The use of stromatolite zones for local correlation in the Belt series provides the stratigrapher with an additional tool in an otherwise unfossiliferous sedimentary sequence. Eight zones have been recognized in the Glacier National Park region. The same zones do not appear to extend throughout the outcrop area of the Belt series in northwestern Montana and adjacent States. However, certain zones may extend from a local area into an adjacent one. By using the interfingering zones, new zones in adjacent areas can be correlated in a standard section and the age relations of the Belt series over the entire outcrop belt can be worked out.
In addition to their use as stratigraphic markers, stromatolites may be utilized to distinguish the tops and bottoms of strata. Collenia and Cryptozoon are composed of laminae that are invariably convex upward. In Newlandia the laminae are concave upward, but the genus can be easily distinguished from Collenia on the basis of the arrangement of the laminae. In most occurrences, Conophyton is attached to the substratum by the apex of its conical basal terminus. However, because of the apparently inverted nature of some of the specimens found in the U. S. S. R., caution should be exercised in using this genus in attempts to determine tops of strata.
With the aid of the key, descriptions, and illustrations, the distinctive and stratigraphically significant stromatolites can be recognized. Forms that do not fit the classification may be found. Only those forms that are consistent in their characteristics over wide areas have been described here. In some places an algal limestone may be suspected without identification of any distinctive structures. As has been pointed out, algal deposits may vary from mere intensification of regular sedimentary lamination to such complex structures as Collenia multiflabella n. sp.
Many stromatolites of the Belt series closely resemble certain structures that are forming at the present time. Until the environmental factors to which these structures are related are better understood, it would be in advisable to attempt exact correlations between fossil and modern environments. As previously observed, the range in salinity of the water in which these organisms grow is extreme. My own limited survey of the published record and of modern environments suggests that the dominant forms in marine or salt-lake environments are stromatolitelike structures, whereas in the freshwater lakes and streams the dominant structures are arborescent masses or unattached spheroidal concretionary masses. Just how far this tentative generalization can be extended remains to be seen.
Last Updated: 18-Jul-2008