DEPOSITION AND GAMMA-RAY STRATIGRAPHY OF THE FILLMORE FORMATION (LOWER ORDOVICIAN), WEST-CENTRAL UTAH

The Fillmore Formation, approximately 1,800 feet thick in western Utah, accumulated on a rapidly subsiding miogeoclinal platform. This unit records a change from dominantly carbonate sedimentation during the Skullrockian Stage to mixed carbonate and siliciclastic sedimentation during the Stairsian Stage. We consider that the Fillmore Formation was deposited during an extensive interval of sea-level lowstand with episodes of higher-frequency relative sea-level rise and fall. Hintze recognized five informal members in the Fillmore Formation. Essentially, major cycles of sedimentation correspond to three intervals of recessive and ledgy strata that crop out in the southern House and Confusion ranges; the uppermost ledgy strata are overlain conformably by thicker-bedded cyclic carbonates of the Wah Wah Limestone.

A plethora of different carbonate facies are present in the Fillmore Formation, but it contains mostly high-energy deposits interbedded with shale, siltstone, and lime mudstone. Depositional facies indicate that bathymetry never reached great depths, nor did it remain near sea level. Abundant intraclast conglomerates and a few grainstone beds, including 2D megaripples with a 2.5-3.0 foot wavelength, demonstrate the influence of storm deposition. Laminae in fine-grained skeletal grainstones indicate traction deposition. The carbonate SAGE model applies to these strata: influxes of shale and siltstone suppressed carbonate productivity so that sedimentation essentially kept pace with accommodation but rarely was above normal wavebase.

Many parts of the gamma-ray outcrop profiles from Section C and Square Top in the southern House Range can be correlated confidently with the American Quasar Horse Heaven-State 16-21A gamma-ray log from the Confusion Range. Thin intervals of mixed shale, siltstone, and carbonates display a high-frequency signal that averages between 40-70 counts per second with a maximum of 108. The well log averages between 60-100 API units with a maximum of approximately 180. The most distinctive feature of the log and profiles is a negative-positive excursion couplet in the uppermost Fillmore Formation. A sequence boundary corresponds with the top of this positive shift.