Cretaceous oceanic red beds (CORBs) are mainly pelagic red shales, marls, or fine-grained limestones. These facies have been the subject of two closely related International Geosciences Programs GCP 463 and 494. They are a significant facies of deep-water pelagic deposits and pelagic-hemipelagic sedimentary systems. A major contribution of these two three-and five-year projects is that CORBs are globally distributed in outcrops in Europe, Asia, Africa, New Zealand, Caribbean, and at DSDP and ODP sites in the Tethyan Atlantic, Pacific, and Indian oceans. CORBs experienced two paleogeographical expansions, (1) in Aptian nannofossil zone CC7 shortly after the OAEla and (2) in Turonian zone CC11 after the OAE2, respectively. Lower Cretaceous CORBs are much less common than Upper Cretaceous. Above OAE2, CORBs in zone CC11 crop out in 25 basins or tectonic zones; CORB distribution is greatest in Coniacian-Early Santonian zones CC13-CC15 in up to 34 basins or tectonic zones in the world. The ages of CORBs are constrained by paleontological data including planktic foraminifera and nannofossils in calcareous CORBs, and agglutinated foraminifera and dinoflagellates or radiolaria in noncalcareous CORBs. The biostratigraphic data were assembled into an integrated, testable data base by graphic correlation. The data base was then correlated to GSSPs and reference sections of Cretaceous stages. Three general facies types of CORB are defined using the end members clay, carbonate, and chert: deep-water red claystones deposited below the calcite compensation depth, red hemipelagic and pelagic carbonates, and red cherts and radiolarites. The depositional environment of most CORB units was in relatively deep oceanic basins. Deposition was generally far from shoreline and only locally associated with coarse terrigenous clastics such as turbidites. Significant controlling factors of CORBs were slow sediment accumulation rates at great paleo–water depths. Like most marine sediments, CORBs are a complex mixture of terrigenous detritus and seawater-derived material. According to data reported from the studied localities, the common geochemical properties of CORBs are their extremely low organic-carbon content and high level of ferric oxides. The ratio between ferric oxides to the total iron is not only higher than the level of adjacent non-red sediments, but also higher than that of Phanerozoic normal marine oxic sedimentary rocks. These chemical properties indicate that CORBs were deposited in environments that were highly oxic at or below the sediment-water interface. Other major and trace elements and isotopic data suggest an oxic, oligotrophic water mass having overall low productivity. The paleoceanographic and paleoclimatological conditions are corroborated by carbon stable-isotope data, phosphorus burial records, dissolved-oxygen index, and comprehensive geochemical modeling results. Paleoclimate, paleogeography, ocean currents, and nutrient flux, among other processes, were related to the deposition and wide distribution of CORBs during the Late Cretaceous. The development of the paleogeographic configuration and Late Cretaceous climate cooling provided the basis for increasing ventilation of the deep ocean. The behavior of redox-sensitive nutrient elements like phosphorus further stimulated the development of oligotrophic conditions. All of these factors contributed to the global distribution of CORBs.

The increasing proportion of unconventional worldwide energy demands have consistently promoted the necessity for exploring a precise, high-resolution, objective, and quantitative stratigraphic division method for macroscopically homogeneous mudstone successions. The chemostratigraphy can resolve this problem well, although it has been applied successfully in North America, but not systematically studied in China for shale gas exploration and development. This work has conducted a chemostratigraphic analysis of Wufeng and Longmaxi Formation on the Changning section of Sichuan Province, southwestern China, to testify its applicability for shale gas exploration in China. Principal component analysis (PCA) was first employed to reduce the dimensionality of datasets. Three chemofacies, including detrital (K, Ti, Fe, Al, Na, Mg, Cr, Zr, Rb), authigenic (Ca, Sr, Mn, Si, S, Ba), and redox-organic (P, V, Ni, Zn, Cu, TOC), were found. Subsequently, constrained clustering analysis was utilized for the zonation of each chemofacies into chemozones. Consequently, the whole Changning section was divided into twelve chemozones (CZ I–CZ Ⅻ). The geochemical interpretation for these chemozones can be resolved from the regional changes in paleogeography and paleoceanography during the Late Ordovician to Early Silurian period. Thus, a three-stage geochemical evolution along the Changning section can be classified: (1) the siliceous and anoxic deposits of Wufeng Formation (CZ I–CZ III) with high TOC contents; (2) the siliceous and anoxic sedimentary rocks of bottom Longmaxi Formation with still higher TOC (CZ Ⅳ); (3) the calcarous-detrital and oxic sediments for the rest of Longmaxi Formation (CZ Ⅴ–CZ Ⅻ). In considering their high content of TOC and abundant brittle siliceous minerals, the CZ (I–Ⅳ, 0 m–33.6 m) are thought to be the most preferable sweet spot for shale gas exploration.