ABSTRACT

Ripple-mark-like structures with rounded crests and tight 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 ripple marks, show that these peculiar structures are modified wave-ripple marks. Ripple indices and the internal ripple 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 ripple marks by pressure solution.

The "tight trough ripple-marked" sandstones lie near the center of a broad, open syncline. The tight trough morphology exists only in ripple 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 ripple marks." Apparently, the troughs of the ripple 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 ripple 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.