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In Tertiary and Quaternary marine environments, temperature, salinity, temperature stability, and substrate probably were the most important natural forces influencing the distribution of benthonic organisms. The notable sensitivity of organisms to the interaction of a multiplicity of environmental controls results in distribution patterns which generally may be related to water depth. Temperature and "water depth" are, at present, the most important factors which lend themselves to objective statistical procedures for use in stratigraphic paleoecology and for interpretation of paleoeustatic changes.
Environmental changes occurring during deposition can be recognized and used for constructing one or more curves that diagrammatically represent local variation in the several controlling factors in the ancient environments. Detected cyclical phenomena are useful as aids to correlation. Separate relative paleo-water-depth and paleotemperature curves may be derived similarly from analyses of sequential assemblages collected from a single section.
Relative paleo-water-depth curves reflect the local balance between sedimentation and subsidence rates and any eustatically controlled variation in sea-level. Such curves are useful in correlating separate sections within a "basin" of deposition, however complex the distribution of facies.
Relative paleotemperature curves mainly reflect current shifts, changes in landmass configuration, and major alterations of world climates. Paleotemperature curves are useful aids to correlation within a "basin" of deposition and probably are useful between adjacent basins.
If eustatic control is known to dominate, the cyclical phenomena can be used in correlating for relatively great distances. The major control, however, can not be determined accurately from either paleo-water-depth or paleotemperature curves alone. The degree of coincidence between the paleo-water-depth and paleotemperature curves for the marine Pleistocene section suggests the derivation of a third curve (paleoeustatic-change curve) that more nearly reflects relative paleoeustatic changes. The proper use of paleoeustatic-change curves will improve interbasinal correlations and should aid in intercontinental correlations.
The application of these proposed procedures to subsurface sections in southern Louisiana suggests several generalizations concerning the marine Pleistocene. The lowest temperatures were recorded for Nebraskan and late Wisconsin (Woodfordian) glacial phases, whereas the highest temperatures for the Quaternary occurred during Yarmouth interglacial time. Higher temperatures than exist at present are indicated for the Aftonian, Yarmouth, and Sangamon stages and during late Wisconsin (Twocreekan) time. Marine faunal evidence, although limited, suggests that the "late Wisconsin-early Recent" section, as commonly defined, has definite characteristics of a fifth interglacial stage.
Seemingly, evidence from marine sections confirms the importance of the interglacial Twocreekan Stage, as recently defined, and suggests that the effects of this stage may have been more widespread than those of the Farmdalian or "Bradyan" of authors.
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