Sequence stratigraphy and platform evolution of Lower–Middle Devonian carbonates, eastern Great Basin

Abstract

Lower–Middle Devonian carbonates (270–400 m thick) of the eastern Great Basin were deposited along a low-energy, westward-thickening carbonate platform. Six regional facies representing peritidal, shallow subtidal, stromatoporoid biostrome, deep subtidal, slope, and basin environments are recognized. Four third-order (≈1.5–2.5 m.y. durations), transgressive-regressive sequences are identified across the platform-to-basin transition based on deepening and shallowing patterns in regional facies, intensity and stratigraphic distribution of subaerial exposure features, and stacking patterns of fourth- to fifth-order, upward-shallowing peritidal and subtidal cycles. Transgressive systems tracts along the basin/slope are characterized by upward-deepening 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. Maximum flooding zones along the shallow platform are composed of stacked peritidal cycles dominated by subtidal facies, noncyclic deep subtidal facies, or distinct deeper subtidal units within successions of restricted shallow subtidal or peritidal facies. Highstand systems tracts along the basin/slope are composed of hemipelagic deposits overlain by distal through proximal turbidites. Highstand systems tracts along the shallow platform are characterized by upward-shallowing succession of cyclic peritidal through shallow subtidal facies. Sequence boundary zones (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 intensity of cycle-capping subaerial exposure features. Sequence boundary zones along the basin/slope (6–20 m thick) are composed of upward-shallowing successions of proximal turbidites or by platform-margin peloid shoal 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 than tectonic subsidence rates. Sequence stratigraphic correlations between contrasting facies belts of the basin/slope (section NA) and the edge of the shallow platform (section TM) were independently verified with high-resolution conodont and brachiopod biostratigraphy. Correlation of sequences 1–4 with transgressive-regressive sequences of similar age in the western, midwestern, and eastern United States, western Canada, and Europe indicates they are eustatic in origin. Systems-tract scale correlations across the study area indicate that the platform evolved from a homoclinal ramp to a distally steepened ramp, then into a flat-topped platform (sequences 1–2). An incipiently drowned, intraplatform basin developed during sequence 3 as the result of third-order sea-level rise and differential sediment accumulation rates between the platform margin and intraplatform basin. During deposition of highstand systems tract 3, progradation infilled the intraplatform basin, resulting in a flat-topped platform. A distally steepened ramp developed during transgressive systems tract/maximum flooding zone 4 and evolved into a flat-topped platform during highstand systems tract 4 deposition. The four sequences stack in an aggradational to slightly progradational pattern (“keep-up” style sedimentation) and are bound by sequence boundary zones rather than unconformities, suggesting that greenhouse climate modes and second-order accommodation gains related to the lower portion of the second-order Kaskaskia sequence controlled sequence-scale stacking patterns.