Abstract

Excellent coastal exposures of middle to upper Eocene deep-water deposits of the Brito Formation (Sandino basin) allow a detailed geometrical and architectural analysis of coarse-grained, channel-levee complexes. Interpretations are based on photomosaics and lithological logging of the up to 1000-m- (3280-ft)-thick sedimentary succession in southwestern Nicaragua. Two types of channel geometries have been observed, showing U-shaped or lenticular forms. Channel aspect ratios range between 1:1 to 2:1 and up to 86:1, respectively. Individual channels occur as isolated features or are organized into larger scale channel complexes showing an offset (nested) or vertical stacking pattern. The channels are filled with coarse-grained volcaniclastic conglomerates, pebbly sandstones, and sandstones deposited from cohesive debris flows, high-density turbidity flows, and sandy debris flows. Associated levee deposits consist of very thin- to medium-bedded, sandy turbidites with climbing-ripple cross-lamination, convolute bedding, and rip-up clasts. Channel geometries and facies characteristics indicate three different channel styles, which relate to phases of erosion, deposition, and channel abandonment. Some channel-levee systems have been affected by gravitational faulting. Two different types of faults that are not compatible with the regional stress field have been recognized: 1) faults parallel with channel margins; and 2) faults perpendicular to the channel axis, probably causing a segmentation and compartmentalization of channel fills.