Recent Sediments of Lake Maracaibo

Abstract

ABSTRACT This paper describes the Recent sediments of Lake Maracaibo, which attain a thickness of as much as 30 meters and consist predominantly of very fine-grained, loosely deposited, dark greenish-gray muds. Median grain diameters of the muds are of the order of 1 micron; densities vary between 1.20 and 1.40; and porosities (water contents) vary between 80 and 85 per cent. Interbedded with the muds is a widespread layer of diatomite. Peat occurs in the lower part of the Recent sediments, and fine sands, generally micaceous, are developed in some of the near-shore shallow areas. The Recent sediments rest unconformably on the brownish, tough clays of the Milagro Formation. C-14 determinations on the peats indicate ages of 7,000 to 10,000 years. Paleontological evidence shows that the lake throughout its Recent history has fluctuated from a fresh to a brackish body of water. The maximum salinity was reached during the rise of sea level that accompanied the postglacial climate optimum, when temperatures were warmer than they are today. The presence of swamp gas in the lake sediments was determined in samples taken with a specially designed core barrel, which secured samples under pressure. Ultrasonic velocities of the lake muds are three to six percent below the velocity of the lake water. On the other hand, the Recent sands and the Milagro clays have velocities more than seven percent greater than that of lake water. Ultrasonic velocities were found to be closely related to porosities. This relationship is described by Wood's equation for the velocity of sound in mixtures. Less scattering of points was observed in the acoustic impedance-porosity correlation than in the velocity-porosity correlation. Porosity of the unconsolidated sediments generally increases as the median grain diameter decreases.