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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Relation of Eustasy to Stacking Patterns of Meter-Scale Carbonate Cycles, Late Cambrian, U.S.A.
David Osleger, J. Fred Read
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.
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