Beyond black shales: The sedimentary and stable isotope records of oceanic anoxic events in a dominantly oxic basin (Silurian; Appalachian Basin, USA)

摘要

Episodes of instability in the global climato-oceanic system have become the hallmark of the Silurian Period. These events are marked by six large positive carbon isotope excursions associated with major extinctions. The widespread environmental consequences of these perturbations of the global carbon cycle remain poorly documented. A high-resolution chronostratigraphic reconstruction of the Appalachian foreland basin (AFB) provides a framework to evaluate environmental signatures of two of these carbon isotope excursions (Valgu and Ireviken events; Telychian-Sheinwoodian) at a regional scale. Integration of published biostratigraphic range data, mapping of sequence stratigraphic surfaces and facies, and generation of high-resolution carbon isotope profiles for several transects provides precise correlation of the basin stratigraphy yielding a high-resolution temporal framework. This chronostratigraphic reconstruction of the AFB highlights the importance of time-specific facies (TSF) during these intervals. For instance, packages of red, green, and gray marine rocks separately occupied discrete time intervals-cutting across "local" facies belts indicative of shallow to deep marine environments. Similarly, widespread iron mineralization occurred during discreet time slices as "bathtub rings" rimming the margins of the basin. Changes from one time-specific facies to the next are coincident with changes in carbon isotope values, indicating a genetic link between fractionation of the global carbon reservoir and redox changes from generally oxic to anoxic oceanic conditions in parts of the AFB. The basal part of the study interval was deposited during the Valgu Event. This first event begins with an unconformity marking global lowstand. This fall in sea level was coincident with rising carbon isotope values as recorded in adjacent basins. A slight sea level rise and flooding of the subaerial unconformity occurred near the peak in carbon isotope values. Oxidized iron minerals were deposited in a zone that extended from the lower shoreface into the basin center-pinching out into shallower parts of the basin. Thin pyrite-phosphorite-glauconite layers, associated with black shales in the basin center, were deposited over these ferruginous strata. As the event ended and carbon isotope values shifted negatively to a more stable baseline, temperatures warmed, sea level reached a Silurian high, and oxic conditions associated with very low rates of organic carbon (OC) burial were established. These conditions were relatively stable for nearly four million years. This long period of stasis was punctuated by the Ireviken Event, which displays a strikingly similar progression of TSF patterns, yet more intense and longer lived than the preceding Valgu Event. With the first positive shift in carbon isotopes at the onset of the Ireviken Event, the basin redox system changed from largely oxidizing to reducing, coincident with a rapid drop in sea level and an abrupt reappearance of iron mineralization on the basin margins. The sea level fall associated with the shift to peak values of the carbon isotope excursion is marked by the progradation of a significant body of sand well out into the basin. During this sea level drop, conditions in the basin center became highly reducing and periodically sulfidic. Widespread ironstones and ankeritic carbonates were deposited isochronously, rimming the shallow basin margins where they alternated with light gray pyritic shales, indicating fluctuating redox conditions. A large negative carbon isotope excursion of ~. 3‰ is coincident with a shift from regression to transgression, suggesting glacial melting via methane release. Continued deposition of ironstones through this time interval suggests a primary link to shallow marine redox processes rather than large-scale transgression or regression of the shoreline.