ABSTRACT

Paleogeographic reconstruction involves determining the spatial and temporal relationships of strata that can represent diverse ancient environments of deposition. The production of a paleogeographic map at a given temporal horizon generally requires that a time datum be drawn between spatially separated stratigraphic intervals that often have different lithofacies which represent the same time interval. This method of reconstruction is often hampered by the fact that chronostratigraphic resolution provided by the most often used tool (biostratigraphy) is commonly inadequate when used in paleogeographic reconstruction across lithologically diverse strata that represent different environments of deposition at a specific temporal horizon. Unconformity-bounded depositional sequences determined from sequence stratigraphic analysis can be used as a basis for paleogeographic reconstruction. Major surfaces associated with depositional sequences present a physical stratigraphic framework that provides relative chronostratigraphic datums which may be used to construct paleogeographic maps. Surfaces associated with a depositional sequence that have chronostratigraphic significance include upper and lower sequence bounding unconformities, transgressive surface, and surface of maximum sediment starvation/maximum transgression. These bracketing surfaces are used as constraints to interpret relative time lines at various points within the sequence. Interpreted time correlative points are then used in conjunction with lithostratigraphic and biostratigraphic relationships to produce paleogeographic maps illustrating the regional distribution of paleoenvironments and rock types at various time horizons within relative sea level cycles.

The Tejas A Gulf Coast (TAGC)-4.3 and TAGC-4.4 depositional sequences of Alabama and Mississippi provide excellent examples of the sequence stratigraphic method of paleogeographic reconstruction. The TAGC-4.3 sequence is a type 2 sequence that includes the Cocoa Sand (shelf margin systems tract), Pachuta Marl (lower transgressive systems tract), and Shubuta (upper transgressive systems tract/lower condensed section) Members of the Yazoo Clay the Red Bluff Clay/Bumpnose Limestone interval (lower highstand systems tract/upper condensed section), and the Forest Hill Sand (upper highstand systems tract). The TAGC-4.4 sequence includes the Mint Spring Marl Member of the Marianna Limestone (lower transgressive systems tract), the Marianna Limestone (upper transgressive systems tract/lower condensed section), and the Glendon Limestone Member of the Byram Formation (upper condensed section/highstand systems tract).

Application of the methodology outlined above to these sequences indicates that sequence stratigraphy can be a useful tool in paleogeographic reconstruction and can greatly enhance the understanding and interpretation of chronostratigraphic relationships within a depositional basin that might not be resolved using other methods.