Subsurface studies in many sedimentary basins around the world reveal that widespread onlap has occurred several times since the Precambrian. The existence of onlap indicates deposition on an unconformity surface that had topographic relief. There are several worldwide onlap unconformities. These began to form during (1) early Oligocene, (2) early Paleocene, (3) Late Jurassic (pre-Portlandian-Purbeckian), (4) latest Triassic (pre-Rhaetian), (5) Permian (pre-Leonardian), (6) latest Mississippian (post-type-Chesterian), (7) Early Devonian, and (8) Middle Ordovician times. In addition, several less important, but worldwide, periods of onlap, restricted largely to basin margins or actively rising areas, have been observed. These occurred during (1) late Miocene, (2) early Mio ene, (3) early Eocene, (4) Early Cretaceous (pre-Cenomanian), (5) Middle Jurassic (pre-Dogger), (6) Early Triassic, (7) Pennsylvanian (pre-Desmoinesian), (8) Late Devonian, and (9) Early Silurian times.

Earlier workers recognized many of these unconformities in the United States and Canada on the bases of truncation (overstep) and onlap. In this paper onlap is emphasized as the better indicator of unconformities, because onlap is much more widely prevalent than truncation. Unconformities identified only by truncation usually occur in regions which have undergone a local period of uplift.

Several factors may obscure the presence of major unconformities. If the underlying sediments were relatively flat at the time of onlap or if the basin was subsiding differentially, yet rapidly, at the depositional site, detailed correlations across large areas usually are required to reveal the presence of an onlap unconformity. The unconformity may be missing in basin centers because of continuous deposition. In such a situation, the strata which were deposited while an unconformity developed toward the basin margin may be identifiable because of a change in depositional rate within the time-equivalent sediments of the basin center. Highly mobile belts commonly have many unconformities; only a few of these may be worldwide. In addition, continental sediments deposited above sea-leve also may contain unconformities that formed entirely as a result of local factors.

Characteristically, the sediments of an onlap cycle are deposited relatively rapidly at the beginning of the cycle, and are deposited less rapidly later in the cycle. At the conclusion of the cycle, onlap at the basin margins is scarcely noticeable. During the next succeeding cycle, onlap commonly is relatively rapid again, but the area of onlap is nearer to the depositional basin center. This shift of onlap cycles through time from basin margin to basin center is believed to represent a fairly rapid drop in sea-level. The initiation of onlap is then interpreted to be the result of a gradual rise in sea-level, and local basin subsidence and sedimentation play the dominant roles in determining the positions and amounts of onlap and sedimentary thickening.

Although the basic causes of changes in sea-level are not well known, they may be related to changes in the configuration of ocean basins resulting from large-scale

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mountain building. Some changes in sea-level may be related to the size of ice accumulations at the poles.

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