Geochemical and mineralogical records of late Albian oceanic anoxic event 1d (OAE-1d) in the La Grita Member (southwestern Venezuela): Implications for weathering and provenance

Abstract

In this paper, stratigraphic distributions of major, trace and rare earth element (REE) concentrations as well as the bulk and clay mineralogy of organic-rich calcareous rocks of the La Grita Member (Capacho Formation) in southwestern Venezuela were studied to constrain sedimentary depositional conditions during the late Albian-early Cenomanian oceanic anoxic event 1-d (OAE-1d; 103 to 99.5 Ma). The statistically significant correlations observed between total REEs and Al, total organic carbon (TOC) and phyllosilicates suggest that REEs mainly occur in clay minerals and organic matter. Ce anomalies are mainly controlled by detrital input and cannot be used as redox proxies in this stratigraphic sequence. Detrital proxies, notably phyllosilicate/(K-feldspar + plagioclase), K-feldspar/(K-feldspar + plagioclase), and kaolinite/illite, suggest that greenhouse warming established during the OAE-1d intensified the hydrologic cycle and consequently increased continental chemical weathering rates. The Zr/Sc ratio and Al2O3–TiO2–Zr ternary diagram suggest detrital input dominated by recycling of older sedimentary rocks, while the La–Th-Sc ternary relationship indicates that most sediment samples were deposited between a passive margin and a continental island arc setting. Sedimentary discrimination diagrams (including La–Th-Sc and Co/Th vs. La/Sc), combined with chondrite-normalized REE patterns, suggest that siliciclastic sediments of the La Grita Member were mainly supplied by metasedimentary and intracratonic rocks of the Guayana Craton, notably the Roraima Formation, suggesting that this area was the main source for siliciclastic sediments during the Cretaceous. The Santander Massif and Merida and El Baúl Arches likely acted as secondary provenance areas, confirming previous work using U–Pb detrital zircon geochronology.