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