Ocean Drilling Program, United States

Ocean Drilling Program Hole 990A penetrated 131 m of subaerially emplaced Paleocene flood basalts on the Southeast Greenland margin with a recovery of 74%. Shipboard P-wave velocity (V (sub p) ), density, and magnetic susceptibility were measured with 2- to 15-cm intervals on the core. Individual flow units were divided into four zones based on the observed petro-physical characteristics. From the top, these are Zone I (<7 m thick with a V (sub p) of approximately 2.5 km/s), Zone II (3-5 m thick with a strongly increasing V (sub p) from 2.5 to 5.5 km/s), Zone III (up to 20 m thick with a V (sub p) of approximately 5.5-6.0 km/s), and Zone IV (<2 m thick with a strongly decreasing V (sub p) from 6.0 to 2.5 km/s). Eighteen samples were selected from three of the fourteen penetrated basalt units for geochemical, petrological, and petrophysical studies focusing on the altered, low-velocity upper lava Zones I and II. Zone I is strongly altered to >50% clay minerals (smectite) and iron hydroxides, and the petrophysical properties are primarily determined by the clay properties. Zone II is intermediately altered with 5%-20% clay minerals, where the petro-physical properties are a function of both the degree of alteration and porosity variations. Shipboard and shore-based measurements of the same samples show that storage permanently lowers the elastic moduli of basalt from Zones I to III. This is related to the presence of even small quantities of swelling clays. The data show that alteration processes are important in determining the overall seismic properties of flood basalt constructions. The degree and depth of alteration is dependent on the primary lava flow emplacement structures and environment. Thus, the interplay of primary emplacement and secondary alteration structures determine the elastic properties of basalt piles. Rock property theories for sand-clay systems are further used to model the physical property variations in these altered crystalline rocks.