Classification and Environmental Significance of Algal Stromatolites

Abstract

Stromatolites are laminated structures that have been previously termed fossil algae. It is now recognized that such structures may be formed by a number of different processes and organisms. Recognizable algal stromatolites are more validly treated as organosedimentary structures than fossil organisms. This paper proposes a new classification of algal stromatolite structures based upon their geometric forms and relates these forms to their sites of growth relative to sea-level environments. The classification uses the arrangement of the basic geometric units (hemispheroids and spheroids) from which common stromatolites and oncolites are built. Three main arrangements of these geometric structures occur in Recent algal stromatolites and oncolites; (1) laterally linked hemispheroids (LLH), (2) discrete, vertically stacked hemispheroids (SH), and (3) discrete spheroids-either as randomly stacked hemispheroids or concentrically arranged spheroids (SS). The abbreviations are used to designate the various structures conveniently. Since certain restricted environments are associated with modern stromatolites, ancient environments may be interpreted by recognition of fossil stromatolite forms. For example, protected intertidal mud flats, where wave action is slight, may be inferred by the presence of type-LLH stromatolites (Collenia). Exposed, intertidal mud flats, where scouring action of waves and other interacting factors prevent growth of algal mats between stromatolites, are inferred by the presence of type-SH structures (Cryptozoon). Low intertidal areas that are exposed to waves and agitated shallow water below low-water mark are inferred by the presence of type-SS structures (oncolites). In addition, the arrangement of laminae in these structures reflects frequency of movement, frequent motion resulting in concentrically arranged spheroids and occasional motion in randomly stacked hemispheroids.