which the profiles were anchored. Because sea level regressed over a continental interior, these buffer profiles likely changed little during sea level rise and fall. Cyclic deposition also likely spanned the entire period of exposure, including falling stage. Sea-level change can thus be assumed to promote the exposure required for deposition of fluvial "lowstand" sandstones, but climatic variability more determines sandstone thickness and internal architecture.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.