ABSTRACT

Upper Smackover strata in Alabama commonly consist of one or more upward shoaling cycles ranging from 15 to 50 feet in thickness. These are fourth or fifth order cycles within the third order upper Norphlet to lower Haynesville depositional sequence. Multiple forcing functions (subsidence, salt halokinesis, and autogenic sediment aggradation) and position relative to sea level at the start and end of each cycle generated an array of sedimentary responses.

The Brittain No. 1 well illustrates nucleation of an offshore bar. Bar deposits are capped by anhydritic sabkha deposits, gradationally overlain by subtidal lagoonal strata. Varying rates (and directions?) of halokinesis controlled this succession and created as many as five sabkha-capped cycles in the eastern Mississippi interior salt basin. The International Paper Company 20-5 No. 1 well contains three upward shoaling cycles capped by evaporites. Because of the limited aggradational potential of supratidal evaporitic settings subsidence caused immersion, which eventually permitted reactivation of the carbonate factory and formation of the next cycle. The Chatom Unit 20-14 No. 1-04 well contains four different cycles. The lower cycle consists of subtidal lime mudstone, capped by a 5 foot thick paleosol. The paleosol underlies an intraclastic storm deposit followed by a deepening-upward lagoonal succession. A thin ooid grainstone containing exposure surfaces caps the second cycle. The third cycle consists of a deepening upward peloidal carbonate succession capped by closely spaced exposure surfaces. In the upper cycle, peritidal carbonate strata underlie sabkha deposits. The first and third cycles were probably caused by halokinesis; the second and fourth could have been autogenic.

Some fourth or fifth order upper Smackover cyclic depositional sequences in southwest Alabama may represent parasequences formed in response to eustatic sea-level change. However, many cycles were caused or influenced by other factors as outlined above.