Site 1001 of Ocean Drilling Program (ODP) Leg 165 was drilled in the Caribbean Sea on the Hess Escarpment of the lower Nicaragua Rise (Fig. 1) where Neogene sediments are thin and a continuous Cretaceous-Paleogene sedimentary section overlying basaltic basement could be recovered. One of the objectives at this site was to recover igneous basement and test models for the formation of the Caribbean Oceanic Plateau. The top of the basaltic sequence at Site 1001 was encountered at a depth of 485.4 meters below seafloor (mbsf) at a water depth of 3259.6 m. A 37.65-m-thick section of basaltic rocks, primarily of extrusive origin, was drilled in Hole 1001A. The basalts are overlain by Campanian age limestones and clayey limestones containing nannofossils that indicate a minimum age of 77 Ma (Sigurdsson, Leckie, Acton, et al., 1997).
The basaltic sequence in Hole 1001A was divided shipboard into 12 units (Units A through L), which represent individual lava flows and associated hyaloclastite breccias (Sigurdsson, Leckie, Acton, et al., 1997) (Fig. 2). Sinton et al. (Chap. 15, this volume) describe the petrology of the basalts and discuss their geochronology. These basalts are interpreted to represent sheet flows associated with volcanic events of high mass eruption rates (Sigurdsson, Leckie, Acton, et al., 1997). The hyaloclastite breccias on top of these flows, which are the boundary between the sheet flow and seawater, are expected to have been an area of vigorous thermal convection where the transfer of heat out of the lava may result in extensive mineralization.
The basaltic sequence is characterized by a pervasive system of carbonate-cement-filled fractures and vugs, which are particularly well developed in Core 165-1001A-54R (Fig. 2). Studies of secondary carbonate minerals found in oceanic basalts at Deep Sea Drilling Project and ODP sites provide constraints on the relative timing of carbonate mineral precipitation. Low-temperature (generally <80°C) basalt alteration in seawater takes place in three stages: (1) palagonite formation, (2) smectite formation, and (3) carbonate mineral formation (Boehlke et al., 1980; Staudigel et al., 1986).
Macroscopic observations on the samples recovered from Hole 1001A reveal that the stage of "carbonate mineral formation" in reality contains a variety of generations of carbonate internal sediments and cements characterized by complex paragenetic sequences. The purpose of this article is to describe the textures and the relative timing of occurrence of the carbonate internal sediments and cements deposited within the basalt sequence with particular focus on the origin of the internal sediment. Micritic internal sediment has often been described with geopetal textures within basalt sequences (Bernoulli et al., 1978) and ophicalcites (Folk and McBride, 1976; Bernoulli and Weissert, 1985), but its origin has remained controversial. Folk and McBride (1976), on the basis of spatial distribution, increasing abundance toward topographic highs, and the resemblance of features to caliche soils, have suggested a pedogenic origin for the micrite found in ophicalcites from the Italian Apennines. Bernoulli et al. (1978) recognized four different generations of carbonate internal sediment all of marine origin within Miocene basalts from Hole 373A, in the Tyrrhenian Basin. More specifically, they identified both an internal sediment derived from pelagic oozes and a diagenetic sediment precipitated and redeposited within the cavities in the presence of seawater.