Role of Thermal Subsidence, Flexure, and Eustasy in Evolution of Early Paleozoic Carbonate Platforms in Appalachian and Cordilleran Miogeoclines

Gerard C. Bond, John P. Grotzinger, Michelle A. Kominz

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. Initiation of the carbonate platforms in the Middle Cambrian followed reduction in supply of Lower Cambrian coarse siliciclastic material to the passive 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.

Evidence of controls on subsequent growth of the platforms is obtained from one-dimensional analyses of post-rift subsidence of the margins. The effects of sediment loading and lithification are removed from cumulative subsidence curves, producing reduced cumulative curves, designated R1. Model cooling curves (exponential) are then fit to the R1 curves and subtracted from them, producing a second set of curves, designated R2. The form of the R2 curves is not significantly affected by uncertainties in time scales, delithification factors and models of post-rift subsidence. Therefore, R2 curves contain evidence of deviations from the thermal-controlled subsidence of the margins. R2 curves from the southern Canadian Rockies, Utah, and Virginia Appalachians indicate two orders of "events" superimposed on the thermal-controlled subsidence of the margins. One event is a single long-term rise and fall of sea level more than 40 m.y. in duration, and the other consists of multiple short-term sea level changes lasting from 2 to 10 m.y. The long-term event has the same timing in all three areas, and it coincides temporally with the Sauk transgression-regression on the craton. The evidence strongly implies that it is a eustatic event. We suggest that the marked expansion in Upper Cambrian strata of carbonate shoal facies together with increases in percentages of cryptalgal laminites and dolomites in both passive margins were caused by a significant reduction in net subsidence rates resulting from a slower rate of thermal-controlled subsidence combined with the long-term eustatic all. The short-term sea level events correspond directly to the Grand Cycles of the southern Canadian Rockies and have correlatives in the Great Basin and the central Appalachians. The evidence suggests a eustatic control for these events as well. The short-term events are comparable to the third-order cycles of the Vail sea level curve. They constitute basic elements in the stratigraphic framework of the platforms and are potentially excellent chronostratigraphic markers that should be readily apparent in Cambrian platform strata around the world.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.