Recent Developments in Siliciclastic Sequence Stratigraphy
A two-dimensional stratigraphic forward modeling program simulates basin subsidence and uplift, sea level change, and changing volumes of sediment input for terrigenous clastic, carbonate, and mixed clastic/carbonate regimes. In this chapter, the model is used to evaluate and illustrate fundamental controls on depositional sequence geometries and to test the significance of synchroneity of sequence boundaries.
Changes in paleobathymetry commonly are confused with changes in relative sea level. A simple model with eustatic sea level held constant illustrates coeval shallowing and deepening in a single basin. Coarsening-upward fringe and shoreface sequences should be interpreted as shallowing upward, but should not necessarily be interpreted as evidence of relative sea level fall.
Changing rates of sediment supply with sea level held constant produce stratigraphic geometries significantly different from those produced by sea level variation. Model results predict that for a subsiding basin, the critical features useful for differentiating between sea level fall and an increase in rate of sediment influx are toplap and downward shifts in coastal onlap (for sea level fall) versus concordance and simple updip thinning of sediments (for increased rate of sediment influx). Downlap, transgressions, regressions, and slope bypass are not by themselves diagnostic of sea level change. Subtle toplap also may be produced during simple progradation because the maximum point of load-induced subsidence is transferred downdip as the system progrades. This effect may be pronounced on elastically weak crust or on systems prograding over a mobile substrate. Load-induced toplap may be differentiated from sea level-induced toplap by anomalously steep dip of topset sediments. A downward shift of coastal onlap cannot be produced by simply changing sedimentation rate in the absence of a change in the rate of subsidence or sea level fall.
For subsiding basins, maximum transgression typically will take place prior to a eustatic sea level highstand, and maximum regression will occur after a sea level lowstand. Preservation of thick topset sequences implies rising relative sea level even during major regressions. Erosional sequence boundaries are most likely to form during the maximum rate of sea level fall.
In basins with high and variable subsidence rates and sediment ponding, the geometry of depositional sequences is controlled mainly by paleobathymetry and subsidence, whereas gross shelfal and nonmarine facies distribution is determined mainly by the rate and magnitude of sea level change.
The age of sequence boundaries will be synchronous (within the limits of biostratigraphic resolution) only if rates of sea level fall are much greater than rates of subsidence in all basins being correlated. The geographic and temporal extent of unconformities is much greater in slowly subsiding basins than in rapidly subsiding basins. The slower the rate of sea level rise and fall, the greater the disparity in the age of unconformities between two basins with different subsidence histories, even though the sea level signals are identical. The near synchroneity of many sequences implies rapid rates of fall. Basins with different rates of sediment supply will have nearly synchronous sequence boundaries if the relative sea level histories are the same for the two basins.