ABSTRACT

Scanning electron microscopy/energy dispersive X-ray analysis, X-ray diffractometry, and petrographic study of siliceous mudstone samples from the Tallahatta Formation in Clarke County, Alabama, reveal abundant diatom molds and casts along with original valve fragments. Molds and casts are opal-CT lepispheres, whereas rare valve fragments consist of biogenic opal-A. These are set in a matrix of opal-CT lepispheres and minor authigenic minerals including heulandite, clinoptilolite, pyrite, microquartz, and chalcedony. Minor detrital clay, quartz, feldspar, muscovite, biotite, glauconite, heavy minerals, and calcitic skeletal debris are also present. Whole-rock chemical analyses average: SiO₂, 81.9%; Al₂O₃, 4.9%; CaO, 2.5%; MgO, 0.6%; Na₂O, 0.1%; K₂O, 0.8%; Fe₂O₃, 1.6%; MnO, 0.01%; TiO₂, 0.2%; P₂O₅, 0.05%; and LOI, 6.5%.

Scanning electron photomicrographs show a high density of whole, large diatom valves. Possible taxa include the open-water forms Coscinodiscus radiatus, Nitzschia, Stephanopyxis, Triceratium, and rare neritic chain-formers such as Paralia and Melosira or Aulacosira. Diatom abundance and paucity of terrigenous mud indicate that the original sediment was a diatom ooze, which probably accumulated in high productivity areas of the outer shelf and slope. Absence of relict volcanic textures, pyrogenic minerals, or diagnostic alteration products supports Wise and Weaver's 1973 interpretation of a primary biogenic origin for these deposits.

Our studies indicate a progressive diagenetic change from original diatom ooze to cristobalitic mudstone and chert. We recognize the following sequence: 1. blades of opal-CT formed through direct replacement of diatom opal-A along micron-scale solution films; 2. opal-CT blades coalesced to form incipient lepispheres that replaced diatom frustules while maintaining original areolae pattern; 3. detachment and continued growth of lepispheres in intraskeletal pores filled areolae and obliterated the original lattice pattern; and 4. metastable opal-CT recrystallized to microcrystalline and chalcedonic quartz.