BLUM, MICHAEL D.
University of Nebraska-Lincoln, Lincoln, NE

Abstract: Importance of Falling Stage Fluvial Deposition: Quaternary Examples from the Texas Gulf Coastal Plain and Western Europe

Past and ongoing research on large rivers systems of the Texas Gulf Coastal Plain and Western Europe demonstrate that fluvial deposition is common to the falling stages of glacio-eustatic cycles.

For the Texas Coastal Plain, the lower reaches of river systems that drain unglaciated hinterlands most commonly have at least 3 falling stage to lowstand stratigraphic units that can be identified in both outcrop and core. Each is bounded by unconformities at their base and paleosols at their top, and occur within the lower half of valley fills as a flight of downward-stepping high net-to-gross sandbodies, distinctly lower in elevation than alluvial plain surfaces from the last interglacial highstand.

For Western Europe, large rivers such as the Rhone, Garonne, and Rhine were glaciated in their headwaters, and coarse-grained glaciofluvial deposits have been mapped from former ice margins down to points where they are buried by highstand alluvial-deltaic (Rhine and Rhone) and/or estuarine (Garonne) facies. Similar to the Gulf Coast examples, these falling stage to lowstand deposits occur as a flight of unconformity- and paleosol-bounded, downward-stepping gravel- and sand-dominated stratigraphic units, inset within surfaces constructed during the last interglacial highstand.

The deposits described here represent the manifestation of "forced regression" within incised valleys, challenge the long-held view that fluvial systems should be characterized by falling stage erosion and sediment bypass, and indicate that fluvial deposition is closely linked to upstream controls on sediment flux, as opposed to a strict deterministic relationship with base level change per se. Falling stage to lowstand stratigraphic units may be partially cut out by subsequent channel migration or wave / tidal ravinement processes, or passively onlapped and buried by transgressive and highstand strata. But their position deep within the valley fills makes preservation in the stratigraphic record more likely, and falling stage sandbodies may comprise the bulk of reservoirquality sands within many incised-valley fill depositional sequences.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas