Continental Shelves as the Lowstand Fluvial Longitudinal Profile: Possible Implications for Icehouse vs. Greenhouse Stratigraphic Records

Mike Blum
Louisiana State University, Baton Rouge, LA

This paper suggests that lowstand fluvial profiles represent the first-order morphodynamic state for continental shelves, and river long profiles are graded to shelf margins. Over the last 10^6 yrs mean sea level has been -60 to 65 m, with a mode at -85 to -90: for most of this time, the majority of shelves would have been subaerial. Long profiles for river systems have equilibrium times > 10^4 to 10^5 yrs: they should be adjusted to mean states over time scales that are ≥ equilibrium times, and insensitive to the anomalous and infrequent highstands. On a global scale, shelf gradients and widths correlate to onshore fluvial gradients and drainage areas.

In an icehouse world, high-frequency climate changes are coupled to changes in ice volume, and unsteadiness of sediment supply due to climate change is modulated by the transit of river mouths across the shelf. Moreover, the transit of river mouths across a broad shelf results in the merger of river systems that discharge separately to the coastal oceans during highstand: merging of drainage basins increases the magnitude of individual point-source sediment supply, but there will be fewer river mouths and delta systems at the shelf margin than there are during highstand time. These relationships should be fundamentally different in a Greenhouse world: high frequency, long distance transit of river mouths and deltas, and merger of drainage basins should not occur to the same degree.

In an Icehouse world, then, major high-frequency (time scales < 10^6 yrs) changes in fluvial-deltaic, shelf-margin, slope, and basin-floor stratal packages will reflect fluvial responses to sea-level change. In a Greenhouse world, high-frequency stratigraphic packaging should be closely coupled to unsteadiness in sediment supply due to climate change, rather than modulated by fluvial transit of the shelf, and merging of drainages.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas