Tight Trough Ripple Marks: Pressure-Solution Modification of Primary Structures

Abstract

ABSTRACT Ripple-mark-like structures with rounded crests and troughs are preserved in deltamargin sandstones of the Cerro Huerta Formation (Upper Cretaceous) in northeastern Mexico. The structures resemble substratal ripple-mark impressions and mullion structures. However, the internal laminae, symmetry, and lung, parallel crests, as well as an association with normal marks, show that these peculiar structures are modified wave-ripple Ripple indices and the internal stratification indicate that the structures were not produced by syndepositional modification or by soft-sediment deformation. Rather, these bedding-surface structures resulted from modification of oscillatory marks by pressure solution. The tight trough ripple-marked sandstones lie near the center of a broad, open syncline. The trough morphology exists only in marks aligned parallel or subparallel to the local orientation of the regional axial-plane cleavage. Pressure-solution cleavage folia extend along the troughs of the tight trough marks. Apparently, the troughs of the marks controlled the location of cleavage. In this structural setting the maximum compressive stress would be oriented along layering normal to the fold axis. During folding stress (force/area) would be highest at the troughs (least cross-sectional area), and, therefore, cleavage would initiate at the troughs. Thus, volume loss during formation of the pressure-solution cleavage was localized in the troughs, acc unting for the tight trough ripple morphology.