CARBONATE SPONGE-ALGAL BUILDUPS IN THE FIRST SEQUENCE OF THE FILLMORE FORMATION (LOWERMOST ORDOVICIAN) OF WESTERN UTAH; A KEY TO UNRAVELLING STRATIGRAPHIC SEQUENCE

Lower Paleozoic carbonate sponge-algal buildups (stromatolitic mounds) are sensitive environmental indicators. Such mounds were stenotopic; they were likely restricted to shallow sunlit waters with low turbidity and sedimentation. Such restricted environmental tolerances suggest that these mounds may mark significant boundaries. The stratigraphic distribution of the stromatolitic mounds in the Fillmore Formation supports this conclusion.

Two orders of cyclicity are recognized in the Fillmore Formation: Small scale 3 to 5-meter cycles, and larger (sequence)-scale 100 to 150 meter cycles. Within the small-scale cycles, algal horizons occur in a position suggestive of shallow-water deposition followed by rapid deepening. A typical cycle contains a basal, thick, coarsening-upward shale- or siltstone-rich interval, with beds of flat-pebble conglomerate and calcarenite grainstone, capped by an algal accumulation. The algal layer generally has an erosive upper surface draped by a thin layer of encrinite grainstone which is covered by the shaley interval of the next cycle.

Numerous thin (1-5 cm) algal accumulations occur throughout the Fillmore section, but a few occur as mounds up to 4 meters in thickness. Thus algal accumulations are not rare, but optimal conditions rarely persisted long enough for mounds to form. The mounds are made of relatively pure, fine-grained limestone, are stromatolitic, and contain calcareous sponges and, in some cases, Calathium. Typically the upper surfaces show evidence of synsedimentary lithification with diagenetic silicified rinds and distinctive sculpting suggestive of exposure.

In the first sequence, mounds delineate sequence architecture by marking breaks in the stacking patterns of smaller-scale cycles. At 54 m (175 feet) a mound horizon (informally "Miller's reef") in the center of the terrigenous basal part of the sequence may mark the maximum flooding surface. The thickest mound horizon lay at 116 m (385 feet; informally "Hintze's reef") and marks the break between progradationally stacking cycles below and a closely-bedded limestone flat-pebble-conglomerate-rich non-cyclic interval, the uppermost beds of the first Fillmore Sequence. Other occurrences of mounds in three overlying sequences parallel these occurrences.