SEQUENCE STRATIGRAPHY AND PLATFORM EVOLUTION OF LOWER-MIDDLE DEVONIANCARBONATES, EASTERN GREAT-BASIN

Lower-Middle Devonian carbonates (270-400 m thick) of the eastern Grea t Basin were deposited along a low-energy, westward-thickening carbona te platform. Six regional facies representing peritidal, shallow subti dal, stromatoporoid biostrome, deep subtidal, slope, and basin environ ments are recognized, Four third-order (approximate to 1.5-2.5 m.y. du rations), transgressive-regressive sequences are identified across the platform to-basin transition based on deepening and shallowing patter ns in regional facies, intensity and stratigraphic distribution of sub aerial exposure features, and stacking patterns of fourth- to fifth-or der, upward-shallowing peritidal and subtidal cycles. Transgressive sy stems tracts along the basin/slope are characterized by upward-deepeni ng successions of proximal through distal turbidites overlain by fine- grained, hemipelagic deposits. Shallow-platform transgressive systems tracts are composed of stacks of thicker-than-average peritidal cycles overlain by subtidal cycles or noncyclic deep subtidal facies, Maximu m flooding zones along the shallow platform are composed of stacked pe ritidal cycles dominated by subtidal facies, noncyclic deep subtidal f acies, or distinct deeper subtidal units within successions of restric ted shallow subtidal or peritidal facies, Highstand systems tracts alo ng the basin/slope are composed of hemipelagic deposits overlain by di stal through proximal turbidites. Highstand systems tracts along the s hallow platform are characterized by upward-shallowing succession of c yclic peritidal through shallow subtidal facies. Sequence boundary zon es (2-16 m thick) along the shallow platform are composed of exposure- capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and increases in the intensi ty of cycle-capping subaerial exposure features. Sequence boundary zon es along the basin/slope (6-20 m thick) are composed of upward-shallow ing successions of proximal turbidites or by platform-margin peloid sh oal deposits; the absence of exposure features and meter-scale cycles within basin/slope sequence boundary zones indicates that the combined rates of third- through fifth-order sea-level fall rates were less th an tectonic subsidence rates. Sequence stratigraphic correlations betw een contrasting facies belts of the basin/slope (section NA) and the e dge of the shallow platform (section TM), were independently verified with high-resolution conodont and brachiopod biostratigraphy. Correlat ion of sequences 1-4 with transgressive-regressive sequences of simila r age in the western, midwestern, and eastern United States, western C anada, and Europe indicates they are eustatic in origin. Systems-tract scale correlations across the study area indicate that the platform e volved from a homoclinal ramp to a distally steepened ramp, then into a flat-topped platform (sequences 1-2), An incipiently drowned, intrap latform basin developed during sequence 3 as the result of third-order sea-level rise and differential sediment accumulation rates between t he platform margin and intraplatform basin, During deposition of highs tand systems tract 3, progradation infilled the intraplatform basin, r esulting in a Bat-topped platform, A distally steepened ramp developed during transgressive systems tract/maximum flooding zone 3 and evolve d into a flat-topped platform during highstand systems tract 4 deposit ion, The four sequences stack in an aggradational to slightly prograda tional pattern (''keep-up'' style sedimentation) and are bound by sequ ence boundary zones rather than unconformities, suggesting that greenh ouse climate modes and second-order accommodation gains related to the lower portion of the second-order Kaskaskia sequence controlled seque nce-scale stacking patterns.