Throughout Late Cretaceous and earliest Tertiary times, large masses of volcanic material were deposited adjacent to and contemporaneously with thick limestone sequences in the Puerto Rican part of the Caribbean island arc. The detailed study of one of these limestones, the Parguera Limestone (Upper Cretaceous Mayaguez Group), and a study of available regional data, suggest a direct relation between tectonism and these lateral changes in rock type.

Late Cretaceous and Eocene deformation in western Puerto Rico consists of northwest-trending open folds cut by west-trending strike-slip faults. The regional overthrusting commonly ascribed to island arcs is not found in this area. Furthermore, the overthrusting initially suggested as a mode of emplacement for the Parguera Limestone is eliminated because of good correlation between the variations in rock fragments

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included in the basal limestone and the variations in the underlying volcanic rocks and serpentinite complex.

Tectonic isolation of the Parguera Limestone is suggested by the internal consistency of the unit, as opposed to the internal abrupt changes which characterize other rock types surrounding the Parguera, and by the contemporaneity of adjacent volcanic units. Contemporaneous tectonism and sedimentation as shown not only by the volcanic flows included in the Parguera but also by the thinning of the Parguera Limestone toward structurally high areas.

Vertical and lateral shifting of upper crustal blocks probably took place in response to deeper crustal movements. In a carbonate-producing environment, carbonate-rich sediments would accumulate on the higher standing blocks while lower areas would be flooded with volcanic debris. Such tectonic isolation of neighboring crustal blocks may have permitted adjacent and contemporaneous development of limestones and volcanic sequences throughout the developing Caribbean island arc.

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