Peter B. Flemings

The processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy. We present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry, topography, and facies patterns. In the model, steady crustal shortening occurs according to a wedge‐thickening model, erosion and deposition follow a diffusive process, and the lithosphere is compensated elastically. Erosion and deposition are controlled by the transport coefficients κ of the diffusion equation. For a range of thrust velocities and lithospheric rigidities, transport coefficients are of order 10 2 –10 3 m 2 /yr in the mountain belt; these values are much higher than those derived from the study of scarp degradation. In the sedimentary basin, transport coefficients of order 10 4 m 2 /yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition. Rapid thrusting results in a narrow underfilled basin, while slow thrusting results in a wide overfilled basin. In addition, by varying the erosional and depositional transport coefficients while holding other parameters constant, we generate both overfilled and underfilled basins. These results suggest that changes in the rate of thrust loading, the climate, or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere. Clearly, to understand either the long‐term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata, the manner in which a basin was filled must be considered. We apply the model to the evolution of the modern sub‐Andean foreland and find that an erosional transport coefficient of 3000 m 2 /yr and a depositional transport coefficient of 20,000 m 2 /yr successfully predict the observed basin geometry.

Miocene through Pleistocene sediments on the New Jersey continental slope (Ocean Drilling Program Site 1073) are undercompacted (porosity between 40 and 65%) to 640 meters below the sea floor, and this is interpreted to record fluid pressures that reach 95% of the lithostatic stress. A two-dimensional model, where rapid Pleistocene sedimentation loads permeable sandy silt of Miocene age, successfully predicts the observed pressures. The model describes how lateral pressure equilibration in permeable beds produces fluid pressures that approach the lithostatic stress where overburden is thin. This transfer of pressure may cause slope failure and drive cold seeps on passive margins around the world.

Log and core data document gas saturations as high as 90% in a coarse‐grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas‐saturated bed is defined by a strong, negative‐polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse‐grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.

We incorporate a process‐based depositional model with basin subsidence models to predict stratigraphic records. This allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins. The models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin, the record of episodic tectonism is distinct from that of eustatic sea level change. In the model, sediment transport is approximated by slope‐controlled diffusion; nonmarine transport is treated as more efficient diffusion than marine transport. Three different subsidence and sediment supply models are examined: a simple passive margin basin, a simple foreland basin, and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief. Predicted passive margin stratigraphies, for cases of varying eustatic sea level, are similar to those of natural basins and include progradational packages and subaerial unconformities, which are used to define sequence boundaries that form during sea level fall. We explore the timing relationships between stratigraphic features and a sinusoidal sea level history, showing that the phase relationship depends on subsidence, sediment flux, efficiency of sediment transport, and period and amplitude of sea level. When the basic geometry of the basin is inverted, placing the sediment supply on the side with maximum subsidence as is the case in foreland basins, the sequence character changes markedly: subaerial erosion does not generate unconformities. In the models of a dynamic foreland basin, sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant. For steady thrusting and variable sea level, unconformities that define sequence boundaries form only on the distal or forebulge side of the basin, and the ages of the sequence boundaries correlate to limes of rising sea level. In cases of constant sea level but variable thrusting, subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin, yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle: erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side.