Genesis of Carbonate Fabrics
Seafloor Precipitates
More than a decade ago, recognition of crystalline precipitates in Archean carbonates suggested that carbonate saturation in the Precambrian may have been significantly higher (Grotzinger 1989). Seafloor precipitates have now been recognized in a wide variety of Precambrian basins. In the Archean and Paleoproterozoic, precipitates are abundant in both shallow and deeper-water successions. In Mesoproterozoic successions, with few exceptions (e.g. parts of the Dismal Lakes and Atar groups), precipitates are restricted to nearshore evaporative environments. By the Neoproterozoic, except in post-glacial deposits, precipitates are generally absent.
 Carbonate precipitates in outcrop, northern Baffin Island |
 Carbonate precipitates in outcrop, northern Baffin Island |
 Seafloor precipitates, from the Dismal Lakes Group, NWT |
 Aragonitic fans preserved in chert, Baffin Island |
 Pseudomorphs after aragonite, Mauritania |
This gradual shift in the distribution of seafloor precipitates may reflect an overall decrease in carbonate saturation state through the Precambrian, and may have had a significant affect on sedimentary substrates. It appears, for example, that substrate lithification may have controlled the distribution of certain cyanobacteria (Kah and Knoll, 1996). In another example, rapid lithification of the substrate may have influenced the development of sedimentary parasequences (Kah, in press). Current research is focussed on learning to identify more subtle changes in carbonate saturation and to understand the mechanisms behind these changes.
 Both modern and ancient Entophysalis colonize only lithified substrates |
 Both modern and ancient Polybessurus are restricted to non-lithified sunstrates |
Molar-tooth Structures
Molar-tooth structure (MT) is an unusual carbonate fabric characterized by variously shaped cracks and voids within non-lithified substrates that are rapidly cemented by a characteristically uniform, equant microspar cement. MT is particularly intriguing, first because of the enigmatic juxtaposition of rapidly lithified void-fills in a non-lithified carbonate sediment, and second, because this unusual fabric occurs worldwide, but is primarily restricted to Mesoproterozoic and early Neoproterozoic shallow marine successions.
 Subspherical MT voids, Avzyan Fm., Russia |
 Horizontal MT cracks, Helena Fm., Montana |
 Complex MT voids, Helena Fm., Montana |
Most MT research has focussed on possible mechanisms of void formation. In a different approach, my students, colleagues and I are working to test possible models of MT genesis anddecipher the environmental implications of MT microspar via detailed petrographic analysis (Pollock et al., in revision).
 MT crack following coarse-grained lithologies, while remaining in bleb-form in fine-grained lithologies |
 CL analysis of MT microspar shows that microspar is composed of 2-3 micron balls surrounded by luminescent cement |
Continuing research by M.S. student Curt Crawford is focused on examining the structure of MT microspar in detail. CL analysis shows that MT microspar consists of subrounded cores (with polygonal overgrowths) that show a strong similarity to experimentally precipitated vaterite. A variety of techniques (SEM, Raman spectrography, precipitation experiments) are being designed to determine the original mineralogy of MT microspar, its diagenesis, and implications for carbonate precipitation in the Mesoproterozoic.
Herringbone Carbonate
Herringbone carbonate is an unusual cement morphology, consisting of elongate crystals in which the c-axis rotates through growth, from parallel to perpendicular to crystal elongation. It generally occurs as a primary marine cement in Archean successions and as void-filling cement in younger successions. In both cases, it is believes to reflect carbonate precipitation in the presence of a high concentration of growth inhibitors (particularly Fe2+) present in anoxic marine waters.
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| Successive photomicrographs of herringbone carbonate under crossed-polarizers. Rotation of the c-axis in herringbone carbonate causes a sweeping extinction along the crystal | ||
Two examples of herringbone carbonate cement are known from Mesoproterozoic successions. In the ~1.3 Ga Dismal Lakes Group, herringbone carbonate is restricted to a single main flooding surface within a large stromatolitic reef, where it occurs as a first generation cement. Stratigraphic relationships suggest that herringbone cementation reflects an incursion of anoxic(?) deep water onto the shelf during maximum transgression. Herringbone cement also occurs in reef facies of the Atar Group, where it represents both first and second generation cements within an extensive stromatolitic reef.
 Herringbone cement from the 1.3 Ga, Dismal Lakes Group, Arctic Canada |
 Herringbone cement from the ~1.25 Ga, Atar Group, Mauritania |
Mapping the time-stratigraphic distribution of these cements in reef facies and surrounding strata, followed by petrographic and geochemical analysis, is integral to understanding the possible roles of geochemically different water bodies in the genesis and distribution of unusual carbonate fabrics.
Linda Kah
Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-6399
Email: lckah@utk.edu