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The AAPG/Datapages Combined Publications Database
Sedimentation and Tectonism in the Upper Cretaceous Puerto Rican Portion of the Caribbean Island Arc
Charles C. Almy, Jr.
Throughout Upper Cretaceous and Lowermost Tertiary time, large masses of volcanic material were deposited adjacent to, and contemporaneously with, thick limestone sequences in the Puerto Rican portion of the Caribbean Island Arc. The detailed study of one of these limestones, the Parguera Limestone (Upper Cretaceous Mayaguez Group), combined with available regional data, suggests a direct relationship between the lateral changes in rock type and tectonism.
Upper Cretaceous and Eocene deformation in western Puerto Rico consists of northwesterly trending open folds cut by westerly trending strike-slip faults. The regional overthrusting commonly ascribed to island arcs and specifically to the emplacement of the Parguera Limestone is not found in this area.
Tectonic isolation of the Parguera Limestone is suggested by the internal consistency of the unit versus abrupt changes to other rock types on its margins and by the contemporaneity of adjacent volcanic units. Contemporaneous tectonism and sedimentation is shown not only by the included volcanic flows but also by the thinning of the Parguera Limestone towards structurally high areas.
Orientation of major geologic structures in Puerto Rico suggest that southwesterly directed horizontal compressive stresses were resolved along vertical shear fractures rather than on low-angle thrust faults. Such stresses might arise from a North American-Atlantic plate of crust and upper mantle moving west-southwesterly relative to the Caribbean block in accordance with the theory of global plate tectonics.
Shifting of upper crustal blocks in a carbonate-producing environment would cause carbonate-rich sediments to accumulate on the higher-standing blocks while lower areas would be flooded with volcanic debris. Such tectonic isolation of neighboring crustal blocks permitted adjacent and contemporaneous development of limestones and volcanic sequences throughout the developing Caribbean Island Arc and may have resulted from Cretaceous-Tertiary continental drift.
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