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Clastic Dikes in Eutaw Formation (Upper Cretaceous) of Southwestern Georgia Coastal Plain

William J. Frazier

The Eutaw Formation is the second oldest stratigraphic unit of the southwestern Georgia coastal plain, and it introduced marine and paralic deposits into the eastern Gulf coastal province during the Late Cretaceous. Penecontemporaneous deformational structures are common in middle and upper sections of the Eutaw, notably large, well-exposed clastic dikes. This report describes these clastic dikes and speculates on the mechanics of their emplacement.

Eutaw clastic dikes range up to tens of meters long and up to 1-5 m wide. Their vertical extent is indeterminate because their bases have never been observed; maximum observed vertical extent is 7.5 m. Dikes invariably occur in sets and intersect each other at 30° and 150° angles, suggesting that the dikes formed in conjugate sets. Sediment within dikes is well to very well-sorted, silty to very fine, micaceous, quartzarenitic sand. Micas tend to be oriented parallel to dike walls, and indistinct, wispy, laminar structures within dike sand also parallel the walls. Both observations are interpreted to imply that sand emplacement occurred while sand was in a fully fluidized state.

Eutaw dikes typically occur near the top of the unit, in dark-gray, fissile, clayey shale. These shales have a restricted fauna, represented principally by Lingula; they are interpreted as restricted lagoonal and poorly drained marsh deposits. Dike margins are extremely sharp and often show brecciation and sand injections into minor apophyses within the shale. Organic-rich muds of the wall rock are interpreted to be well compacted and to have behaved in a brittle manner during strain. Therefore, sand injection occurred at some time considerably after the mud was deposited, and the sands contained interstitial water under relatively high hydrostatic stress. However, I infer that sands were emplaced from below, as a result of passive injection during which sands behaved like Newtonian f uids. Also, a stress field, perhaps the result of seismic activity, fractured the compacted muds and concomitantly caused the sand injection.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.