ABSTRACT

An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large-scale depositional sequences and the component peritidal to subtidal meter-scale cycles that comprise them. The focus of this paper is on the small-scale cyclicity, its probable control by Milankovitch-forced sea-level oscillations, and how stacking patterns of meter-scale cycles can be used to define internal components of larger-scale sequences and estimate variations in relative sea level.

Fining-upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening-upward subtidal cycles which lack evidence of emergence and form a continuum across the Cambrian carbonate platforms. Eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on Late Cambrian carbonate platforms. Evidence for Milankovitch forcing of glacio-eustatic sea-level oscillations is provided by a 4:1 bundling of fifth-order meter-scale cycles ( 96 ky) within fourth-order cycles spanning tens of meters ( 440 ky) within the Big Horse Member of the Orr Formation in the House Range of Utah. The 4:1 bundling may manifest the short e centricity to long eccentricity ratio of the Milankovitch astronomical rhythms.

Systematic changes in the stacking patterns of meter-scale cycles can be used in conjunction with Fischer plots to define long-term sea-level cycles. On Fischer plots of peritidal cyclic successions, long-term relative sea-level rises are characterized by thick, subtidal-dominated cycles with thin laminite caps. Long-term relative sea-level falls are defined by stacks of thin, laminite-dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand. On Fischer plots of dominantly subtidal cyclic successions, long-term sea-level rise is characterized by storm-dominated, open marine carbonate cycles or thick, deep ramp, shale-based cycles. Falling segments of the Fischer plot are characterized by thin, shallow subtidal cycles compos d of restricted lithofacies. Cycle stacking patterns (parasequence sets) provide the crucial link between the meter-scale cycles (parasequences) and the larger scale sequences and their component systems tracts.

One- and two-dimensional models of peritidal and subtidal cycle development indicate that, whereas peritidal cycle thickness is primarily controlled by accommodation space, deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level; lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm-wave reworking that oscillated in harmony with sea-level fluctuations and may have acted as a subtidal limit to upward aggradation. The 2-D modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses, compositions and stacking patterns similar to the Late Cambrian of North America can be generated by ilankovitch-driven composite eustasy.