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Geochemistry and chemostratigraphy of the Colón -mito juan units (campanian-maastrichtian), Venezuela: Implications for provenance, depositional conditions, and stratigraphic subdivision

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 paieoenvironmental 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, P 2O5, 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 ol 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. Copyright © 2013 by The Geochemical Society of Japan.