THE CARBONATE SUPPRESSION-ACCOMMODATION GENERATION (CARBONATE SAGE) MODEL: SEA-LEVEL LOWSTANDS CAN MIMIC MAXIMUM FLOODING ZONES ON RAPIDLY SUBSIDING MARGINS

A new carbonate suppression-accommodation generation (carbonate SAGE) model illustrates the possibility of depositing upward-deepening strata during regressions induced by relative sea-level falls, just the opposite of what normally would be interpreted with standard sequence-stratigraphic models. Rocks of the Late Cambrian Steptoean Stage provide an example of this phenomenon.

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.