THE ONSET OF EXTENSIONAL FAULTING IN WESTERN PUERTO RICO AT 15 MA

Western Puerto Rico is marked by a basin and range topography underlain by an East-West striking, South dipping half graben system. The Northern portion of the island includes a Late Oligocene to Middle Miocene limestone cover sequence. The Late Oligocene units of the sequence are absent in extreme Western Puerto Rico where previous workers have interpreted the Miocene Cibao Formation to rest unconformably on Eocene arc basement rocks.

The fossil mollusk Kuphus incrassatus is abundant in the Cenozoic limestones of Puerto Rico. Strontium (Sr) is present as a trace element in CaCO₃ and replaces Ca in the Kuphus incrassatus skeleton. The ⁸⁷Sr/⁸⁶Sr value from an unaltered skeleton can be used to determine the approximate age of the limestone where the fossil is present. Kuphus skeleton tubes, oriented in growth position, were collected in the Cibao Formation in extreme Western Puerto Rico to determine the age of deposition, which is interpreted to mark the onset of half graben development. Petrographic thin sections were made to identify un-recrystallized areas within the specimens. After micro-drill sampling, stable isotopic analyses (δ¹⁸O, and δ¹³C) were carried out to identify suitable samples for Sr isotope analysis.

Stable isotopic analysis of three Kuphus incrassatus tubes produced δ¹⁸O and δ¹³C values acceptable for Sr analysis. Using a published seawater Sr isotope curve, the resultant analyses of two samples correspond to an age range of 15.10 - 15.20 Ma (Middle Miocene, Langhian). We interpret this age to represent the lowering of the land surface in extreme Western Puerto Rico below sea level caused by the onset of normal faulting and the development of the half graben geometry, which persists in the modern topography.

The Cibao Formation dated herein also locally includes a basal conglomerate unit. Careful study of cobbles at this horizon shows that their provenance likely lies to the south. This interpretation is consistent with concurrent deposition over a northward tilted land surface of a half graben just prior to limestone deposition.