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In the central and western Gulf Coast the Tertiary, whose maximum thickness at any one place is probably about 30,000 feet, consists almost entirely of alternating marine and non-marine fine-grained terrigenous clastics. Some of the marine formations extend to the outcrop, but many others are now deeply buried and are represented in the outcrop by non-marine deposits. All formations grade eastward into shallow marine carbonates.
Numerous local and many regional transgressive and regressive sequences of sediment are present. The local fluctuations in the strandline were caused by delta building and abandonment; the regional shifts are believed to have been caused by variation in the rate of subsidence of the basin or to variation in the amount of sediment transported to the area. It appears that sedimentation was faster during the regressive periods than during transgressions. However, the progradation was in most cases slower than the movement inland (transgression) of the sea.
The generalized sedimentation history of the Tertiary in the central and western Gulf Coast is explained. The only rhythmic or cyclic sedimentation patterns in this thick section are a result of shifting strandlines which may have no relation to eustatic changes in sea-level.
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