Simulation of Continental Margin Sedimentation

Christopher G. St. C. Kendall, Ian Lerche, Kazuo Nakayama

Interpretation of stratigraphic sequences is aided by a 2-D simulation that reproduces the onlapping and offlapping geometries of marine and near coastal alluvial deposits of basin margins. It tracks thickness and geographic location of sandy and shaly facies as eustatic sea level varies. The depositional surface changes position under the effects of compaction and tectonic movement and, when a critical angle of deposition is exceeded, bypass occurs.

The simulation involves a series of equal time steps at which sediment is deposited in an array of columns across the depositional surface. Height of columns is determined when: (1) The height of a right-angle triangle,whose area is a 2-D expression of the quantity of sediment deposited at each time step (length = maximum basin margin slope; height = length × sediment volume), is matched to sea level. If the sediment column reaches above sea level, its height is reduced to sea level and the area of left over sediment is added back to the triangle by increasing its height. (2) The surface of the alluvial fan is projected back at an angle to intersect the landward depositional surface. This area is filled with sediment subtracted from the triangle. This process is repeated until th triangle's heights match sea level, at which time the remaining area of the sediment triangle is deposited. When the bypass angle is exceeded during any of these steps, deposition does not occur.

This model can be used as a complement to seismic stratigraphy, or can be used alone as an inexpensive alternative to test stratigraphic models independent of seismic.

In addition, rules for determining sea level variations from observed sections can be tested by supplying a synthetic section to the protocol and seeing how well the rules satisfy the known input values. Results based on the coastal onlap technique of P. R. Vail and others are presented.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.