The Upper Cambrian of the Appalachians is dominated by peritidal cycles consisting of a grainstone lag overlain by cryptalgal bioherms and ribbon rocks capped by laminated dolomites. Fifth-order (10,000-100,000 years) small-scale (1-10 m) shoaling-upward cycles are evident throughout both miogeoclinal sequences. Superimposed scales of cyclicity can be recognized within the Upper Cambrian carbonate sequences of the early Paleozoic Appalachian and Cordilleran passive margins. The long-term event has the same timing in all three areas, and it coincides temporally with the Sauk transgression-regression on the craton. in duration, and the other consists of multiple short-term sea level changes lasting from 2 to 10 m.y. One event is a single long-term rise and fall of sea level more than 40 m.y. R2 more » curves from the southern Canadian Rockies, Utah, and Virginia Appalachians indicate two orders of events superimposed on the thermal-controlled subsidence of the margins. Therefore, R2 curves contain evidence of deviations from the thermal-controlled subsidence of the margins. The form of the R2 curves is not significantly affected by uncertainties in time scales, delithification factors and models of post-rift subsidence. Model cooling curves (exponential) are then fit to the R1 curves and subtracted from them, producing a second set of curves, designated R2. The effects of sediment loading and lithification are removed from cumulative subsidence curves, producing reduced cumulative curves, designated R1. Evidence of controls on subsequent growth of the platforms is obtained from one-dimensional analyses of post-rift subsidence of the margins. Two-dimensional modeling of palinspastically restored cross sections implies that significant reduction in relief of onshore sediment sources would have occurred in the Middle Cambrian when increased time-dependent flexural rigidity extended the area of subsidence into the craton. Initiation of the carbonate platforms in the Middle Cambrian followed reduction in supply of Lower Cambrian coarse siliciclastic materials to the passive margins. Modeling of early Paleozoic passive margins (miogeoclines) in the Cordilleran and Appalachian orogenes suggests that factors controlling growth of early Paleozoic carbonate platforms were thermal-controlled subsidence, time-dependent flexure of the lithosphere, and at least two orders of eustatic sea level changes.
0 kommentar(er)
