MIXED SILICICLASTIC AND CARBONATE SEQUENCES OF THE LOWER ORDOVICIAN FILLMORE FORMATION AT IBEX, WESTERN UTAH

The Fillmore Formation has 1 m-scale cycles,10 m-scale cycles, and three 100 m-scale, sequences with their 4 sequence boundaries. Some sequence boundaries may represent eustatic events and have broad correlation potential.

The 1 m-scale and 10 m-scale cycles show an upward decrease in siliciclastics. They contain 4 facies in various proportions: 1) basal shale-rich facies; 2) nodular bedded lime mudstone/calcisiltite with shale facies; 3) upper pure lime-mudstone, stromatolitic or sponge reef facies, frequently capped by a silicified erosion surface and echinoderm-rich grainstones; and 4) distributed grainstone and packstone beds, probably tempestites. The tempestites are mostly flat pebble conglomerates (FPC) near the base of the Fillmore and mostly bioclastic near the top.

The 100 m-scale sequences display the 4 facies in 3 phases: 1) basal shale-rich phase with thin carbonate beds and occasional reefs, interpreted as a lowstand deposit; 2) middle nodular bedded lime mudstone-calcisiltite-rich phase with reefs and FPC beds becoming thicker and more frequent up section, interpreted as a transgressive deposit; 3) a capping limestone-rich phase where FPC beds are thicker, better washed, and more frequent, interpreted as a highstand deposit. Thin sand/chert-rich carcarenites of the next sequence boundary zone occur in the upper parts of this phase.

The stratigraphic positions of 4 sequence boundaries are: SB1, near the base of the formation at 10 ft (3 m), at the base of the Low Diversity Interval (LDI), in lower trilobite Zone D; SB2, base of slope-forming shaly siltstone member at 485 ft (148 m), in the Acodus deltatus-Oneotodus costatus Zone, Zone F; SB3, base of the brown slope and ledge-forming member at 999 ft (304 m), Oepikodus communis Zone, middle part of Zone G-2; and SB 4, shale near base of the Calathium calcisiltite member at 1640 ft (500 m), Reutterodus andinus Zone, base of Zone I.

SB 1appears to be eustatically controlled. The LDI is associated with an underlying unconformity and a negative carbon isotope excursion in Colorado and New Mexico; it is marked by sandstones in the carbonate dominated mid-continent, e.g. the Roubidoux Sandstone in Missouri and the New Richmond Sandstone in Iowa-Minnesota-Wisconsin. SB 2 may be roughly the same age as the basal “Arenig” unconformity in Europe.