ABSTRACT

A geochemical and chemostratigraphical study was undertaken on Campanian-Maastrichtian sedimentary rocks (the Colón-Mito Juan sequence and the upper La Luna Formation) in the southwestern Maracaibo Basin, Venezuela. The objectives of this work were to determine the paleoenvironmental and physico-chemical characteristics of the Colón-Mito Juan sequence and its possible subdivision into chemofacies and to study the main chemical differences between the Colón, Mito Juan, and La Luna Formations within the study region. One hundred and ninety-one rock samples were collected, and bulk inorganic geochemistry (TiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, P₂O₅, C, S, Rb, Cs, Ba, Sr, Th, U, Y, Hf, Mo, V, Cr, Co, Cu, Ni, Sc, La, Ce, Nd, Sm, Eu, Yb, Lu, As, Sb, Zn, and Be) was analyzed by instrumental neutron activation analysis or inductively coupled plasma-atomic emission spectroscopy; total sulfur and carbon analyses were performed by a LECO SC-432 apparatus and coulometry, respectively. Multivariate statistical techniques were applied to evaluate correlations within this group of variables. Using cluster-constrained analysis, eight subdivisions, or chemical facies, were defined: two chemofacies differentiating the intervals controlled by biogenic deposition and by the predominant clastic contribution; three chemofacies correlating with the lithologic units (La Luna, Colón, and Mito Juan); and another three chemofacies related to changes in the paleoredox conditions along the stratigraphic column. All of the units studied were deposited under a relatively constant climate regime, and the composition of the sediment source showed no significant changes. The prevailing physico-chemical regime was disoxic-oxic, with a trend of increasing oxygen concentrations towards the top of the column.