Stromatolites in Earth History
Architecture of Stromatolite Reefs
The architecture of modern reefs reflects, in large part, reef growth in response to changes in sea-level. Several of my recent and ongoing studies are aimed at deciphering the growth history of Precambrian stromatolite reefs to help understand environmental controls on stromatolite growth and lithification. In particular, my colleagues and I have focussed on the ~1.3 Ga Dismal Lakes and Atar reefs (NWT, Canada and Mauritania, respectively), because these reefs are composed largely of unusual conical stromatolite forms, which are thought to grow in deep waters.
 Meter-scale conical stromatolites in the Dismal Lakes Group, NWT Canada |
 Giant conical stromatolites in the Dismal Lakes Group, NWT, Canada |
In the Dismal Lakes Group, m-high cones are abundant directly above an unconformity, indicating growth during initial transgression. Upon a second, more significant marine transgression, a new series of cones developed that reach up to 15 meters in synoptic relief. The wide spacing of cones and the presence of herringbone cement at the transgressive surface suggest that anoxic basin waters flooded the shelf and inhibitied carbonate nucleation.
 Stacked, prograding domal stromatolites extending as "arms" from giant cones |
 Plan view of conical stromatolites with evidence of exposure and karsting |
Inter-stromatolite space is occupied by a series of stacked, prograding stromatolite "arms." Examination of growth laminae suggest (1) that arm growth initiated only after termination of conical growth, (2) that upward growth of arms was limited by accommodation space, and (3) that up to 4 different sea-level cycles can be accounted for by the stromatolite package. This unusual architecture suggests tha original cone tops may have been exposed above sea-level several times during reef history - an interpretation recently confirmed by the discovery of cone-tops that show evidence of internal karst formation. Our understanding of reef architecture is being further developed by tracing stromatolitic boundaries down-dip into the basin, to see how sea-level changes affect stromatolite growth in different environments along the reef.
 Conical and branching conical stromatolites that make up reef facies in the Atar Group |
 Close-up of a branching cone showing branch initiation along a single lamina |
Deep-water strata of the Atar Group, Mauritania, are also dominated by conical and branching conical stromatolites. In this case, low depositional dips result in little lateral change in facies. However, satellite and aerial imagery indicate that reef growth ends abruptly over a distance of several kilometers, and is replaced by a shale basin that contains neither in situ or transported carbonate. My colleague, Julie Bartley (State University of West Georgia) and I are currently examining the nature of this reef-to-basin transition in detail.
Genesis of Microbialite Morphologies
 Domal to branching-columnar stromatolites in the ~1.3 Ga Dismal Lakes Group, NWT |
 Uniform columnar stromatolites in the ~1.2 Ga Atar Group, Mauritania |
Stromatolite morphology has long been recognized as a potential environmental indicator - stromatolites with high synoptic relief commonly occur in deeper-water settings, and flat-laminated mats are typically restricted to shallow-water environments. It's often overlooked, however, that only the uppermost (living) layer responds to environmental conditions at any given time. Overall stromatolite morphology, therefore, is a cumulative effect of changing environmental conditions through time. In 1994, Hans Hofmann (Université dé Montreal) proposed a mathematical description of lamina morphology that permits categorization of synoptic (growth) shape in terms of a vertical versus horizontal, scale-independent measure of laminosity. If environmental conditions exert a dominant control on lamina growth, laminosity change through sequential growth layers should define trends that correspond reliably to particular changes in growth environment. Although demonstration of these trends is still in it's infancy, my colleagues and I hope that we can test this relationship quantifying stromatolite lamina shape in individual bioherms that occur across different environments and using sequence stratigraphy as an independent means of inferring the magnitude and direction of environmental change.
 Unusual, deep-water cuspate stromatolite, Dismal Lakes Group, NWT Canada |
In another example, my colleagues and I are using growth morphology and depositional environment of an unusual cuspate microbialte form to infer initial microbial composition. Cuspate microbialites occur along a transgressive surface offshore from the giant conical stromatolite reef in the ~1.3 Ga Dismal Lakes Group, Arctic Canada. Constraints from the onshore reef suggest that water depths may have exceeded 45 meters. At these depths, it is unlikely that any significant light reached these deep-water stromatolites, suggesting that they may not have been composed of photosynthetic bacteria. This suggestion is supported by microbialite morphology, which indicates the presence of an upright stalk (< 500 microns thick) that supports several horizontal, "tented" layers. It is unlikely that the stalk was composed of phototrophic bacteria because overlying layers would have kept the lower stalk from getting sufficient light. These prelimiary analyses suggest that chemotrophic bacteria, responding to chemical gradients in the water column, may have been the primary mat composer.
Linda Kah
Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-6399
Email: lckah@utk.edu