A total of 339.30 m of basement was cored during Leg 185 (Cores 185-801C-13R through 52M), deepening Hole 801C to 935.70 mbsf. Recovery averaged 47.28%, with six cores having 80% recovery and six cores having 20% recovery (Figs. F5A, F5B, F6; Table T4). Cores 185-801C-14R through 17R at the top of the cored interval had good recovery (average = 71.85%), and Cores 18R through 26R had relatively poor recovery (average = 41.75%). Recovery improved again from Cores 27R through 31R (average = 66.44%) and then deteriorated toward the base of the hole. In general, the intervals of good recovery correlated with the presence of thicker massive flow units in the cores, whereas the intervals of poorer recovery coincided with zones of thinner and more fractured or altered units dominated by pillow basalts, interpillow material, and breccia (see Fig. F5A, F5B).
The rocks recovered from Hole 801C during Leg 185 are primarily aphyric basalts. Minor amounts of plagioclase and olivine were observed as phenocryst phases, but they were rarely abundant enough to permit the addition of a mineral modifier to the rock name. Rarely basalts contained 1%-2% phenocrysts, and these were classified as sparsely aphyric. The aphyric basalts include a mixture of thick massive flow units, thinner (<50 cm) pillow-dominated units, and some intermediate thickness (50 cm to 1 m) units that could represent either pillows or flows. In addition, a hydrothermal unit was encountered in Core 185-801C-16R, and several intervals of breccia, hyaloclastite, and interpillow or interflow recrystallized sediment were obtained throughout the cored section (Fig. F7).
The lithology in Hole 801C (see Fig. F5A, F5B) has been summarized, on the basis of basement stratigraphy and geochemical and logging data, into major eruptive sequences (numbered in roman numerals) comprising an upper section of alkali basalts (I) and tholeiitic massive flows (III) separated by a hydrothermal zone (II) at 510-530 mbsf. Beneath these massive flows, in the material drilled during Leg 185 (>590 mbsf), there is a section consisting of pillows and flows (IV) and a lower hydrothermal zone (V) at 625 mbsf. Between Cores 185-801C-26R and 27R, at 720 mbsf, there is a major change in the cooling unit thickness, which correlates with a break on the gamma and resistivity logs (Fig. F5B). This change marks the top of the lower sequence containing massive flows (VI). Beneath these flows (840 mbsf) there is a thick breccia sequence (VII), and toward the base of the hole (850-930 mbsf) the cooling unit thickness decreases again in the lower section, which consists of pillows and thin flows (VIII).
The last core retrieved from Hole 801C during previous drilling at this site, Core 129-801C-12R, is characterized by a 2.5-m-thick aphyric basalt flow. This flow was given the unit number 32. Lithologic units assigned during Leg 185 are numbered beginning with Unit 33 (i.e., numbering is continued from the end of Leg 129). In total, 28 units were identified, although most of these contain subunits of breccia or individual cooling units. Lithologic units were assigned on the basis of a major change in lithology or a change in basalt mineralogy. Some of these, for example Unit 50, are continuous over many cores (Cores 185-801C-17R through 28R); in this case subunits were identified on the basis of the presence of glassy contacts or fining of grain size toward the margins of flows. Thin intervals of sediment or interpillow material and breccia were noted as boundaries between subunits, whereas thicker intervals were distinguished as individual subunits when they were >5-10 cm.
The basement stratigraphic log (Table T4; also see 801_MIN.XLS in the "Supplementary Materials" contents list) includes all the igneous core description data for Leg 185 Hole 801C: unit or subunit number, depth, interval thickness, lithology, texture, structure, color, presence or absence of chilled margins, comments, and the location of shipboard samples. The log also includes the same information for Cores 129-801C-1R through 12R recovered during Leg 129. The igneous stratigraphy for all of Hole 801C (data from both Legs 129 and 185) is plotted as a stratigraphic column (Fig. F6; also see the Igneous Mineralogy Description Logs in the "Supplementary Materials" contents list). Primary igneous structure was described on the basis of the size of the units: units or subunits >1 m in thickness were denoted as flows, those between 50 cm and 1 m in thickness were noted as either pillows or flows, and those <50 cm in thickness were noted as pillows. Descriptions of the more important characteristics of Leg 185 Hole 801C lithologic units are presented in the following paragraphs.
