A record of long- and short-term environmental and climatic change during OAE3: La Luna Formation, Late Cretaceous (Santonian–early Campanian), Venezuela

Abstract

The La Luna Formation was deposited under anoxic/dysoxic conditions in a tropical epicontinental sea on the northwest South America margin. Sedimentological, micropaleontological and geochemical evidence provides insights into factors that influenced the sedimentation and controlled the accumulation of organic-rich deposits at decimeter and meter scales during the youngest of the Cretaceous oceanic anoxic events (OAE). The La Luna Formation consists of an alternation of black marlstones interbedded with black limestones and black marly limestones. The benthic foraminifera assemblages indicate sedimentation in the upper neritic to upper bathyal environment. These rocks contain large amounts of organic matter. It is interpreted that a combination of warm global and rainy climate and the presence of bathymetric barriers caused poor circulation and low rates of water column ventilation during a high sea level in the early Santonian leading to the preservation of carbon-rich deposits in this region. During the late Santonian, a cooling-trend in global climate increased wind strength and upwelling; this change probably reduced runoff causing a weakening of the pycnocline and destabilized the stratification in the water column providing a progressive increase in oxygen in the water column and on the sea floor and a decrease in total organic carbon preservation in a shallower basin. These changes and the establishment of full mid- and deep-water exchange in response to the deepening and widening of the Equatorial Atlantic Gateway could have been important mechanisms for ending the epeiric sea anoxia. Changes through time in the vanadium–nickel fraction, planktonic and benthic foraminifera assemblages, productivity proxy elements, and lithological characteristics support this model. Superimposed on the general trend, variations in calcium carbonate and total organic carbon percentages at the scale of tens of centimeters reveal high frequency cyclic variations, which apparently correspond to the main frequencies of orbital forcing. This cyclicity is interpreted as a primary depositional signal and is the result of orbital-controlled fluctuations in terrigenous dilution and variations in oxygen concentration at the base of the water column. Variations in the intensity of upwelling can be observed, but they did not control the cyclicity.