ABSTRACT The Upper Devonian Rhinestreet black shale of the western New York state region of the Appalachian Basin has experienced multiple episodes of overpressure generation manifested by at least two sets of natural hydraulic fractures. These overpressure events were thermal in origin and induced by the generation of hydrocarbons during the Alleghanian orogeny close to or at the Rhinestreet's ∼3.1 km maximum burial depth. Analysis of differential gravitational compaction strain of the organic‐rich shale around embedded carbonate concretions that formed within a metre or so of the seafloor indicates that the Rhinestreet shale was compacted ∼58%. Compaction strain was recalculated to a palaeoporosity of 37.8%, in excess of that expected for burial >3 km. The palaeoporosity of the Rhinestreet shale suggests that porosity reduction caused by normal gravitational compaction of the low‐permeability carbonaceous sediment was arrested at some depth shy of its maximum burial depth by pore pressure in excess of hydrostatic. The depth at which the Rhinestreet shale became overpressured, the palaeo‐fluid retention depth, was estimated by use of published normal compaction curves and empirical porosity‐depth algorithms to fall between 850 and 1380 m. Early and relatively shallow overpressuring of the Rhinestreet shale likely originated by disequilibrium compaction induced by a marked increase in sedimentation rate in the latter half of the Famennian stage (Late Devonian) as the Catskill Delta Complex prograded westward across the Appalachian Basin in response to Acadian tectonics. The regional Upper Devonian stratigraphy of western New York state indicates that the onset of overpressure occurred at a depth of ∼1100 m, well in advance of the Rhinestreet shale's entry into the oil window during the Alleghanian orogeny.
The Lomagundi-Jatuli Event (LJE) represents perhaps the largest magnitude and longest duration positive carbon isotope excursion of Earth history. However, the synchroneity, scale, and linkage of the LJE to Earth's early history of atmospheric oxygenation remain controversial. Strata of the Paleoproterozoic Jingshan Group of the North China Craton that preserve the isotopic record of the LJE excursion in marble layers and banded iron formation and abundant graphite deposits provide an opportunity to elucidate the significance of the LJE in early Earth history. Stable carbon isotopic values of the LJE based on analysis of 20 samples of the Lugezhuang Formation, lower Jingshan Group, range from -0.8 to +9.6 ‰ and display positive co-variance with stable oxygen isotope values. The positive carbon isotope excursion is constrained to 2140.6 ± 8.5 Ma (MSWD = 0.82, n = 25) based on magmatic zircon U-Pb geochronology of biotite granulite. Variation of facies-dependent carbon isotope values, the presence of graphite deposits of the Douya Formation, upper Jingshan Group, and the absence of a Ce anomaly in PAAS normalized REE patterns of marble samples suggest that the positive carbon isotope excursion is not linked to a marked increase of organic carbon burial and associated significant atmosphere oxygenation. Elevated concentrations of iron and PAAS-normalized middle REE enrichment of analyzed Jingshan Group marble samples point to anoxic and ferruginous oceanic conditions during accumulation of Jingshan Group carbonate. A positive Eu anomaly (average = 1.58), low La (average = 0.23), (Nd/Yb)N (average = 1.27), and Y/Ho (average = 36.6) anomalies, and negative iron isotope values (average δ56Fe = -0.12 ‰) are consistent with accumulation of Paleoproterozoic Jingshan Group carbonate in a restricted marine setting that was affected by high temperature hydrothermal fluids. Enrichment of the studied samples in heavy carbon suggests elevated bio-productivity in the restricted, redox stratified marine setting in which Jingshan Group marble accumulated. Thus, it is likely that the positive stable carbon isotope excursion associated with Jingshan Group strata as well as other contemporaneous isotope excursions are local signals that are not linked to a global perturbation of the carbon cycle.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)