LIMESTONES FROM GUAM: A PREDICTABLE PATTERN OF DIAGENESIS?

The Miocene-Pleistocene limestones of Guam were studied to determine whether the diagenetic condition of the rocks is related to their age, stratigraphic position, or proximity to karst features. Analysis of more than 100 samples from outcrops and quarries revealed that diagenetic alteration is advanced. There is no apparent relationship between degree of alteration and the age of the rocks, or proximity to karst features or paleosols.

Fifty four thin sections were obtained from a deep core (Ex5-a) drilled to 183 meters depth in the Miocene-Pliocene Barragada Limestone in northern Guam. Guam is tectonically uplifted with near-vertical cliffs up to 183 meters high. The Barragada limestone represents the early stages of the shallowing-up of an emerging submarine plateau. Point counts of the thin sections reveal a trend of greater porosity and less diagenetic micrite with depth. However, the trend is not statistically robust. Preservation of allochems is greatest in samples from near the top and bottom of the core. Moldic porosity developed by dissolution of corals and forams plays an important role in porosity development, but vuggy and interparticle porosity is also common. It is only in the middle of the core where precipitation of spar crystals infilling the voids is evident, and that is most likely the result of fluctuating freshwater lens position.

The degree of diagenesis is generally related to the time the rock has spent in the freshwater phreatic and vadose environments. Lesser diagensis of the upper core may have resulted from rapid uplift early in the exposure that quickly moved those rocks above the zone of diagenesis near and within the freshwater lens. However, because of widespread karst development (from micropores to caves) the degree of alteration is not simply a function of depth.

One cannot predict the degree of diagenetic alteration of the rocks of the Northern Guam aquifer. The porosity and permeability are not simply related to any known variables. The preferred pathways for infiltration of freshwater, the rate of infiltration, and the rate of discharge from the margins of the freshwater lens are probably locally controlled and highly variable. In order to protect the Northern Guam Aquifer, Northern Guam should be developed in an environmentally conservative manner.