PARAGENESIS OF FRACTURE- AND CAVITY-FILLING CARBONATE SEDIMENTS AND CEMENTS

The distribution of the different carbonate phases at a microscopic scale varies greatly from sample to sample and also within the same thin section. Although a detailed study of these features was beyond the purpose of this paper, the main phases of fracture and cavity filling by carbonate minerals are briefly described here with the purpose of better constraining the timing of formation of the micritic internal sediment and cement relative to basalt alteration and later phases of calcite precipitation. They are discussed in order of paragenesis (Fig. 3).

1. The first phases included initiation of fractures and alteration of basalts. Basalts in contact with the sediment preserve quenched margins with remnant spherulitic structures. Glasses in the contact zone have been completely altered to palagonite and/or smectite. Macroscopic observations indicate that fracturing events happened at different times, as evidenced by crosscutting and superposition of different generations of carbonate cements (Fig. 3). New fractures formed and continued to expand and propagate throughout the paragenetic sequence; they are postdated by progressively younger generations of carbonate cements (Fig. 4).
2. Pelagic internal sediment (micrite 1 [m1]) was infiltrated either during or immediately after the emplacement of the basalts (Fig. 4A). This is suggested by the presence of Campanian carbonate beds that are interbedded between Units E and F, which contain a large number of thick-walled benthic foraminifers typical of upper bathyal environments of the Late Cretaceous (see Sigurdsson, Leckie, Acton, et al., 1997).
3. The basalt was fractured, and fractures were filled by equant sparry calcite ("sparry calcite 1"), which occurs as the first phase of carbonate lining the fracture walls (Fig. 5D). This cement is rare and was observed in only a few samples. Crystals are typically 20-30 µm in size and are inclusion rich (Fig. 5D). This phase is nonsymmetrical in fractures and forms preferentially on the downward side of the fractures.
4. Further fracturing took place with subsequent precipitation/deposition of laminated internal sediments (micrite 2 [m2]) either as the first phase at the base of cavities (discussed in detail below) clearly postdating the alteration of basalts in seawater, or as a second phase in the older fractures containing equant sparry calcite as the first precipitate (Fig. 5A-D).
5. Finally, precipitation of different phases of sparry calcite cements, all referred to for simplicity purposes as "sparry calcite 2," took place. This cement postdates all previous generations and is the latest phase of cavity filling (Fig. 5A-D).