Effects of Basin Type on Coastal Plain-Shelf-Slope Systems during Base-level Fluctuations: An Experimental Approach

WOOD, LESLI J., Colorado State University, Fort Collins, CO

Sequence stratigraphic models emphasize the importance of basin type on the reactions of Coastal Plain-Shelf-Slope systems to base-level changes. A series of experiments were performed in a 4.5 m by 7 m flume to examine the effects of a passive margin basin with a shelf/slope break versus a ramp margin basin on coastal plain-shelf-slope deposits that result from base-level fluctuations.

Results indicate that basin type has a strong influence on the erosional features and deposits that develop in response to base-level fluctuations. Fluctuations that occur along a shelf/slope break margin result in well-defined, deeply incised valleys, which develop early in lowstand time and have low width:depth ratios. Rivers may incise into and deposit over outer-middle shelf deposits of the previous highstand. Late lowstand deposits are coarser grained than early lowstand deposits and include fine- and coarse-grained slope and basin floor fans. During subsequent base-level rise early transgressive deposits are confined to incised valleys. A significant lagtime exists between initial flooding of incised valleys and deposition of thick, laterally confined valley-fill deposits, and t e subsequent transgression of the shelf.

Fluctuations along a ramp margin result in shallow, wide incised valleys with high width:depth ratios, which develop late in lowstand time. Incision occurs into shelf deltaic deposits and these are overlain by valley fill deposits. Deposits of the lowstand systems tract do not coarsen upward significantly and contain only sand-rich, small, thin delta front fan deposits. During subsequent base-level rise transgressive deposits are not confined to incised valleys. Transgressive deposition within the valleys occurs over a short time interval and is followed closely by flooding of the adjacent shelf.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)