The middle Tertiary sedimentary regimen of the Gulf Coast is related directly to Rocky Mountain events, and most of the terrigenous sediments came from the Rocky Mountains. A relatively minor amount of terrigenous material was derived from Mid-Continent, Appalachian, and Piedmont sources, and from Mesozoic rocks in the Gulf coastal region. Carbonate rocks and bioclastic rocks in the eastern Gulf region are autochthonous or were derived locally.
Criteria are given for identifying transgressive and regressive trends in both surface exposures and subsurface sections. Juvenile (freshly arrived) clastic materials are winnowed in transport to coastal areas during the early part of transgressive phases. Sand with small quantities of reworked gravel is the first juvenile material deposited in the marine environment. Gravel arrives later and tongues out downdip above the basal sand. Clay and bioclastic material are deposited downdip at the same time that juvenile sand is deposited updip. Lag in transport causes both juvenile sand and gravel to transect time lines upward downdip.
The sedimentation cycle is related to the normal inverse relation between sea-level and stream gradient. Periods of non-deposition in the marine environment (diastems) that have been attributed to oscillation of sea-level, but which do not correspond to regressive maximums, are attributed to unknown and undeterminable lapses in sediment transport, to submarine scouring, and to tectonic activity.
Disconformities are classified as eustatic or tectonic; either type may coincide with subaerial erosion. Major events in geologic history can be deciphered by recognition of these disconformities and with consideration of the terrigenous-carbonate (or bioclastic) ratio in sediments, the aggrading or degrading behavior of streams, and the transgressive-regressive sedimentary cycle.
Correlation between the surface and the subsurface is based on equation with transgressive and regressive phases. Gulf coastal stages are delimited by regressive maximums. Transgressive and regressive trends are believed to have been gradual or irregular, not oscillatory as is commonly supposed. The outer-shelf shale tongues which usually are identified by distinctive foraminiferal assemblages, such as the Hackberry, Abbeville, and Harang assemblages, are regarded as regressive maximums, not transgressions as frequently stated. The Textularia warreni tongue of the subsurface Vicksburg and the Anahuac tongue represent transgressive peaks.
Pure bentonite, as found in strata of the Vicksburg Group, provides the only means of direct correlation with middle Tertiary Rocky Mountain events. Pyroclastic and terrigenous debris, which required fluvial transportation to the Gulf, is delayed one stage as a result of the transgressive-regressive sedimentary cycle.
Difficulty in locating the Oligocene-Miocene boundary around the world results from the fact that the Aquitanian Stage is defined as a time-stratigraphic unit based on a type section (stratotype) by some authors and as a biostratigraphic unit by other authors. The base of the stratotype section is at about the middle of the biostratigraphic unit. Some European authors recently have regarded Chattian-Aquitanian as a single stage, thus agreeing with the inclusion of both in one transgressive-regressive cycle.