Porosity and Cement Development In Pleistocene Key Largo Limestone, Key Largo Florida

Abstract

<p> A detailed petrographic study of the Pleistocene Key Largo Limestone Formation, Key Largo, Florida, was undertaken observing three units of varying ages. From oldest to youngest, these are: Q4A (age unknown); Q4B, (180,000 years B.P.), and Q5, (125, 000 years B.P.). Fifty thin sections from four cored wells were point counted. Porosity remained constant for all three units at 28.5% as did total cements at 27.5%. These are still very porous rocks relative to ancient carbonates, which generally show 0-2% porosity. Q5 is diagenetically less mature than Q4B and Q4A, with the persistence of aragonite (which is being leached) and solution-enhanced interparticle porosity. Q5 has been exposed to a vadose environment. Multiple generation cements in Q4B indicate two periods of vadose exposure coincident with known Pleistocene eustatic sea level fluctuations. Moldic porosity is predominant in Q4B. Vague horizons of macroscopic vuggy porosity act as pathways of directed flow for percolating fluids precipitating late stage cements and stains rich in iron. Q4A has been interpreted as a beachrock in part, deposited at pH's greater than 9, resulting in dissolution of quartz grains. Of special interest in Q4A is the probable exploitation of a quartz substrate by endolithic borers. </p>