Computer simulation of stratigraphy
Bowman, Scott Andrew
Vail, Peter R.
Doctor of Philosophy thesis
Simulation of stratigraphy coupled with sequence stratigraphic and backstripping analysis quantitatively defines the timing and magnitude of geologic events, including the history of sediment supply, tectonism and eustasy. This provides a quantitative basis for interpreting the mechanisms causing these variations. This computer simulation comprises algorithms that model subsidence and uplifts, eustasy, flexural response of sediment and water loads, compaction, traction- and suspension-load deposition, gravity-flow sedimentation, carbonate production and redistribution, and erosion. Backstripping analysis can provide a geohistory, burial history, sediment accumulation history, porosity history, and a first approximation of the tectonic subsidence or uplift history. A backstripping analysis of a stratigraphic section produced by the two-dimensional simulator demonstrates the error due to overcompensating for the flexural response to sediment loading with a calculation that assumes local isostasy. These errors reinforce the necessity to use a two- or three-dimensional simulation or backstripping technique to accurately define the eustatic and tectonic history of a region. Simulation results of the Last Chance Canyon study show that documented stratal patterns are a product of the interaction of a dynamic depositional system, with constant parameters, fed by alternating siliciclastic sand and carbonate production, a constant subsidence rate of 0.4 cm/ky, and a eustatic sea-level history that contains "third-order" and higher periodicity cycles. A hierarchy of stratigraphic packaging is presented that include continental encroachment megasequences, transgressive-regressive facies-supersequences, complete and incomplete sequences, component groups and components. Sediment supply, tectonism, and eustatic fluctuations produce these packages by changing the accommodation space with characteristic rates and patterns. The geometry of the substrate and bathymetric changes strongly influence the geometry of stratal surfaces and distribution of lithofacies. The response of these variables is simulated independently to identify their unique stratal signatures. Examples from different settings (passive margin, mixed carbonate-siliciclastic sediment supplies, carbonate ramp, and steep carbonate platform margin, and others) demonstrate how siliciclastic and carbonate depositional systems interact with the bathymetric conditions produced by these variables. Simulation results show that the relative change of sea level is the sum of total subsidence (tectonic subsidence, flexure loading and compaction) and eustasy.
Geology; Geophysics; Physical oceanography