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Filling in Gaps in the Sedimentary Record: An Integrated Study of Discontinuity Surfaces in Devonian Epeiric Sea Carbonates, Iowa

Abstract

Discontinuity surfaces develop in carbonate successions due to environmental changes that result in a hiatus in sediment accumulation or erosion of previously deposited material. Each discontinuity surface therefore, represents a gap in the sedimentary record. Although the depositional record is missing at an unconformity, the diagenetic record is continuous and contains information about the environment and climate leading up to, during, and after a depositional break. By studying discontinuity surfaces, it is possible to gain insight into the cause of these breaks in sedimentation and fill in these gaps in the sedimentary record. The Middle-Late Devonian Cedar Valley Group was deposited in the Iowa Basin during three third-order transgressive-regressive cycles that flooded the Laurentian craton, depositing subtidal and peritidal carbonates across Iowa and adjacent states. The apparent lack of systematic parasequence stacking patterns and evidence of missing depositional cycles in these cratonic ramp carbonates are thought to be the result of lowered sedimentation rates and frequent depositional hiatuses. Periodic oceanographic changes that resulted in inhospitable conditions in the shallow epeiric sea may have stressed the carbonate factory and contributed to these unique and condensed strata. To constrain the environmental factors that contributed to possible suppression of the Middle-Late Devonian carbonate factory in the Iowa Basin, this study integrates microfacies with geochemical analysis of discontinuity surfaces in the Little Cedar and Coralville Formations. Diagenetic overprints of past environments and processes can shed light on what variables control sedimentation in ancient epeiric seas.