Paleocene-Eocene Boundary in Eastern Gulf Coast: ABSTRACT

Abstract

The Paleocene-Eocene boundary in Alabama has been placed at various levels within the Tuscahoma Formation and the overlying Bashi Formation. The location of this boundary is important because both lignite- and petroleum-bearing deposits occur within this sequence in the Gulf Coast, and the region appears to be a good locality to test the local coastal-onlap models. The middle beds of the Tuscahoma Formation are upper Paleocene (calcareous nannofossil Zone NP9 and Morozovella End_Page 1429------------------------------ velascoensis Internal zone). The entire overlying Bashi Formation is lower Eocene (Zone NP10 and lower part of the Morozovella subbotinae Interval zone). No calcareous fossils are known from the upper part of the Tuscahoma. Therefore, the boundary problem was investigated, using abundant sporomorphs in cores of the Tuscahoma and the Bashi at Ozark, Alabama, and other cores and outcrops in Alabama and western Georgia. The sporomorph assemblages of the Tuscahoma and the Bashi differ considerably, but the assemblages within each formation are generally quite uniform from base to top. According to sporomorph data, a small hiatus appears to exist between the Tuscahoma and the Bashi. In the Oak Grove core hole in Virginia, the shallow-marine Aquia Formation contains Tuscahoma-like sporomorphs in beds within Zone NP9. The overlying lagoonal Marlboro Clay is either entirely upper Paleocene or may span the Paleocene-Eocene boundary. Above the Marlboro is the marine-transgressive Nanjemoy Formation (Zone NP10), containing Bashi-like sporomorphs. Thus, sporomorphs from the top of the Tuscahoma in Alabama are most probably of late Paleocene age, as are the similar sporomorphs from the upper Paleocene middle part of the Tuscahoma and upper Paleocene Aquia Formations. The uppermost beds of the Tuscahoma in eastern Alabama and western Georgia are marginal-marine to nonmarine deposits. The overlying Bashi Formation is of inner neritic origin. Therefore, in the Atlantic and Gulf coastal plains, a regression marked the end of the Paleocene, and a fairly rapid, sea-level rise occurred at or just after the beginning of the Eocene; this agrees with published global sea-level curves. End_of_Article - Last_Page 1430------------