GEOCHEMISTRY, PETROLOGY, AND SEQUENCE STRATIGRAPHY OF THE LOWER ORDOVICIAN GARDEN CITY FORMATION IN NORTH-CENTRAL UTAH

The lower Ordovician Garden City Formation and the Pogonip Group, deposited in the Northern Utah Basin and the Ibex Basin of north-central and west-central Utah, respectively, are coeval sedimentary rock units. Due to differences in basin history, the stratigraphic expression of sea-level fluctuations differs markedly between the two units. The Ibex Basin experienced high subsidence rates and large amounts of clastic input. This resulted in the deposition of the Pogonip Group, which ranges in thickness from 343 – 1067 m, and consists of sandstone, shale, lime-mudstone to grainstone, and flat-pebble conglomerate, which are readily grouped into third-order sequences. The Northern Utah Basin did not experience the high rates of subsidence or the large amounts of clastic input of the former. This resulted in the deposition of the Garden City Formation, which ranges in thickness from 366 – 549 m. The Garden City Formation consists largely of calcisiltite, argillaceous limestone, lime-mudstone to packstone, and flat pebble conglomerate which are not easily grouped into sequences. Construction of an accurate depositional model is also hindered by the muted facies contrasts within the Garden City Formation. A variety of techniques, including δ¹³C and δ¹⁸O stable isotopes ratios, elemental concentrations, and petrologic analyses, coupled with Markov Chain analysis and the use of Fischer plots has resulted in the delineation of repetitive packages of coarsening-upward meter-scale cycles, which may be grouped into larger-scale packages or sequences. Two measured sections of the Garden City Formation, separated by 27 km, were analyzed, which resulted in the identification of seven sequences in a nearshore locality and five in an offshore locality. Global sea-level curves suggest that 7 – 9 sequences fall within the geologic time frame of the Garden City Formation (c. 488.3 – 471.0 Ma). The nearshore locality was more sensitive to eustatic events, as evidenced by the greater number of higher-order cycles displayed and exhibits unconformities at the bottom and top of the section, as indicated by δ¹³C and δ¹⁸O stable isotope ratios and elemental concentrations.