CYCLOSTRATIGRAPHY OF MIDDLE DEVONIAN CARBONATES OF THE EASTERN GREAT-BASIN

Middle Devonian carbonates (250-430 m thick) of the eastern Great Basi n were deposited along a low energy, westward-thickening, distally ste epened ramp. Four third-order sequences can be correlated across the r amp-to-basin transition and are composed of meter-scale, upward-shallo wing carbonate cycles (or parasequences). Peritidal cycles (shallow su btidal facies capped by tidal-flat laminites) constitute 90% of all me asured cycles and are present across the entire ramp. The peritidal cy cles are regressive- and transgressive-prone (upward-deepening followe d by upward-shallowing facies trends). Approximately 80% of the periti dal cycle caps show evidence of prolonged subaerial exposure including sediment-filled dissolution cavities, horizontal to vertical desiccat ion cracks, rubble and karst breccias, and pedogenic alteration; local ly these features are present down to 2 m below the cycle caps. Subtid al cycles (capped by shallow subtidal facies) are present along the mi ddle-outer ramp and ramp margin and indicate incomplete shallowing. su bmerged subtidal cycles (64% of all subtidal cycles) are composed of d eeper subtidal facies overlain by shallow subtidal facies. Exposed sub tidal cycles are composed of deeper subtidal facies overlain by shallo w subtidal facies that are capped by features indicative of prolonged subaerial exposure (dissolution cavities and brecciation). Average per itidal and subtidal cycle durations are between approximately 50 and 1 30 k.y. (fourth- to fifth-order). The combined evidence of abundant ex posure-capped peritidal and subtidal cycles, transgressive-prone cycle s, and subtidal cycles correlative with updip peritidal cycles indicat es that the cycles formed in response to fourth- to fifth-order, glaci o-eustatic sea-level oscillations. Sea-level oscillations of relativel y low magnitude (< 10 m) are suggested by the abundance of peritidal c ycles, the lack of widely varying, water-depth-dependent facies within individual cycles, and the presence of noncyclic stratigraphic interv als within intrashelf-basin, slope, and basin facies. Noncyclic interv als represent missed subtidal beats when the seafloor lay too deep to record the effects of the short-term sea-level oscillations. Exposure surfaces at the tops of peritidal and subtidal cycles represent one, o r more likely several, missed sea-level oscillations when the platform lay above fluctuating sea level, but the amplitude of fourth- to fift h-order sea-level oscillation(s) were not high enough to flood the ram p. The large number of missed beats (exposure-capped cycles), specific ally in Sequences 2 and 4, results in Fischer plots that show poorly d eveloped rising and falling limbs (subdued wave-like patterns); conseq uently the Fischer plots: are of limited use as a correlation tool for these particular depositional sequences. The abundance of missed beat s also explains why Milankovitch-type cycle ratios (similar to 5:1 or similar to 4:1) are not observed and why such ratios would not be expe cted along many peritidal-cycle-dominated carbonate platforms.