The first core recovered, Core 185-801C-13R, contained only one section consisting entirely of debris that was probably dislodged from the walls of the hole at the end of Leg 129 or when logging during Leg 144. The debris included several pieces of aphyric basalt, some interpillow material, and a piece from the distinctive hydrothermal unit in Core 129-801C-4R (Unit 8). These pieces were assigned to Unit 33. Unit 34, observed in Core 185-801C-14R, consists of a 2-m-thick aphyric basalt flow, similar in character to Unit 32, which was drilled during Leg 129. The upper contact of this flow was not observed, and the lower contact of the flow in Unit 32 was not observed in Core 129-801C-12R; therefore, it is possible that these two units are continuous. Beneath this flow, still in Core 185-801C-14R, a sequence of pillow lavas (Units 35 and 37) was encountered, separated by cherty pelagic sediments (Unit 36). Unit 38 is also characterized by a sequence of pillows and thin flows. The pillows and/or thin flows vary in thickness from 11 to 68 cm. Curved and chilled or glassy pillow rims were commonly observed, and recrystallized cherty sediments (Fig. F8), hyaloclastite, and breccia intervals (Fig. F9) were commonly encountered between pillows. This sequence of thin pillows and interpillow materials (Fig. F10) continues into Core 185-801C-15R, Units 39-43. Units 44 and 45 both represent single flows; Units 46, 47, and 49 are mixed units containing thin flows, pillows, interpillow sediments, and breccias. Unit 48 in Core 185-801C-16R is a distinctive marker horizon useful for correlating the recovered core section with the downhole logs. It consists of a highly silicified yellow-brown hydrothermal deposit (Fig. F11), very similar in character to the hydrothermal deposit Unit 8 in Core 129-801C-4R. Both above and below the lower hydrothermal deposit, there are intervals of highly altered and recrystallized interflow sediments.
Unit 50 contains a sequence of aphyric basalt pillows and flows that vary in thickness, although thinner flow units and pillows predominate. The recovery of this unit is relatively poor, with the recovery possibly biased toward thicker units. Thus, there may be even more pillows and interpillow material within the cored section than we recovered. Varying proportions of interpillow recrystallized sediment (e.g., see Figs. F12, F13), hyaloclastite (Fig. F14), and breccia are observed between some of the cooling units, and where these intervals are thicker than 5 cm they are identified on Figure F6 as individual subunits. Contacts were not always observed; therefore, cooling units were defined using chilled or glassy pillow or flow margins (Fig. F15), or obvious changes in grain size. The subunits in Unit 50 of which there are 76, range up to 2 m in thickness with one thicker, 4.5-m flow found in Core 185-801C-27R.
Unit 51 (Cores 185-801-29R through 31R) is distinctive in that it contains two massive flows, 7.0 and 9.3 m thick, respectively. Units 52 and 53 are again mixed units, consisting of thin flows, pillows, and breccias (e.g., see Figs. F16, F17), although flow thickness appears to increase progressively toward the base of Unit 53. Unit 54 in Cores 185-801C-36R through 38R contains the thickest flow, 12 m in thickness. It is also the coarsest flow, microcrystalline at its margins grading to medium grained in the center. This flow contains an unusual distribution of vesicles (see Figs. F18, F19), a number of thick veins, and intensely altered patches (see "Basement Alteration"). Units 55, 58, and 60 are again mixed units, dominated by thinner flow and pillow units. Units 57 and 59 are single flows. Unit 56 is a 5.0-m-thick breccia interval, where the basalt clasts are angular to subangular, ranging in size from <1 to 40 cm. The larger clasts, however, are often fragmented and cut by veins. The cement is carbonate rich toward the top of the unit and saponite rich toward the base of the unit. Because of its thickness, we interpreted the breccia as tectonic and possibly fault related, as opposed to being a brecciated flow top or base. Slickenlines were observed on some vein surfaces within this unit, which lend support to the tectonic interpretation.
The contacts of many of the thicker and more massive flows were not recovered, and in these cases, cooling units were identified on the basis of the presence of chilled margins (e.g., Fig. F15), changes in grain size from fine grained to microcrystalline or hypocrystalline at the flow margins, or changes in lithology. Contacts between pillows were rare, although for some adjacent pieces both had glassy rims, as would be expected at the margins of stacked pillows. Fresh glass is present in several places (see Table T5), in particular in Cores 185-801C-18R, 28R, and 42R, nearly always associated with pillow rims or hyaloclastite intervals.
