BAHAMIAN HOLOCENE STROMATOLITES AND THROMBOLITES MIMIC MORPHOLOGY AND STRUCTURE OF PRECAMBRIAN AND CAMBRIAN EXAMPLES

The morphology and structure of a large (ca. 1.5 m) Holocene stromatolite and that of an equally large thrombolitic column show remarkable similarities to specimens from the Precambrian and Cambrian respectively. Both come from the same oceanic tidal channel some 7 m deep in the Exuma Islands and both consist of ooid sand trapped and bound by cyanobacteria. In cross-section, the stromatolitic column that widens upward consists of a central part with distinct convex-upward, millimeter scale laminations. Surrounding this central compound stromatolite are a series of centimeter-thick petals arranged like leaves of an artichoke. The successive petals are steeply inclined upward; they also consist of ooid sand and several show convex-upward laminatons. This petalloid morphology mimics the form and scale of Jacutophyton from the late Precambrian of Siberia, Mauritania, Norway and South America.

In section, the thrombolitic column has three parts: a basal section 20 cm thick with two stromatolites; a middle section a meter thick with complex and variable fabrics: and a cap 20 cm thick of convex-upward stromatolitic laminations. The thick middle section includes unlaminated and vaguely clotted ooid sand, draped stromatolitic margin on one side, several centimeter-scale stromatolites, disrupted and wispy laminations, vugs and cavities and a period of no accretion marked by sponge borings. Similar successions and alternations of stromatolitic and thrombolitic fabrics are described from the Cambrian of Australia (Kenard and James,1986). These arresting similarities between Holocene and fossil fabrics in both specimens testify to up to a billion-year-persistence of growth mechanics in microbial structures.