BIZARRE PATTERNS FOUND IN UPPER CAMBRIAN STROMATOLITES

Upper Cambrian stromatolites of the Notch Peak Formation in southwestern Utah are often furnished with bizarre patterns in their inner core as seen in plan-view. Patterns can be round, sub-round or elongate, and either standalone within the larger stromatolite structure, or connect with the laminated outer portion of the stromatolite. Slabbing of the stromatolites, as well as thin-section work, reveals a composite structure consisting of three main parts: 1. A round to sub-round inner core composed of small, mound-like masses separated by cm-scale channels filled with coarser-grained, bio-clastic material. 2. An outer, finely laminated micritic zone enclosing the inner core. 3. An interspatial zone separating the stromatolites that consists of coarse-grained, bioclastic material. Cross-cutting and growth related relationships seem to suggest four phases of deposition: 1. An initiation phase whereby small, concave-up mounds randomly grow at the sediment-water interface and are separated from each other by cm-scale channels. 2. A consolidation phase whereby a small group of mounds, complete with channels, coalesce into a single round to sub-round core. 3. A distinctly stromatolitic phase whereby the consolidated core becomes encased in fine micritic laminations. 4. A burial stage whereby the stromatolite is completely buried in coarser-grained interspatial material. Stage 4 represents a simple depositional process. The processes responsible for growth in stages 1 – 3, however, are as yet unknown.