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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 4. (April)

First Page: 480

Last Page: 480

Title: Multiple Depositional Cycles in Bashi and Hatchetigbee Formations (Lower Eocene), Alabama: ABSTRACT

Author(s): Thomas G. Gibson, Laurel M. Bybell


The marine Bashi Formation and its updip facies equivalent, the Hatchetigbee Formation, may represent the most complete earliest Eocene shallow-marine section in the world. These two coeval formations contain as many as 4 transgressive-regressive cycles, probably reflecting several significant sea level changes during the first 2 million yr of the Eocene. The most transgressive, deepest water marine deposits, which suggest inner to middle neritic depths, are at or very near the base of each cycle, and consist of abundantly fossiliferous glauconitic sand, silt, and clay. The uppermost sediments in each cycle usually consist of clay containing brackish to freshwater palynomorph assemblages. This upward transition in each cycle from neritic to freshwater environments within ess than 6 m (20 ft) of section can most reasonably be explained by rapid changes in relative sea level rather than by progradation.

Discoaster diastypus, Tribrachiatus bramlettei, or Tribrachiatus contortus occur in the lower 3 cycles. First occurrence of these 3 species has been used to define the base of calcareous nannofossil Zone NP10 of Martini or the Discoaster diastypus Zone of Bukry, both considered to mark the base of the Eocene. In central Alabama, strata above a scour surface at the top of the third cycle do not contain T. bramlettei or T. contortus, which become extinct at or near the top of Zone NP10, or Discoaster lodoensis, which first occurs in Zone NP12; they do have Tribrachiatus orthostylus, which first occurs in upper Zone NP10. We tentatively have placed within Zone NP11 these sediments which probably represent a fourth cycle.

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