ABSTRACT

The Tertiary of the central and western Gulf Coast, with maximum thickness of approximately 30,000 feet, is composed almost entirely of terrigenous clastics (interbedded sand, silt, shale), and is characterized by alternating marine and nonmarine strata. Some of the marine formations extend to the outcrop, but the shorelines of many other formations are now deeply buried, and the outcrop equivalents of these subsurface marine formations are mainly of continental (alluvial) origin. The formations grade eastward into shallow open sea marine carbonates.

Sea level deposits of Tertiary age are now buried at least 20,000 feet so there can be no doubt that the coastal area of Louisiana and Texas has been subsiding. The sequence of alternating marine and nonmarine sediments in this area resulted from advance and retreat of the strandline; this shift of the marine shoreline was due to differential rates of subsidence and to change in the amount of sediment brought to the area. Eustatic changes in sea level were not necessary to cause the transgressions (inland advance of the shoreline) and regressions (seaward advance of the shoreline). In fact, the mainly carbonate section in the eastern Gulf of Mexico region does not reflect the shallowing and deepening shown in the central and western Gulf Coast section.

Most of the numerous local transgressions and regressions in the Gulf Coast Tertiary were very likely due to shift of the large deltas. Streams find the easiest path to the sea and, once a depocenter is established, the stream will build its delta there so long as subsidence is sufficient to take care of the sediments. The delta can also be built seaward if the sediment supply is greater than subsidence. This progradation in a marine body of water is regression, and the numerous regressive sequences in the Gulf Coast, which contain nearly all of the oil and gas, are a result of deltas built into shallow water of the continental shelf. The areas between deltas continue to subside so that sooner or later the stream is diverted to a shorter course to the sea with a more favorable gradient. The abandoned delta continues to subside and the sea transgresses it.

It is obvious that both transgressions and regressions take place simultaneously in a subsiding basin such as the Gulf Coast during periods of delta building.

The more regional transgressions of the Paleocene, Eocene, Oligocene, and Miocene suggest a small sediment supply, possibly the result of dry climate in the sediment source areas. Continued sinking of the Gulf Coast permitted the sea to transgress over the coastal plains of the preceding periods of abundant sediment supply. The interbedded regressive sections were deposited when the source areas were more elevated or there was more precipitation, or both. It is believed that the rate of sedimentation was much greater during the regressive periods than during transgressive periods. However, the progradation (regression) of the shoreline was in most cases slower than the movement inland (transgression) of the sea.

Some of the numerous transgressions and regressions, both local and regional, are pointed out. The only rhythmic or cyclic sedimentation patterns in this thick section are a result of shifting strandlines which may have no relation to changes in sea level.