THE CARBONATE SUPPRESSION-ACCOMMODATION GENERATION (CARBONATE SAGE) MODEL: SEA-LEVEL LOWSTANDS CAN MIMIC MAXIMUM FLOODING ZONES ON RAPIDLY SUBSIDING MARGINS
Trilobite extinction horizons and faunal zones of the Steptoean are useful biostratigraphic constraints on the timing of relative sea-level changes. In the eastern Great Basin, mixed carbonate and shale strata, 400-1,200+ feet thick, were deposited over a faulted miogeoclinal platform. The succession typically consists of four depositional units: 1) shale and interbedded carbonates with laterally equivalent carbonate shoal strata, 2) a carbonate shoal or tidally influenced carbonate mudstone, 3) shale and shallow-water carbonates with a distal wedge-shaped carbonate interval, and 4) cyclic carbonates that are overlain by a thick succession of meter-scale carbonate cycles or carbonate turbidites of the Sunwaptan Stage.
One shale package (unit 1) bears sedimentary structures and biota indicating deeper-water deposition; the other (unit 3) is associated with shallower-water sedimentary structures. These shale-rich zones correlate to missing faunal zones on the craton (unit 1) or karstification (unit 3), so they must be products of relative sea-level fall and regression. In unit 1, greater water depths for shales must have been achieved through rapid subsidence. Regression induced by relative sea-level fall would have delivered siliciclastics that suppressed carbonate productivity. In one area protected from siliciclastic influx, carbonate shoal deposition continued. In unit 3, the rate of relative sea-level fall may have been more rapid, creating karst in the unit, depositing interbedded shales and carbonates in shallow water, and shifting the locus of clean shallow-water carbonate accumulation to the west. The uppermost cyclic strata record the transgression and ultimate flooding of the Transcontinental Arch and much of Laurentia.