ABSTRACT: Two-Dimensional Stratigraphic Simulation Model of Sediment Volume Partitioning and Stacking Patterns in Sequences of Different Scales: Can We Identify Forcing Functions from Stratigraphic Attributes?

LESSENGER, MARGARET A., Colorado School of Mines, Golden, CO

A two-dimensional stratigraphic forward model simulates the geometries and lithologies of coastal plain through shelf siliciclastic strata. Process parameters include eustasy, tectonic movement, sediment supply, initial basin topography, and lithospheric compensation of surface loads. This model calculates and displays distributions of erosional, nondepositional, and bypass surfaces, and facies between these surfaces. Spatial scales of model runs were as great as hundreds of kilometers, and temporal scales ranged from thousands

to a few million years. This model was developed and tested by incorporating results from field studies conducted by colleagues at CSM.

The model simulates accumulation and preservation of sediment in different facies tracts of progradational units during base-level transit cycles. Model output shows that sediment is partitioned into different facies tracts during different portions of a base-level transit cycle. More preserved fluvial sediment accumulates during base-level rise than during base-level fall, and more preserved shoreface and marine shelf sediment accumulates during base-level fall than during base-level rise. This partitioning of sediment volumes into different facies tracts during a base-level cycle produces lateral variations in cycle symmetries within a progradational unit. Dominantly fluvial cycles tend to be asymmetric, with sediment preserved during periods of base-level rise. Shoreface and marine shelf cycles are strongly asymmetric, with sediment preserved only during periods of base-level fall. Transitional environment (nearshore and coastal plain) cycles are more symmetrical, with sediment preserved during both fall and rise periods.

Multiple progradational units are arranged in landward-stepping, seaward-stepping, and vertically stacked geometries that simulate Vail-type depositional sequences. In model output, landward-stepping units are preserved preferentially during long-term base-level rise in more landward positions, and seaward-stepping units are preserved preferentially during base-level fall in more seaward positions of a basin. Like the differential partitioning of sediment volumes into facies tracts within progradational units, progradational units within depositional sequences are geographically partitioned within basins such that in some areas the depositional sequence will be symmetrical and contain all systems tracts, whereas elsewhere the depositional sequence will be asymmetrical and contain only one or two systems tracts. Thus, at two scales there is self-similarity in volumetric partitioning and symmetry of stratigraphic units.

Relationships between the values and combinations of process parameters on the one hand, and the volumes, geometries, and times of deposition of facies tracts within sequences of multiple temporal and spatial scales on the other, were examined with the model. Results suggested that there is a moderately determined coupling between particular elements of the stratigraphic response and particular values and combinations of process parameters. Lateral variations in symmetries and distribution of surfaces within progradational units, and stacking patterns of progradational units, are most strongly influenced by eustatic changes and depositional topography. By contrast, basin-scale stratal geometries and types of symmetry of depositional sequences within basins are most strongly influenced by tectonics and depositional topography.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)