Parasequence Stacking Patterns, Third-Order Accommodation Events, and Sequence Stratigraphy of Middle to Upper Cambrian Platform Carbonates, Bonanza King Formation, Southern Great Basin

Abstract

The Bonanza King Formation of the southern Great Basin is composed of 150 to 250 carbonate parasequences (0.5 to 7 m thick) that provide a remarkable strip chart of Middle to Late Cambrian third-order accommodation history. Six superb exposures of the Banded Mountain Member of the Bonanza King Formation were logged on a decimeter scale and correlated to generate a detailed platform-to-basin transect. These cyclic carbonates were deposited on a flat-topped, fully-aggraded platform that extended approximately 250 to 300 km across the early Paleozoic passive margin of southern Nevada and eastern California. The Banded Mountain Member ranges from 400 to 1330 m in thickness and forms a westward-thickening wedge from craton margin facies in the eastern Mojave to shelf-edge an base-of-slope facies in the Last Chance Range. Thick successions of meter-scale carbonate parasequences show systematic changes in parasequence type, dominant lithofacies, and thickness vertically through the cyclic interval that were governed by long-term, third-order changes in accommodation. Fischer plots provide a graphic illustration of changes in accommodation space through time, and appear to be a valuable tool for correlating sequence boundaries and systems tracts. Similar patterns of positive and negative slopes on Fischer plots of the five platform sections of the Banded Mountain Member define four major sets of long-term increases in accommodation followed by long-term decreases in accommodation that correspond to four distinct depositional sequences. Translating sequence stratigraphic principles originally defined for seismic-scale siliciclastic systems to outcrops of cyclic carbonates requires a de-emphasis of stratal geometries and an increased awareness of correlative End_Page 305------------------------ vertical changes in stacking patterns of component parasequences. Sequence boundaries and transitions between systems tracts on flat-topped platforms, such as the Bonanza King, are believed to be zones rather than distinct horizons because of the effect of the higher frequency sea-level signal superimposed on the long-term, third-order event. Sequence boundary zones bracketing the four depositional sequences within the Banded Mountain Member are characterized by thin, tidal-flat dominated parasequences exhibiting abundant evidence for repeated episodes of exposure. Transgressive systems tracts in the Banded Mountain Member are characterized by a lower succession of parasequences composed of subequal amounts of tidal-flat and subtidal lithofacies passing upward into thicker pa asequences with higher percentages of deeper subtidal facies. Highstand systems tracts in the Banded Mountain Member are characterized by thinning-upward stacks of parasequences showing progressively shallower peritidal conditions within the component lithofacies.