ENVIRONMENTAL CHANGE RECORDED BY STROMATOLITE MORPHOLOGY – QUANTITATIVE APPROACHES

Stromatolite morphology has long been recognized as a potential environmental indicator for Archean and Proterozoic depositional environments. For example, stromatolites with high synoptic relief commonly occur in deeper-water settings, whereas flat-laminated mats are typically restricted to very shallow environments. It is 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, Hofmann proposed a mathematical description of lamina morphology that permits categorization of synoptic (growth) shape in terms of a vertical versus horizontal, scale-independent laminosity plot, where laminosity relates the length of a lamina to its height (vertical laminosity) or its width (horizontal laminosity). On such a plot, observed variation in lamina shape from flat-laminated to strongly convex is recorded as a distinct laminosity trend.

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. This relationship is testable by quantifying stromatolite lamina shape in individual bioherms that occur across different environments. Studied bioherms may reflect either transgressive or regressive cycles but must occur in time-equivalent strata. Specifically, bioherms are predicted to show similar upward trends in lamina shape, reflecting successive environmental changes, regardless of position on platform (i.e. absolute water depth) or gross stromatolite morphology. The 1.3 Ga Dismal Lakes Group, arctic Canada, contains several recognizable stromatolite horizons that span a range of water depths. Sequence stratigraphy, meanwhile, provides an independent means of inferring the magnitude and direction of environmental change. In this study, we quantify lamina shape and attempt to relate laminosity trends to observed environmental changes in an effort to determine how temporal trends in lamina shape and environment ultimately produce stromatolite morphology.