MILANKOVITCH CYCLES IN THE MOOREVILLE CHALK (CAMPANIAN) IN THE NORTHEASTERN GULF OF MEXICO AREA

A thick chalk section was deposited in the northeastern Gulf of Mexico area during the Campanian Stage. The lower part of the Campanian chalk section consists of the Mooreville Chalk. Decimeter scale chalk/marl cycles are well developed in this section. Walsh Transforms were conducted on the chalk/marl cycles from a core of the Mooreville section in Dallas County, Alabama. Three Milankovitch frequencies (eccentricity, obliquity and procession) were identified from the resulting power spectra. The eccentricity band and obliquity band are very prominent throughout the section. Therefore, the controlling frequencies are different from the eccentricity-procession rhythms observed previously (mainly in Europe) in which five couplets of chalk/marl layers were observed to form one bundle (Schwarzacher°¯s rule). The fluctuations of atmospheric and marine energy regimes at the surface of the Gulf of Mexico that controlled marine siliciclastic sediment transport along the shoreline are proposed as the major reason for the formation of the decimeter scale chalk/marl cycles. The main source for the siliciclastic sediments is interpreted to be nearby deltas. When/where the energy regime was high, the alongshore currents transported large amounts of siliciclastic materials, thereby diluting the carbonate input, and carbonate-poor layers (marl) resulted. On the contrary, when/where the energy regimes were low, marine transport of siliciclastic sediment was at a minimum. Therefore, in nearshore areas between delta lobes, purer carbonate-rich layers (chalk) were deposited. This hypothesis is supported by stratigraphic analysis of the Campanian strata in the northeastern Gulf of Mexico area.