Use of Cycle Stacking Patterns to Define Third-Order Depositional Sequences: Middle to Late Cambrian Bonanza King Formation, Southern Great Basin

MONTANEZ, ISABEL P., and MARY L. DROSER, University of California, Riverside, CA

The Middle to Late Cambrian Bonanza King Formation (CA, NV) is characterized by superimposed scales of cyclicity. Small-scale cycles (0.5 to 10 m) occur as shallowing-upward peritidal and subtidal cycles that repeat at high frequencies [10(4) to 10(5)]. Systematic changes in stacking patterns of meter-scale cycles define several large-scale (50-250 m) third-order depositional sequences in the Bonanza King Formation. Third-order depositional sequences can be traced within ranges and correlated regionally across the platform.

Peritidal cycles in the Bonanza King Formation are both subtidal- and tidal flat-dominated. Tidal flat-dominated cycles consist of muddy

bases grading upward into thrombolites or columnar stromatolites all capped by planar stromatolites. Subtidal-dominated peritidal cycles may contain intraclastic basal lags overlain by subtidal limestones consisting of ribbon carbonates, digitate-algal bioherms, and biohermal thrombolites. Subtidal limestones grade up into dolomitic, unfossiliferous ribbon carbonates, pelletal packstones, thick laminites, and cryptalgal laminites with erosional tops.

Subtidal cycles in the Bonanza King Formation consist of grainstone bases that commonly fine upward and contain stacked hardgrounds. These are overlain by digitate-algal bioherms with grainstone channel fills and/or bioturbated ribbon carbonates with grainstone lenses. Cycle caps consist of unfossiliferous ribbon carbonates, pelletal packstones, ooid grainstones or rarely of thrombolites that exhibit scalloped tops typical of tidal-rock platforms. Well-developed regoliths and vadose diagenetic profiles in these meter-scale cycles indicates periodic exposure and relative sea-level fall below the platform. Smallest scale cycles defined on the basis of centimeter to decimeter alternations in ichnofabric (from ii1 to ii4) and in argillaceous content, are superimposed on meter-scale subtid l cycles and likely reflect higher frequency fluctuations in salinity.

Transgressive depositional facies of third-order depositional sequences consist primarily of stacks of subtidal-dominated peritidal cycles and subtidal cycles, whereas regressive depositonal facies are dominated by stacks of tidal flat-dominated peritidal cycles and regoliths developed over laminite cycle caps. The use of high frequency cycles in the Bonanza King Formation to delineate regionally developed third-order depositional sequences thus provides a link between cycle stratigraphy and sequence stratigraphy.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)