Ash layers, hyaloclastite, and alteration of basaltic glass at DSDP Leg 65 Holes

Three types of tephra deposits were recovered on Leg 65 of the Deep Sea Drilling Project (DSDP) from three drill sites at the mouth of the Gulf of California: (1) a series of white ash layers at Sites 483, 484, and 485; (2) a layer of plagioclase- phyric sideromelane shards at Site 483; and (3) an indurated, cross-bedded hyaloclastite in Hole 483B. The ash layers in (1) are composed of colorless, fresh rhyolitic glass shards with minor dacitic and rare basaltic shards. These are thought to be derived from explosive volcanoes on the Mexican mainland. Most of the shards in (2) are fresh, but some show marginal to complete alteration to palagonite. The composition of the glass is that of a MORB-type tholeiite, low in Fe and moderately high in Ti, and possibly erupted from off-axis seamounts. Basaltic glass shards occurring in silt about 45 meters above the basement at Site 484 A in the Tamayo Fracture Zone show a distinctly alkalic composition similar to that of the single basement basalt specimen drilled at this site. The hyaloclastite in (3) is made up chiefly of angular sideromelane shards altered to smectite and zeolites (mainly phillipsite) and minor admixtures of terrigenous silt. A very high K and Ba content indicates significant uptake of at least these elements from seawater. Nevertheless, the unusual chemical composition of the underlying massive basalt flow is believed to be reflected in that of the hyaloclastite. This is a powerful argument for interpreting the massive basalt as a surface flow rather than an intrusion. Glass alteration is different in the glassy margins of flows than in thicker glassy pillow rinds. Also, it appears to proceed faster in coarse- than fine-grained sediments.

(Table 1) Age and depth of reflectors of the 3.5-kHz and airgun seismic record and their corresponding depth of ash layers, carbonate peaks, and transition zones from DSDP Hole 83-504B

DSDP cores from areas of low (Site 505) and high heat flow (Site 504 B) near the Costa Rica Rift, together with seismic profiles from the Panama Basin, have been studied to determine the relationship between: (1) carbonate content and physical and acoustic properties; and (2) carbonate content, carbonate diagenesis and acoustic stratigraphy. Except for ash and chert layers, bulk density correlates strongly and linearly with carbonate content. Velocity is uniform downcore and only small variations at a small scale are measured. Thus an abrupt change in carbonate content will cause abrupt changes in acoustic impedance and should cause reflectors that can be detected acoustically. A comparison of seismic profiler reflection records with physical properties, carbonate content and reflection coefficients indicates that the main reflectors can be identified with ash layers, diagenetic boundaries, and carbonate content variations. Diagenesis of carbonate sediments is present at Site 504B in a 260 m-thick ooze-chalk-limestone/chert sequence. These diagenetic sequences occur in areas of higher heat flow (200 mW/m**2). Seismic profiler records can be used to map the extent and depth of these diagenetic boundaries.