Grain size is fairly uniform downhole. Most of the basalts are microcrystalline, occasionally with hypocrystalline or glassy margins, whereas thicker units or more massive flows tend to grade toward fine grained in the center. Only the thickest flow, in Unit 54, reaches medium grain size (>1 mm). Small vesicles (up to 1 mm in diameter) are present in most of the samples, whereas larger vesicles (>5 mm in diameter) are more rare. Again, only Unit 54 contains a significant proportion of these larger vesicles, which are occasionally aligned along a specific horizon (e.g., see Fig. F18). The distribution of vesicles is often quite variable over short distances. For example, in Section 185-801C-37R-2, there is a change from an absence of vesicles to a significant proportion (5%-10%) of vesicles over a distance of only 20 cm (see Fig. F19).
Brecciated intervals are fairly common within the cored section. Both cement-supported and clast-supported varieties are present. Breccias were only recorded as units or subunits if the interval thickness was >10 cm. In most cases highly fractured and broken pillows with filled fractures were considered largely as in situ pillows; the term breccia is reserved for cement-supported breccias (see Fig. F17). Hyaloclastite intervals are encountered less commonly and over smaller intervals, particularly within pillow sequences (e.g., in Section 185-801C-19R-2). The hyaloclastites are often preserved on the outer edge of chilled or glassy pillow rinds, and they consist of glassy shards and fragments in a carbonate and saponite cement (Fig. F14). Some of the hyaloclastite breccia intervals (e.g., interval 185-801C-28R-3, 53-59 cm) contained fresh glass (Fig. F16).
Interflow or interpillow sediments occur in the upper part of the cored interval. The last appearance of chert as interpillow material downhole is in Core 185-801C-17R, at 637.2 mbsf. The interflow or interpillow sediments (see Fig. F8) are described in "Sedimentology and Biostratigraphy" and are, therefore, not included here. Interpillow material is distinguished from breccia by the absence of angular clasts of basalt and/or glass. Interpillow material (including metasediment, metavolcaniclastics, and alteration phases) occurs in Cores 185-801C-14R through 18R (Figs. F10, F11, F12, F13), 20R to 22R, 25R to 27R, and 32R. Below Core 185-801C-32R, interpillow materials were not observed, but volcanic breccias and hyaloclastites are present down to the bottom of the hole. Interpillow material is documented in detail (see the "Site 801 Vein Log," also in ASCII format) and is also noted in the igneous core description log (Table T4) for intervals >5 cm. Interpillow material makes up 2.5% of material cored in Hole 801C during Leg 185. The volume percent of interpillow material per core is shown in Figure F7 and is plotted with the volume percent of breccia intervals. Overall, the volume percent of interpillow material decreases downhole, from 19% to 1% for Cores 185-801C-15R to 18R. Beyond Core 185-801C-18R, until it disappears after Core 32R. Interpillow material ranges from none observed to 5% per core. The nature and occurrence of interpillow material is discussed in further detail in "Basement Alteration".
A massive, yellow-stained, silicified, Fe-rich hydrothermal unit (Unit 48) occurs in Sample 185-801C-16R-3, 0-43 cm (see Fig. F11). This unit is very similar to Unit 8 in Core 129-801C-4R (Shipboard Scientific Party, 1990), which was interpreted to be a hydrothermal deposit formed by precipitation of iron and silica from low-temperature hydrothermal fluids. However, sedimentary laminations were also observed (Shipboard Scientific Party, 1990), suggesting a sedimentary contribution. Sublinear, contorted laminations observed in Unit 48 also suggest a sedimentary contribution to this material, which was later cemented by silica.
The major part of the cored basement consists of aphyric basalts having either flow or pillow structure. Numbered from the base upward, the summarized volcanic sequences (Fig. F5A, F5B) in Hole 801C basement are pillow basalts and minor flows (VIII; >850 mbsf); a breccia sequence (VII; 840-850 mbsf); thick lava flows (VI), some of which approach 15 m in thickness with well-defined rubbly bases (720-850 mbsf); a pillow-dominated zone (IV) with well-developed interpillow horizons (595-720 mbsf); and an ocherous, Fe-Si-rich, low-temperature hydrothermal unit (V) occurring within Sequence IV (630 mbsf). The upper pillows and flows above the lower hydrothermal unit have abundant interpillow sediments, some of which contain radiolarians. The presence of pelagic sediments in the igneous units above signals quiescence in igneous activity, or possibly off-axis eruptions. Three sequences were defined for the section drilled during Leg 129. These are upper massive flows (III), upper hydrothermal sequence (II), and alkali basalts (I). The ocherous hydrothermal units are unique to Hole 801C. Although similar types of deposits exist near the modern East Pacific Rise and elsewhere on the seafloor, they have never been drilled elsewhere in oceanic basement. Fresh glass was recovered in more than 20 cores and is the oldest fresh glass recovered from the oceans.