To browse Academia. These stratal architectures are here simulated by geometrical models that are governed simply by bounding slope gradients and the ratio of coeval slope to basinal aggradation rates. In cases of no coeval deposition on bounding slopes, pure onlap results. In cases of high slope aggradation rates relative to basinal aggradation rates, a feathered geometry results with successive sandstone beds onlapping against successively younger slope drapes.
The cyclicity of alternating sand-rich and sand-poor packets seen at Chalufy results in a stepped climbing trajectory of successive slope base positions, with climb angle equal to bounding slope angle when mud-poor sandy flows deposited sandstone bodies with abrupt onlap, and climbing more steeply when slope drapes are deposited. An implication of this result is that any zones of facies change and palaeocurrent swing that are related to distance from bounding slope will exhibit a comparable vertical trajectory through time.
The two end member termination types can be compared with the convergent-baselapping and convergent-thinning seismic facies used in the Gulf of Mexico subsurface and elsewhere. The convergent-baselapping seismic facies may include both abrupt onlap and intervals of feathering aggradational onlap if the presence of slope drapes is not resolvable by the frequency content of the available seismic data.
Unconformity-bounded depositional sequences represent the fundamental building blocks of sedimentary successions. They are typically characterized by onlap at the base and by offlap at the top, and they tend to be markedly asymmetrical, with onlap accounting for a larger part of any cycle of sedimentation than offlap. Offlap cannot be attributed solely to erosional truncation, but instead implies that sequence boundaries develop over a finite interval of time.
Depositional sequences are commonly associated with a cyclic arrangement of facies, but transgressive-regressive cycles are out of phase with respect to sequence boundaries, which in down-dip locations are both overlain and underlain by progradational deposits, and hence form during times of regression of the shoreline. These observations are used to develop some ideas about the origin of unconformity-bounded sequences, with reference to the inter-related roles of changes in depositional base level and sediment supply.
